Last week, DigiKnow explained the concept of Depth of Field (DOF) and mentioned three factors affecting DOF:
- Focal Length
This weeks post looks more closely at Apertures and the technical side, along with how apertures are used to control DOF.
What are apertures?
Apertures are the size of the opening at the end of the lens (much like the pupils in our eyes). By changing the size of the aperture, this opening/closing increases/decreases the aperture to let more/less light enter in to the camera.
In camera, apertures are controlled by F-numbers. These are square root numbers. It would be nice if the F-numbers ranged from 1 to 10, but they don’t. Examples between apertures would be F3.5, F5.6 and F8.0.
Apertures are controlled by the camera but apertures live in the lens. Different lenses have varying ranges of aperture available.
Apertures and Depth of Field
The diagram below shows a number of apertures, there are others available. Please note the size of the openings available and the F-numbers. Larger apertures (toward the left) have smaller F-numbers and smaller apertures (toward the right) have larger F-numbers.
Small F-numbers result in small (or shallow) DOF. Imagine F2.8 (small F-number but large aperture) as the relaxed elastic band (i.e., at 2.8 inches) and whatever fits within it is in focus. Shallow DOF is often used in creative photography.
Thus large F-numbers result in large (or broad) DOF, i.e., F16 (large F-number but small aperture) would be the stretched band (i.e., at 16 meters) where more detail is captured within the scene. This could be for street, documentary or landscape photography but to name a few.
Using F-numbers (Apertures)
To use F-numbers, change the camera to Aperture (A) or Aperture Variable (AV) mode on the mode dial. Whilst using Aperture priority, this is considered a semi-automatic mode.
There are many features you can change and we covered Exposure Compensation and White Balance previously. However, when using Aperture priority, its beneficial as the camera compensates some settings to ensure correct exposure.
Thus, we will only concentrate on aperture affecting DOF, not exposure.
There now follows several images to demonstrate depth of field and apertures. Note, the camera is always the same distance from the subject, the only changing factor was the F-number (aperture).
In the first image, this is a shallow DOF as only the foreground ducky is sharp. This was captured at F4.0.
The second image is still shallow DOF but note how the two duckies in the background are less blurred. This was captured at F8.0.
The final image is the broadest DOF. This was captured at F16.0.
As the F-number gets bigger, the depth of field broadens out (and vice versa).
When do I use what aperture?
A fair guide is to split the available apertures in to groups:
Close subjects (i.e. subjects 6-12 inches away, intentional shallow DOF)
Mid distance subjects – documentary/portraits (i.e. subjects 1-5 meters away, mid-DOF)
Distant subjects – landscapes (i.e. subjects 5+ meters away, broad DOF)
You might notice as you increase F-number, shutter speeds get longer (slower). This makes it impossible to hand hold the camera and get a sharp image.
Using aperture priority (A or AV on the Mode Dial), set up a small subject 8-10 inches away from the camera. Make sure the camera doesn’t move during the task and also ensure you are shooting straight at the subject and can see the background behind it (not downwards).
- Focus on the subject.
- Take 2 photos – one at F3.5 and one at F22.
- Listen to the camera as you capture the images.
- Playback the images.
The image at F3.5 should result in a blurred background and when capturing the image, the shutter speed should sound quick.
The image at F22 should result in a sharper background and when capturing the image, the shutter speed should have slowed down.
Repeat task 1 but move the subject an arms length away from the camera. Don’t zoom in.
How do the results differ?
Repeat task 2 with the subject at arms length, but this time zoom in using the lens.
How do the results differ?
The three factors affecting DOF are:
- focal length, and
Task 1 – When the subject was close (distance) to the camera, at F3.5 (aperture), the background is blurred. At F22 (aperture), the background is sharper.
Task 2 – When the subject is an arms length away (distance), F3.5 and F22 (aperture) look reasonably similar. It would be hard to tell what aperture was used when viewed.
Task 3 – When the subject is an arms length away (distance) and the lens is zoomed in (focal length), F3.5 (aperture) looks softer. F22 (aperture) still looks acceptably sharp.
- Apertures control depth of field via F-numbers
- Smaller F-numbers are large apertures (and vice versa)
- Smaller F-numbers results in shallow DOF (and vice versa)
- The closer a subject is at F3.5, the quicker the background blurs
- DOF broadens with distance regardless of F-number
- Aperture Priority mode is a semi-automatic mode and the camera compensates shutter speed to ensure correct exposure
Next week, we look at the aperture/shutter speed relationship in more detail. We look forward to to you joining us next week.