Testing shutter speeds with a digital camera

I wanted to write a quick "how-to" about how I recently tested a couple of shutters before using them for an important shoot.

One of the issues inherent with shooting older large or medium format lenses in their leaf shutters is accuracy of the shutter. I have heard that even newly manufactured shutters were said to be "within specifications" even if the shutter speeds were up to 30% off. And it's a known fact that the fastest speeds on these shutters are never even close to spec.

So while I recently pre-ordered a newly-designed shutter-speed tester, I needed one to test my 15cm and 21cm APO Lanthar lenses that I knew were shooting slow.

It's quite simple to test using a newer digital camera with "Live View" and a couple of tripods. Set up the digital camera, sans lens, behind the camera with the lens being tested mounted. The digital camera should be in a somewhat dark room but aimed at a suitably bright subject - I usually just shoot out of the house into my yard. The film camera will have the ground glass back removed and the DSLR placed as close to the camera as possible.

The lens can be set to T or locked open otherwise to focus. Then I set the DSLR to bulb. Using two hands, I trigger the DSLR on bulb and then immediately shoot the lens at whatever setting I am testing.

I set the DSLR ISO and the lens aperture at a proper setting to get a decent exposure. Then, having a baseline result, I again lock the shutter open and shoot the DSLR at the same shutter speed. If the results match, the shutter is shooting at the written speed. Otherwise, I change the DSLR shutter speed around in 1/3 stop increments till I match the exposure.

At first I wasn't sure if I could differentiate between 1/3 stop increments, but after trying this it was easy to see when the exposure was exactly the same vs. off by just a fraction of a stop. I tested each individual shutter speed and matched it to the "actual" speed as demonstrated by my DSLR. Shooting these two lenses later, my exposures were dead-on accurate using the "corrected" shutter speeds.

If you already have an appropriate DSLR to do this with, this method can be a cheap (free!) and effective way to check exact shutter speeds. Admittedly it's a but cumbersome compared to a dedicated tester - which is why I am buying one and will more thoroughly test all of my lenses. Also, this method may not work for wide-angle lenses since you can't focus accurately with a DSLR due to the flange-focal distance vs. the mirror box (a mirrorless camera could work).

I used these two lenses almost exclusively for my wedding images, hence my need to get perfect exposures!!