Why a 15mm f4.5 Lens Can Capture the Same Amount of Light as a 50mm f2 — cameraville

October 19, 2018 in Lenses, Film, Tips/Resources/Guides

On a trip to Serbia to attend a camera show I had brought with me my Leica IIIg and Elmar 50mm 3.5 lens and loaded up with my favorite film Agfa Vista 200. What was interesting about this roll is about half of it was shot at dusk or night (very low light). With an f/3.5 lens a low speed 200 film, it’s not a situation you would expect to get good pictures. Should be dark right? Well I shot many at 1/15th of a second – so should be blurry right? Not entirely.

As you may know you shouldn’t shoot a shutter speed less than the focal length number of your lens, or you will in theory get blurry pictures without a tripod. There is no name for this rule as far as I’m aware but I call it the Shutter Sharp Rule which I first mentioned here in 2015. The safe zone is shooting a speed double your focal length but most I have found take it to the edge and just match it.

On a 50mm lens this means that the slowest shutter speed for sharp pictures on a 50mm lens is 1/60th of a second (closest shutter speed to the hypothetical “1/50th” where 50 matches the focal length of the lens). So to make this setup work at night we need to add more light, within the restrictions of our lens specs and hand-held limits of our slowest shutter speed.

Lets first get more light into the camera, and figure out how to solve the blurry problem after. I cannot shoot with a larger aperture of f/3.5 because that’s the best the lens can do. I can only make the shutter speed slower. By shooting 1/15th instead of 1/60th I let 2 stops more light in than “the limit” (1/50th, 1/30th, then 1/15th).

Now I’ve broken the Shutter Sharp Rule so how do I keep the subtle hand-held shake from adding blur? Well firstly with any rangefinder there is no mirror inside like an SLR. As the mirror quickly flicks up and slaps the inside of the camera on an SLR to reveal and open the shutter, that momentum causes shake. Rangefinders do not have this extra movement as they do not have a reflex mirror inside and are inherently more stable hand-held than SLRs. On top of that if you stay as still as possible, hold your breath, and shoot between heartbeats (like any good sniper), you can usually overcome the barrier of the Shutter Sharp Rule. That’s it.