Scale-focus calibration

[tom williams]

Greetings. Can anyone advise on a procedure for re-calibrating focus on a scale-focus folder? After cleaning the elements of the Vaskar f4.5 105mm lens in my Bessa 1, I followed a procedure that I found online to re-calibrate the focus/distance scale. That method involved using a dslr, focus set to infinity, staring directly into the Vaskar lens. I drew very fine lines on a transluscent piece of paper, and taped that sheet into the folder's film plane, with a bright light source behind. I was able to bring the image of the lines in the folder's film plane into focus on the dslr screen by turning the outer lens housing on the Vaskar (thus extending the lens farther from the film plane) - but there was a substantial range in rotation through which the target lines remained in focus (near as I could tell). I'm thinking that I should consider the focus to be calibrated to infinity at the smallest rotation (least extension) that brings the projected pencil lines into focus. I wanted to get this clear before spending a bunch of film experimenting. And perhaps there is another method known to someone here?

 

[BrianShaw]

Stop down.

 

[gone]

I'm thinking that I should consider the focus to be calibrated to infinity at the smallest rotation (least extension) that brings the projected pencil lines into focus.

That is correct. You really need a better (more primitive) method to set the focus. A piece of ground glass on the film rails and using a loupe to ck things closely is the usual method. That gives a much brighter image than your current setup. Did you take the lens off to clean it? If so, you'll need to experiment to find the correct place to start threading it on the helical.

Ck eBay for a small ground glass. Its possible you could get a small piece of plexi or glass and scrub it w/ something to create your own ground glass. Just google DIY ground glass and you should get numerous methods for this.

Make sure the ground side is towards the lens. If you stop the lens down as suggested you won't be able to see much. I've used an ordinary camera lens in the past as a loupe substitute, it worked fine.

 

[xkaes]

I'm not exactly sure about your setup, but you need a subject at infinity to focus on.

Then by changing the distance between the lens and the film, see when it is in focus. This can be done with the naked eye -- if you shield everything from light -- or you could use any camera (digital or not) with a good macro lens that is focused on the translucent paper taped to the camera film plane. One problem with digitals is that they usually have a simple matte screen. Many none digital SLRs have a split-image rangefinder in the middle of a micro-prism collar -- which would be much easier to use.

Sounds like a job for TWO tripods -- and maybe two people, one at each camera.

 

[snusmumriken]

If you think the screen of the DSLR might be contributing to your uncertainty, you can manage without the DSLR (yeah). Wait for a full moon, and focus its image on the translucent paper in your folder using the calibration adjustment. A thin piece of lightly frosted glass is actually better - I made my own from picture glass using fine emery paper.

 

[tom williams]

That is correct. You really need a better (more primitive) method to set the focus. A piece of ground glass on the film rails and using a loupe to ck things closely is the usual method. That gives a much brighter image than your current setup. Did you take the lens off to clean it? If so, you'll need to experiment to find the correct place to start threading it on the helical.

Ck eBay for a small ground glass. Its possible you could get a small piece of plexi or glass and scrub it w/ something to create your own ground glass. Just google DIY ground glass and you should get numerous methods for this.

Make sure the ground side is towards the lens. If you stop the lens down as suggested you won't be able to see much. I've used an ordinary camera lens in the past as a loupe substitute, it worked fine.

momus, I have a broken ground glass that I can trim and use, I think. Thanks for the suggestion. Another something I wonder about is being able to calibrate with a target nearer than 'infinity'. If I place a target 10 feet away (say), and use a ground glass to focus on it, then set the distance scale mark at 10 feet, wouldn't I have achieved the same thing as with a target at 'infinity'? Too simple to be true?

 

[takilmaboxer]

After you've done the static test, load film in the camera, point it at angular objects at actual infinity, and take several shots at f/8 with the lens systematically set at intervals near infinity (like 24 feet, 48 feet and in the area between 48 feet and the infinity mark, then the infinity mark itself). Examine the negs under high magnification and the truth will be revealed. And be aware that the marks on the lens ring corresponding to closer distances are often wrong. That's life with a folder!

