Depth of Field

You have to realise that the image on the film is only truly sharp with the light from the point you are focused at.

A lens has a constant bending effect on light rays coming from the subjects. When we focus at a mid-distant object, rays of light from a more distant object are bent the same amount and come to focus in front of the film, while light from a closer subject focus behind the film.

Circle of Confusion, is precisely that, a circle of confused image that looks sharp to you ...like the sun through a magnifying glass or your lens if you take it out of the camera.

There is an acceptable limit to this .....drawn as a circle. From the red and green lines you can see that objects closer and further away will 'appear' sharp.since the rays fall within the circle of confusion ..... the distance between the subject is the depth of field while in the camera, it is called depth of focus. An accepted standard for 35mm work is 1/1000 inch diameter.

Many say that for most shots depth of field stretches from 1/3 in front to 2/3 behind the focus point. However, this only applies, with a 50mm lens for example, from 25ft out to infinity. With distances closer the proportion gradually changes to about 50/50 at around four feet. As you get even closer to the subject it is pretty academic since depth of field reduces to just millimetres either side of the subject.

If you have a digital camera much of the following will be of no use so I hope you have an SLR or similar ..... This shows the effect of closing down the aperture .. the f numbers f2 > f2.8 >f4 >f5.6 > etc etc Exaggerated in the drawing where when stopped down the blue line goes off the drawing ... for clarity, only half of the rays from near and far points are drawn..

OK now some photos ... how it works on the camera using lens from my Pentax.

Here is my 35mm lens focused at 5'6" If I was to close the aperture down to f/22 I would have depth of field from beyond infinity back to about 2'6" ... the entire range of the scale.

But working at f/11 [the aperture setting ring has 11 clicked to the orange dot] I have to look for the pair of '11' in the middle row which tell me DOF ranges from perhaps 10'6" down to 3'7"


Here with the 55mm lens, also set at f/11 there is no f/11 indicated just the mark between f/8 and f/16 [ That little orange line to the left of the diamond is the focus point when using infra-red film]

So DOF ranges from perhaps 40ft back to 9ft when focused on 15ft.


Now with the 135mm the indicating lines are compressed together and say I was shooting a portrait close-up at 6'6" at f/11 it would be a matter of inches either side. Even at f/22 it doesn't amount to a foot of DoF

Hyperfocal distances

In the Ilford Manual of Photography there is a set of tables calculated for a circle of confusion of 1/1000 inch.

If the camera is focused at infinity then everything from the HD out to infinity will be acceptably sharp. The smaller the aperture being used the shorter the distance out to the HD

HD for a 2 inch lens at 1/1000" Circle of Confusion

F/1 168ft

F/2 84ft

F/4 42ft

F/8 21ft

F/16 10' 6"

You can see a definite relationship in the above.

Depth of field table extract for HD of 20ft .....

Inf. 20' 10' 6'8" 5'

Focus on 10ft and DOF ranges from 6'8" to 20ft

Likewise on 6'8" DOF is from 5ft to 10ft.

If you know the HD for the lens and aperture you are using, a table will give you the DOF since if you focus at the HD you will be 'sharp' from half that distance out to infinity. In the above at f/8 from 10'6" out to infinity.


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