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What is Aperture?

When you press the shutter on your camera to take a photograph, light passes down your lens, past several blades which form a hole, and onto your camera’s sensor.


This hole is known as the aperture, and looks something like this:

In many senses, it is much like the iris of your eye. In dark situations, the iris opens fully, and in bright light, it closes to a much smaller size.

How does this affect my image?

The size of the hole can be adjusted; by the camera if you are in an automatic mode, or by

yourself, if you are using a semi-manual, or fully manual mode.

A small hole will obviously let in very little light, and a wide hole will let in a lot of light.

Roughly speaking, a small hole will keep more of your image in focus. A large hole will have a smaller area of your photograph sharply in focus. This is known as Depth of Field.

How is the Aperture size measured?

By aperture size is indicated by an ‘f number’, such as f11. Confusingly, a small number, such as f5.6 indicates a large hole, while a large number, such as f22, is showing a very small hole.

It helped me to picture the number as a measure of the thickness of the blades, from the edge of the lens, although this is not technically accurate.

 

The f number is typically shown on the lens, to indicate the minimum hole size your selected lens is capable of. This is a key consideration for many photographers when purchasing a lens.

Tell me more about Depth of Field

Your camera can precisely focus at only one distance at a time;

thus, there is actually a very small area which is truly in focus.

Whilst the rest of the image may not technically be in focus,

non-sharpness is imperceptible under normal viewing conditions.

Thisis generally known as acceptably sharp.
 

The distance from the nearest to the furthest which is sharp,

is known as your Depth of Field.

Controlling this gives you certain artistic control over your images. A shallow depth of field is common in portrait images, as it allows the viewer to focus on the main subject within the image.  A large depth of field is beneficial in landscapes, where the viewer would normally want to see all of the image in focus

.
Look at the following two images, taken via CameraSim.com (a great tool for practicing, by the way). Which image do you find sucks you in?

This portrait has a shallow depth of field. Notice the blurred background. ISO 100 ž f2.8 ž 43mm ž 1/400 sec

This portrait has a large depth of field. Notice the sharp background. ISO 800 ž f22 ž 43mm ž 1/1000 sec

Is there a trade-off?

You bet! Photography is always a compromise.

First, a large aperture. Because the large hole lets in more light, it’s easier to over-exposure your image. For example, imagine a bright summer’s day, you’re outside taking a portrait.  In these circumstances you could quite easily get too much light.

 

And with a small aperture, you’re letting in a lot less light. This can cause the shutter speed to lengthen, which may in turn produce camera shake.

The large aperture has let in too much light, giving an overexposed image. ISO 800 ž f2.8 ž 43mm ž 1/4000 sec

Now we have a small aperture, which has reduced the light, but caused a knock-on effect with the shutter speed. ISO 200 ž f22 ž 43mm ž 1/10 sec

In both cases, adjusting the ISO may help.

If you are shooting in bright daylight, perhaps moving the subject into a shaded area, or waiting for the sun to move behind a cloud, may help. In extreme cases, buying an ND filter may be beneficial – this screws onto the front of your lens and reduces the amount of light coming into your camera.

If you want a large depth of field, and are struggling with camera shake, using a tripod will probably almost certainly help. Most landscape photographers won’t go anywhere without one!

 

Not every lens will be capable of opening the aperture as wide, or as small, as other lenses. Typically the lens which comes with a Canon camera, will open to a maximum of f3.5 – and even this is only at certain levels of zoom.

How do I adjust the aperture?

Many photographers will keep their camera in Aperture Priority mode

(marked as AV on Canon cameras, and A on Nikon), as this gives

quick control over the depth of field, whilst the camera automatically

sets the shutter speed.


On a Canon camera, you can select AV on the mode dial. Then you

can turn the Main Dial left or right, to change the aperture.


Your selected aperture will be displayed on the LCD display, and in your viewfinder.
 

The LCD display on a Canon camera shows your selected aperture size (f5.6).

This image replicates what you see in your viewfinder, here the camera has has an aperture of f16

Anything Else?

Depth of field can also be affected by your focal length (ie the zoom of your lens). If you are zoomed into your subject, you will magnify a shallow depth of field.


Also, if you want a shallow depth of field, move the subject further away from the background where possible.
 

Study these examples