How to Calibrate Infinity Focus on a Rangefinder

6 min read by Dmitri.
Published on . Updated on .
This is a side-by-side comparison of the two 35mm Ilford Delta 100 frames — one of the sharpest and most fine-grained emulsions on the market. Shot on Voigtländer Ultron 2.0, scanned with PrimeFilm XA and enlarged/cropped 4x. Top-left: the lens is properly calibrated using the method described in this guide. Bottom-right: the same lens before the adjustments.

Few things are worse than having every single shot being slightly out of focus. If your film rangefinder camera is making your distance (infinity focus) scenes appear blurry, it’s time to calibrate its infinity lens focus.

This is different than calibrating the rangefinder patch as the adjustments described here calibrate the lens’ infinity focus mark.

Infinity focus calibration on analogue rangefinder cameras is particularly tricky as the image seen through the lens isn’t immediately visible like with SLRs or digital cameras. Short of taking your camera to the shop, there were just two methods to accurately make those adjustments properly: using an autocollimator or via the cumbersome technique of fastening an SLR to your rangefinder.

In this guide, I’ll show you a new simple technique using cheap, easily available tools that will help you adjust the infinity focus of your film rangefinder camera. No autocollimator, no sending your camera to the shop, no awkward dual-camera setups. Here’s what you’ll need:

1) Figure out how to change the infinity focus on your camera. Typically, it is a matter of moving the stopper where your lens rests at the ∞ mark slightly closer/further to your camera’s film plane.

2) Some frosted, translucent tape — about $5.

3) A digital microscope — about $50.

You will also need a laptop/tablet to connect to your microscope, ample workspace, all the tools to adjust your lens, and a target that’s at least 20m/65’ away (like a telephone pole or a building in the distance).

Note: I assume that you have some experience fixing film cameras and take no responsibility if you accidentally break something.

Calibrating rangefinder infinity lens focus.

Rangefinders with interchangeable lenses have a little arm that touches the lens and feeds that position into the rangefinder and/or a stopper to fix the lens a certain distance from the film plane. Some lenses may have you make those adjustments on the barrel itself. In this guide, I will be using my Vitessa A as an example. It is a fixed lens rangefinder with a metal stopper on the focusing arm that I will be working on.

Figure 1: CORRECT: carefully stretch your frosted tape so that it sticks to the rails meant to keep the film flat against the camera’s pressure plate.

Whatever your camera is, the main issue at hand here is the ability to preview your lens’ renderings on the film plane without having to develop film or using complicated tools. Here’s how to do that:

1) Stretch your frosted tape over the horizontal film pressure rails as flat as possible; see Figure 1. Understand that there could be dramatic changes in focus if the film isn’t held at an exact distance from the lens. The pressure plate and the rails are precise, with tolerances of a fraction of a millimetre. If you mount your tape incorrectly, as seen in Figure 2, this method will not work.

Figure 2: INCORRECT TAPE MOUNTING: the tape should instead be stretched vertically over the pressure rails. However, this is how the image will generally look (inverted) once projected onto your tape.

2) Once you have your camera’s back exposed and the frosted tape accurately placed (using Figure 1 as the guide) where the film would typically be, you will need to situate all your gear into position in front of a far-away target. I don’t own a tripod, so I simply placed my Vitessa on top of a box, aimed at a window.

3) Your microscope should be focusing directly on the tape — see Figure 2.

I rotated it to have the image on my laptop appear right side up and adjusted the focus so that I can see the maximum amount of detail, up to the point of being able to discern a single grain on the tape. While most lenses appear sharpest at 𝒇4-8, I set my Ultron wide-open to 𝒇2 for the maximum amount of light; because the point of interest is smack in the middle of the plane, we do not need to stop the lens down. If you have a trigger/bulb cable that can hold your camera’s shutter open for you, you’re set — I simply held the button down while in Bulb setting to let the light in and preview the results.

4) Now that your microscope can transmit the magnified image from your frosted tape onto your screen, set your lens to infinity and make adjustments until the desired sharpness is achieved.

This is pretty much all there is to it. You can now assemble your camera back and shoot it confidently, knowing that your infinity focus is on point. Do ensure that your close focus has not drifted after adjusting the infinity — this may happen with certain cameras/lenses.

Uncropped 35mm Ilford Delta 100 film scans. The unadjusted lens’ blurring, top-left, isn’t noticeable at an “Instagram” (click here to preview) resolution but is plain on laptops tablet-sized screens, and in print.
400% crop.

To get the above split-image focus demo, I used a single roll of Ilford Delta 100 film and documented each test exposure before making adjustments to the camera, totalling about ten.

I then rolled the film back, careful to leave the leader exposed, calibrated the lens, and made eleven exposures in complete darkness at 𝒇16, 1/500th of a second to avoid double-exposures. The next ten frames were exposed from the exact same location. A trip to the lab, precision scanning session with PrimeFilm XAs, some Photoshop magic, and that’s that.

There you have it, yet another way modern technology makes things previously inaccessible in the past easy to do in 2021.

By the way: Please consider making your digital microscope (and other tools) purchase using this link  so that this website may get a small percentage of that sale — at no extra charge for you — thanks!