Depth of Field for a 35 mm Camera

When you look at photos, sometimes everything from the nearest object to the most distant object in the image is sharp. In other photos the subject is isolated from objects in front and behind because they are out of focus while the subject of the photograph is sharp.

To find out more about this, look very closely at the ''depth of field" scale marked on the barrel of your lens. The marks indicate the distance of the nearest object in focus and to the most distant object in focus. This is the "depth of field".

The depth of field comes about because when light passing through the lens to focus on the film, it can come by a variety of paths. With a large aperature, the path of differnt rays of light can converge to the image from a wider range of angles. When the aperature is closed down the light can only come from a limited range of angles. This means that the "circles of confusion" are smaller when the light passes through a smaller aperature. In a photograph, the range of objects in focus will be greater when the aperature is smaller. We can use this to good effect in our photography.

The following tables show the depth of field for a 35 mm camera with at 50 mm focal length lens.

The first table shows the depth of field for the camera focused on a subject at a distance of 2.0 m.

The following is a guide to the the distance to the objects that would appear in focus. As you can see, the depth of field increases as the aperature decreases in size.

f-stop	closest	most distant	depth of field
f1.4	1.94 m	2.07 m	0.13 m
f2	1.91 m	2.10 m	0.19 m
f4	1.83 m	2.21 m	0.38 m
f8	1.68 m	2.46 m	0.78 m
f11	1.59 m	2.69 m	1.10 m
f22	1.32 m	4.12 m	2.80 m
pinhole	0.64 m	to the horizon	a very long way

The next table shows the same f-stops, but this time the camera is focused at a distance on 10 m.

f-stop	closest	most distant	depth of field
f1.4	8.57 m	12.01 m	3.44 m
f2	8.70 m	13.14 m	5.06 m
f4	6.77 m	19.14 m	12.37 m
f8	5.11 m	223.21 m	218.10 m
f11	4.32 m	to the horizon	a very long way
f22	2.76 m	to the horizon	a very long way
pinhole	0.85	to the horizon	a very long way

So, if you want a wide depth of field you must use a small aperture, that is, the largest number possible f-stop.
The only drawback to this is that you must use long exposure times, and this will probably mean you will have to use a tripod to hold the camera steady.

If you want to use selective focus to completely isolate your subject from both the foreground and the background, then you will use the largest aperture, with the smallest depth of field.

If you have a zoom lens attached to your camera, you may find that there is no depth of field scale.

Using the Depth of Field Scale

I find the depth of field scale very useful when taking photographs outdoors which require a great deal of depth. You can see from the above tables that if you are interested in photographing an object at a distance of about 10 m with an aperature of f8 there is a great deal of latitude. You don't need to focus at 10 m, but you can focus closer to achieve as much of gthe scene in focus as possible.

The procedure I use is as follows:
a. I work out what objects I want included in the photograph by looking through the view finder.
b. I then select the smallest aperture which I can use without the shutter speed being too slow. (I try to use a speed greater than 1/60 s.)
c. I focus on the nearest object to be included in the photograph and carefully note that point on the distance scale.
d. Find on the depth of field scale the mark corresponding to the nearest limit of the depth of field.
e. Adjust the focus of the lens until the distance of the nearest object to be in focus is beside the near limit of the depth of field scale.
f. Check what distance is next to the far- limit of focus on the depth of field scale. (If infinity is included within the range, then everything in the photograph will be in focus.)
g. Take the photograph. When you print the picture, everything you wanted in focus will be seen clearly in the print.

Example

Suppose you want to photograph a person standing 5.0 m away using a camera with a 50 mm lens. You also want to have the statue at 2.5 m in focus, in front of the person. You would also like the swans on the lake behind to be in focus. This sounds like a tall order, but if you adjust the lens stop to f16, you can adjust the focus so that 2.5 m is beside the f16 near limit of depth of field and then you will find that everything from 2.5 m to infinity will appear to be in focus.

You will rapidly learn to use this property of lenses to get the most out of your depth of field. When you put in the effort, you will find that the results are very rewarding.

Buy a tripod and use it. Buy a camera- tripod quick release as well.