How To Properly Determine Infinity For a Lens?

[LFLarry]

Thanks Corran. I was able to get this all setup today! I do have a mismatched cam and it works great. I even exposed and developed a couple sheets of film to confirm what I was seeing in the rangefinder and on the ground glass. Thanks for your detailed reply and help!

If you have the Technika, cams, and lens, first get the lens on the camera and pull the front standard out to middle of the rail (if you have a wide-angle that needs to have the rail pushed back or a telephoto with it moved forward on the extension, set it as such).

Next, focus in the rangefinder on a very clear target at close to infinity. For the purpose of infinity, anything farther than a couple hundred feet is fine, you aren't going to be able to discern anything past that in the RF. After that is set, lock the focus.

Then, put the infinity stops on the camera and let them move freely on the rail with the tabs flipped up. Move the lens on the rail till it is in focus on the ground glass. Snug up the infinity stops and tighten them down slightly.

Now focus on something about 25 feet away. Check and see if the RF and GG match up. If not, you may want to move the stops till it is, and focus on infinity second - choose whatever works best for your shooting style, but most likely they will both be in focus at all ranges unless you have something seriously wrong with the camera, lens, or cam. Once you are satisfied, you can tighten down the stops more tightly. They can move from the impact of the front standard if not tightened enough.

You shouldn't need the factory to do this. I've done it, even with mismatched cams/lenses which often are "close enough" for most situations.

[Bob Salomon]

Thanks Corran. I was able to get this all setup today! I do have a mismatched cam and it works great. I even exposed and developed a couple sheets of film to confirm what I was seeing in the rangefinder and on the ground glass. Thanks for your detailed reply and help!

What happens when you focus at other then infinity?

[Daniel Casper Lohenstein]

Hi Bob. Yes, I have a properly cut cam for my lens.

I know the default answer is to send the camera to an authorized service center for setup and calibration, but I simply don't have months to wait, so I want to at least try and understand what is involved so I can make an educated attempt at trying to setup my lens on my Master Technika.

I believe the first step is to set the lens at infinity and that is what I am asking the original question, but I want to make sure I know what that means so I start off with the lens in the right place on the rails.

Any pointers are much appreciated.

You can buy new cams and sharpen them yourself. I would prefer this solution. You don't have to "adjust" anything. You build yourself the cams you need, depending on the things you focus. You only need a file and a vice.

Tschau zäme

[LFLarry]

Hi Bob, it works perfectly at all distances. I shot 3 sheets of film to confirm. 1 at infinity, 1 at about 100 ft. and 1 at about 25 ft. All three were tack sharp on the film as they were on the ground glass and in the rangefinder.

What happens when you focus at other then infinity?

[LFLarry]

Any tips or insights into how to go about doing this?

You can buy new cams and sharpen them yourself. I would prefer this solution. You don't have to "adjust" anything. You build yourself the cams you need, depending on the things you focus. You only need a file and a vice.

Tschau zäme

[Daniel Casper Lohenstein]

Hi Bob, it works perfectly at all distances. I shot 3 sheets of film to confirm. 1 at infinity, 1 at about 100 ft. and 1 at about 25 ft. All three were tack sharp on the film as they were on the ground glass and in the rangefinder.

Any tips or insights into how to go about doing this?

Well, if your cams are tack sharp, everything is fine.

Otherwise, I imagine that you mount a new cam, focus on the ground glass at a certain distance, 1m, with an aperture of 5.6, and then check whether the range finder is also sharp. If not, move the cam to see if something is missing (bad luck) or if there is too much on it. If there is too much, file it off carefully, but only at the region where the focusing slider bottom bolt touches the cam.

This again is done successively for all distances, I think. I suggest not to finish the near point right away, but to file the near and far point and the middle evenly. This way you should slowly approach a sharpness in all areas. It surely takes time and requires intuition. Maybe that's why this is something that is done in the factory, by trained specialists. You don't tune pianos yourself, do you?

