Thursday, April 03, 2014

New Thoughts About Photography

I figured out today why all my pictures are so lousy.  I have been going on the theory that the object of photography is to get everything in focus.  It isn't. The object of photography is to get everything out of focus except the subject.

I have read about depth-of-field and assumed it was a study in getting as much as possible into the field. For single subjects one wants shallow depth of field not deep, so that everything behind the subject, and even things in front of it, are blurry.

Which explains why there is such a fetish about fast lenses with wide apertures. For otherwise identical lenses, the faster one might sell for four, eight, or ten times as much as the slower one. A Nikon 70-300mm f/2.8 VR (vibration reduction) lens sells for $2160. A Nikon 70-300mm f/4.5-5.6 VR lens sells for $465, both from B&H of New York.

I had been told, and believed, that the purpose of fast lenses was to enable one to shoot in dimmer light. It isn't.  At least not primarily. The wider the aperture, the shallower the depth of field.  With a wide aperture lens, one opens to full aperture, focuses on the conductor, and voila!  Michael Tilson Thomas with his hair flying and each hair in focus and a bunch of soft focus or blurry musicians barely noticed behind him. With a slower lens one does the same thing and gets a whole bunch of people on a stage, one of whom might be Thomas.

Notice that the picture above is of Michael Tilson Thomas. Had it been taken with a slower, smaller aperture lense, the depth of field would have been greater and the guy behind Thomas would have been in focus too. Which would have made it a picture of both of them which was not what one wanted.

The problem here is that I am not about to spend two grand for a lens. Indeed, I am not crazy about spending four hundred odd dollars for one either.

Fortunately, depth of field shrinks with lens length. Wide angle lenses have deep fields. A longer, i.e. narrower field of view lens, also has a shallower depth of field at the same aperture. A 500mm lens has a much shallower depth of field at f/8 than a 35mm lens has at f/8. 

The problem is that telephoto lenses are prohibitively expensive. A Nikon 500mm f/4 lens costs a quick $8000.  It is 16 inches long and weighs over 8 pounds, not fun to hang from a strap around one's neck. The solution is thorough shopping. One can occasionally find an old Nikon 500mm f/8 reflex lens, which is not a lens at all but a parabolic mirror disguised as one. It is 7 inches long, fairly light, and can be had for about $200 if one can find one.

One should note that f/4 is not twice as fast as f/8 but four time as fast.  And by 'fast' we mean amount of exposure time to get the same exposure.  With a long lens this is a big deal because the longer a lens is and the narrower of an angle it sees, the more the jiggle of one's hands blurs the image.  Which is crucial if one is shooting hand-held.  But shooting handheld with that long a lens is folly.  One mounts the camera and lens on a tripod which holds it perfectly steady absent high winds or earthquakes.  So the four-fold difference in speed matters far less.

The great increase in "film" speed from ISO 400 in the old days to fancy modern cameras that shoot at ISO 25,600, or 640 times as fast, is more than 18 stops.  That diminishes the need for the lens to be so fast as to dim light.  There are still cavils about electronic "noise" at such high ISO's.  Since noise can now be fixed in software, the objections are now themselves just noise.

Which means that the remaining advantage of a fast lens is that it has a shallower depth of field.  Which as I realized today is all about picking out one's subject, not about shooting a black cat in a coal chute at midnight, as the saying is.

The best-known reviewer of lenses is a guy named Ken Rockwell and he dumped on the Nikon 500mm f/8 Reflex lens - because a) it is too light which makes the camera back-heavy, and b) because if there are bright things in the background blur, they will appear as blurry circles which is distracting.  Poor baby!  How could such terrible things happen to him?

Rockwell, being a well-known reviewer, gets his lenses free from the distributors. So the $7800 difference in price matters not at all to him. He does note in passing the same thing I found, that the mirror lens, because the light does not pass through any glass, is sharp and free of distortion.

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