what is a stop

marky.b

marky.b

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ok, so ive been doing photography for a long time, and i guess when you think about it most of us just go through the motions, and don't really understand a lot of what we do. we just do it
like stops for example.
what is a stop
for example on the standard kit lens

app 3.5 4 4.5 5.0 5.6 6.3 and so on

speed 4 5 6 8 10 13 15 20 etc

i take it each one is not a full stop but a part stop
i read somewere about halving or doubling but i cant get it to sink in
 
The word 'stop' can be a bit confusing. It actually dates back to the early days of photography when apertures were just plates with different sized holes inserted in a slot behind the lens and it was thought of as 'stopping' light.The word 'stop' had now been adopted into camera terminology to mean a halving or doubling of exposure, whether it's achieved via the aperture, shutter speed or ISO setting. Doubling the shutter speed halves the exposure time by one whole stop. Halving the aperture size (say from f5.6 to f8) lets in only half as much light and decreases exposure by one whole stop.

Have a look at this tutorial and see if it helps

CLICKY
 
It is a measure of half the amount of light moving down a stop or twice the amount of light going up so a shutter of 1/50 lets in half the amount of light then 1/25 or twice the amount of 1/100. Aperture works the same way as does the sensor sensitivity with iso. Lots of help on the web if you google "photography understanding exposure"
 
The confusing bit is lens apertures which are measured in f/numbers, which don't appear to adhere to the halving/doubling relationship, but actually they do. They are related by the square root of two (ie 1.41, there is a reason for this, but it's not obvious) and each full f/number is either twice or double the light passing area of the one either side.

The range of full f/number that are commonly seen is: f/1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32.
 
The reason apertures are confusing is because it involves an area rather than a linear measurement. If you double the diameter of the aperture, you are increasing the area by four fold, letting in 4 times as much light rather than double.

That's why the progression of aperture stops seems a little confusing when compared to the progression of shutter speeds or ISO.
 
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I was going to write a reply, but I think CT's reply covers it fantastically!

I didn't realise where the name 'stop' came from, but now it's nice to know it comes from the plates which 'stopped' light from reaching the light sensitive material. Thanks CT!

I've been reading Bryan Peterson's book 'Understanding Exposure' which goes through stops and aperture in an easy to understand way. It was recommended on another thread on Talk. Again, I liked finding out that 'f' stands for focal length.

Peterson said f-stops are normally written with / e.g. f/4 or f/22.

If you take a lens e.g. 55mm and divide it by the stop e.g. 4 (i.e. 55/4) that gives you the diameter of the aperture e.g. 13.75mm and that is the actual size of the opening for light to be let through.

I love learning about the technical in a non-techie way!
 
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