Kitz Forum
Chat => Chit Chat => Topic started by: scottiesmum on September 19, 2010, 10:29:30 AM
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'Allo 'Allo to all you lazybones >:D from chez nous .... I expect you are all actually busy, busy really :no: :D
I know Unc is back, from his post yesterday evening, welcome back Unc :flower:
I've done the ironing out on the terrace this morning ( and no, I've not ironed the terrace ;D ) .... so now feel quite justified in tootling off to an antique fair later.
Have a lovely Sunday everyone, especially Chrissie on her Pearl Anniversary ...enjoy the family tea party :)
Take care all ! :)
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Good Morning Scottiesmum and the rest who follow later,
busy it has been, such a toil to raise the duvet. :D Greeted by :rain: when I opened the curtains.
I have a pile of ironing waiting to be done, actually it's been waiting for about a week now. Must get down to doing it.
Welcome back UncleUB, I hope you and Sue had nice time away.
Not much else on today.
All the best
Colin No.2
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Good afternoon scottiesmum, ss44 and any late comers who may call in. ;D
Cloudy but very mild, and TD has just had a lovely ramble along the riverbank.....spotted a kingfisher.. :thumbs:
Feeling tired now so off for a wee nap.
Enjoy the ironing and the antique fair, and a good day to all. :)
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Good afternoon all :)
Its nice to back. :)
We had a good week,the weather was a bit hit and miss,but it never rained while we were out.always when when we were in the tent.The new tent stood up well to the wind and rain and not one single drop got in.
We visited some lovely places,one being Flatford Mill.Had a few camera problems which spoiled some shots.Will post some later for analysis.I think the problem was there could have been some dust on the lens. :-[
I trust everyone is well and have had a good week. :)
The smudge seems to be on the church tower (St Peter and St Paul Church,Lavenham)
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Flatford Mill with smudge to the right
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Not too bad here
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Flatford Mill again....looks ok?
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Lovely photos, unkyUb.........but lets come back to them later, and in the meantime lets think of the blobs..
Were you using the Sony for these shots, and did you change lenses between the shots.
I ask because the problem does not manifest itself on the closer shots but it does on the wider shots.........so, were the first two taken on your short zoom, and the others on your longer zoom.
If so, you had a smudge or fleck of dust on the short zoom.......and if you cannot spot that on the front element of the lens, what about the rear element.
If you used the same lens all the time, then the culprit may be a spot of dust inside the camera and landing occasionaly on the sensor.....but such a problem would normally show itself as distinct black/grey blobs rather than the diffused effect you can see here.
Anyway, one step at a time...Sony, and did you change lenses for some of these shots ?
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Hi TD
I used the same lens all the time for those shots.The church ones were taken on Tuesday and the Mill ones were taken on Friday.
Its got me baffled as it doesn't show up on all my shots,only a few.I did twist the lens to zoom in/out,but didn't swap to my longer lens.
I am thinking I need to invest in a cleaning kit...any recommendations? The Sony does have a self cleaning mode,but I think also I need one of those air blower thingys
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These church shots were taken the same day as the above church image
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This was very interesting..Holy Trinity Church,Long Melford.
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Coo-Cou Unc :) Lovely photos ! I have one of those blower brushes and I use hubby's spectacle lens cleaner tissues to wipe the lens from time to time ....and then give the whole camera a wipe over with the same tissue, so as to get full use from it ;) I find that they do the trick OK.
Ooooo just spotted the latest photos . I love the topiary too ..... :)
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I'm probably way off here, but I suppose those smudges aren't slight condensation on the lens, are they? The sort of thing you get if you breathe on the lens?
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I'm probably way off here, but I suppose those smudges aren't slight condensation on the lens, are they? The sort of thing you get if you breathe on the lens?
That's just what Sue has said Eric.Although she had noticed a finger mark on the lens,but that wouldn't tie into to some being ok and some having marks on them.....would it?
