Which aperture should I use?

Previously I have spoken about choosing your shutter speed, so now I will talk about aperture! :)

What is aperture?

Aperture refers to the size of the lens opening in terms of how much light it can let in. It is measured in f/numbers, where the lower the value corresponds to the wider the aperture and the more light that it can let in. For lore on this, I suggest you read my post on what is a stop of light.

Controlling how much light can pass through your lens (and therefore hit your sensor), is one of the three ways you can control your photographic exposure.

How does aperture effect your image?

Other than controlling how much light can pass through your lens, and therefore effecting both your ISO and shutter speed, the aperture also determines the depth of field (DOF), or how much of the image will be in focus. The wider the aperture (smaller the f/number), the smaller the depth of field and less of the image in front and behind your focus point will be in focus.

For example, imagine you are shooting a portrait of someone 2 metres away with a 50mm lens at f/1.8 on a full-frame camera. The DOF will be 17cm, 8.5cm in front and behind of the point you focused on (which for a portrait should be the eye). Now if you were to take the same image at f/8, the DOF will increase to 78cm!

So, the lower the f/number the wider the opening of the lens, the more light it lets in, and the smaller the DOF.

Knowing how to use your aperture to get the best DOF will dramatically improve your photography. If you want to play around with theoretical DOFs here is a handy online calculator!

So which aperture should I use?

Choosing your aperture can be tough, and there are many considerations including available light and desired shutter speed and ISO constraints, but here is a table of suggestions for different types of photography. These are not law, but is where I start off in each of these situations. However, there is no such thing as a generic best aperture, and in each case I can think of many times where I have ended up using apertures far different from those stated that better suit the situation in front of me at the time. So please remember that these are just suggested starting points for you, and you should adapt them as need be. As you will find out in the advanced section that follows, apertures are not straight forward... :)


The more advanced section

Above I have talked in generalities, and covers the basics, but there are some other considerations that you should be aware of that might impact your choice of aperture.

Focal length

The DOF is also affected by the focal length of your lens. f/4 on a 600mm lens will have a narrower DOF than a 24mm lens at f/4 (assuming they are focusing at the same point). This is one reason why many portraits are taken with longer lenses, to get that narrow DOF.

Focal distance

The closer you are to the subject, the smaller the depth of field, and the further away you are from the subject, the greater the depth of field. This is why macro photographers use narrower apertures, as they are trying to compensate for the very small depth of field they get by focusing so close.

Hyperfocal Distance

This is a combination of the two above really. Basically, for every focal length and aperture, there is a focal distance that will produce an infinitely large DOF. This is called the hyperfocal distance. At this distance you will have a DOF spanning from, the midpoint between you and the focal point all the way back to infinity. This is a good thing to be aware of for landscapes. This value is different for every aperture and focal length, so to check you lenses out, use this online calculator!

NOTE : The fact that the hyperfocal distance will only encompass 1/2 its distance in front of the focal point means that no matter how high the f/number, you will never get everything in focus, just almost everything! :)

The crop factor

There are two types of DSLR cameras, full-frame (FF) and cropped (APS-C). This is basically in reference to the size of their sensors, FF being the same size as a frame of "6mm film, and cropped being 1.6x smaller (in Canon cameras, 1.5x for Nikon). Different sensor sizes have different qualities, which I am not going to into here, but it also affects the DOF. The short of it is, the smaller the sensor, the larger the DOF at the same relative focal length, focal distance and aperture.

*This is equivalent focal length. In this case a 100mm lens, as on a APS-C camera, this is equivalent to a 160mm lens on a FF camera.

This affect is directly linked to the crop factor. an f/2.8 lens on an APS-C sensor camera will produce the same DOF as a f/4.5 aperture on a FF body (at equivalent focal lengths and same focal distance). 2.8 * 1.6 = 4.5.  So your equivalent aperture in terms of DOF is multiplied by your crop-factor. However, this does not change the amount of light the lens can let in, just the DOF.


The bottom line

  • Aperture controls the amount of light passing through your lens and the DOF
  • The larger the aperture, the smaller the f/number
  • The smaller the f/number the smaller the DOF
  • DOF changes with different focal lengths and focusing distances, so you will need to factor these in when choosing an aperture
  • There is a point for every lens and aperture that your DOF will be infinitely big
  • It is impossible to get EVERYTHING in focus
  • Your sensor size has an impact on the DOF
  • You can use this DOF calculator to test your gear out

4 Comments

Blog RSS