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Lens Brightness: Camera Makes an Adjustment?

Bruce D writes in reference to my Sigma/Canon 35mm comparison, in which I found that the Canon lens was brighter by ~1/3 stop:

Is it possible that the difference in apparent brightness results from Canon's fast-lens correction? It seems that all camera manufacturers silently boost the camera's ISO to compensate for sensor's inability to detect peripheral light from fast lenses.

The effect has been widely reported (initially by DXO Labs - see
http://www.dxomark.com/index.php/Publications/DxOMark-Insighs/F-stop-blues )

I did a brief test of the effect with my 5D MkII and an OM 55/1.2 and confirmed the effect.

May I suggest you determine whether the Canon lens really is brighter than the Sigma - this is easy to test by partially unlocking the lens so that the camera cannot identify which lens is mounted. I'm fairly confident the third of a stop correction will magically disappear. :)

DIGLLOYD: This diffference is at all apertures. Perhaps the camera compensates for wide open, then maintains a fixed staggering for consistency?

I have confirmed a difference at ƒ/1.4, tested under tungsten and daylight. I shot the comparison four times to be sure. The difference is real (Canon 5D Mark III + 35/1.4L). I have not confirmed a difference at smaller stops (e.g., ƒ/5.6).

In terms of Adobe Camera RAW exposure compensation to match two frames of the Canon 35/1.4L, I am seeing difference of ~0.13 stop, numerically small but easily visible in an A/B comparison. The color appears to be subtly different also, but this I have not confirmed.

Variation in brightness — Canon auto-lens tweak 
Variation in brightness — Canon auto-lens tweak

 

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