What is it?
What affects it?
Why do I care / How can I use it?
A Wikipedia definition:
“In optics, particularly as it relates to film and photography, depth of field (DOF), also called focus range or effective focus range, is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.”
Let me emphasize ” … a lens can precisely focus at only one distance at a time …”
Here’s an image with a foreground and a background clearly out of focus, and a 2 foot deep area that appears to be in focus. I focused on the bird’s eye and at f/11.0 had enough depth of field for the whole bird to appear sharp.
Three things affect Depth of Field
1) Lens Focal Length
2) Lens aperture (f-stop)
3) Camera to focus point distance
Below are three examples showing the impact of focal length. All three were shot at an aperture of f/8.0 and all three were shot from a fixed location. All three had the rock out in the water as the focal point.The first was shot at 16 mm and has apparent sharpness from foreground rocks all the way across the river to the trees on the other shore – more than 100 yards away. The second shot was at 70 mm. The foreground rocks are gone. The focus point rock is sharp for it full depth but the trees on the far shore are getting pretty fuzzy. The third was shot at 300 mm. Both the foreground rocks and the distant shore are gone. The focus point rock is not sharp for it’s full depth. So at 16 mm we had apparent sharpness for over 100 yards. At 70 mm the sharpness was down to less than 10 feet. At 300 mm the apparent sharpness was further reduced to less than 3 feet.
Aperture – what is it,
how does it affect
Depth of Field?
Aperture is stated as the f-stop or f-number. The f stop is the focal length in millimeters divided by the diameter of the diaphragm opening. Here are two images of a lens at different f-stops. You can clearly see the diaphragm blades and the difference in the size of the opening.
The bigger the hole in the diaphragm the lower the f-stop. The smaller the hole the bigger the f-stop. In general the bigger the f-stop number (smaller hole) the more depth of field. But as shown in the previous example the shorter the focal length the greater the depth of field so the effect of changing f-stop is less pronounced at shorter focal lengths. Here are two sets of four images. One was shot at 16 mm and one was shot at 70 mm. The f-stops used at 16 mm were f/3.5 (lens wide open), f/7.1, f/16.0, and f/22.0. If you study the example closely you will see slight differences in depth of field – but only slight. The four shots at 70 mm were shot at f/5.3 (lens wide open), f/8.0, f/16.0. and f/22. In this series you will see a more pronounced change in depth of field as the lens is stopped down. [Note, the 16 mm f/7.1 was supposed to be 16 mm f/8.0 but I accidentally adjusted it as I was struggling to keep balance in a precarious position – chalk it up to operator error! ]
Here’s the 16 mm series.
And here’s the 70 mm series – watch the distant trees go from quite blurry to almost in focus.
So far we’ve looked at focal length and f-stop and how they affect depth of field. No lets look at something that is not as apparent to the casual photographer – camera to subject distance.
Here are two image shot with the same lens at 300 mm. In the first the camera to subject distance is less than a foot and the apparent depth of field is only a few inches. The upper twig in the image is less than 2 inches behind the lower twig on the Aspen tree. The second image was used earlier and shows a rock where the depth of field is at best 3 feet. The rock is a good 10 to 15 feet away from the camera. So in this example an increase of camera to subject distance on the order of 10 to 12 times has resulted in an increase of apparent sharpness on the order of 10 to 12 times. I’m not sure that you can expect such a linear correlation of depth of field to subject distance to hold.
Here are two more examples. The same bird as used in the opening image shot at a distance of less than 50 feet resulting in a depth of field of about two feet. The second, shot with the same 500 mm lens has a subject distance of about 8/10ths of a mile and has a depth of field of more than a mile.
The really long shot!
How can this be? Here’s the church shot with a much shorter lens.
And here’s an annotated aerial shot showing the spatial relationship of the elements in the long shot.
Think about it this way. With a subject at 8/10ths of a mile away the lens is effectively focused at infinity and anything farther away is still infinity.
So where do you focus? When Carl Heilman is trying to maximize depth of field for an image he is making, he says to manually move your focus back and forth until the foreground and background are equally out of focus. I find that easier said than done. Other sources advise you to focus 1/3rd of the way into your image. That’s also easier said than done in a lot of situations – like where you have a shore bound rock with featureless water behind it for quite a distance. Most modern DSLR cameras have something called a “Depth of Field Preview Button.” This button stops the lens down so that in theory you can see the actual depth of field when looking through the view finder. In practice this is also sketchy at best as stopping down usually darken the viewfinder to the point that is difficult to see much detail in the viewfinder. I usually pick some object past where I want the near edge of the acceptably sharp area to be and then review the image by using a magnifying loop that will block stray light from hitting the display. (See https://www.bhphotovideo.com/c/product/899482-REG/Hoodman_ch32_Compact_Hoodloupe_3_2_Viewing.html for an example product made by Hoodman.) For wildlife the choice is easy – focus on the eyes.