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Aldeburgh,Suffolk coast......45 mph winds that day.(Wednesday)
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TD Relative? ;D
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What is odd about the blobs is that they are appearing out of sequence.....there on dsc00505 and dsc00570....not there on dsc00569 and dsc00582 .....so it appears to be comong and going.
It certainly looks like the sort of thing you would get with either condensation or a spot of rain......but even if you had a spot of rain on one frame..it dries...you would still get a residual bit of flare on the next frame, and yet here we have it not on 569 but it is on the next 570.
Of course it may have been spitting a bit on and off........but the blurred bit appears the same size and shape and in the same position on the occasions that it does appear.
TD trundles off to put on thinking cap again.
Aha...........just noticed your post, unkyUb, and Sue's mention of a smudge >>but that wouldn't tie into to some being ok and some having marks on them.....would it?<<< Yes, yes it could !!........Now the, were you using the lens hood ?
In the photos where the very bright sky predominates, the blob is there......where the sky does not predominate, such as the closer shots and the interiors, it is not there. Sooo,.............. I think what we have here is lens flare.
The smudge has been there all the time but the light has caught it and caused flare only under specific lighting circumstances....ie where the bright sky has predominated.
If that is the case, a good clean with a lens cloth is the thing
Don't use any of the liquid cleaners.....liquids can seep round the edges of the lens elements into the lens and onto the rear of the element....just rub in a circular motion with piece of fine, soft cotton fabric.
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Thanks TD,
I didn't use the lens hood(should I have).Tbh when the church shots were taken (Tuesday)it was very grey ,with rain threatening,but when the Flatford Mill shots were taken (Friday)it was lovely and sunny all day.
One type of shot I seem to struggle getting right is inside churches.
Church Interiors
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Sue lit a candle for Blue. :'(
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>>>I didn't use the lens hood(should I have) <<< .........yes, yes, yes, always.
It shades the front element of the lens from stray light and greatly reduces the risk of flare. Also, it protects the element from sticky fingers and other things that you want to keep well away from the glass.
Will be back. :)
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>>>I didn't use the lens hood(should I have) <<< .........yes, yes, yes, always.
It shades the front element of the lens from stray light and greatly reduces the risk of flare. Also, it protects the element from sticky fingers and other things that you want to keep well away from the glass.
Will be back. :)
I have 2 lens hoods......the Sony SH0006 which is16mm deep and the SH0007 which is 49mm deep.Which ones do you use in different shooting modes. :-[
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The shooting mode is not a factor, unkyUb, the important thing is the correct hood for the two lenses..........too short and it is ineffective, too long and you get vignetting ( darkening on the corners of the image )
You will have been supplied with one for each lens, I'm sorry, I don't know the item codes.........the one for the wide lens will be shorter and wider (the 16mm one), the long lens one a bit longer and a bit narrower (the 49mm one)......the filter thread on the front of the lenses will indicate the correct diameter to use.
What do you think is the problem with your church interiors ? They look fine. :)
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I think the Church interior look fine too Unc .. and what a beautiful interior it is ! ( the one thing that does bother me is the inscription about the rose ..... "each year a payment of a red rose is made by the Mayor" ...... does he put that on expenses I wonder >:D )
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The shooting mode is not a factor, unkyUb, the important thing is the correct hood for the two lenses..........too short and it is ineffective, too long and you get vignetting ( darkening on the corners of the image )
You will have been supplied with one for each lens, I'm sorry, I don't know the item codes.........the one for the wide lens will be shorter and wider (the 16mm one), the long lens one a bit longer and a bit narrower (the 49mm one)......the filter thread on the front of the lenses will indicate the correct diameter to use.
What do you think is the problem with your church interiors ? They look fine. :)
Hi TD :)
Both the lens hoods will fit both lenses.....The Sony SH0006 is the 16mm deep one and the SH0007 is the 49mm deeper one.The 49mm one is slightly wider than the 16mm even though the screw on fitting is the same.Now I know I should use the lens hood all the time I'm just not sure when you use the deeper one and when you should use the shallow one.
