Controlling depth of field

Everyone will know that depth of field is reduced as the aperture is enlarged. I often use my Canon EOS 5D in aperture-priority (Av) mode, and set the aperture to achieve the depth of field I want. One of the reasons I bought the 5D was that it has a full-frame sensor and the bigger the sensor the easier it is to get a narrow depth of field.

One can use the depth of field preview button to stop the lens down to the taking aperture so that one can see in the viewfinder what is in focus and what isn't. However, this is rather hard to do if the taking aperture is small, as it is likely to be for landscape photos where one wants everything in focus from near by to the horizon. At f16 or f22, the viewfinder is likely to be so dark when stopped down that it is hard to discern what is in focus.

Another way of achieving the widest possible depth of field is to set the lens manually to its hyperfocal distance for the given focal length and taking aperture. A lens set to its hyperfocal distance should be sharp from half that distance out to infinity. A program to calculate hyperfocal distances - and more complex matters related to out of focus blur - can be found on Bob Atkins web site here. Alternatively, if you have Microsoft's EXCEL program, you can download this spreadsheet to calculate hyperfocal distances using whatever Circle of Confusion you like.

A problem that I have found in attempting to set hyperfocal distance in the field is that the distance scales on Canon's lenses are rather vague. For example, here is the distance scale for the 24-70mm f2.8 EF L lens:
As you can see, there are no markings between 3 metres and infinity. To take an example, at a focal length of 50 mm and an aperture of F11, the hyperfocal distance one wants to set on this scale is 11.4 metres. I cannot see any practicable way of doing that given the vagueness of this scale.

Of course, there is not a problem if you are using prime lenses, because they have a depth of field scale for each aperture marked on the lens. This is a reason for using a prime rather than a zoom that I hadn't thought of before.

The only other way one could set a hyperfocal distance on a zoom lens would be to use a tape measure and the autofocus capability. One would measure out the required hyperfocal distance on the ground, autofocus on that point - thus setting the correct distance - and then switch the lens to manual focus (MF) before taking the shot.

Because of this problem, I was quietly thinking to myself that it would be nice if there was some way of saying to the camera "please focus the lens on the hyperfocal distance for the currently-set focal length and aperture". This would be useful for landscape and architectural photographers, and it seems an easy thing for the focussing microchip in the lens to do.

But a chance conversation with a budding young photographer (see his web site here) alerted me to the fact that Canon has completely solved this problem by another, and more powerful feature.

The Canon EOS 1V is Canon's top-of-the-range film SLR camera, launched in the year 2000. In addition to shooting modes like aperture-priority (Av), shutter-priority (Tv) and manual, it has another mode called "depth of field AE" or DEP for short. In this mode, you can autofocus on a subject close to the camera, and then on a subject further away, and the camera will then automatically set the focus and aperture so that these two subjects are sharp and things outside that range are blurred. Thus it offers a quick and accurate means for the photographer to tell the camera what depth of field is wanted, and then the camera sets the aperture and focus appropriately. (It also sets the shutter speed appropriately, but that is just the same as it would be if you were in aperture-priority mode.)

The only disadvantage of the EOS 1V's DEP mode that I can see is that you are stuck with Canon's own idea of what is sharp but, if that is sensibly chosen, that is fine.

This DEP feature seems to me to be rather useful, and the curious thing is that it is not available in any Canon digital camera.

The EOS 5D, 1D mark II and 1Ds mark II all have nothing more than a depth of field preview button to stop the lens down to the taking aperture.

The Canon EOS 30D, launched in February 2006, has a shooting mode called "automatic depth of field AE" or A-DEP for short. In this mode, the camera uses its nine autofocus measuring points to detect the nearest and farthest subjects within the viewfinder area covered by the autofocus points. It then sets the focus, aperture and shutter speed to achieve that depth of field. That is clearly an even faster method than the EOS 1V's, at the cost of a slight loss of control because the photographer is not expressly identifying the near and far subjects. This is the nearest Canon comes to addressing this problem in a digital SLR.

I wonder why Canon has not carried this feature forward into its digital SLRs. It seems that DEP was provided in the EOS 1D and 1Ds but Canon removed it for the mark II versions. Perhaps they did some market research and discovered that most professionals regarded the feature as too automatic, too fiddly or beneath their dignity.

I suppose it is fair to say that, if we are talking about portraits or groups, in most situations the subject will be within a few metres of the camera, a fairly large aperture (low F number) will be used, and hence there is no difficulty in assessing depth of field through the viewfinder. So for those situations the feature is unnecessary. However, it does seem to be a good way of controlling depth of field in other situations - such as landscapes and architectural photographs - where the desired aperture is rather small, and hence the stopped-down viewfinder is rather dark.

Peter Facey, Winchester, England