Author Topic: [ANSWERED] What are the raw shutter and aperture values exactly?  (Read 2656 times)

Marsu42

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* Shutter: I know the raw shutter value doubles every 8, so for example 120 = 1/250s and 160 = 1/8000 ... but what the intermediary values, i.e. what is 1/180s and 1/200s?

* Aperture: What are the raw aperture values & what are the steps?

* As I read in the source code you cannot set any value but have to stick to 1/3 or 1/2 steps, but this is covered by the functions in lens.c, correct?

Thanks!

eduperez

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Re: What are the raw shutter and aperture values exactly?
« Reply #1 on: July 29, 2013, 08:52:27 AM »
[I hope all Canon cameras work the same: this is what I learned from the 400D, but from my browsing of the ML source code, it seems to use the same method]

Both shutter speeds and aperture values (and even the ISO values, at least at the 400D) use the same algorithm: raw values are simply expressed in 1/8EV steps; and that means 1EV (or double exposure time) equals to increasing the value by 8, as you already know. Exposure compensation values follow the same pattern; and this is very convenient, as you can use plain arithmetics to work with exposures.

For the intermediate values, +1/2EV is naturally expressed as (base + 4), +1/3EV is (base + 3), and +2/3EV is (base + 5). Other values (base + 1, 2, 6, or 7) for exposure or aperture are just rounded to the nearest supported values; but they all work for ISO values (thus we have 7 intermediate ISO values for each EV increment in the 400D).

Hope this helps.

Marsu42

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Re: What are the raw shutter and aperture values exactly?
« Reply #2 on: July 29, 2013, 03:47:06 PM »
Hope this helps.

Yes, it does - thanks! It's great to be able to ask around here rather than painstakingly wading through the ml source and trying to figure it out myself.