One of the most confusing photography terms is the word "stop". What is it?
The simple answer - it is a unit of light. If this answer is enough for you? Stop reading (pun intended). Still confused? Read on.
When we refer to aperture openings in photography we refer to them as an "f/stop". This is likely the origin of the term stop, but the origins of f/stop are a bit more convoluted, and not totally relevant, to understanding what a stop is. What you need to know is that when you change your aperture from one f/stop to another you are either gaining or losing a unit of light. If you move from f/16 to f/11 you have gained one full stop of light. On the other hand if you go from f/16 to f/22 you have lost one full stop. Also when you double your ISO, you have also gained a stop of light. Likewise, if you halve your ISO, you have lost a stop of light. The same is also true for shutter speeds. If you double your shutter speed, you will gain 2 x the amount of light etc.
You need to be aware that most digital cameras now move f/stops not in full stop increments but rather in 1/3 stop or 1/2 stop increments which, of course, complicates the calculations.
One other point to understand is that in photography stops are exact units of measure. It's not a guessing game. When you move your aperture, shutter speed, or ISO you are moving it in EXACT increments. When you are buying a neutral density filters the manufacturer will tell you exactly how many stops of light you will be loosing when you use that particular filter. For instance 6 stops or 10 stops etc. Therefore when you are using one of these filters you must move either your aperture, shutter speed, or ISO (in combination or independently) exactly the same number of stops in order to compensate for that loss of light.
Here is a good example. Your are photographing a landscape and your camera gives you a reading of aperture f/16, 1/125 (shutter speed) and ISO 100. Your Lens is an f/2.8. You then attach a 10 stop neutral density (ND) filter to your lens which means that you are loosing 10 stops of light. You must now compensate for that loss. If you only move your aperture you must come to f/2.8, however going to f/2.8 means that you have only moved a total of 5 stops and are still in deficit of 5 stops because here f/16 goes to f/11 to f/8 to f/5.6 to f/4.0 to f/2.8.
To compensate for this deficit you may move your ISO; which means your ISO would have to be 3200 to compensate. Why? Because 100 goes to 200 to 400 to 800 to 1600 to 3200 [the equivalence of 5 stops]. If you would prefer not to loose the resolution then you would need to move your shutter speed.
Your starting point was 1/125. To gain 5 stops via shutter speed you would then move to 1/4 second; because 1/125 goes to 1/60 to 1/30 to 1/15 to 1/8 to 1/4; again the equivalence of 5 stops]. Therefore, if you left your ISO at 100 and moved your shutter speed and aperture your new setting would be f/2.8 & 1/4 of a second.
Having said this the usual reason for using a 10 stop filter is to acquire a very long shutter speed. Therefore if you leave your f/stop and ISO where it is (f/16 and ISO 100). And simply move the shutter speed. Another 5 stops would mean 1/4 goes to 1/2 to 1 seconds to 2 seconds to 4 seconds to 8 seconds. Your final setting is f/16, ISO 100 and 8 second shutter speed.