Understanding depth of field
Depth of field and its use and control in your photography is crucial. When used effectively, depth of field can make the difference between a good photograph and a great one.
Depth of field can be used to isolate your subject in the scene, draw the viewer’s focus to specific areas of the photograph, or even blur your subject entirely for artistic effect. See below for an example of an identical subject photographed at both f8 and f2 using slide film. Notice the wooden planks have been isolated from the front and rear elements of the photograph.
In simple terms, depth of field (DoF) is the zone of sharpest focus in front of, around and behind the subject being photographed. A shallow DoF will isolate your subject, whereas a deeper DoF will bring other elements in the picture into focus.
An added result of DoF is bokeh. Taken from the Japanese “暈け (boke)”, meaning “blur” or “haze”. Bokeh is a term used to describe the out of focus areas which sit outside of your DoF. An image taken using a shallow DoF will generally have more bokeh than an image taken with a medium, or large DoF.
Additionally, the bokeh captured by your camera is influenced by the shape of your lens’ aperture diaphragm. Round bokeh, pentagonal bokeh, hexagonal bokeh, it’s all possible. See the image below for an example of both hexagonal and round bokeh from the same lens, simply by using different apertures.
How do I use DoF in my photography?
If you are new to photography, it’s likely that you won’t yet be taking advantage of how DoF can enhance your photos. I find the best way to describe how to use DoF in photography is as follows:
- The closer you are to your subject, the shallower the DoF will be.
- The wider your aperture (smaller f-stop number), the shallower the DoF.
- When shooting from a distance, your DoF will generally extend more behind your subject, than in front. Roughly speaking this works out to 1/3 being in front of, and 2/3 behind your subject.
- If you are using a shallow DoF, you can better isolate your subject the further they are from objects in the background.
Another thing to remember for SLR cameras is that your viewfinder will nearly always be showing you an image using the maximum aperture of the lens. These cameras will also most likely have a depth of field preview function, which will allow you to “stop down” to the aperture you’ll be taking the photograph at. This feature will vary from camera to camera but you can normally find it on the lens, or camera body.
You can use the DoF preview on your camera to verify that there’s enough coverage for your entire subject (the product you are shooting, the person you are shooting, or the area of the subject you want to keep in focus).
Most Rangefinder, Point-and-Shoot and TLR cameras (with one or two exceptions), do not have DoF preview, so you’ll have to use your best judgement!.
Factors affecting depth of field
As I mentioned previously, you can use DoF to totally isolate your entire subject, or only bring elements of it into sharp relief. An example would be portrait photography, where you may want to use a very shallow DoF to keep only one eye in focus. While this would have an artistic effect, it wouldn’t be appropriate for say, a passport, or driver’s license!
There are two main factors that affect DoF and how you control them will determine the results of your photography. These factors are:
- The aperture (f-stop) of your lens
- The distance of the subject to the camera
The focal length of your lens will also have an effect on your DoF but I’m (stupidly?) assuming that most readers will have lenses of focal lengths ranging from 20-150mm (35mm equivalent), so I’ll leave this factor for a future article.
Aperture number (f-stop) is the term used to the amount of light your lens is able to “throw” onto your film. The greater the light being allowed in, the shallower the depth of field you can create. In short:
For more isolated subjects use a large aperture:
smaller f-number = shallower depth of field
For less isolated subjects use a small aperture
larger f-number = deeper depth of field
If we’re shooting landscapes and street scenes, we’ll normally try to use smaller apertures such as f8 or f11 in order to try and keep as much in focus as possible. With portraiture, we may choose much larger apertures, such as f1.4, f2, or f4 in order to isolate our subjects, or aspects of them.
Another factor to consider when deciding on your DoF is the light conditions around you. If you want to create a very deep depth of field for a landscape photo, you will have to use a smaller aperture and slower shutter speed. Remember that if you use a slower shutter speed, you may also have to deal with motion blur, or camera shake.
Sometimes this can be used to artistic effect, so the key message here is to experiment.
Distance and depth of field
The closer your subject is to your camera, the shallower your depth of field becomes. Lets say you have a DoF of about 30cm at a distance of 1m at f8. If your lens’ max aperture is f1.4, then you may find your DoF shrinking to only a few centimeters. When using extension tubes, or macro bellows equipment to get even closer to your subject, you may find that your DoF is mere millimeters deep, even at f16!
Can’t I just get everything in focus?
Yes, and it’s quite simple. Every lens has what’s called a “hyperfocal distance”. This means that when you set your lens to this distance, the depth of field will extend from about half the selected distance all the way to infinity. As a rule of thumb, I normally use f8 at about 2-3 meters. It works for me but you may want to be more specific.
Depth of field isn’t a science, it’s an art (ok, so it’s part of the science of microscopy, apparently). To put it another way, depth of field is a great tool to add to your creative arsenal and will help you to bring out the best in your photography.
Understanding how and when to use it as well as how and when NOT to use it will help you to create stunning, inviting images that draw the viewer in for more.
Finally, once you develop a mastery of depth of field, it becomes another tool you can exercise mentally before taking that shot and burning that film.
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