Understanding Aperture

Continuing on our start-of-the-busy-season refresher courses, this week we’re talking about aperture.

Aperture refers to the hole or opening through which light passes through a camera lens.  Aperture is often referred to as f-stop and is written as a f/# value such as f/8, f/1.4, f/22, and so on.  The larger the f-stop value (eg: f/22), the less light passes through the lens compared to a smaller f-stop value (eg: f/1.8) at the same shutter speed and ISO.  For example, at a 1/40th of a second shutter speed at ISO 100, less light comes into your camera at f/22 than at f/1.8.

This f-stop business confuses a lot of new photographers:  “Why the heck does a bigger number let in less light???  MAKE IT SIMPLER!!”

APERTURE MATH (THERE WON’T BE A TEST)

The terms F-stop and aperture are often used interchangeably and have effectively come to signify the same thing, but they actually aren’t exactly the same thing.  That may contribute to the confusion new photographers have with the numbering system.

Technically, aperture is the opening in a camera lens.  The term f-stop refers to the focal length of the lens divided by the actual diameter of the opening of the lens.  For example, a 100mm lens with an aperture of 25mm has an f-value of f/4.  100 / 25 = 4.  If you make the opening size smaller, for example 10mm, the f-value goes to f/10.  100 / 10 = 10.  That’s why a larger f-number refers to a smaller aperture.

You don’t have to know all that.  You can just memorize the fact that as your f-stop number goes up, less light comes through the lens.  Smaller f-stop number, more light comes in.

What real effects does aperture have on photography?

APERTURE AND DEPTH OF FOCUS

The larger your aperture (remember: this means a SMALLER f-number, like f/1.4), the shorter the depth of focus.  Translation:  if you want lots of background blur in your image, use a small f-stop.

Here are some rules of thumb (Remember: rules are made to be broken) for which f-stop to use in different photographic situations:

PORTRAITS: f/2.8-f/4  If you’re only photographing one person, you can use a smaller f-stop, but it’s a good idea to use a bit of a larger f-stop when photographing larger groups in order to make sure everyone is in focus.

LANDSCAPES:  f/8-f/32  If you’re shooting a big expansive landscape that stretches back to the horizon, stop that aperture down as small as you can get it so that everything in the picture is tack sharp!

NO DEPTH:  f/8-f/11  If you’re shooting a scene that doesn’t have any depth, like a wall of graffiti shot straight-on or a footprint on the ground shot from overhead, use an aperture between f/8 and f/11.  That range of f-stops is optically the sharpest range in nearly every lens.

One thing to keep in mind when shooting at a wide open aperture like f/1.8 is that f/1.8 gives you a very small area that will be in focus in your picture.  Be careful–it’s easy to have everything be out of focus when shooting wide open like that because the area in focus is so small and it’s easy to miss your focus.  Stopping down to f/3.2 or so will give you a bit more “room” to play with while still giving you nice background blur.

DEPTH OF FOCUS 2:  APERTURE PLUS FOCAL DISTANCE

Depth of focus is also determined somewhat by how close to your subject you are when you photograph it (this is called focal distance).  How close or far your background is from your subject also determines how in focus your background will be.  You can cheat and get great background blur using a larger f-stop if you photograph your subject in front of a background that is very far away.

Give it to me straight, dude.

Okay.  If you shoot a head shot at f/8 and get very close to your subject, the background will be more out of focus than if you stand back.  You can get even more background blur if you stand close to your subject and put them in front of a background that is very far away.  Try it.  You’ll love it, and you’ll probably think I’m awesome for showing this to you.  I won’t argue with that.

APERTURE PLUS FOCAL LENGTH

The longer the focal length of a lens, the more background blur you can get in your pictures.  If you love background blur and want to get as much of it as possible, shoot at a wide aperture and a very long focal length.  For example, a 70-200mm lens at 200mm and f/2.8 has TONS of smooth background blur and is a great choice for photographing head shots.

CONCLUSIONS

There is a time to use a wide open aperture like f/1.8 and there is a time to use f/22.  Every aperture has a use.  Too often, people just shoot wide open all the time.  Think about how much depth of field you want.  Don’t be afraid to use a wide range of f-stops to achieve different looks and to keep your photography looking fresh and varied.

I hope this was helpful.  Get out there and experiment.  Be creative.  Good luck, and have fun!

-Josh Lamkin, PWD Editor