 

[tom williams]

If you think the screen of the DSLR might be contributing to your uncertainty, you can manage without the DSLR (yeah). Wait for a full moon, and focus its image on the translucent paper in your folder using the calibration adjustment. A thin piece of lightly frosted glass is actually better - I made my own from picture glass using fine emery paper.

I don't understand why the method I described should work, which contributes to my uncertainty. I had expected the 'target at infinity' to come in and out of focus rapidly, which didn't happen. Equals more uncertainty.

The Nikon D750 that I used lets the user magnify the image on the lcd screen, a significant advantage for me. Using that feature, the thin lines on the target screen were very clear and detail was easy to make out, so judging in-focus was easy. Using the full moon seems like an excellent idea too, with a ground glass.

I was attracted to the method I used because I wouldn't have to place a loupe against a piece of paper - and didn't want to fabricate a GG for this one test. But that's probably what I'll do. I had to replace a 4x5 ground glass at an earlier time, and found it quite easy and painless to fabricate one. I may just make a permanent screen for my folders.

 

[tom williams]

.... And be aware that the marks on the lens ring corresponding to closer distances are often wrong. That's life with a folder!

That sucks! But thanks for the tip, and the test procedure. I just shot, but have not yet developed, several frames of a target at 5', 10', 15' and 20', on a Bessa 1 just returned to me freshly CLA'd. I guess I should have taken more shots at greater distance, to differentiate apparent calibration error from built-in scale inaccuracy.

 

[takilmaboxer]

There are many variables that can affect the folder's usability with respect to setting focus. Such as, How good was the lens originally? Has it been removed and replaced? Are the distance settings correct? Has the camera been knocked around? I have Ikonta folders that have never been worked on since 1938 and they show surprising variation. So I'd recommend doing a careful infinity setting, and then doing more tests to see how accurate the closer distance markings are. I have one folder that is accurate at infinity, but the focal point corresponding to 3 meters distance, actually occurs at the 4 meter setting.

 

[tom williams]

Thanks takilma(Oregon?)boxer - from a former Eugene and Tijeras resident.

 

[MattKing]

I'd set it for infinity, and then check how much it is off at ten feet. You may wish to set it for ten feet instead, if that is the camera to subject distance you like to use a lot.

 

[OAPOli]

I don't understand why the method I described should work, which contributes to my uncertainty. I had expected the 'target at infinity' to come in and out of focus rapidly, which didn't happen. Equals more uncertainty.

I've used this method a few times. It should work well if the DSLR/mirrorless lens is longer then the testing lens and you are certain the infinity mark on the DLSR lens is correct. It's akin to using an autocollimator. I usually use a clear piece of glass on the film rails with a sharpie mark on it, pressed on with a rubber band. Essentially what happens is that when the testing lens is at infinity, it "collimates" the image to infinity (i.e. parallel rays). The image is then properly refocused to the DLSR sensor as long as its lens is also at infinity.

The image usually falls out of focus rapidly. How does the range you observe compare to the marks on the focus scale (from infinity to the next distance)?

 

[Helge]

Make a clean hole in a roll of sacrificial film through backing paper and film, smaller than the opening in the red window and right on a printed number.
Make the hole a bit bigger in the paper, so you get a clean sharp edge of film to look through.
The precise shape doesn’t matter too much. A line is fine, a round sharply punched hole too.

Load the film and wind to the hole. Leave the red window open and backlight it with a powerful diffuse light.
Now you have an exact film plane gauge to use the SLR focus check method in.

Ground glass focusing will never ever be accurate on a folder. A. The film plane is not where the ground glass rests. B. The ground glass is too coarse and the contrast and image too small to accurately focus on. Also fiddly as hell.

