How to "Focus" at Infinity

Gary892 · Jan 14, 2009

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).

Thanks
The Old Dog
Gary

Nick Zentena · Jan 14, 2009

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.

Frank Szabo · Jan 14, 2009

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?

pelerin · Jan 14, 2009

Gary892 said:
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).

Thanks
The Old Dog
Gary

Hi,
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.
Celac

Steve Smith · Jan 14, 2009

Frank Szabo said:
VIOLA! Infinity focus.

What's an oversized violin got to do with it?

I think you meant VOILA!

Steve.

Pinholemaster · Jan 14, 2009

"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.

John Koehrer · Jan 14, 2009

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.

Ralph Javins · Jan 14, 2009

Good morning;

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.

Frank Szabo · Jan 14, 2009

Steve Smith said:
What's an oversized violin got to do with it?

I think you meant VOILA!

Steve.

Picky, picky, picky. How much fly poo did you get out of the pepper? <grin>

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.

Chuck_P · Jan 14, 2009

Gary892 said:
So, if one has a 210mm lens the bellows draw would be 8 1/4 inches for infinity.

Click to expand...

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 I humbly ask the question; For a 155mm lens on a 4x5 camera, how does one set it to infinity

Click to expand...

Infinity for this lens would be:

155 divided by 25.4 mm/inch = 6 inches-----------locate the aperture plane 6 inches from the film plane and you will have infinity.

Q.G. · Jan 15, 2009

The "aperture plane"? You mean the one the exit pupil is in? Or the one the entrance pupil is in? Or where the physical diaphragm is?
None of the three is the correct thing to measure from.

You need to measure from the rear principal plane. Where that is located (and where the entrance and exit pupils are) depends on the lens design. When data are provided by the manufacturer, they usually state the position relative to other bits of the lens, like the last lens vertex. So you need to do some calculations (involving the exact focal length, not just the nominal focal length) to find a number you can use.

But that's all too technical. Not practical.
Cameras have a groundglass screen. Use your eyes.
Reduce the lens to film distance/bellows draw, until everything starts getting out of focus. Go back a bit, "et voila"! Infinity focus!

Steve Smith · Jan 15, 2009

CPorter said:
Gary892 said:

155 divided by 25.4 mm/inch = 6 inches-----------locate the aperture plane 6 inches from the film plane and you will have infinity.

Click to expand...

I like imperial measurements as much as most people of my age and over who didn't have the metric system drummed into them at school.

However, why do a conversion? Why not just use the metric measurement?

Your conversion has already put it 0.1" out as 155mm is 6.102". 6" is 152.4mm.

I find that I use imperial for large things like building/restoring houses and metric for smaller engineering work.

Steve.

Chuck_P · Jan 15, 2009

Q.G. said:
The "aperture plane"? You mean the one the exit pupil is in? Or the one the entrance pupil is in? Or where the physical diaphragm is?
None of the three is the correct thing to measure from.

Well, I'm not going to pretend to know the ends and outs of lens construction and lingo--it's more than I care to think about. I can only tell you that I measure from where the aperture is and it works beautifully.

Chuck_P · Jan 15, 2009

Steve Smith said:
CPorter said:

Why not just use the metric measurement?

Click to expand...

IDK, I get there just the same and without delay.

EASmithV · Jan 15, 2009

When I wanted to find out where my lens was when focused at infinity, I took it outside at night and focused on some stars...

Chuck_P · Jan 15, 2009

EASmithV said:
When I wanted to find out where my lens was when focused at infinity, I took it outside at night and focused on some stars...

I simply found the distance from lens to film plane by measuring with a ruler just once, then noted the distance in mm from inside to inside of the front and back standards. Anytime I want infinity, I simply separate the standards by 126mm inside to inside on the monorail------very quick.

How to "Focus" at Infinity

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