It’s a common misconception that the f-stop you use will control depth of field (DOF). Aperture setting certainly has an influence, but there are other factors to consider.
DOF is the area in a photograph which is acceptably sharp. Lenses can only focus at a single point. There is always a certain amount in front and behind the focus point which is acceptably sharp. This varies depending on:
- Aperture setting
- Lens focal length
- Camera distance to the subject
- Sensor size.
The transition between what’s sharp and what’s not is gradual. It’s important to learn how to manage the variables to create the look you want in your photographs.
How Sensor Size Affects DOF
The physical dimension of the sensor in your camera affects DOF. Unlike the other variables, it’s not possible for you to change, unless you use a different camera.
Small sensors, such as in phones and compact cameras, give you the most DOF. This is one main reason people upgrade from a phone to a camera. Because they are not able to achieve a shallow depth of field with their phone.
Phone manufacturers are trying to mimic shallow DOF in various ways. But as yet it appears to be little more than a poor gimmick. There is no substitute for size.
Basically, cameras with smaller sensors make photos with more DOF at the same aperture and distance settings. To make comparisons of DOF from different-sized sensors, you must calculate the same effective focal length and aperture settings.
Larger sensors in DSLR and mirrorless cameras have made them popular with video producers. This is because of their capacity for shallow DOF. Traditional video cameras contain small sensors so therefore generally have deeper DOF.
How Camera To Subject Distance Affects DOF
The closer you are to your subject, the less DOF you will have at any given aperture setting, with any lens on every camera. Move further back, and your DOF increases.
This is why it can be challenging when taking close-up photos to have enough DOF. Being very close to your subject may mean you do not get it all in focus. Using macro lenses and close up attachments amplifies this problem.
So if you are still only using your kit lens, you’ll need to move in close to achieve a shallow DOF. This is because these lenses do not have a very wide maximum aperture or long focal length.
Remember that from the point you are focused on 1/3rd of the DOF will be closer to you and 2/3rds of it will be further away. Knowing this can help you choose your point of focus to control you DOF more precisely.
How Lens Focal Length Affects (Apparent) DOF
The longer focal length lens you use the shallower the DOF appears. But it doesn’t actually change.
If you take photos of the same subject with two different focal length lenses, the images made with the wider lens appear to have a deeper DOF. The aperture should remain constant. When you crop the image made with the wider field of view, so the elements in the images are the same size, you will see no real difference.
The idea that longer focal lengths produce a shallower DOF is a myth. Peter West Carey has already written an article for DPS about this based on Matt Brandon’s experimentation. Matt’s images prove the point clearly. It can be a difficult concept to comprehend. Especially if you are predisposed to the popular idea that focal length affects DOF.
How Aperture Affects DOF
The aperture is an adjustable opening within a lens. The primary function is one of the controls used to control the amount of light entering the camera. A narrow aperture setting lets in less light than a wider setting. The settings are measured in f-stops.
Adjusting the aperture setting, (changing the f-stop value,) not only controls the amount of light entering, but also the DOF. Changing the aperture is the most common way photographers choose to control DOF. The wider aperture the shallower the DOF. So the lower f-stop number you choose (eg. f/1.4), the less of your image will be acceptably sharp. Choosing a narrower aperture, a higher f-stop number (eg. f/22), will render more of your photo in focus.
Lenses are made with differing maximum apertures. Typically a kit lens will have a widest aperture value of f/3.5 when the lens is zoomed to its widest focal length. This value changes the more you zoom in. So the widest f-stop at the longest focal length may only be f/6.3. For information please read the article ‘What The Numbers On Your Lens Mean.’
Prime lenses usually have a wider maximum aperture. This is why they are often favored by photographers who like creating photos with a shallower DOF. Popular 50mm lenses have f-stop settings of f/1.8, f/1.4 or even wider.
How Can You See The DOF When Composing A Photo?
Cameras with digital viewfinders or monitors will display the DOF as it will appear in the photo. Because of the small size, it can be difficult to see clearly unless you zoom in.
Cameras such as DSLRs with optical viewfinders will not allow you to see the effect of the DOF unless you use the DOF preview button. This is because the aperture is automatically set to the widest possible. It is adjusted to the f-stop you’ve chosen as you press the shutter release button. If the f-stop were able to be altered while composing, at narrow apertures, the image would appear dark in your viewfinder. You can see this when you use the DOF preview.
Manage Your DOF Well
Keeping all these variables balanced may seem complicated. But it’s important to know how each of them affects DOF so you can manage it well in your photos.
To help you learn how each aspect of DOF works try setting up a few photos and experimenting with them. Not for the sake of making great pictures, but to understand how changing each one affects the look of your images. It will be good to set your camera on a tripod or stable surface for this exercise.
Line up a few objects in your frame which are at different distances from your camera. Set your aperture to its widest – the lowest f-stop number (eg. f/1.4). Get as close to the first object as you can so that your lens will focus on it.
Take a photo of it, then focus on another object further away from you and take another photo. Repeat this with each object further away from you as you have in your frame.
Now repeat this process with a middle range aperture setting and then the narrowest your lens has. Try this with different focal lengths as well.
Then move back and make another series of photos the same way. Repeat this process as you move further back from your subject.
Compare the photos side by side on your computer and take note of the differences in DOF between them. Look at the EXIF data so you can see what your aperture and zoom settings were.
Working through an exercise like this will help you learn to control depth of field. As you can see the effects in your photos it will become less complicated.
originally posted on digital-photography-school.com by Kevin Landwer-Johan