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Colin Melhuish
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 08, 2004 - 01:34 pm: | 
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Something I've been pondering on recently: If a lens is adjusted at it's infinity setting to a position which is actually just slightly beyond infinity will the lens perform OK when focussed at infinity (due to depth of field having an effect), or does the whole depth of field effect break down in this situation? (Does that all make sense :-/
I ask because I have an Olympus 35SP rangefinder that produces "soft" images when focussed at infinity (at f8 or so), but acceptably sharp images at closer focussing distances. I suspect that maybe the infinity setting is actually slightly beyond infinity. Any Ideas?? |
Jon_Goodman
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 08, 2004 - 02:49 pm: |
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Well...unless something is wrong that we don't know about, a fixed lens on a rangefinder doesn't normally get out of adjustment, unless maybe the mechanism that stops its travel breaks. On the other hand, the rangefinder itself can slip out of adjustment, and it is not uncommon for it to do so. Assuming nobody has monkeyed with the lens, I would think when you set it to infinity, it will be correctly focused (regardless of what the rangefinder may say), and the pictures should be ok. Furthermore, if the pictures at closer focus are sharp, I think the rangefinder may be ok and so might the lens. Have you checked to see if a distant phone pole lines up straight when the camera/lens is adjusted to infinity? Back to your original question...if a lens is adjusted for a point just past infinity will the depth of field effect take care of the discrepancy? My gut tells me it would be better if the lens was not quite reaching infinity, but either way, it might be ok. I think it depends on how far from infinity the lens was off, how small the aperture setting was and how wide the angle of the lens was. |
Peter Wallage
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 08, 2004 - 05:33 pm: |
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I think you're correct about a lens not quite reaching infinity being the lesser of two errors, Jon. As I understand it, the point on which a lens is focused is not in the middle of the depth of field. The depth of field for a given degree of definition actually extends less in front of the point of focus than it does behind it. A diagram in an old copy of the British Journal of Photography Almanac gives the ratio as approximately 1 to 1.5.
Peter |
snap30
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 24, 2004 - 08:59 pm: |
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Though I have not done the research into rangefinder's specifically, I am curious if there is something comperable to what on video cameras is known as "backfocus"? This is the setting that ensures that the OTHER side of the lens is in focus, i.e. where the image hits the CCD or film plane in the case of a still camera. If the rear of the lens assembly was out of whack, or the pressure plate and guide rails that keep the film flat were too far forward or back, this could cause aberrant focusing.
Also, Peter is correct in that the actual focus point lies about 30%-40% into the range called depth of field. A greater range of distances appear in focus beyond the focal point than before it. There's a decent write up on DOF here:
http://www.rakoczyphoto.com/words/dof.html |
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