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I am a software developer writing a software interface for a Bosch mic 9000i camera. I have an interface to change the focus of the camera and I'm writing documentation for an end user. The numeric focus values that the camera accepts are 0-255. I've been referring to the value range as the magnitude. What should I call a numeric focus value for a camera?

When I increase the focus value, the camera shifts focus to closer objects, decreasing the number brings further objects into sharper focus. Is there a better way to phrase this than 'Focus Near' and 'Focus Far'?

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  • Having written code for stuff like this I feel your pain. In my system there is a preset "near focus" button and "far focus" with a pair of in and out buttons. Adjust the focus using near and far. Then a long press on the "far focus" sets it, and a brief press recalls it. My boss always sets them the wrong way round - focusing on an object and then setting the wrong button and then gets confused. (Near & Far Focus buttons should just be called preset A & preset B)
    – D Duck
    Mar 25 at 21:04

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Photographers don't care about the 0 to 255 values, they care about the actual focal distance of the lens in real world units (i.e. metres in the vast majority of cases). If you are creating a product for photographers, you should find out how to translate arbitrary numeric values into real-world units and use those.

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  • If you're not creating a product for photographers, then the question is off-topic.
    – Philip Kendall ♦
    Mar 24 at 20:14
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    Agreed. Nobody other than the users of the software interface care about 0–255 limitations of computer number representation. They care about what the input represents. As a user (and even as a software developer), I want to input a useful real-world number, such as focus distance. The software library, API, driver, or whatever, should handle the conversion from human-useful numbers to system-specific digital values, IMO. I've written dozens of such types of specs, and used hundreds, and I always appreciate the ones that abstract the computer-oriented details away from real-world use cases.
    – scottbb ♦
    Mar 25 at 1:14
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Minimum and maximum focus (focal) distance. As others have indicated, you'd be appreciated for indicating in a metric like feet or meters. Regardless, increasing and decreasing focus distance seems appropriate.

At the very least, you'll want to change 0 to be the minimum focus distance for that lens, and 255 the maximum. According to BH photo's specs for the model you mentioned, the range is 276 feet - infinity. The values between are an exponential function, so that probably 50 is roughly 2x your minimum distance, but you probably will want to know the exact function.

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    "You'll want 0 to be the minimum focus distance for that lens, and 255 the maximum." The OP clearly indicates that the internal system he must deal with is the opposite: "decreasing the number brings further objects into sharper focus," so 0 is focus at infinity or as close to it as the system can get. I see no reason to bring the "0" through "255" numbers out to the user either way; a photographer wants to know the focus distance in meters (or possibly feet if he's so unfortunate as not to be familiar with metric).
    – cjs
    Mar 25 at 5:31
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    "The values between are an exponential function" Are you sure? Typical linear focussing systems would give a hyperbolic function, assuming that the "focus values" correspond linearly to a displacement of the optics.
    – Ralf Kleberhoff
    Mar 25 at 10:06
  • Good catch @cjs that OP is treating 0 as infinity. Since they are a software developer writing an interface for end user, I made more clear that the polarity should be flipped.
    – Mark K
    Mar 25 at 15:20
  • @RalfKleberhoff Admittedly I used the term exponential to refer to any changing rate of change, sorry for sloppy wording. Would a hyperbolic function suggest a different (kind of) rate of change than the example I had provided immediately after the quoted line?
    – Mark K
    Mar 25 at 15:27
  • The word you're looking for may be "non-linear".
    – Ilmari Karonen
    Mar 25 at 16:29

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