Optimum focus distance is far more important than infinity focus with a front cell focuser. Unless you plan on only using the camera for horizon shots. Even at fifteen meters accurate focus is important with 6x9.
The optimum focus distance is often found by dividing the focal length by 20.
So a 105mm lens like the Vaskar should be optimized for about 5.25 meters (5 is fine too as long as you’re consistent).

 

[reddesert]

Couple comments on auto-collimation: 1. It's really best to have a rigid surface like a glass or a piece of plastic at the film plane. Paper and tape aren't flat enough.

2. (Speculative) There is a possibility of your eye focusing on the aerial image and not the image on the DSLR ground glass. This can lead to a larger range where the image appears to be in focus. Normally, the point of a ground glass is so that your eye is forced to focus at that point. However, many DSLR focusing screens are very fine matte, because they're not primarily intended for manual focusing, and you might be able to "see through" the screen to the aerial image. If you try to superimpose the target lines onto the fiducial marks of the DSLR screen (like focus point or grid line marks), this should keep you from focusing on the aerial image. Also, on some DSLRs, the camera needs to be powered on to use the screen properly. And auto-collimation may work better with a longer than normal focal length lens.

 

[Bill Burk]

Try making a lens cap with two holes in it (like a smiley face) ( : ) then your out of focus image will be a pair of lines that come together and spread apart.

 

[nosmok]

So far I've had good luck with using ordinary Scotch Magic Transparent tape stretched across the film rollers. Since the film is a flexible material going over the rollers, I figure the tape is a good stand-in for it. Sticky side toward the lens, of course!

 

[tom williams]

I'd set it for infinity, and then check how much it is off at ten feet. You may wish to set it for ten feet instead, if that is the camera to subject distance you like to use a lot.

No doubt that focus accuracy in the 6'-30' range is most important for my typical scenes, and I favor shifting focus inaccuracy to ranges I use least. There are good suggestions here. I like the simplicity of Helge's and Bill Burke's suggestions (posts 14 and 16)). Since I have a piece of ground glass that I can fit to the camera, I'm thinking that I'll use that, and simultaneously vet the lens cap and hole-in-the-film methods. One will surely appeal to my idiosycracies.

Thanks to all you folks for your good ideas and savvy.

 

[Helge]

No doubt that focus accuracy in the 6'-30' range is most important for my typical scenes, and I favor shifting focus inaccuracy to ranges I use least. There are good suggestions here. I like the simplicity of Helge's and Bill Burke's suggestions (posts 14 and 16)). Since I have a piece of ground glass that I can fit to the camera, I'm thinking that I'll use that, and simultaneously vet the lens cap and hole-in-the-film methods. One will surely appeal to my idiosycracies.

Thanks to all you folks for your good ideas and savvy.

Using film, as described in my post, is far simpler and far more accurate.
Just saying.

 

[JPD]

When I recalibrate the scale focus on folders, I first use a groundglass on the film plane and focus on something at infinity. Then I print a numbered scale with lines about 1,5 mm apart, that I tape on the lens mount, and make a scratch on the front lens mount, like on this picture, so it points to a line in the middle of the printed scale. Then I load the camera with a test roll of film and take pictures, with wide open aperture, of something at "infinity", turning the front element from 1 to 8 (or 12), develop the film and check which one nailed the focus, let's say it's #5. Then I turn the front element so the scratched mark points to the corresponding number on the taped scale. Remove the scale, and put back the focus ring and tighten the lock screws.

https://live.staticflickr.com/65535/51184059283_611c2f99ef_w.jpg

This has worked very well for me.

 

[Kino]

Just a note: you can sand-off the emulsion of some scrap film to make a "ground glass" for focusing.

 

[Donald Qualls]

develop the film and check which one nailed the focus, let's say it's #5.

Okay, I really like this method. Likely to give better accuracy than anything a human can manage on ground glass.

