Light loss on current CMOS sensors at big apertures

Started Oct 27, 2010 | Discussions thread
Andrew Westlake Senior Member • Posts: 2,928
Fast lenses, pixel vignetting and blur circles

Bogdan_M wrote:

I'm so glad I decided to stay with the f 1.5 nokton 50mm, I would've ben very disappointd to know I bought a huge 1.1 chunk of glass only to get f 1.6 "real" light and dof.

This, I think reflects the most questionable idea put forward in the LL article - that somehow a fast lens is 'wasted' due to pixel vignetting, and one that's rather slower will do essentially the same job. Reality is that, because this is a sensor issue, on any given camera the slower lens behave exactly the same at its available apertures as the faster one, but that faster lens will still deliver benefits at larger apertures (the real 'fix' would be to change the camera, but this isn't an option for many users). Yes, there's diminishing returns, but the one thing that's not addressed in the article is what this means in practice.

Here's a little quick'n'dirty experiment I did about 6 months ago investigating the pixel vignetting phenomenon using the Leica Moctilux 50/0.95 on the GF1. This is aimed at looking at DoF and background blur effects, by measuring the blur circle from an out-of-focus point light source (here an LED flashlight about 4m from the camera in a darkened room) in the centre of the frame across a range of apertures. Clearly, the blur circle diameter should be directly proportional to the aperture diameter. Here's the results:

What you see immediately from this is that there's no sharp cutoff in blur circle at a specific aperture - however a little further calculation shows that you don't get the full theoretical benefit of the maximum aperture either. In this I'm assuming pixel vignetting affects are negligible at F2 (which, given the results, seems fair), and calculating what the blur circle diameter should be at larger apertures.

This shows that even at F1.2 there's not a huge effect - maybe 1/6 stop loss. At F0.95, though, the blur circle is considerably smaller than expected, corresponding to an apparent aperture of about F1.1, i.e. maybe a half stop loss in light. However, even then you do still get a bit more blur than when the lens is set to F1.2, so the F0.95 lens isn't completely wasted. (However it does reinforce the idea that the Voigtlander Nokton 50/1.1 may just be better value than the Noctilux, as if we didn't know that all along.)
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Andy Westlake
dpreview.com

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