Defining of a stop of light

Do you remember that doubling and halving thing I mentioned in the previous lessons? Well, that’s a stop of light. A stop of light isn’t any quantifiable measurement it’s a relative one. We define a stop of light as a doubling or halving of any quantity of light.
f/ 5.6 lets in twice as much light as f/8. Another way to say that is f/5.6 lets in one more stop of light than f/8.
1/60 second lets in one less stop of light than 1/30 second.

The word “stop” is pervasive in photography. Mostly we use it in regards to setting exposures. At workshops, you’ll hear me say, “Open up a couple of stops”. That could mean adding two stops of light to the exposure. It could also mean to set a wider aperture for depth of field affect.
If you actually looked at all the shutter speeds and apertures available on your camera, you will have noticed (unless you have an older camera) that there were quite a few in-between numbers. These numbers represent fractions of a stop between the main numbers. Depending on your camera, these are in either 1/2 stop or 1/3 stop increments. My cameras are capable of setting different step increments via custom functions. Yours may be too. Check your manual.
On my Nikon D810, the shutter speed sequence of 6, 8, 10, 15, 20, 30, 45 and 60 is in half-stop increments. The shutter speed sequence of 8, 10, 13, 15, 20, 25, 30, 40, 50 and 60 is in one third-stop increments. Your camera may be slightly different.
The f-stop sequence on my D810 looks like this: 4, 4.8, 5.6, 6.7, 8, 9.5 and 11 for half-stop increments and 4, 4.5, 5, 5.6, 6.3, 7.1, 8, 9, 10 and 11 for one third-stop increments. Just remember that these are not whole stop steps. They may be one click on the dial but they’re only either a third or a half of a stop, depending on your camera or what custom function you've set. Also note that there is a one-stop difference between f/4.5 and f/6.3 or between f/7.1 and f/10, etc.

You’ll also see the word stop associated with filters. Polarizers, for instance, cut between one and two stops of light. There are also filters specifically designed to reduce the amount of light going through the lens. These neutral density filters are available in different stops of neutral density. We’ll discuss filters in more depth later.

The meter display in your camera is also divided into stops. You’ll need to consult with your manual and look through your viewfinder for your specific camera, but in general, most displays show a range of about plus and minus two stops in half-stop or third-stop increments. This is a very important feature of your camera and we’ll be talking a lot about it in upcoming lessons.

Try this. With a lens attached and your camera in the manual exposure mode, point your camera at anything at all and keep it pointed at your chosen object. Now change your shutter speed or aperture until the meter display shows a “correct” exposure, or what we call “zeroing the meter”. You’ll need to consult your manual to know what this should look like. Now change the exposure by one stop by either changing the shutter speed by one stop or the aperture by one stop. It doesn’t matter whether you add or subtract light at this point. What I want you to observe is the change you should see in your meter display. The indicator should move one whole stop. If you’ve added light, it’ll change one stop in the positive direction, if you’ve subtracted light, the change will be in the negative direction.

Ok, now try this: Reset the camera so that the meter is “zeroed” again. Now, add a stop of light with the shutter. This means using a slower shutter speed. For example, if you were at 1/30 second, set the shutter to 1/15. The meter should now show a one-stop overexposure. Now, without changing the shutter speed, subtract a stop of light with the aperture. This means using a smaller opening (a larger number). For example, if you were at f/8, change to f/11.
What happened? You should have ended up right where you started, with the meter “zeroed”.

It’s important that you understand the concept of a stop. This is one of those BIG CONCEPTS in photography. It allows you to master things like exposure, and flash, and using certain filters.

Next, Putting Shutter and Aperture Together....