In the past, I've used a thread stretched across the film plane to let my eye focus on the aerial image without ground glass -- when both thread and aerial image are sharp, the image is focused at the plane of the thread. I find it's better than ground glass, if only because the texture of the improvised focusing screen doesn't interfere with seeing the actual image.

 

[tom williams]

When I recalibrate the scale focus on folders, I first use a groundglass on the film plane and focus on something at infinity. Then I print a numbered scale with lines about 1,5 mm apart, that I tape on the lens mount, and make a scratch on the front lens mount, like on this picture, so it points to a line in the middle of the printed scale. Then I load the camera with a test roll of film and take pictures, with wide open aperture, of something at "infinity", turning the front element from 1 to 8 (or 12), develop the film and check which one nailed the focus, let's say it's #5. Then I turn the front element so the scratched mark points to the corresponding number on the taped scale. Remove the scale, and put back the focus ring and tighten the lock screws.

Good idea, JPD. I'm going to add your method to the other two that I intend to try.

 

[Helge]

When I recalibrate the scale focus on folders, I first use a groundglass on the film plane and focus on something at infinity. Then I print a numbered scale with lines about 1,5 mm apart, that I tape on the lens mount, and make a scratch on the front lens mount, like on this picture, so it points to a line in the middle of the printed scale. Then I load the camera with a test roll of film and take pictures, with wide open aperture, of something at "infinity", turning the front element from 1 to 8 (or 12), develop the film and check which one nailed the focus, let's say it's #5. Then I turn the front element so the scratched mark points to the corresponding number on the taped scale. Remove the scale, and put back the focus ring and tighten the lock screws.

https://live.staticflickr.com/65535/51184059283_611c2f99ef_w.jpg

This has worked very well for me.

Don’t unnecessarily scratch your camera no matter how minuscule the scratch.
Just use tape or use a fine marker drawing arrows. More precise, and completely invisible once you’re done.
Even if they are pretty cheap, these folders are irreplaceable.
With that off my chest, I have a hard time seeing how this would work precisely.
Determining accurate focus at infinity is rather hard. A target 20 meters away is going to be only slightly less sharp than a target at infinity, even wide open. There is much room for interpretation between those two.
Much better to use close focusing, especially the distance the lens was designed to optimally focus at.

 

[blee1996]

@tom williams The youtube/Internet method you attempted was a good and fast method. I use this method all the time, and recently checked/calibrated 6 different medium format 6x9 folders with scale focusing. It is totally legit and accurate. Just adding some details for your next experiment.

1. I used a mirrorless camera Nikon Z6, which can magnify the image which is handy. I read your D750 can do the same in live view. Then we are all good there.
2. I need a telephoto SLR lens that is wide open for critical focus. I used the Nikkor AiS 105mm f/2.8 macro manual focus lens. It has an added benefit of a focus lock knob, so infinity will stay infinity on the lens when I lock it down. And I keep the lens wide open. This way, the depth of field is kept the shallowest to assist in accurate calibration. I could try 200mm or 300mm lens in the future.
3. Make sure the digital camera and SLR lens are actually focus on infinity, and do not always trust the infinity marking on the SLR lens. Once the SLR lens is set on infinity, I point my digital camera to as far an object as I can find, e.g. a TV antenna on a remote mountain top. I use the magnifying function on the digital camera to double-check.
4. In terms of what to put on the film plane of the folder, I think ground glass, blank film, tracing paper are all fine as long as you can keep them exactly where the film will be and keep it as flat and tight as possible. Draw a cross hair on a matte surface, so you can focus both on the line as well as on the grain.
5. On the folder: there should only be a very narrow position of the outer lens housing in relation to the fixed part of the lens, where the infinity stand. If you have take care of all the above steps and there is still a "wide range" of infinity position, can you check if the lens assembly is put back together properly after your cleaning?
6. I only calibrate for the infinity on the folder, and rely on the scale focus lens' existing focusing marks for other distances. So far they work fine with the half dozen 6x9 scale focus folders.