Those are the better images I took.I will post one or two that I think are not quite right.One thing is a dark interior and bright light coming in through windows(not stained glass)
Here's a picture with the hoods fitted to give you a better idea
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Good morning unkyUb ;D
Lens hoods.........the short fat one goes on the short fat lens...all the time, the longer one goes on the longer lens...all the time. The hoods are not interchangeable between the lenses even if both will fit each other in terms of throat diameter.
Each hood is designed to offer flair protection to a specific lens and will be effective on that lens only.
Am trundling through your photos one at a time........there are some very good ones there and I will post some specific comments....there is a lot to look through. :)
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Good morning to you also TD :)
Thank you for your comments,they are very much appreciated.
I am quite annoyed with myself as it is down to poor camera housekeeping on my part. :-[
I will certainly make sure I keep the lenses clean in future and use the hoods all the time.
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I think you're being too hard on yourself, Phil. There are some very good pictures in that collection. A great deal of the enjoyment of photography is in learning from your mistakes and doing even better next time, and we have the benefit of a first-rate teacher here. :)
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I agree with roseway to the extent that you are being hard on yourself, unkyUb........I doubt if there is a lens in the world that has not had sticky fingers or dust or whatever on it from time to time. The TD lenses are a disgrace in this respect.
A good lesson, however, .......give them a wee clean before using them.. ;)
Edit to add..
Just to round off the flair question and why it is apparent in some images and not others.
Lets look at the three Flatford Mill images.
Two of these do not exhibit flare. The ones where you have zoomed in a bit. In these images, the house and ground predominate the scene and there is very little sky showing......when calculated as a percentage of the whole scene.
The third image, where you zoomed back to your widest setting does exhibit flare. The factor here is the huge amount of sky, as a percentage, but more important is the large area of water in the forground. Very shiny water....just look at the huge amount of light being reflected off that water..it almost takes over the image.
The very strong reflection is bouncing back light into and onto the lens....add that to the light from the sky. Furthermore, unlike the light from the sky, the reflected stuff is not hitting the lens squarely...it is coming in at an angle and is raking across the lens element instead of passing through in a straight line.
In that circumstance, any dirt or smudge on the element will bend the light a bit and cause a loss of definition....flair.
Similarly, in the shot of the church tower flair has been picked up because, due to the very nature of the subject, you have in effect pointed the lens at the comparatively bright sky...comparative to the steeple, that is.
Both these instances would not have occured on a smudge free lens.. :)
Will be back.....this is interesting.. :clap2:
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:)
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Much better :clap2:
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:)
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You're working well TD :)
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You're working well TD :)
Isn't he......what a star. :) :clap2:
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I have been having a look again at you images, unkyUb, and all I can say is...........congratulations.
They are lovely, without exception.
In the Flatford Mill ones, I really like the way you have resisted the temptation to plonk the building into the centre of the frame. You have used the "rule of thirds" well and have the building in just the right spot. These images are very pleasing indeed.
The church exteriors are very good, well balanced and nicely composed and are a treat for the eyes.
The interiors are stunning, and you have done very well with exposures in very tricky lighting.
One image is affected by barrel distortion, but that is par for the course with zoom lenses and there is not much you can do....except in photoshop or the like.
The seaside ones are great..........and nice level horizons.
You have an eye for compositions, and your results are top rate.. :thumbs:
Well done............but I would still like to wean you off the auto modes ;D .......but that's just me.. :)
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In the Flatford Mill ones, I really like the way you have resisted the temptation to plonk the building into the centre of the frame. You have used the "rule of thirds" well and have the building in just the right spot.
That is the advice you gave me last year which was logged in the old memory banks. :)
Well done............but I would still like to wean you off the auto modes
I know I should be a bit more adventurous.There are plenty of various options on the dial wheel,or do you mean use the manual setting 'M' and play about with various options,ISO setting etc.
