How to "Focus" at Infinity
I am embarrassed to bring this up, but, how does one set a 4x5 or any other large format camera to infinity.
I have been photographing at least 30 years with a 4x5 and larger cameras, usually in the studio but often out in the field.
I have never given much thought to setting the camera lens to infinity as I have just used swings, tilts, and of course focusing to get the image I wanted or needed. But while I was out and about making photographs a while back, a stranger asked me how to set the focus of a lens to infinity.
I thought about it a minute and replied, the focal length of the lens determines the amount of bellows draw and when at max bellows draw for a specific lens then that leans is at infinity. So, if one has a 210mm lens the bellows draw would be 8 1/4 inches for infinity.
The stranger left with their newly gained knowledge and I felt good about helping someone.
Then I started to think about my answer. I may have given out misinformation, so I searched the internet using Google but could not find a succinct answer.
So, the folks at APUG now have an opportunity to teach an old dog some new tricks.
So I humbly ask the question; For a 155mm lens on a 4x5 camera, how does one set it to infinity, or any size lens for that matter.
I occurs to me that I may have it backwards(dyslexia).
The Old Dog
"He who expecteth nothing,
Shall not be disappointed." Robert Willingham, 1907
It's not really the focal length it's the flange focal distance. It varies with lens design. Telephotos will need less then the focal length. Some wide angles need more.
Just to set at infinity focus requires a bit of prior work.
Make two marks or locate two points on your camera's standards or whatever you can find on the cam to measure reliably.
Looking through the groundglass with the camera aiming at something quite some distance off (a mile or so), focus the 155mm or whatever lens you're interested in setting at infinity - when you're pleased with what you did, measure the distance between those aforementioned points and write that number down - don't lose it.
With the tape measurer or whatever you used to determine that distance, set up your camera and extend the standards - measure off that distance you should have written down.
VIOLA! Infinity focus. The actual number doesn't matter - just return to the measurement you determined.
... or did I misunderstand what you wanted?
"Beer is proof that God wants us to be happy."
Originally Posted by Gary892
Yes, bellows draw increases as the point of focus approaches the lens. (easy to test y/self w/ a yard stick) As to a the exact amount of bellows draw being = to the nominal focal length of the lens this is not likely to be true in practice. Your len's actual focal length is likely to differ from the nominal and most probably the nodal point sits in front of where the bellows terminates. Cameras with infinity stops (the little flip-up doo-hickeys on the bed of a Linhoff etc.) have the placed by practical trial. The repairman uses a collimator to provide a reference for infinity and places the stops where appropriate for the lens. I think the short answer here is that your method is working so keep on keeping on... however, ( http://www.trenholm.org/hmmerk/download.html ) if you want to delve into this so you can answer random questions w/ enough detail to make people's eyes go glassy.
What's an oversized violin got to do with it?
Originally Posted by Frank Szabo
I think you meant VOILA!
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"when at max bellows draw for a specific lens then that leans (lens) is at infinity"
Isn't is 'minimum' bellows draw? An 8 1/4" lens is at infinity at 8 1/4". Maximum bellows draw is for macro work.
Anyway, one measure bellows draw from the nodal point of the lens. Frankly, I don't even worry about it. Put the lens on the camera, and move the back standard to the point where infinity is in focus.
So the OP is correct. At infinity the film plane to nodal point is 8 1/4" approximately. It's not going to be precise but is a good answer.
Consider it a rule of thumb.
Heavily sedated for your protection.
I agree with PinHoleMaster; the minimum bellows draw should be small enough or short enough to allow the lens to come back close enough to the ground glasss to allow the image of the object/subject a very long distance away ("infinity") to be "in sharp focus" on the ground glass.
The bellows extension out along the rail would allow the lens to move out away from the ground glass to allow an object/subject very close to the camera to be "in sharp focus." This is what we do when we are trying for a large image on the film or "close focusing" or "macro" imaging with our camera. As I recall, for a lens to be called a "macro lens" it had to be able to bring the image on the film plane up to a 1:1 size ratio. If it did not do that, it was only a "close focusing lens." I think "micro" lens labeling did not begin until around a 10:1 or 10X size ratio. The optics section in physics class was a long time ago.
Ralph Javins, Latte Land, Washington
When they ask you; "How many Mega Pixels you got in your camera?"
just tell them; "I use activated silver bromide crystals tor my image storage media."
Picky, picky, picky. How much fly poo did you get out of the pepper? <grin>
Originally Posted by Steve Smith
You're right - tried to type too fast.
They are a bit easier for me to play, though, with the arthritis and all. My fiddle is giving me problems anymore - painful to play.
Last edited by Frank Szabo; 01-14-2009 at 02:08 PM. Click to view previous post history.
"Beer is proof that God wants us to be happy."
Yep, it is at infinity when the lens is at a distance from the film plane that is equal to its focal lenth. I go by the info given in "The Camera" and measure that distance from the aperture plane. So I position my 210mm lens 8.25" from the aperture plane to the film plane.
So, if one has a 210mm lens the bellows draw would be 8 1/4 inches for infinity.
Infinity for this lens would be:
So I humbly ask the question; For a 155mm lens on a 4x5 camera, how does one set it to infinity
155 divided by 25.4 mm/inch = 6 inches-----------locate the aperture plane 6 inches from the film plane and you will have infinity.