To infinity but not beyond . . .
May 13, 2011 11:37 AM   Subscribe

∞ focus at ∞ distance = out of focus?

In attempting to take some moon pictures I noticed something wacky - on my manual reflex/mirror lens I must focus WAY pre-infinity to get the moon in focus. It's the moon - it's infinite distance right? Is this normal or is there something wrong with the lens and/or pairing of lens and camera?

Camera: Nikon d90 (APS-C sensor, lens is film-era)
Lens: (super old) Osawa 650mm mirror lens
posted by datacenter refugee to Sports, Hobbies, & Recreation (10 answers total) 1 user marked this as a favorite
Best answer: Camera are set up differently, and if the focus plane of the sensor is not where the designers of the lens expected it to be, then you could end up with focus issues. On the Canon, the APS-C sensor cameras have a special line of lenses (-S) that have the objective element of the lens closer to the sensor (and thus can't be used on a full frame body, because the element will hit the sensor). Or so I've been told.

But, so long as you can find focus on the old lens, regardless of where the lens thinks infinity is, I don't think there's anything wrong.
posted by Admiral Haddock at 11:43 AM on May 13, 2011

Though, having just bopped over to Flickr through your profile, I think the moon shot is a little soft at 100% (but it's a nice shot!). Are you just going by the focus markers on the lens, are you using the LCD to focus? I've shot the moon both with my 100-400 and with a 70-200 (with a 1.4x converter), and I've ended up just manually focusing using the zoomed LCD, and they come out a bit sharper.
posted by Admiral Haddock at 11:50 AM on May 13, 2011

Response by poster: I just picked up how to focus using the LCD last night, it helped significantly but even so that's the sharpest I could get (after 3 sessions of shoot, download, review). I Agree that it's most likely an incorrect focal plane issue - this lens was made a long time ago in a galaxy far far away from digital sensors photography. Thanks!
posted by datacenter refugee at 11:55 AM on May 13, 2011

Best answer: Yeah, I have a couple lenses that needed to be shimmed at the mount to correct the infinity focus. It's absolutely possible to be focusing beyond infinity - sometimes way beyond.
posted by fake at 12:13 PM on May 13, 2011

I find LCD focusing hard, myself, particularly if you're using a shorter tripod that leaves the camera below your normal line of sight. The EXIF data on that image shows a shutter speed of 1/100. Is the camera totally bolted down? I can imagine that at 1/100 and at 650mm, and shake is really going to set your focus off. Are you using a remote/cable release/self timer for the picture?

Also, as an alternative to what you're doing, can you just find something on the near side of focus and take 25 pictures in a row, shifting the focus each time? Hopefully one of the images would hit the sweet spot.
posted by Admiral Haddock at 12:18 PM on May 13, 2011

Sorry, but as a final thought, have you tried locking up the camera's mirror? I normally would think 1/100 is fast enough to isolate that vibration, but I don't shoot at 650mm...
posted by Admiral Haddock at 12:36 PM on May 13, 2011

Best answer: Common problem. The infinite focus point on your lens is more of a suggestion - that "real" infinite focus is around here somewhere Actual infinite focus is a much more complicated problem that relies on things like the mechanics of your lens and camera, temperature, and atmospheric conditions.

Most lenses let you focus further than infinity just for this reason.
posted by fremen at 1:31 PM on May 13, 2011

Best answer: datacenter refugee writes "I Agree that it's most likely an incorrect focal plane issue - this lens was made a long time ago in a galaxy far far away from digital sensors photography. "

You may never be able to focus this lense as sharp as you'd like. Mirrors could be made long and cheap (and light but that was rarely a determining factor) and so they were. So they tend not to create the greatest pictures. And because of their fixed aperture they were often made with universal threads to which you attached an adapter mount to fit your camera in order to maximize the market of the tooling. If the adapter couldn't be exactly right (either from installation or production) it was better for the focus at indicated infinity to be too long rather than end up with it too short.

Not all mirrors suffer from these problems but the ones that don't tend to be just as expensive as long glass lenses if not more expensive and were bought only by those who needed either the reduced length or mass or both.

Finally I don't know how much IR photography is done with mirrors but it requires the ability to focus farther away.
posted by Mitheral at 1:56 PM on May 13, 2011

Best answer: I had the same problem trying to focus for this photo. I figured it would be simple to just focus all the way out, but it wasn't, the lens goes right past infinity. it's a completely different lens than yours. For all I know, nearly every lens does this. This seems to be what other commenters are saying, and it would make sense. You'd rather be able to focus just past infinity than have slightly out-of-spec manufacturing mean that you can never quite focus to infinity at all.
posted by tylerkaraszewski at 3:09 PM on May 13, 2011

Best answer: They have the ability to focus beyond "infinity" at normal temperatures, so that 1) no matter how the tolerances of the individual components stack up, you can still get "infinity" focus, and 2) so that no matter what temperature you're shooting at, you can still focus at infinity.
The coefficient of thermal expansion for an aluminum lens frame is .0000123/degree f. If you assume a 100F range, 100mm focusing distance, the focusing distance can change .1 mm over that range. It's "only" 1%, but think about how little the focusing elements move in and out inside a lens (not the zoom elements).
posted by notsnot at 4:17 PM on May 13, 2011

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