Welcome to Part 2 of “Keys to Properly Exposing a Photograph.” To start at the beginning, CLICK HERE
Understanding exactly how f/stop work behind the scenes is VERY complicated. I’ve spent countless hours throughout my career studying it. If you’re looking for the most comprehensive, in-depth, intense explanation of f/stop. It lives here. Really, follow that link. It will tell you everything you could possibly want to know.
Now, if you’re reading this, you either have returned with an amazing understanding of the complex inner-workings of your SLR camera, or your back here waiting for me to give you the simplified version. Well, here it is.
The f/stop, as you will use it, is a range of seemingly random numbers from 1 to 22. (1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.) (Some lenses will go higher than f/22. Thanks for the correction Rob) They represent how open your shutter (eyelid) will be when your sensor (eye) captures the picture. So why the weird increments? The answer is complicated. The simplest way I can say it is the f/stop is a ratio which describes the relationship between the diameter of the aperture (how wide your eyelid is open) and the focal length of the lens (how long your lens is.) The longer your focal length, the bigger your aperture needs to be to have a low f/stop. This is why a 70-200mm f/2.8 is so much bigger around than a 50mm f/1.4.
So again, why the weird increments. Now we’re getting to the bottom of this. We know an f/stop is the relationship between the diameter of the aperture and the focal length. Let’s say a 200mm lens, is set at f/4. The diameter of the aperture is going to be 200 divided by 4 which equals 50. (200/4=50) So, the diameter (distance across) of our aperture will be 50mm. Remember ? Long story short, the area of the hole letting the light in for our 200mm lens at f/4 is about 2000mm. Solve that for one stop down, f/2.8, you get about 4000mm, f/2 and you get about 8000mm. Summary, every stop down doubles the area of the hole letting the light in. This effectively doubles the light on your sensor, and this is what makes your picture brighter!
- Remember when I told you your shutter goes in increments of doubles and halves too? Theoretically, if your image is properly exposed at (1/125 and f/2.8)… It will also be properly exposed if you double your f/stop (open the aperture twice as wide) and double shutter speed (leave the shutter open half as long) i.e. (1/250 and f/2.0) or if you halve your f/stop (open the aperture half as wide) and halve your shutter speed (leave the shutter open twice as long) i.e. (1/60 and f/4)
The other catch is cost and weight. To get a low f/1.4 on a 50mm you need an aperture diameter of about 35mm. If you wanted a 200mm f/1.4 your diameter jumps up to 142mm. That aperture hole would need to be nearly 6in across. Such a lens does not exist that I’m aware of but it would cost a pretty penny. Just for a 70-200mm f/2.8 you’re getting into a $1000-2000 lens that weighs in excess of 3lbs. If you’ve got the money, (and the upper body strength) go for it. Here my equipment review post where I talk about mine along with the rest of my gear.
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