Bob Blakely wrote: > > Many folks have erroneous beliefs which they "know" to be true Indeed. > So that all can see that DOF is dependent on the focal length of the lens as > I said, I will provide my example from: > > http://www.silverlight.co.uk/resources/dof_calc.html > > 100mm lens at f/4 set at 10 meters... Total DOF = 2.659 meters. > 300mm lens at f/4 set at 10 meters... Total DOF = 0.285 meters. That is true so long as you wish to produce two images which have a different field of view. And that is fine. On the one hand you may wish to isolate a face from a croud, on the other you may wish to show the crowd, both from the same vantage point, both with the same aperture, and incidentally both with the same shutter speed, but with different focal lengths. What will you notice: 1) the one taken with the shorter lens shown a greater DOF 2) the one with the shorter lens shows less motion blur 3) the one with the shorter lens shows fewer efects from camera shake 4) shadow detail differs 5) the perspecive is unaltered 6) grain effects may be more apparant with the longer lens 7) the image is a totally different one. 8) both images appear to be in focus But how much of this is due to the focal length, and how much is due to our imagination? If we blow up a bit of the image taken with the shorter lens to match the other image we will find: 1) there is no difference in DOF 2) there is no difference in motion blur 3) there is no difference in the effects from camera shake 4) shadow detail differs 5) the perspecive is unaltered 6) grain effects will be hugely more apparant in the image from the shorter lens 7) the image is essentially the same. 8) The image from the shorter lens is out of focus So, although we can use a shorter lens to easily get an image with greater DOF, that greater DOF is not because the lens is somehow magical, but because the image is rendered at a lower magnification. The identical circles of confusion are printed smaller such that many of them are unable to be resolved by the eye. Given that we want to take a particular image from a particular perspective, it matters not whether we : a) use a wide angle lens and crop the centre b) use a long lens and create a montage c) use a focal length that fits the desired image to the film format. However I know which one I'd choose. But the only reason is ease of use. It's a pain to always use a super fine grained film, and tools to help with critical focussing, then have to cope with huge enlargements. Likewise it's a pain to have to try to seamlessly join several images together. It makes a lot more sense to use an "appropriate" focal length. In fact it makes so much sense that many people start to thing that it is the focal length itself which we are choosing. Indeed, I have been guilty of telling people to use a longer lens when photographing people. I have been guilty of not taking the seconds to explain that this is the same as moving back with a shorter lens and cropping. And then taking hours to explain why. Yes Bob, if it's a different cropping you desire, or a different DOF (with no other constraints) then changing to a shorter lens will do it. Steve
