Originally posted by Class A Please visit a DOF calculator and change the "lens speed" and see what it does to the DOF. .
I said that DOF is not involved in the definition of lens speed (or aperture) at all; not that DOF is independent on aperture (DOF is dependent on aperture, focal lenght, focusing distance and subject magnification). A fast lens is equally fast regardless of format you use it on. You don't get faster lens by going larger in format and if you go large enough there are no fast lenses at all due to cost/weight issues. Lens speed is tied to exposure by definition, not DOF.
One need to have a clear grasp of the definitions of aperture, lens speed, focal lenght, angle of view and DOF or else the discussion becomes meaningless.
---------- Post added 08-13-14 at 02:38 PM ----------
Originally posted by ElJamoquio Hmm. I even put fastest EQUIVALENT glass in the text, and I still get the same reply. 'Speed' refers to the faster shutter speed. We both know this.
Since you have higher IQ with larger formats, you can shoot at the same SPEED with the same image quality when the larger format is at a higher ISO.
You might not like equivalence, Pal, but I think you should get used to other people considering it a useful tool.[COLOR="Silver"]
You mean thinner DOF. This is not the same as equivalent speed. Equivalent speed gives equivalent exposure by definition. Equivalent DOF do not.
"Equivalency" compensate rigidly this reduction in light by boosting the ISO correspondingly. However, no photographer respond to light reduction by boosting ISO exclusively. Hence, it is no law but an option.
In addition, the "same DOF" argument is only partly true as far as similar images are concerned. It is only so at certain focusing distances. Formats are simply not DOF equivalent. Note that smaller formats have higher magnification (with comparable lenses). Magnification is one of the factors that define DOF.