Lens Fastness and Sensor Size

Created by tomtakespictures on Nov. 21, 2014, 6:56 a.m.
  • Hey guys! I've got a broad question about lenses and their fastest available aperture. I think it has more to do with physics and optics than photography. So, this seems to be the best place to post the question because everyone here seems to be coming from a different background.

    Help me understand if you're able to.

    I work at a camera shop and come across a wide variety of lenses all of the time. Lenses for large format (sometimes with image circles covering up to 11x17), DSLR, 35mm SLR, micro 4/3, t.v. lenses, etc.

    I have noticed something with these lenses. As the light sensitive area gets larger, the largest aperture gets smaller. Not in the sense that the size of the diameter of the hole gets smaller, but in terms of stops of light. For example, I just came across a Fuji CL mount lens that has a zoom of 15-153mm and it is f/1.9 opened up all the way through. However, you would never be able to have this type of zoom with that large of an aperture on a full frame DSLR camera let alone a 4x5 camera. The fastest zoom that comes close in terms of range and aperture for a full frame DSLR is a 24-105mm f4 and that doesn't even come close.

    I guess what I'm asking is what exactly is causing this? Does it have to do with sensor size? Seeing as how you can fit a 50mm f/.95 Canon TV lens onto a Leica camera, makes me think it doesn't have to do with sensor size. I know that this is probably more of a physics and optics question rather than a photography question, but I can't be the only photographer in the dark here.

  • Yes, it all has to do with sensor size. Let's have a look back at the fundamentals. We typically think of the f-stop as the intensity of the light hitting our sensor. While that's true, it's also defined as the focal length divided by the physical diameter size of the aperture. So, for example a standard lens on micro 4/3 has a focal length of 25mm. To get an f-stop of f/1.8, you it would need a aperture diameter of 13.9mm. On a full frame camera, a standard lens has a focal length of 50mm. To get the same f-stop, you need an aperture with a diameter of 27.8mm. Or another way to think about it. If you put the aperture of the micro 4/3 lens on the full frame lens, you'd end up with f/3.5 lens (two full stops slower).

    But aperture size isn't everything. The elements of the lens are almost always larger than the aperture, meaning that lenses get even larger. Another thing to keep in mind is that smaller sensors typically don't require as good image quality. That C-mount lens you were looking at was made for shooting video. At, most 2Mpx (1080p) and most probably around 0.3 Mpx (standard res video). Photography lenses are held to a much higher standard of sharpness and correction. To get a better image quality, you typically need to add glass lens elements which lead to a larger lens.

    If you want a comparison for your Fuji C mount lens, have a look a large range cinema zooms. Some have a comparable zoom range at a fast aperture but they are comically large and cost several tens of thousands of dollars.

  • Thank you so much for the info! You know of any good place to start looking more into optics and the physics behind them? Any books/websites you'd recommend? I've been shooting for years and I'm really interested in what what's going on behind my camera in terms of light, physics, and optics.

    Thanks again!!
  • This is a very cool topic and something that I have wondered about ever since I started getting into photography and picked up my first "Fast" lens... the humble 50m 1.8. I would love to learn more about the physics behind how these optics work..

  • It may help to think about light in some standardized units - say Watts (proportional to # of photons) per square cm (W/cm2, for here). So imagine putting different film formats behind a lens with a fixed opening size in a dark room, with only one candle as a source of light. Put a tiny (much less than 1 cm2) sensor behind that lens and it would be swamped with light. Put a 11"x8" large format piece of film behind that lens (it'll have to be farther away) and the same number of photons have to spread out to illuminate something that's now hundreds of square centimeters. Now it's going to be very underexposed.

    For "stops" to be useful for photographers, then, who may be using different formats, the f/ratio of a lens has to be stated relative to the size of the chip/film/wet plate it's intended to illuminate.

  • Thank you very much for the info guys!

    Very Helpful!