Depth-of-Field In Depth
Written by: steve
By Steve Russell
For the beginning photographer, especially when using a DSLR, depth-of-field can make absolutely no sense. For the accomplished photographer it’s an invaluable tool for creating the desired result when photographing landscapes, portraits, wildlife, etc.
In its simplest form, depth-of-field is the area, or depth of area, in front of the lens that will be in focus when the photo is taken. Depending on the focal length of the lens and the aperture being used, it begins a determinable distance from the lens and ends at a determinable distance from the lens. Okay, for all you “Picky Patty’s” out there, it’s actually measured from the sensor or in film cameras from the film plane. Knowing that it’s actually the distance from the sensor instead of the front of the lens can be very important in macro photography; in landscape photography not so much.
You might ask, “Why do I care about depth-of-field if my subject is in focus? Because there are times when you want or need part of the environment to be out of focus or blurred. Knowing and understanding depth-of-field gives you the ability to manipulate it and create the image you desire.
For example, the photographer capturing this image of a village nestled in the landscape of central Malawi was able to keep both the mountains on the far horizon and the boulder in the foreground in focus.
In this image of a thistle, I was able to keep the subject in sharp focus while the entire background is completely blurred and only provides background color and some texture to the image.
There are three variables for determining the depth-of-field in a specific situation, the aperture you’re using, the focal length of the lens and the size of the sensor in your camera.
Let’s get sensor size out of the way first. What the sensor size actually does is affect the apparent focal length of the lens. For example, if I use an 85mm lens with my Canon 5D Mark IIcamera, the apparent focal length of the lens is 85mm. However, if I attach that same 85mm lens to my Canon 7D(Body Only)camera, because the sensor is smaller in the 7D, the apparent focal length of the lens becomes 136mm. Therefore, if I’m using my 7D and want to calculate the depth-of-field, I have to use 136mm instead of 85mm.
Next is the focal length of the lens. This is relatively simple because the focal length, in millimeters, is indicated on the lens. If your lens has a fixed focal length, like the Canon EF 85mm f1.2L II Lens, all you have to do is apply the sensor multiplier, if applicable, and you know the focal length you’re using. It’s a little more complicated if you’re using a zoom lens, like the Canon EF 24-105mm f/4 L Lens, but if you know the depth of field at either extreme then you can make a reasonable guess. I’ll come back to this later. The important thing to remember here is that longer lenses (telephoto lenses) have less depth-of-field than shorter lenses (wide angle lenses), all other settings being the same.
Aperture is the last variable in depth-of-field. The smaller the aperture (opening of the diaphragm of the lens that allows light to reach the sensor) the greater the depth-of-field; the larger the aperture the smaller or narrower the depth-of-field. This becomes more confusing for relatively inexperienced photographers because the indication of the size of the aperture is counter-intuitive. The larger the number or f-stop, the smaller the aperture. In other words, at f/16 the aperture opening is small, while at f/1.8 the aperture is very large.
Here are two examples using the 85mm lens:
• Using my Canon 5D Mark II or a full frame sensor camera, with the camera 25 feet from the subject and using an aperture of f/1.8, the depth-of-field (or the distance between the near focusing limit and the far focusing limit) is 2.8 feet.
• Using the same equipment and distance from the subject but an aperture of f/16, the depth-of-field is 33.4 feet.
As you can see, the depth-of-field can vary greatly when only one of the variables is changed.
I’m going to add one more bit of confusion before I try to make it clearer. Using the second example above with the depth-of-field of 33.4 feet, if you focus on a particular subject the depth-of-field begins at 8.3 feet in front of the subject and extends to 25.1 feet beyond the subject or focus point. Thus, if you focus on a subject and without changing anything except your position, you can move up to 8 feet closer to your subject and the subject and everything 33.1 feet beyond your subject will be in focus or conversely, if you move further away from your subject more of the depth-of-field is in front of the subject and less is behind. In other words, once the focal length, aperture and focus are all set on the camera, the depth-of-field is fixed and moves with you and the camera as you move. Knowing and becoming comfortable with this can be invaluable
I recognize that this seems really complicated to most beginning photographers and many intermediate ones as well, but here are some things to keep in mind that should help.
• Most DSLR cameras have a depth-of-field preview button. Refer to your owner’s manual to find where it’s located. By pressing or releasing the depth-of-field preview, the lens diaphragm is stopped down to the aperture you have set on the camera which will allow you to see what’s in focus and what isn’t. I’ve never been a great fan of depth-of-field preview because the image you see is really dim at smaller f-stops and it’s difficult to see clearly. This is because when taking a photo, the sensor collects additive light as long as the shutter is open. When you’re looking through the viewfinder, the light your eye sees isn’t additive so it doesn’t get brighter the longer you look.
• With experience you’ll become more and more familiar with depth-of-field and as you do you’ll be able to capture the effect you want by guesstimating. Except for sometimes in macro-photography, you almost never have to be accurate down a foot or a fraction of a foot. Most of the time “close enough” works and as long as you’re familiar with depth-of-field and how it works you can develop a comfort in working with it.
• I believe that if you know a subject somewhat in depth, then it’s easier to generally apply the principles and with practice you can become very proficient with it. With that in mind, I want to refer you to a web site I found that has a depth-of-field calculator, tables, and more information than the vast majority of photographers will ever need. There are many websites and books that will provide the same information but I found this one entitled DOFMaster to be very helpful.
While it may seem that way to some of you, this article isn’t intended to be a Master’s Dissertation on photography optics and the effects of defocus blur and diffraction blur. There is much more to the subject than is presented here. Hopefully, there is enough information contained herein to help you understand the subject and to help you become a better photographer.
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