5D3 vs D800 - A Technical Analysis (UPDATED & EXTENDED) - With Samples

Started Mar 30, 2012 | Discussions thread
OP Legion5 Senior Member • Posts: 1,047
Re: Sigma 70mm f2.8 IS f2.8

Jan Madsen wrote:

Legion5 wrote:

If imaging resource really doesn't vary the amount of light then, it's likely that the Canon and Nikon varieties of the Sigma 70mm f/2.8 have apertures that are a third of a stop different, which is fairly reasonable in retrospect. In some of Sigma lenses because they lied so much about the maximum aperture of a lens, when you stop down their lenses they only go down a half stop. In this case perhaps Sigma opted to keep the stops uniform and each stop on the Canon version of the Sigma 70mm f/2.8 is a third of a stop less than the Nikon version of the Sigma 70mm f/2.8. Due to the different packaging and focusing motor requirements for Canon and Nikon this sort of difference makes a lot of sense.

Now THAT is ridiculous! All lenses cheat a little with the max aperture (zooms often more than primes), but to postulate that the Nikon version of a lens should be different from the Canon version is far out. What would Sigma obtain with that? All lenses from third part manufacturers have identical inner workings between the various camera mounts, it is only the electronic/mechanical interface that differs.

I just measured the entrance pupil of my Sigma 70mm f2.8 (Canon mount), a quick measurement says around 24½mm in diameter, that gives 70mm / 24½mm = f2.85, which is correct. And the 70mm focal length is also close to the mark, I tested that earlier. This is of course at infinity, at closer distances both focal length and aperture changes a lot.

Another quick test a few minutes ago with various f2.8 lenses in the 70~180mm range gave the same exposure - 1DsII camera selected 1/10 or 1/8 randomly from measurement to measurement, even with the same lens, with focus set at infinity onto a diffuse target only 30 cm from the lens. Including the Sigma 70mm f2.8.

I did more digging and found that the Nikon 70mm Macro is well known to over expose by about a half stop, but the Canon version of the lens exposes normally.

I also e-mailed Imaging-Resource about this issue where Nikon cameras are getting 50% more light than Canon ones and they are aware of it. Imaging resources put forward that the the Sigma 70mm Macro has a software bug which causes it to overexpose on most Nikon bodies. On newer Nikon cameras the Sigma reports the aperture incorrectly and lower than it actually is. So if you tell the lens to go to f/8.0 it's actually going to f/7.0. The explanation for this issue is this is a result of Sigma's poor reverse engineering of Nikon's lens communication protocols. (Sigma didn't buy or license Nikon's lens protocols they just tried to figure them out on their own). On older Nikon cameras the Sigma 70mm f/2.8 exposes fine, but on most newer ones it overexposes by half a stop through incorrectly reporting the aperture to the camera.

So there's our answer, the shutter speed is different because the Sigma 70mm f/2.8 for the Nikon mount has a software bug related to the way it reports it's aperture on newer Nikon bodies and needs to be patched.

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