How to Calibrate Infinity Lens Focus

On a Film Camera

7 min read by Dmitri.
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 cropped by 400%. Top-left: the lens is properly calibrated using the method described in this guide. Bottom-right: the same lens before the adjustments.

This guide provides specific instructions on how to preview lens rendering on a film plane in real-time, making film camera calibration much more practical/simple than most other methods suggest. This approach is compatible with virtually any film camera. However, it is up to you to find the proper technique for adjusting/shimming your particular lens/body combo. Here, I am using my experience adjusting 1957 rangefinder-type 35mm film camera as a practical example.

Note: I assume that you have some experience fixing film cameras and taking no responsibility if you accidentally break your expensive toy after reading this article.

Over time, my obsession with Voigtländer Vitessa cameras yielded a small collection of L- and A-bodies. Foldable, no-frills, high-quality metal construction with a lens that, according to Amateur Photographer (13-02-1952), is capable of resolving details finer than a single grain on film.

Left: two Vitessa A’s, one measuring distance in feet, the other: meters. Right: Vitessa L3. Other than these variants, Voigtländer also sold N-types, which are similar to A’s, and T-types, which aren’t foldable.

Having plaid with a few Vitessas, I found that not every lens performed the same. While some created incredibly-detailed renderings, others did not. All of the copies I bought are in good condition, which made me wonder: are the inconsistencies in resolution fixable?

Vitessas’ rangefinders are notoriously difficult, though not impossible, to calibrate. Even then, having perfeclty-adjusted patches did not solve the lens resolution inconsistencies I’ve been observing. Having examined the photos a little closer, I realized that there were indeed areas that look reasonably sharp, just in the wrong places. Seeing that the rangefinder isn’t the problem, my next order of business is figuring out how to properly calibrate the lens focus on the film plane.

Previewing lens renderings on a film plane in real-time with a digital microscope.

Factories build thousands of camera components on a mass-scale. Tiny imperfections are often shipped with those parts and require skilled intervention to ensure the device is adjusted to perfection before hitting store shelves. Time, humidity, and temperature variations often throw mechanical equipment off-balance, which calls for another tune-up. This is the case with most complex physical devices.

Tuning instruments require feedback, as we are typically guessing which way to turn a screw, or whatever, while reviewing the results and adjusting until perfection. A faster feedback loop means that we can make more adjustments, quicker. Unfortunately, when it comes to seeing the results from a film camera, it may take weeks until the test emulsion comes back, which is highly impractical.

Few techniques exist that help seeing how sharp the image would be on a film plane without having to expose and process an emulsion. The most “proper” method is to use an autocollimator, which costs about a thousand dollars on average. Others will have you hunt for a tiny ground glass square and a powerful loupe. Yet another technique requires fastening two cameras together, one of which has to be an SLR with perfect focus. The last two options aren’t necessarily that precise; all three require additional, often hard-to-find equipment.

The good news is that today, digital microscopes are quite cheap and easy to find. Combined with regular frosted tape from the school supplies department, one can do an excellent job of adjusting the lens. I bought mine online for $30 — it came with a 1,000✕ magnification sensor, a chap-o stand, and a way to see the results on both mobile devices and laptops. This tool will be our key to the tuning success.

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

Here are all the things that you will need to begin calibrating your camera: a digital microscope, frosted sticky tape, clear work surface, a way to mount the camera and microscope in place, and a target that’s at least 20 metres/60 feet away. Plus, a set of micro screwdrivers and whatever else tools you need to make adjustments to your lens — remember, that part is up to you.

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 the tolerance of a fraction of a millimetre. Thus you’ll need to be careful to stretch your frosted tape over the horizontal film pressure rails, as flat as possible, see Figure 1. If you mount it incorrectly, as seen in Figure 2, this method will not work.

Figure 2: the tape isn’t mounted correctly in this image; it should be stretched vertically over the pressure rails. However, this is how the image will generally look (inverted) once projected onto your tape.

Now that you have your camera’s back exposed and the frosted tape accurately placed (using Figure 1 as your guide) where the film would typically be, you will need to situate all your gear into position infront 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.

Your microscope should be pointing directly at the tape — see Figure 2 and the image below. 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 chord 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.

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 have effectively transformed your film camera into a clumsy digital contraption, good enough to make accurate adjustments to the lens.

On my Vitessa, there’s a stopper on the focus wheel that lets me calibrate the infinity a little. This time it wasn’t enough, so I removed a few shim rings wedged between the lens and the body. A few screw turns later, I finally got a much better infinity focus. I took some extra time to also match my rangefinder patch for the nearest distance as well, essentially ensuring accuracy across the entire focus range.

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 and tablet-sized screens.
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 XA, 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 2020.