When discussing this topic you might hear the term “Hyperfocal Distance.” Hyperfocal distance is the distance between a camera lens and the closest object that is in focus when the lens is focused at infinity. Alas, back when cameras were mostly manual and there were no connections for sending lens information to body computers, most lenses had marking to facilitate focusing at the hyperfocal distance. Here are three images of lenses with and without such markings.
The first lens is a 50 mm prime lens. There are a series of short colored lines on the lens barrel between the f stop ring and the focus ring. The colors correspond to the colors on the numbers on the f stop ring. So if the f stop ring is set for f/16.0 (blue colored number), you would line up the infinity mark on the focus ring with the left blue line and you are at the hyperfocal distance. You can then read the focus ring value lined up with the right blue line and see what the distance to the near end of the depth of field area is.
The second lens is a zoom lens and the lines are all white but are curved. This lens is a push/pull zoom. You zoom by sliding the end of the lens barrel back and forth. You basically read the focus ring numbers the same as on the prime lens but the curved lines clearly show the decreasing depth of field as you zoom to longer focal lengths.
The last lens is a modern zoom with no hyperfocal distance indicators and no f stop ring. You set f stop with a dial on the lens body. The computer in the body tells the lens what f stop to use. So you are back to having to figure out on your own where to focus.
Bokeh? (pronounced bōˈkā )
It’s not exactly part of depth of field but it can be a consideration when deliberately going for a shallow depth of field. You’ve seen it on Christmas cards. The kid standing in front of the tree. The kid is sharp but the tree isn’t and the lights on the tree are rendered as soft globs of bright color. What we’re talking about is how a lens renders out of focus highlights. This is pretty much a product of the number and shape of blades in the lens diaphragm. It is not unheard of for a photographer to make a lens purchase decision based on how it renders bokeh. Here are two versions of the same subject. In the first I focused sharply to show where the shiny spot on the rocks was. In the second I switched to manual focus and deliberately racked the lens out of focus to to generate the nine sided spot in the second image.
Why should you care about Depth of Field?
Well, for sure you probably want enough depth to capture all of your subject. On the other hand you might want to reduce depth to reduce the distraction of extraneous items in your frame. Here’s an image where I wanted some foreground rocks and the far shore to both be in focus. The sheer expanse of the subject dictated a wide lens and I then made sure to stop down enough to get the desired depth.
Here are a couple of bird shots where the distance to the subject dictated a fairly long focal length. I then had to pick an f stop that would give me enough depth to get the whole bird in focus. Once those two decisions were made I then selected an ISO high enough to get the shutter speed high enough to eliminate blur resulting from camera movement as I was shooting without a tripod.
Here’s an image where I wanted enough depth to capture the mushrooms but wanted to limit it so that the distracting background was rendered out of focus.
When shooting at extremely close distances, depth of field becomes very shallow. There are a couple of techniques that can help. One is to make sure as much of your subject as possible is in one plane. Here are two flies. By shooting side on I only needed enough depth to capture the width of the flies. If I shot head on I would have needed twice as much depth.
Here’s a tiny lilac blossom that does not have enough depth to capture the whole blossom.
After taking it I read up on a technique called “Focus Stacking.” The camera need to be locked down and the subject need to be motionless. So this isn’t something you do hand held on a windy day! I have tried it exactly once and clearly need to refine my process. Here’s the camera and subject that I used for my test of the technique.
I had an extension tube between the camera body and the lens. This allows closer focus than with the lens alone. It also reduces depth of field. The items to the left of the camera are a radio frequency remote release that allows triggering the shutter without touching the camera thus reducing the likelihood of camera movement during exposure. The other item is my Hoodman loupe. Looking through the viewfinder in this position was difficult and also increased the risk of moving the camera during focusing. So using live view with the loupe on the screen allowed a more delicate focusing process. I took three shots, each focused on a different mushroom. I combined the three shot in Photoshop resulting in more depth of field than I could get in a single frame. Here are the original three shots.
Here’s the combined final image.
Here are links to a few articles of interest
And finally, I highly recommend checking out a free application called DOF Master available for multiple platforms. The web version has a series of boxes that are either filled from drop down menus or filled with numerical data about you settings. When the form is filled you will be presented with a report showing the near limit, far limit and total depth. Check it out at http://www.dofmaster.com/dofjs.html