Proper focus for TLR's and other questions.

I have read numerous statements about setting focus to infinity but yet for some reason I can't see the logic. The argument is that if infinity is focused then everything closer will be focused since both lenses move the same distance when focusing. So why doesn't that apply going the other way? I would think if both lenses matched it wouldn't matter the distance. My problem is that with my eyes and using a loop I really can't tell if infinity is focused dead on but I can tell if something 3-10 feet is in focus. My other problem is using the finder without the magnifier I can't be certain I am in focus but when I use the magnifier and then use use the gg for framing I think my focus is changing from me standing up to take the shot. Most of my shots look slightly soft but I can't tell if my focus is off or my scanning is off. I look at the negatives but lack the experience and confidence of what a very sharp negative looks like as well as properly exposed and developed.

First, all digital images need sharpening. A scan is a digital image, therefore it needs sharpening. Light and simple sharpening. High pass filter sharpening is actually nice for this kind of preliminary first step sharpening.

With an SLR, you are right, get screen and film aligned at one point and the rest will fall into place. The problem with any other system is that you need both the focus system and the imaging system to be exactly the same. So for a Leica, there are, I believe, 3 distances that you are supposed to set the focus, with adjustment points at each one. the assumption being that any variance at other points will fall well within tolerances.

With a TLR, the issue is that the viewing lens and taking lens could have different focal lengths. So no matter what distance you set at, the two lenses will drift apart if they are not exactly the same. How much and how fast and if it matters is subject to a variety of factors, such as how big the different is, or what depth of field will handle at different distances. I scabbed a pre-WWII 75mm onto an Autocord body with a 75mm viewing lens, but focus wouldn't follow over a large range. I set focus to match at 8 feet and accepted that I rarely shoot at winder than f/8 and it worked out fine.

The Kodak Medalist is a rangefinder, but there is not an adjustment on the cam response and the cam itself is linear in action. The Kodak repair manual says to set the focus at 15 feet (get rangefinder and ground glass in agreement), and that other spots will fall into place. Someone had me work on their camera and knew that he was going to be doing portraits so I set focus at 10 feet. I also checked it at 4 feet. Both points were in agreement with the rangefinder. It worked out well for him. I now do a standard 10 foot set point and confirm four feet. At longer distances, well, no complaints yet.

Infinity has a specific theoretical mathematical existence and practical phsical existence in optics. Other points introduce a variety of new variables. So 'infinity' travels well from lab to factory to neighborhood repair shop. And a factory level collimator provides obvious and clear feedback on hitting infinity, and being off in either direction.

Set your camera and focus screen wherever you want. Check other points. See what you can live with. Take a magnifier to your negatives directly if you aren't certain about what your scanning is doing to the focus and sharpness you see.

First of all, get or make a focus target. Find one on google and print it out. Place it 3 meters away from the camera and parallel with the film plane. Put the camera on a tripod so it doesn’t move. Put a piece of groundglass on the film rails with a loupe and confirm focus agreement. Put some film in and shoot a few frames so you can check them after processing. If it’s off there are adjustments to be made or there is something wrong with the taking lens.

Celestal objects make excellent infinity targets as they produce a nice circle on the view screen.
A sheet of 100 ISO film left out in the sunlight for 30 minutes or more then processed or not makes a usable filter for using the sun as an infinity target. If you do not have sheet film then open the shutter on B or T with the aperture wide open for the same time. Do not look at the sun for more than a few seconds (5 to 15) even with a filter. Once you achieve a sharp circle you're in focus.

shutterfinger said:

Celestal objects make excellent infinity targets as they produce a nice circle on the view screen.
A sheet of 100 ISO film left out in the sunlight for 30 minutes or more then processed or not makes a usable filter for using the sun as an infinity target. If you do not have sheet film then open the shutter on B or T with the aperture wide open for the same time. Do not look at the sun for more than a few seconds (5 to 15) even with a filter. Once you achieve a sharp circle you're in focus.

Click to expand...

