Photo Friday – Intro to Aperture

Last weeks Photo Friday covered shutter speed, this week we turn our attention to aperture.

Simply speaking, the aperture of a lens is the size of the hole that the light passes through. A decent analogy is your eye, shine a light in it and the pupil constricts, so less light reaches the back of your eye. The same with a lens, reduce the aperture and less light reaches the camera sensor.

The challenge with understanding aperture is that a smaller aperture is a larger f-number. WTF?

The key is to know that the f-number is the *ratio* between the focal length and the diameter of the aperture. For example, a lens focal length is at 100mm and the aperture is 25mm in diameter, therefore the f-number is 100/25 or f/4. This also explains why in most consumer- or entry- level lenses, the aperture changes if you zoom in or out, because you’re changing the focal length.

Last week I talked about halving or doubling the amount of light hitting sensor based on shutter speed changes (1/200 sec captures double the light as 1/400 but half as much as 1/100, for example). Unfortunately, because we’re dealing with the area of circles, the numbers aren’t as straight-forward.  Here is the basic f-number sequence

1.0, 1.4, 2.0, 2.8. 4.0, 5.6, 8.0, 11.0, 16.0, 22.0

Moving left 1 place doubles the light, moving 1 place right halves the light reaching the sensor.

Do you see how this would interact with shutter speed? If you cut the shutter speed in half and double the aperture, the effective exposure hasn’t changed, the same amount of light is reaching the sensor. So f/4.0 at 1/200 sec yields the same exposure as f/2.8 at 1/400 sec.

A quick word on terminology- “fast glass” or a “fast lens” is one that is capable of a low aperture (in my mind, f/2.8 and lower), thereby allowing for a *faster* shutter speed while maintaining exposure. My fastest lens is my 35mm f/1.8 prime lens, which cost me less than $200- a fast lens does *not* necessarily mean expensive!

Now why do we care about being able to adjust aperture instead of just locking it wide open? Depth of field.

The ability to explain the optics/physics are beyond me, but I know that a fast aperture will result in a shallower depth of field.

Remember this photo from last weeks post? I shot it at f/5.6, which is the fastest aperture for the lens & focal length I was using at the time. As a result the depth is field is fairly shallow, so the bikes leaning against the tree are blurred. That’s because they are out of the depth of field that is in focus.

Contrast the above with this photo, shot at f/29. The fountain and the water in the foreground are nice and sharp and while the bikes are still a little blurred, they’re much more in focus. Click to open in a new window and zoom to full sized, you might be able read the brand name of the bike on the downtube.  Then try that with the above photo.

Hopefully you’re starting to see how shutter speed and aperture are interrelated.

Quiz time- I want to take a photo of something moving (requiring a relatively fast shutter speed) with a deep depth of field (ie small aperture), both of which limit the amount of light hitting the sensor. So what can I do?

That, dear readers, will be the subject of next weeks post on ISO, or the sensitivity of the sensor.

Questions, comments, anything I need to clarify?
That’s what the comments section is for, so use it!

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