Ricoh GXR A12 Mount compared with Leica M8/M9

jankapp said:
The problem with these correction tools is, that they are dependent of the f-stop. See as an example:
http://de.leica-camera.com/photography/m_system/lenses/139.html
Technische Daten and then Vignettierung.

Jan
Click to expand...

Jan,

I looked at the vignetting data for the 24mm f/1.4. (By the way, do you actually own that gorgeous lens?) The Y axis shows light falloff as you move away from the center, where "100%" means "no vignetting." Doesn't that mean that 0% equals 1 f/stop of falloff?

Anyway, the X value of interest would be 10-12 mm, right? The edge of an APS-C sensor is about that far from the center. Anyway, difference between f/5.6 and f/1.4 is about 25%. I'm not sure how may stops that equals--1/4?

If my reasoning is valid, which it may not be, it seems like a vignetting correction for wide open would be reasonably good for a couple of stops down, too.

Tom
 
No, no, this is just an example.
The diagonal of full format is 42mm (43,2mm to be exact). This gives an X value of 14mm (radius and crop factor).
The Y value of 5.6 at 14mm is 60%, that is almost one f-stop from the ideal value (100%).
At 1.4 it is 30%, that is another full f-stop less.
Jan
 
jankapp said:
No, no, this is just an example.
The diagonal of full format is 42mm (43,2mm to be exact). This gives an X value of 14mm (radius and crop factor).
The Y value of 5.6 at 14mm is 60%, that is almost one f-stop from the ideal value (100%).
At 1.4 it is 30%, that is another full f-stop less.
Jan
Click to expand...

1. OK. In that case I can see the reason to aim the vignetting correction at whatever aperture you use most frequently. With something like the 24mm f/1.4, I'd probably be shooting wide open a lot, because that's the reason for owning the lens!

2. Just checking my math: is the factor of light falloff the antilog of the percentage of falloff? If so, then "60%" means a falloff of 2.5x (antilog .4), even more than one stop. Please set me straight if this is not the right formula.

Tom
 
Isn't it like exposure times, f-stops and ISOs? Double or half.

One f-stop is 50% of the light (-intensity), two f-stops are 25%, etc.
At the top: 100% means no f-stop compensation necessary. And 0% means absolutely no light in this area, say an infinity number of f-stops 🙂.

I propose the following formula: y = 1/log(2.) * log(1/x)
where
x is the light fall-off in parts (in stead of 25% -> 0.25).
y the vignetting in f-stops
log function at base 10
constant log(2.)=0.30103

These compensations (M9 and Ricoh) stay emergency measures. In these areas the electric current is amplified simply, giving the same effect as in Photoshop, namely more noise. The light (the information) is dimmed in these areas.
Jan
PS
A light fall-off of 60% gives 0.74. So between 2/3 and 1 f-stop.
 
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