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
......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.
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
) 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..