Do not look at the sun, with or with a filter. Instead find a very distant object and focus on it.

btaylor said:

First of all, get or make a focus target. Find one on google and print it out. Place it 3 meters away from the camera and parallel with the film plane. Put the camera on a tripod so it doesn’t move. Put a piece of groundglass on the film rails with a loupe and confirm focus agreement. Put some film in and shoot a few frames so you can check them after processing. If it’s off there are adjustments to be made or there is something wrong with the taking lens.

Click to expand...

I would also do this at infinity. Once you confirm focus using the piece of groudglass on the film rails, you could take a gander at the image on the camera's groundglass to see if it's in focus. Use the built-in magnifier or a loupe and you should be able to determine if the projected image is in focus. Other issues that could be causing bad focus with a TLR are misaligned/out-of-adjustment mirror, incorrect replacement of the focus screen, or out-of-adjustment focus system.

Dan Daniel said:

With a TLR, the issue is that the viewing lens and taking lens could have different focal lengths. So no matter what distance you set at, the two lenses will drift apart if they are not exactly the same.

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I remember reading that Rollei was very rigorous in selecting taking and viewing lenses to have very close actual focal lengths. And during assembly, they were fastidious in aligning and mounting these lenses on the lensboard. A danger many decades later in buying a used Rolleiflex or Rolleicord is that a repair person has switched lenses from another body. The viewing and taking lenses may no longer be the right focal lengths.

Take a look at a DOF chart, plug in your film size, focal length and aperture you wish to use and it tells you the closest and furthest distance that will be
in ACCEPTABLY SHARP FOCUS.
https://dofsimulator.net/en/

Most cameras may not have the precision of the Rollei but they will have "matching"
focal length viewing and taking lenses or they would be useless

shutterfinger said:

A sheet of 100 ISO film left out in the sunlight for 30 minutes or more then processed or not makes a usable filter for using the sun as an infinity target. If you do not have sheet film then open the shutter on B or T with the aperture wide open for the same time. Do not look at the sun for more than a few seconds (5 to 15) even with a filter.

Click to expand...

This is a scary recommendation. I recommend anyone planning to do this research it carefully. There are filters out there that make it possible to look at the sun, but B&W film is not one of them. Nor color negative film. Nor color chrome film.

Maybe Shutterfinger can provide a link that confirms that using his recommended material is an effective and safe filter.

As a counterargument-
https://gizmodo.com/what-happens-when-you-stare-directly-at-the-sun-5926497

TLDR:

Sirius Glass said:

Do not look at the sun, with or with a filter. Instead find a very distant object and focus on it.

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A 100 ISO sheet of film left in daylight has the same density as a #11 welders glass. NOT for extended viewing of the sun but for a few seconds it takes to achieve or verify infinity focus it worked for me.
Don't mix Vodka with your lighter fluid, drink a few shots, then look at the sun either.

IDK, but I know I wouldn’t do that...

infinity focus doesn’t need to be set using a target at exactly infinity. There are other things far enough away that aren’t dangerous like the sun.

The sun as viewed through a shht of film left out in the day light.

Yes its overcast from the SCU and SCZ fires.
The sheet of film


The sun shot took all of 3 seconds. I prefer the moon but its not always available and loading up the camera, tripod, and tools to set up infinity and driving several miles to get a good infinity target is not a viable option. I have set up infinity on cameras and rangefinders using a target about 3/4 to 7/8 mile away and celestrial objects are not in perfect concidence when viewed through such a camera or rangefinder.

Again do not look at the sun directly and only for less than 15 seconds with a very strong filter. 2 layers of this film completely block the sun on a perfectly clear day.

If you are unsure which part of the system (eg camera, your focusing technique, scanner, etc) is off, you can take a picture of a subject oblique to the camera, like a picket fence running diagonally near to far, or a yardstick placed with one end nearer the camera and one end farther. Focus on some identifiable feature near the center and write down what you focused on. Use a sturdy tripod, obviously. Use a fairly wide aperture or depth-of-field will cover up the effect. Develop the film and examine both the negative with a magnifier, and zoom in on your scan.

If the point of sharpest focus is somewhere different than intended, the camera has a focus offset. If the negative is sharp at correct focus but the scan isn't, something's wrong with your scanning. And so on.

