Originally posted by wed7 Another Question about DoF. They say DoF or openning of lenses varies as per focal length. Now which has more DoF? For example, F/8 of a 50mm lens or F/8 of 200mm?
Remember that an f-stop is a ratio, the ratio of the maximum aperture opening to the lens' focal length. And DOF is a complex function of photography, presentation and perception. If we look just at the photographic elements, just at lenses, we get this: At the same f-stop, a shorter/wider lens will have thicker DOF than will a longer lens. And at the same focal length, a lens with a tighter f-stop (smaller iris opening) will have thicker DOF than a lens with a wider f-stop. Those are the basics.
There is a simple way to compare DOF of various lenses and settings, with all else being equal. (Of course, all else is NOT equal, but we'll ignore that right now.) This simple metric is the DOF INDEX and it goes like this:
divide focal length by f-stop. So a 50/2 lens has an index of 25. So does a 100/4 lens. This index doesn't say what the DOF is, just that 50/2 and 100/4 and 200/8 and 400/16 will have similar DOF, and that will be thinner than 50/3.5 or 100/5.6 or 200/11. The larger the index number, the thinner the DOF. Which has thinner DOF, a very expensive 58/1.2 or a very cheap 135/2.8? Do the math and see.
Originally posted by Lowell Goudge What is worse, is that in today's environment where every one views an image on a different size monitor, DOF is almost irrelevant.
Amen. At least, DOF as an objective calcuable factor. We can still judge *relative* DOFs, that one image may seem to have thicker or thinner DOF than another, but it can't be pinned down. Too many variables. So when shooting, just go with the basics:
* For thinner DOF, use longer focal lengths and/or wider apertures and/or larger frames and/or closer working distances.
* For thicker DOF, use shorter focal lengths and/or tighter apertures and/or smaller frames and/or further distances.
Those are the DOF ROTs (rules of thumb). They handily ignore the presentation and perception areas, and all the minutiae of diffraction and Circles of Confusion and all that. They ignore what can be done with light, or with split-dioptre lenses or massively-tilted lensboards, or the various other tricks that can affect DOF. In a word, they over-simplify. But those are the factors a photographer can most easily control.