How to check focussing on a RF - tips

[DavidK]

Hi

I had a new Bessa R2 with a 75/2.5 lens and was concerned that I was missing the focus a lot. Having trawled the web for tips on how to check this I came across one hint (from where I cannot remember, but thanks to the poster!):

1. Open the lens fully up.
2. Set shutter to 'B' setting.
3. Attached cable release, fire shutter, and screw knob on cable release so as to keep shutter open. [Important unless you have seven hands!].
4. Open back of camera. You should now be able to see through a fully open lens.
5. Put on tripod.
6. Take a small piece of glass or hard plastic (I used a bit from a CD jewel case) and cover one side with the translucent Scotch sticky tape. This acts as a really good ground glass screen replacement.
7. Place it on the film transport rails in the back of the camera, with the tape towards the lens. The tape is essentially your film. Note that (certainly in my R2) there are often TWO sets of rails, and the outer ones are slightly higher. These are edge guides, whereas the film runs on the inner rails. You want to place the tape/plastic so that it rests only on the inner rails, or else it will be too far away. Make sure the tape is resting on the inner rail, not just the edge of the plastic.
8. You should now be able to see an image on the tape. Focus by eye (use a lupe to get a closer look). Compare with what rangefinder split image is saying. Try for subjects at different distances.

I did all this and convinced myself that the camera was fine, and I just needed to improve my technique.

I'd be keen for any comments on the about approach and any improvements or different ways to check focussing accuracy other than setting the lens on infinity and checking that the RF patches are coincident.

Then, of course, you have to find the screw to adjust the RF 🙂

David

 

[Pherdinand]

You can check infinity focus also by drawing some stuff on that Scotch tape, setting the focus to infinity and looking into the lens from the front with another, well-calibrated camera/lens combo set at infinity. It should beperfectly sharp. The longer lens you use on the viewer camera, the bigger the image/easier the procedure.

 

[Bill K.]

Somewhere on this forum I read about using a laser to test for rangerfinder calibration.

I have been experimenting with rangefinder calibration using an inexpensive laser pointer.
It seems to work with most of my RF's (Kiev, Zorki, Retina & Ried).

By shining the laser through the view window on to a light wall in a darkened room, you should see 2 dots on the wall. (one brighter than the other).

By focussing the lens, the two dots should come together. Then check the reading on the focus ring on the lens and compare to the actual distance to the film plane with a tape measure.

It may take some adjustment of the laser to get the two spots to project.

So far, I have experimented with this concept "hand-holding" everything. But the results appear to be encouraging.

Regards to all,

Bill K.

 

[jlw]

I wouldn't use the footage scale for a definitive test, because the level of precision in the RF optics is usually higher than that of the footage scale. But it should be good enough for a quick check to make sure the RF isn't knocked seriously out of whack.

Here are a couple of other things I use:

Indoor infinity: The infinity adjustment is the most important thing to check, but often it's hard to find a good infinity target (which has to be at least a couple of miles away.)

This may sound crazy, but if you know your RF mechanism's exact physical base length (distance between the centers of the RF windows) you can make an indoor "infinity target." Just make two dots on a card, wall, etc., that are this EXACT distance apart. To the RF optics, these dots will always appear infinitely distant, no matter their actual distance (!) So, you can make a quick check anytime by setting the focus distance to infinity, aiming the camera at these two dots, and making sure they appear merged into a single dot.

Making sure of close focusing: The close focus setting is harder to check, but it's also the most critical. If you miss by a couple of percent at infinity it's no big deal, but even a tiny error in a full-aperture portrait can leave you focused on the nose instead of the eyes, for example.

The only test I trust for this is the ol' 45-degree-ruler test. Get a ruler with good clear markings and put an extra-bold mark at the middle point. Mount the ruler at a 45-degree angle to the camera, at about your closest focusing distance.

Focus very carefully on the middle point. Then, make some test shots at full aperture, have them processed, and examine them with a loupe. If the middle mark is within the zone of best focus, you're OK. If the zone of best focus is actually somewhere else, then you've got trouble in RF City.

 

[Jon Goodman]

First, I'll check the lens to film plane, and I find the "backsighting" technique the best (Rick Oleson describes it very well on his website). When I've decided the lens to film plane adj is ok, I'll adjust the rangefinder to infinity and for vertical. For some time, I've been using the moon on a cloudless night as the infinity/vertical target. It seems about as far away as any visible thing I can imagine. If you have a Soviet rangefinder, you may also need to adjust the following cam for close focus.
Jon

 

[jdos2]

I use cat-fur on the scotch tape. I've a couple more-or-less willing donors about.