No. Not withstanding your long post, you are wrong. If in my example, you crop and blow up the image from the 100mm lens to match that of the 300mm lens, and further, if the subject contains a (large) ruler set foreground to background and at an angle such that the scale can be read, and assuming perfect film (we're only talking about the lens) then: 1.) The 300 mm lens will appear to have a depth of field of about 0.29 meters. 2.) The 100 mm lens will appear to have the 2.66 meter depth of field reduced to about 0.89 meters (to the same degree of apparent sharpness on the ruler as the 100 mm lens). 0.29 meters != 0.89 meters. It's not exactly the same. I've done this quite a bit with different macro lenses for industrial photography (hence the reference to the ruler, though it wouldn't actually be practical at 10 meters). What I observe agrees with what I've said and with optical theory. Regards, Bob... From: "Steve Hodges" <shodges@wantree.com.au> > 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 >
