Why calibrate focus at infinity and not at close range?

[dimitri_kalakanis3]

When calibrating the focus on a manual camera (i.e. konica s3,

minolta Autocord), all e-threads and web postings refer to adjusting

the focus for correct 'infinity'. Why not adjusting for 'close

minimum-focus'? Only references I found claim: ﾓit is more

difficultﾔ, but no one explains why?

[Rob_the_waste]

Margin of error is much tighter at infinity. You can set up at close range, but if you're off, you'll notice it much more at infinity.

[pshinkaw]

But any given focal length lens at the same f-number setting will give greater depth of focus (front-to-back) at infinity than it will at, say 6 feet. That's what the DOF scale on the lens will show as well as a DOF table.

 

I calibrate at 15 feet and then check to make sure that I am not focusing beyond infinty at the infinity setting. If it actually focuses at 80 feet on a 100 foot away object, the depth of focus will ususally take care of it.

 

-Paul

[dan_fromm2]

Paul, I think you're confusing depth of field with depth of focus. Look 'em up, they're not at all the same. Words have meanings, let's use them correctly to avoid confusion and looking, at best, sloppy.

[m_elek]

"Infinity" is a known distance -- when speaking of collimating lenses. Sounds crazy, but it's true.

 

You could collimate a lens at 5 feet, for example, but you would have to know that 1) the lens marking was exactly 5 feet and not 5.02 feet and 2) that the distance from subject to film plane is exactly 5 feet, not 5.02 or 4.99 feet. Any error in either or both of the two will result in miscollimation, which would then be compounded at closer or further distances than 5 feet.

 

Basically, if you use the SLR method and a target at the film plane, your SLR will be set to infinity. And that will allow you to precisely collimate your target camera's lens at infinity, especially when using a waist-level finder with magnifier and ground glass viewing screen.

[pshinkaw]

Dan:

 

Thanks!

 

You seem to know what you are talking about. Could you explain?

 

-Paul

[dave_s]

This is, with respect, pedantry. You could use either 'depth of field' or 'depth of focus' in Paul's post. They indeed mean different things, but his statement would be correct regardless.

[dan_fromm1]

Not at all so, Dave, and not sterile pedantry either.

 

Paul, depth of field is the distance around the plane of best focus on the subject end of the lens at which objects are acceptably sharp. Other things equal, the greater the film to subject distance is, the greater depth of field is.

 

Depth of focus is the distance around the plane of best focus on the film side of the lens at which the image projected by the lens is acceptable sharp. Other things equal, the greater the film to subject distance is, the smaller depth of focus is.

 

As a practical matter, we can control depth of field but can't do much about depth of focus. Real people, ULF photographers using non-standard film holders excepted, don't worry about depth of focus.

 

Now, Dove, do you understand better why calling ideas (and people) by the names normally applied to them matters? I tell you, Dive, using the right name makes a difference. Do you agree, Duve? What say, Dale?

 

Cheers,

[dan_fromm1]

http://www.largeformatphotography.info/lfforum/topic/504501.html

[richard_oleson]

You can adjust to see that your viewing and taking systems match at any distance, and it's a good idea to check them at both ends if you have any question whether the camera has been damaged or is somehow out of spec. But this is to check one against the other, not to establish it at a specific distance. The lens only has one absolute position in terms of a specific distance, that's infinity - at all other distances you'd have to compare the engraved distance scale against a tape measure reading or something similar and I wouldn't trust that.

 

Equally or possibly more significant is that the infinity setting can be produced by a collimator so that you have a standard instrument with which to set the infinity point to ensure that it's optically correct and not just a "that light pole looks sharp" approximation. True, a collimator can be adjusted to set a different distance than infinity, but this is not generally what they are made for: they are made to deliver a collimated beam, which is the optical equivalent of infinity. If you have such an instrument, then it's much easier to correct at infinity than to pace off distances and worry over whether the engraving on the lens barrel is correct...