Infinity at 5.6 and 150mm is assumed at about 24m, because of the hyperfocal distance at about 47m, given 0.086mm as circle of confusion at 30cm print width and 35cm viewing distance. 100cm print width at 35cm viewing distance gives 0.026mm as circle of confusion. Then everything will be sharp at 5,6 at 78m, with a hyperfocal distance of 156m. Cf.http://www.erik-krause.de/schaerfe.htm#top Correct me if I get it wrong

[reddesert]

Infinity at 5.6 and 150mm is assumed at about 24m, because of the hyperfocal distance at about 47m, given 0.086mm as circle of confusion at 30cm print width and 35cm viewing distance. 100cm print width at 35cm viewing distance gives 0.026mm as circle of confusion. Then everything will be sharp at 5,6 at 78m, with a hyperfocal distance of 156m. Cf.http://www.erik-krause.de/schaerfe.htm#top Correct me if I get it wrong

This is the second time you've said something like "Infinity at 5.6 and 150mm is assumed at about 24m, because of the hyperfocal distance at about 47m," and I think that's misleading. 24m would be the near limit of acceptable focus when focused at the hyperfocal distance, not when focused at true infinity.

Infinity is still at true infinity. Infinity (or really, something far away, like 100+ meters) is used to calibrate focusing systems because there's no question about where it is, and because it's one limit of the travel of the focusing system. Also, it often appears in photographs as the background. If you only use a rangefinder at 2-10 meters then you should certainly calibrate the rangefinder there, but I think it will generally be easier to calibrate a rangefinder by getting it correct at infinity first.

[Corran]

Yes!

A good RF like the Linhof should be perfectly in focus at the point where you focused at the widest aperture of the lens. Not fudged for "hyperfocal" and/or "good enough" focus for f/22.

[Daniel Casper Lohenstein]

This is the second time you've said something like "Infinity at 5.6 and 150mm is assumed at about 24m, because of the hyperfocal distance at about 47m," and I think that's misleading. 24m would be the near limit of acceptable focus when focused at the hyperfocal distance, not when focused at true infinity.

Infinity is still at true infinity. Infinity (or really, something far away, like 100+ meters) is used to calibrate focusing systems because there's no question about where it is, and because it's one limit of the travel of the focusing system. Also, it often appears in photographs as the background. If you only use a rangefinder at 2-10 meters then you should certainly calibrate the rangefinder there, but I think it will generally be easier to calibrate a rangefinder by getting it correct at infinity first.

Yes!

A good RF like the Linhof should be perfectly in focus at the point where you focused at the widest aperture of the lens. Not fudged for "hyperfocal" and/or "good enough" focus for f/22.

That's a good point (whereas the circle of confusion and the print size / viewing distance play an important role).

But what do you "see" on your tiny ground glass? Every far away positioned object behind the near border will appear sharp, so "true" infinity (what's that?) is no longer recognizable.

Why do I mention this again and again? I don't know what kind of Linhof Technika the OP has. Eg. the Linhof Technika IV is not "zeroed".

BTW the problem is not solved by turning the focusing rail back to zero. You also have to find the zero position of the front standard, which you then fix with the infinity stops. And here the near limit problem plays again an important role.

Do you really know for sure which specification Linhof has for "infinity"? Do you think they focus on the spires of the Frauenkirche in Munich? They've got specs and devices. As long as you don't know the specs you can't "adjust" anything. Inevitability and uniqueness or unique features are part of successful commercial thinking.

As you say we want to take pictures in the range of 1-5m with the range finder. For infinity (landscape, architecture) we don't need a range finder. - Personally, I think a range finder has to work in a close range. There will be near range deviation if you make wrong far distance adjustments, because the depth of field becomes shallower, especially with larger apertures. Therefore a good calibration in the close range seems to be reasonable.

You're right, Corran: after all, it's about focusing with 5.6/135mm in the group range and 5.6 / 210mm in the portrait range - otherwise what's the point of having a 10k$ high-performance precision device like a range finder Technika?

Tschau zäme

[reddesert]

I don't really understand what the argument is here.

To set the infinity stops, Linhof doesn't focus on the Frauenkirche. They presumably use an autocollimator to produce a virtual target at infinity. Most of us don't have an autocollimator (although there is a Rick Oleson article about using an SLR to imitate one, if you have an SLR whose focus is known good). Without one, you can start by focusing a faraway target on the ground glass, like a utility pole, a streetlight, or the Frauenkirche. You focus back and forth to find the best focus, as with any target. It doesn't matter that the focus is "acceptable" at the near distance - you're finding the best focus. Now you know the location of infinity for that lens, so you know where to put the infinity stop.

After that, you can proceed to focus at some other distance, say 2 meters, and check between the ground glass and the rangefinder.

The problem with trying to start at 2 meters is measuring 2 meters from what? From the lens, the film plane, or some property of the lens like the front principal plane? I don't know exactly what the camera maker measured when they engraved the focus scale. However, the camera maker and I can agree on infinity.