Ok, Grasshopper, Monday’s lesson is brought to you by the letter F. F is for Aperture.
Aperture acts like the pupil of your eye (and sorta looks like one too). It expands and contracts to let in more or less light. It opens wider as light decreases to let in more available light; it gets smaller when light increases to reduce the amount of light entering the eye.
Aperture is measured in terms of f-stops (aka. f-number, focal ratio). An f-stop is the focal length divided by the “effective” diameter of the len’s aperture. It gives us quantitative measurements for lens speed.
er . . . what?
The f-stop tells the photographer how much light is present or is needed to expose a photograph properly. Your camera (assuming you have it in auto) is going to look at a scene and attempt to dim or brighten what it sees by letting in more or less light. One of the devices it will use is aperture (it also uses shutter speed, but that will be next Monday’s lesson).
Rule of thumb: The lower the f-stop number, the “wider” or “faster” it is; the higher the f-stop number, the “smaller” or “slower” it is. The main f-stops are: f1.2, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32. A lens which has a maximum aperture of f1.2 is considered very fast and great for shooting in the dark as it lets in the most available light.
[There used to be f1 lenses made back in the old days, but they were massively huge lenses. As you get further down in the f-stop range, quality becomes a major issue. So these days, you won't see a lens being produced that's faster than f1.2.]
But why do we need to know this?
Knowing what f-stop your camera is using is essential to producing a good photograph. When the aperture is wide open it’s field of focus is very limited. Ever go to the eye doctor and they put those horrid drops in your eyes and you walk around not being able to see anything? Yeah, it’s sorta like that. Your pupils are so wide open, it’s hard for you to focus on anything and if you can, it’s range is very narrow (and aside from that, it’s flippin’ bright!). If you walk around shooting on f1.2 all day, don’t expect your scenics to be completely in focus. You’ll have a small amount of it in focus, but not foreground all the way to background.
If you walked around shooting on f22 all day, you will enjoy some very large fields of focus. Your foregrounds to your backgrounds will be acceptably sharp. That’s because your aperture is very, very small and the light is more focused on your camera sensor (or film, what have you).
Here is an example of the same scene, but with different f-stops.
1. Shot at f2 – VERY VERY VERY wide open. Notice only one key is in focus.
2. Shot at f5.6 – getting better. This f-stop is very common when your camera is set on auto.
3. Shot at f11 – starting to tell I don’t own a Steinway.
4. Shot at f32 – the focus in this shot is almost sharp from front to back.
As you can see the wider the aperture, the less you have that’s within the focusing range.
Now, there are advantages to shooting at a fast f-stop. F-stops at this end of the scale allow you to shoot in low light. You can also use it to blur out distracting details – this is especially crucial when shooting portraits outside if you want to blur out the background so the attention is focused (pun intended) on your subject.
Using the other end of the spectrum, say f22, will allow you to capture great scenics. Your foreground will be in focus as well as your background.
Ansel Adams, with his massive 8×10 view camera, often shot at f64 and f124.
Wow.
So, set that camera from “A” for amateur to “P” for professional. You control the aperture and, for now, let the camera decide what shutter speed to use.
Experiment and have fun!
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