Continuously variable aperture

My office mate and I were just talking about the sunny 16 rule, and it occurred to me, "Why don't any widely available lenses allow you to continuously vary the aperture within the mechanically available range? (or are there some that do?)"
If I'm shooting film without a light meter (or digital without a light meter for that matter, but when does that happen?), then I see obvious merits to only having sqrt(2) increments in f-number, and the nice-looking reciprocal-integer shutter speeds that fall close to sqrt(2) increments in time: It makes it easier to quickly calculate the aperture and/or shutter speed that you want in a given situation. You can reckon more quickly if you only have to deal with factors of 2 (or sqrt(2), whatever...). Also it would often be painfully slow to scroll through a "continuous" (or at least much more finely spaced) spectrum of apertures while you're shooting.

However, most cameras now have aperture priority and shutter priority modes. I don't see any reason why I shouldn't be able to dial in an f/2.718 aperture and have the camera calculate the proper shutter speed. Nor do I see why conversely I couldn't set a 0.154 second exposure time and have the calculate an appropriate aperture for me. Of course, the camera could still warn me if the "appropriate" parameters fell outside the realm of possibility for the equipment; that's what my current camera does.

I guess one short answer would be that the current level of discrete f-numbers are finely spaced enough for most purposes, and that maintaining them maintains continuity with the traditions of photography. I wondered though, does anyone (basically meaning Nathaniel 🙂 ) know of a mechanical reason why continuously variable aperture would pose any problem? I can think of only a few highly specialized situations in which this feature would actually be desirable (and that's probably why it isn't generally available), but I thought I'd throw the question out there.



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One thought on “Continuously variable aperture”

  1. So, the short answer... I think all SLR-type cameras do have infinitely variable apertures... Well, at least with the older-style lenses. Most of my Nikon lenses are D type, which means they have electronic innerds but still have an aperture ring you can turn manually. Since I use that to set the aperture, I can change it to anything I want, I don't have to stick at the detents. I don't know what kind of resolution the shutter has, the minimum speed is 1/8000 s, so I'd guess it can step in units that size. Looking at the EXIF information for my pictures in Lightroom though, it looks like I mostly stick to set numbers... f/2.0 and 1/80th sec. I don't know if the EXIF information is rounded though.

    I'm pretty sure I've seen weird apertures in the info from little point and shoot cameras too.

    So, mechanically, I definitely know that aperture setting is continuous (at least the way I do it). The shutter speed though is a bigger question. I mean, I really should be possible to have the camera select the exact right shutter speed. I'm not sure that it matters though. Lets say that you can adjust the shutter is 1/3 stop increments. At any arbitrary aperture, you're never going to be more than 1/6 of a stop overexposed or underexposed with the 1/3 stop stepping. I don't know that 1/6 of a stop is going to be visible in any sort of real-world situation.

    Sidenote: with my large-format camera I usually keep track of exposure down to roughly 1/3 of a stop and I think of it as f/11, f/11+1/3, f/11+2/3, f/16, etc. I went to a photography workshop once though where one guy had calculated that, in terms of exposure, it goes f/11, f/12.8, f/14.5, f/16. It was very odd. (And no, I don't want to admit how much work it just took to figure that out for myself.)

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