I did just take a couple of shots in manual mode and altered the ISO settings,one taken @200and one taken @ 1600.The 1600 one seems a lot more detailed.
I found this image that I took using the hood when we visited Ullswater in June.I saved as I wanted to know why you can see the circle round the edge.I'm wondering if I had the hood on the wrong lens?
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Oooops...............yessiree...wrong hood on the wrong lens.. ;D
Wide angle lenses are thus described because they have a very wide angle of view and gather light and the scene from a wide view. Something like 73 degrees of an arc or there abouts.
The little short hoods for these lenses are built so that the front edge of that hood does not intrude into that wide angle of view.
However, longer lenses have a much narrower angle of view, (which angle decreases the longer is the lens or the more it is zoomed out) and can thus carry a hood which is longer and narrower than the wide one.
However, put a long lens hood onto a wide angle lens....and the leading edge of that hood intrudes into the angle of view cutting of some of the light/scene and preventing it getting through the lens........which is exactly what has happened in this image.
As to being more adventurious.....there is really no "must" about it........take the photos you are happy with by whatever means give you the result you are after. Manual mode will give more creative opportunity but stick with what you are happiest with. :)
As a side issue, interesting on that photo, taken in June, there is the dreaded finger smudge.. :( .........that was the trip then when you poked the lens... :no:
Changing from iso 200 to 1600 makes the sensor more sensitive in that it becomes capable of gathering more light and yes it may appear to give bit more detail when used in good light. However it can cause a lot of digital noise to appear....so use with caution.
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As a side issue, interesting on that photo, taken in June, there is the dreaded finger smudge..
I did notice that :-[
And knew that you would as well :graduate: ;D
Oooops...............yessiree...wrong hood on the wrong lens..
:-[ once again........Saying that,I am very grateful of any criticism,its all helping to make me a better photographer,so don't hold back I can take it :baby: :D
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On the subject of adventurousness, the picture with the wrong lens hood has a certain quality about it which could be quite attractive. It almost looks as though the picture was taken through a telescope (or a porthole perhaps). With the right sort of subject it could produce a very nice result.
I'm obviously no expert, but can I suggest, Phil, that you set the camera dial to P? By default it's the same as fully automatic, but in 'P' mode you can change individual settings one at a time while leaving everything else on automatic. I've got mine set to disable the flash, and I've found it handy for increasing the ISO setting in order to get some natural light pictures in a dark wood.
By the way, I second TD's comments about this batch of pictures - they're lovely. :)
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I have just been having a play at changing the aperture values.Taking the same shot of a plant on the window sill I notice that the higher you set the aperture the more focused the area is around the subject matter.
I took one at 5.6 and the outside sky just looked white,but when I took the same shot upping it to 11.0 then you could make out all the clouds.
Am I talking sense here or just rambling?
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>>> Am I talking sense here or just rambling? <<<
You are talking sense, unkyUb...........and the effect you are seeing is the very reason that we have variable apertures in cameras......creativity
It is all to do with depth-of-field (DOF)..........the amount of back to front sharpness in an image.
The bigger the "f" number used ( ie 11 instead of 5.6 ) the more DOF you get ............the smaller the number, the less DOF you get.
What you are seeing when using f11 is the fact that at that aperture value, the lens can render sharp both the close (or near-to) subject and the clouds in the background.......whilst when using a smaller value of f5.6 the lens can resolve the near to subject as sharp whilst distant objects lose sharpness and become blurred. In the case you mention, the background clouds are blurred out to such an extent that they have become totally featureless and just looked white.
Now this is where you can start to get a bit creative... ;D....lets take two of your recent images as samples.
Flatford Mill...........in this case you want a nice landscape with everything from your toecaps to the horizon to be pin sharp and in focus..use a big f number, 11, 16, 22
Church interior...Sue lit a candle for Blue.......focus on that candle and use a small f number, 2, 2.8, 4, 5.6 ........the candle is rendered nice and sharp but the background goes out of focus and blurs out, giving a sharp candle without anything in the background to distract from it.