 

Also, the lens generally has a stop at infinity. If it is correct there, and incorrect at closer distances, it's still perfectly usable as long as it matches the viewing system (RF or groundglass). But if you correct it at a close distance and it's off at infinity, you may never be able to attain correct infinity focus because you may hit the stop before you get there.

 

:)=

[davecollopy]

Dimitri

 

Rob gave you the general answer and Richard gave you the definitive one. I usually calibrate at infinity unless I have a specific reason not to. I know I have a TLR which I calibrated at fifteen feet because I only use it as a close focusing camera(ten to twenty feet) and the engineers who designed the lens optimized it at that focusing range.

[kelly_flanigan1]

You calibrate and or adjust at both distances with a TLR or rangefinder. Usually many folks just calibrate at infinity, and "hope" the close distance tracks ok at close distances. With a Zorki, signet 35, Kodak Ektra, or a Leica one can adjust the "gain" of the rangefinder so close distances are correct too. With many TLR's; the taking and viewing lenses are matched focal lengths at the factory, and destroyed by hackers who mix up the lenses with other cameras. This makes a good Ebay looker, and rolls the dice on close performance. DO you feel lucky today? With some low cost RF cameras, the gain of the RF is by bending an arm. Probably alot of folks dont really understand alignment, so infinity adjust or checking is all their brain can fathom. :)

[dimitri_kalakanis3]

Thank you all for taking the time to answer. I, too, feel that a small error at infinity is OK. Unless you are shooting stars, a small stop-down and infinity will look fine. It is the close range and wide open (low light; isn't this what rangefinders are best at?) when I need the focus to be perfect.

 

Thanks again!

[profhlynnjones]

Apo lenses for close up use are calibrated at 1:1

 

S.uper wide lenses for large format are calibrated at 1:1. For thsi reason, super wides (that is 95 degrees and wider) must be "focused in", that is focus at the hyperfocal distance at maximum and then carry greater distances by depth of field.

 

A typical 90mm, 100 degree WA at f 32 for 4x5 when focused at 200 feet will be semi sharp and 200 feet, and in focus on the edge of the image circle at about 18 inches. That is field curvature!

 

That very same lens when focused at 35 feet and stopped down to f22 or f32 will be be in focus from a few inches to infinity critically sharp throughout.

 

47mm, focus on the sumbect but no further than 15'(5m)

65mm, focus at the subject but no further than 25' (8.4m)

90mm, focus at the subject but no further than 35' (12m)

120mm, focus at the subject but no further than 50' (17m)

 

I know you won't believe this, but if you will test my hypothesis you will be astounded.

 

Co creator Caltar lenses, co creator Computar lenses, team member Celestron telescopes.

 

Lynn

[krishna_rama]

Since we are already discussing focussing issues with SLR, I'll ask my question here instead

of starting a new topic. I have a mamiya 645 slr with a viewfinder prism. When viewed

through the viewfinder prism, infinity doesn't look sharp at all. I tried this with a

55/80/150 mm lens,

and with a varying degree of inaccuracy, they are all off in sharpness.

 

1. I first thought that this was because the focussing screen is matty, but counter argued

that the same focussing screen can render close objects sharp enough.

 

2. I thought that the lens bayonet is not calibrated (the body has been opened before),

and checked with a transluscent paper (I don't have ground glass), held at the image

plane (of course this itself is approximate), and infinity looks sharp. Nevertheless I get

sharp negs from infinity focus (improving this might be another issue)

 

3. I concluded that my lens-film-plane distance was good enough, but my mirror-

focussingscreen-prism-viewfinder system was the culprit. This is annoying to know, since

I'm not looking at what my film is looking at.

 

4. How do I fix this ? There is no external diopter in my prism,.. is there an adjustment

that needs to be done inside ? Any help is appreciated.

 

5. Thanks for clarifying Depth of Focus v.s. Depth of Field (and that they work against each

other).

 

-Krishna