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 p.1 #7 · 21mm hyperfocal distance? | |
Cableaddict wrote:
-But I'm not sure how this relates to my focus ring. Does this mean that, with the ring turned fully, the point of maximum sharpness should be at 12.1' ? (with the rest being within acceptable limits)
-Or am I SUPPOSED to be able to focus further away, so that distant objects are even sharper, at the expense of closer objects that would have looked OK had I focused at the hyperfocal distance.
I thought I understood this, but the more I think about it, the more confused I get.
[...]
I'm also thinking, why not be able to go "past infinity" a little, if the distance scale allows it?
I've read that some lenses can purposely focus past infinity to allow for thermal expansion. As long as the distance scale accurately shows the "infinity" mark, then why not? You could also use this creatively, to get near objects more OOF, while still having the horizon within acceptable focus limits.
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Sadly, this lens I'm working on now has a distance scale that stops exactly at infinity.
So ......
Don't worry. 
Well, I think most of what you are wondering about is a rather physical, better theoretical problem. There are lots of people discussing the notion of hyperfocal distance and I'm not really an optical expert, but from what I know about hyperfocal distance, it means that when you have your lens's focal length set to 12.1', everything behind your point of focus is in acceptable "sharpness" (defined by size of the point of confusion) as well as everything from 6' towards your point of focus (focus distance).
Still, perfect sharpness of subjects in greater distance, e.g. a mile away, are in perfect focus (meaning smallest size of the circle of confusion) only, when the lens is set to infinity with an appropriate infinity adjustment. And this is usually the purpose of the focus ring stop. It prevents users from focusing past infinity, since optimal sharpness is provided already. The more you focus past infinity, the farther away the "area" of sharpness moves away.
The effect of temperature deviations on lenses this wide is an issue I wouldn't take care of. There is plenty depth of field at f/4, so the effect will be absolutely insignificant, if existent at all.
Since "acceptable" sharpness depends on the size of the point of confusion, you could adjust your lens by means of a borrowed Crop-DSLR with high resolution. They usually are much more demanding with regard to absolute resolution thus requiring a smaller circle of confusion to achieve acceptable sharpness. So what you see as sharp in the images taken with a crop DSLR will eventually be sharp with your 5DII for sure.
Hope this helps.
si
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