In this case you need to experiment a bit with regard to getting the right f number....you may want some background feature showing, some definition there to suit the shot. In time and with practice you will learn just what effect you can expect at any one f number.
There now, get practicing ;)
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Thanks TD (once again) for you excellent help :clap2: :)
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Hopefully I am not jumping ahead here, but I notice the lack of mention of shutter speed. Not having played with these digital cameras I don't know if they have such a control. Would someone enlighten me please.
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@ ss44..........yes, unkyUb's camera has shutter speed controls, manual and auto ans of course you would vary these speeds in parallel with varying the aperture in order to keep to the correct exposure.
Just going step at a time for the moment. :)
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Sorry I didn't mean to go too fast, it's just that shutter speed doesn't seem to be mentioned in other tutorials either.
Thank you for the answer though.
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I did once post a long ramble about DOF,................. the relationships between aperture and shutter speeds, the variances achieved by using different focal lengths, those achieved by varying the camera to subject distance , how and why the normal rules vitually reverse themselves in macro photography etc. etc.
I was hoping to link back to that....but can't find it. It was some time in mid 2008, I think. Am still looking for it.
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Is it in this topic TD ?
http://forum.kitz.co.uk/index.php/topic,7713.0.html
hope so :)
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3rd post down ?
http://forum.kitz.co.uk/index.php/topic,4119.msg97928.html#msg97928
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Good morning scottiesmum and unkyUb.
Thank you both for trying to help me find the "missing" post, but unfortunately neither of the suggestions you post is the one I was thinking about..... :(
Not to worry, will scribble it out again and repost....this may take some time.. ;D
Thanks again for your help. :)
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Before going on to chat about the relationship between aperture and shutter speed, a wee note about apertures.
A lot of newbies to photography get confused about this subject as the aperture scale on a camera is maybe a bit baffling in
that the numbers we talk about don't, at first glance, make any sense.
The number sequence runs 2, 2.8, 4, 5.6, 8, 11. 16, 22, 32 and it is difficult at first to equate that sequence with the fact
that as you move from one number to the next you are either doubling or halving the size ( diameter) of the aperture. Afer
all, the numbers themselves are not doubling or halving so how come they are having that effect on the physiacl dimensions
of the aperture ?
Well, the fact is that these numbers represent not the simple diameter of the aperture but the ratio between that diameter
and the focal length of the lens.
This is why these numbers are usually described as "ƒ " numbers, ƒ being the mathematical symbol for ratio.
(TD will get tired copying and pasting ƒ all the time so will instead call it F ).
So the sequence of numbers in the aperture scale is really F2, F2.8, F4 etc.
Now those of you with a mathematical turn of mind ( and that does not include TD so please comment if I am wrong ) you
will know that if you wish to double or halve a ratio, you multiply/divide the factor not by 2 but by 1.44 ( near enough)
Using this, the sequence of aperture values makes sense........F2 multply by 1.44 gives F2.8 (near enough), F2.8 multiply by
1.44 gives F4 ( near enough ) and so on.
Using that set of values it is now apparent that when you go up and down the aperture scale, you are in fact halving or
doubling the ratio aperture to lens...effectively doubling/halving the size of the hole ( iris or aperture ) though which the light
has to pass.
As these numbers represent ratios between aperture diameter and focal length, what happens when we change the focal
length of the lens....either by zooming in and out or by swapping lenses ?
It means that F2.8 on a wide angle lens and F2.8 on a telephoto lens will give the same exposure value......the focal length
is different, the physical diameter of the aperture will be different...but the ratio invoked in each lens is constant and the
exposure value will be constant between the two lenses
The next thing that can confuse is the fact that as the numbers in the aperture scale increase from say F2 to F2.8 the
aperture value does not increase, as might be expected....it decreases ( again because the F number is a ratio.)
So as you progress through the scale from F2 to F16 say, the aperture diameter is getting smaller..it is halving in diameter
every time you move up from one number to the next ( or of course doubles in diameter if you are moving down the scale
of numbers.)
