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

Shooting a “Slow” Lens at Night

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.


Hand-held Shutter Speed Limits

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.


Breaking the Rules

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.


What Does it All Mean?

After considering all that, on paper, 50mm f/3.5 at 1/15th of a second captures roughly the same amount of light as 50mm f/1.8 at 1/60th of a second.

This understanding changes the way you look at the ability of a lens. Lets take a Voigtländer 15mm f/4.5 and a Canon 50mm f/2. Which one gives you more light? With sniper tricks aside, using the Shutter Sharp rule alone you can see they can offer 1/3rd of a stop difference, not 2+.

Take 50mm f/2 at 1/60th of a second (assuming this is what's needed to capture the image).
Remove two stops of light from the aperture.
That's f/4.
Give the light right back with the shutter speed.
That's 1/15th.
Match the focal length to the speed for sharp handheld pictures.
That's 15mm.

Now the amount of light entering the camera has not changed, we just played with reciprocity to shuffle the specs - and what did we create?

A 15mm lens at f/4 shot at 1/15th of a second. That’s nearly the exact same specs as the Voigtländer! Just 1/3rd of a stop less light capturing ability in this scenario than a 50mm f/2 lens.