Regarding targets for focusing, the accuracy needed depends on focal length and aperture, and so does the infinity target. The extension required to focus at distance D is roughly e = (focal length)^2 / distance. If you want to calibrate the extension of a 35mm film camera with a 50 mm lens to 0.03 mm in lens extension, then you need a target ~ 80 meters away to be close enough to "infinity." If you want to calibrate a 4x5 camera with 150 mm lens to 0.1 mm in extension, then you need a target 225 meters away to be "infinity."

Manufacturers and repair people often use an artificial infinity target created with an autocollimator. Normal people don't have autocollimators. Rick Oleson's website has a method for using an SLR with known good focus to autocollimate light - but you can only get as accurate as is the SLR you are using as a test instrument.

Another note: when I was a kid, the idea of using exposed and developed black and white film as a solar filter for brief solar viewing was common in magazines, books, etc. We (astronomers) no longer recommend this for a variety of reasons. One is that it's not really opaque enough, and another is that with chromogenic B&W film (Ilford XP2, etc) it is possible for a member of the public to have "B&W" film that really isn't any good at blocking infrared light at all.

If you want to make a solar filter, rather than bodging one out of film, buy a sheet of Baader solar filter material from a reputable dealer. B&H sells a 5.5x6" piece for $19, which is a small price to do things right. https://www.bhphotovideo.com/c/prod...mical_asolv_e_astrosolar_safety_film_5_0.html You can also find this from specialized telescope dealers and Baader. I used this stuff to make filters for binoculars, for example. Please buy it from a reputable seller, as in the run up to the 2017 solar eclipse, we had reports of dodgy sellers marketing unsafe eclipse glasses that did not block the necessary amount of light, especially IR.

Thanks for the B&H link. I had tried Edmund Optics, ebay, and a web serch before restorting to using the Arista EDU 100.

campy51 said:

I have read numerous statements about setting focus to infinity but yet for some reason I can't see the logic. The argument is that if infinity is focused then everything closer will be focused since both lenses move the same distance when focusing. So why doesn't that apply going the other way?

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What I usually do is check to see if the taking lens at infinity is at least really close. That way if you do shoot at infinity you don't have to worry if the infinity stop if off. Then as you said, I focus on something about 3 or so meters away and check to see if the ground glass agrees with the focus plane. IMHO I'm more concerned about that distance being spot on than I am infinity as most shots I take that include objects at infinity are hyperfocal focused with the lens stopped down anyway. It's the closer up shots that I shoot with the lens opened up that I need to nail the focus on. With a rangefinder, I do usually calibrate them using a fairly distant tree to line up infinity as a starting point, then recheck them at 3-4 meters and fine tune at the shorter distance if needed. Again personally for the way I shoot, the focus accuracy at 3-4 meters is much more critical than infinity.

btaylor said:

First of all, get or make a focus target. Find one on google and print it out. Place it 3 meters away from the camera and parallel with the film plane. Put the camera on a tripod so it doesn’t move. Put a piece of groundglass on the film rails with a loupe and confirm focus agreement. Put some film in and shoot a few frames so you can check them after processing. If it’s off there are adjustments to be made or there is something wrong with the taking lens.

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Hi btaylor! I know this is an old post but can you explain further what you mean by "Put a piece of groundglass on the film rails with a loupe"? TIA!

Hi Lisa PVA, the important thing is to have something flat and translucent on the film plane, which would be the film rails where the film would be if you were taking a picture. A piece of ground glass, or even Scotch Magic tape or a gray piece of processed film will work. Then focus on the target with the viewfinder, and use a loupe or other magnifying optic to see if the focused image in the viewfinder matches the focus point on your groundglass, or tape, etc. if they are in agreement you’re good to go!

LisaPVA said:

Hi btaylor! I know this is an old post but can you explain further what you mean by "Put a piece of groundglass on the film rails with a loupe"? TIA!

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You have the shutter open on 'B' and open the camera back and put a piece of groundglass on the film rails (matte side towards the lens) so the lens projects the image on it, and then you check the focus with a loupe. You can make a simple "groundglass" by cutting a piece of flat transparent plastic, like from a CD case, and tape matte scotch tape on one side.

Edit: btaylor replied at the same time as I.