Why do we have these varying aperture sizes ? We have talked of it before...it is to adjust and control your depth of field,
the amount of back to front sharpness in an image. In rough terms, the higher the F number used, the more depth of field
you get..............earlier posts have mentioned the effect on depth of field when using different focal lengths/ camera to
subject distance and some other factors such as that.
These F numbers are very often described by photographers as "stops" and the act of going up and down the scale as
"stopping down" or "stopping up"
In the days of very early cameras, internal variable aperture devices had not been invented and the lenses themselves had
only one F number...the size of the lens throat through which the light passed.
Some bright spark, realising the need to control depth of field, created a set of brass plates, each with a different sized hole
punched in the middle with which to represent the aperture. These plates were effectively used to stop some light getting
through and were inserted in a slot between the lens and the camera body....they were called "stop" plates.........and
although we now have built in aperture adjustment..the term "stop" is still used.
Will write out note about shutter speed, then another about the relationship between the two and post in a bit.
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:-[ TD ... I have just realised I have your notes in my TD Photoclass Folder .... would you like me to post them all and you can pick out the bits you might need ?
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Thanks for the offer, scottiesmum, that is most kind :flower:
But no, not to worry and please don't go the trouble of doing that...won't take long to tap out some drivel.. ;D
Thanks again. :)
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I hope you won't mind my trying to clarify the mathematical aspects of the above, TD.
Each successive number in the F number sequence represents a halving of the area of the aperture, not its diameter. So f2.8 has half the area of f2, and so on. The F numbers themselves are related to the diameters; hence f2.8 has 1/1.4 times the diameter of f2. As area is proportional to the square of the diameter, this is equivalent to half the area.
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Ooops.........thank you for that clarification, roseway.........I could remember that the figures were ratios but my maths are such that I could not clearly recall the other bits, :-[ ..............but it has come back after reading your corrections of my waffle.
TD trundles off to read his maths primer again.. :)
Thank you again.
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Nothing to thank me for, TD. Your photographic explanation was excellent, and I've stored it away for future reference. :)
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I think I need a book on maths,not photography. ;D
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I can just picture the scenario :-
Sees train coming, puffing steam, looks great, one minute and it will be just perfect.
Scratches head, now what was that about thirds, where do I focus.
Scratches head, it was all about F's and some fancy maths, wish I had listened at school.
Train getting nearer and I still haven't focussed right.
Scratches head, there's this almighty wind as the train goes past.
I'll get it next time when I've read all them posts on the Kitz Forum.
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@ ss44 .............. :lol:
You well express the problem when trying to write these posts.......all the stuff about DOF and speeds etc etc is so ingrained that you just don't think about, it comes naturally...........and when you do post it is easy to (a) omit a point or (b) assume too much knowledge on the part of a reader.
Still, it's fun and I hope somebody gets something out of it all... :thumbs:
@ unkyUb.....me too.. :D
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The good news about shutter speeds is that there are no ratios or fancy arithmatic to worry about ! :clap2:
However, just to confuse the matter, dslr cameras do not have a shutter.......
Film cameras had a cloth or metal screen across the film to block out the light from hitting the film until you wanted to take the image...at which point you press the button and the screen moved out of the path of the light for the required period and then reclosed.
In a dslr, the sensor is exposed all the time and only becomes "charged" and light sensitive for the duration set as shutter speed.
For the purpose of this...I'll simply talk of shutter and shutter speeds...nice and simple.
The shutter speed scale runs from 1 second, 1/2 second, 1/4..1/8th...1/15...1/30...1/60...1/125...1/250...
Some may give speeds fo 2, 4, 8, seconds and upwards and the scale will continue in the above sequence up to 1/2000
or faster................all these figures being seconds or fractions of a second.
From that scale it will be seen that these figures are doubling or halving as you go up and down the scale. ( I know, 1/8th to 1/15th and 1/60th to 1/125th is not exactly double/half but it is as near as dammit and expressed that way keep the scale tidy.
