Nikkor Adapters for Canon EOS 5D - Not all are equal quality!
 

I've always strived over the years to use same-brand lenses to their intended bodies - Nikkor to Nikon; EOS to Canon; Zeiss to Contax; SMC to Pentax; Minolta to Minolta, Leica to Leica etc., but there are times when a certain opposite brand lens of a certain length or maximum aperture performs better than the same length lens offered by your current camera body brand.

This is when you go for the simple option of buying a lens adapter that will allow you to use that longed-for lens on your favourite camera body. It is rare for all the electronic signals to be passed over so that you can employ all AF and IS/VR options etc, but most adapters allow you to at least use that gem-of-a-lens Nikkor or Contax etc., on your Canon body.

But think carefully before you buy just any adapter - 'but if the adapter contains no additional glass, surely all adapters will be fairly equal?' Not so.

To avoid me writing too long a post, I will point you to a simple series of tests between a sharp Sigma wide zoom and legendary Nikkor 15mm prime. The Nikkor apparently comes out worse during the first test (using a normal Canon EOS to Nikon Nikkor adapter) but a second series of the same tests using a different lens adapter with deeper register actually completely reverses the lens test results!

View the link to read more information of the tests:

Nikon 15mm f3.5 AIS v Sigma 12-24mm: Intro

And here is the final test results where the different adapter made a huge difference:

Nikon 15mm f3.5 AIS v Sigma 12-24mm: f5.6

Do you have an explanation why the the thicker adapter contributes to better sharpness?

Nikon lenses sit further away from the "film" plane or in this case the sensor plane than Canon lenses. This is why they can be adapted to Canon bodies - from memory the reverse is not true. So setting the correct distance between the last lens element and the film plane is critical. I guess that if you get too close the lens can never produce a truly focussed image without a backfocus type adjustment which still camera lenses lack.

If that's the case how do macro extension tubes work? I thought they only affected infinity focus, and assumed it would be the same situation if an adapter was slightly off.

Not sure Evan - could only hazard a guess that macro extension tubes take the lens further away from the film plane not closer. I'd wait for someone with actual optical design knowledge to chime in for the definitive answer. Clearly there is something going on - my best guess is the minimum focal length from the rear element... the effect on that first image reminded me of when I developed some images and accidentally left the condensor lens out of my enlarger (years ago:). Less exagerrated but roughly the same.. centre sort of in focus and directional blur from the centre to the edges.