So by adjusting shutter speeds within that scale you can vary the length of time the shutter is open each increment up or time being measured by a factor of two....double or half.
And we know from earlier chat that apertures. despite their odd looking scale of numbers, are also adjusted incrementally and also adjust by a factor of two...half or double.
That is the corelation between them....adjusting the admission of light to the sensor in factors of two...half or double.
To an example.........You are in manual mode and you just happen to have your camera aperture set at F8 and the camera meter tells you that for that aperture and to get a spot on exposure you need to set a speed of 1/125th of a second. Great, no bother..set that speed without altering the aperture value and take shot.....perfect exposure.
However, you then decide that in this particular image you want more depth of field...and we know that is achieved by "stopping down"...ie making the aperture physiacally smaller which we do by setting a larger F number on the scale.
So we move our aperture scale setting from F8 to F11
Doing that we have halved the size of the aperture through which the light will pass...only half the light will pass through in the given time.....underexposure results......soo we must adjust the shutter speed from 1/125th of a second to 1/60th of a second..........the shutter now stays open for twice as long... and that compensates for the fact that the aperture is half the size and letting in only half the light..
It works the other way, of course...starting again (for this example) at F8 at 1/250th.....you may want less depth of field so you reset the aperture scale to F5.6.............the aperture is now twice as big, letting in twice as much light...so you increase your shutter speed to half the time that the shutter is open by doubling the speed to 1/250th of a second.
Simple as that ;D......when you start with a correct exposure and then make the aperture smaller in increments of two...you extend the shutter speed by a factor of two to compensate...and vice versa if you are making the aperture larger.
Why change aperture values...........said it many times, to control and adjust depth of field. Or in extreme cases such as trying to photograph in extreme light conditons..as an example very low light where you may have to set your slowest speed and largest aperture to get any exposure at all.........or in very bright light where you may need the smallest possible aperture and the fastest speed to make an image.
The occasion you may want to alter aperture values are many...but now you know how to compensate for that by adjusting your shutter speed to suit..............or do you....... :(
..............let us say that you are using a zoom lens (something that TD rarely does).......problems arise, big problems. :swoon:
We have seen that the aperture scale figures of F 5.6 etc etc represent the ratio between the focal length of the lens and the area ( got it right this time :D) of the aperture.........you can see what is coming, can't you ?
We have our zoom lens at its widest setting..we dial in our aperture of choice..we do a quick exposure meter check..we set the appropriate shutter speed value..we take the image. Great !! we decide that the image is good and that we would like to pick out some detail so we zoom in a bit. Can we use the same exposure setting ? Nope, for by zooming you have altered the focal length of the lens....therefore you have altered the ratio between that and the size of aperture...Start again and remeter the scene, alter speed etc to get it right again
Now if your zoom is 28mm at the wide end and 56mm at the long end and if you have zoomed from end to end, the arithmatic to establish the new exposure values is simple.........but it gets a bit complex if you zoom in to the precise composition you want and find that you have landed on a focal length of 43.75mm or some other silly figure
No can to the arithmatic for that.
TD recommends that when using zooms, you do not use full manual mode, simplt because the odds are that you do not what focal length you are using within the zoom range unless it be the top and/or bottom end.
Better to use a half way house between full manual and full auto...........Aperture Priority or Aperture Value mode. In that, you choose and set your aperture of choice and the camera takes care of the shutter speed...simple and effective.
Of course, you can do all this sort of thing in reverse so to speak. If, for instance, you wish to photograph a bird in flight, your priority may not be aperture but speed............you may want to set such a speed as to "freeze" the bird in action, or to partially freeze but have nice blurry wings to show movement. It that case you would set a suitable speed and then adjust the aperture scale to give good exposure.
There is an semi-auto mode for that too, Shutter Priority, Time Priority, Time Value...whatever it is called on your camera.
Off now...........will be back to yak on about ISO ratings..