Tuesday, 19 June 2012

Focus Shift with Voigtlander 35mm f/1.4 Nokton Classic

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I am currently reviewing the Ricoh GXR A12 M-mount body and have the super-compact not to mention super fast Voigtlander 35mm f/1.4 Nokton Classic MC rangefinder lens for review also. Before the reviews are published I thought I would show the focus shift that this particular lens exhibits. On a camera like the Ricoh GXR M-mount (or, for that matter, any mirrorless camera using an adaptor) this particular characteristic isn't really an issue, providing you focus at the taking aperture. It is, however, a distinct issue if you focus at the maximum aperture and then stop down to the taking aperture or if you're using a coupled rangefinder such as the maker's own Bessa models or a Leica M series, which do not compensate for focus shift.

I've used the Michael Tapes Design Lens Align Mk II to demonstrate the focus shift - it's a first class focus calibration tool which was originally designed to highlight issues with DSLRs and AF lenses but it's perfectly suited for this particular purpose. The following images are unretouched JPEGs displayed at 1:1 in Adobe Lightroom 4.

F/1.4

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At maximum aperture, this is about as accurate as we can get (at a distance of around 3 1/2 feet), using focus peaking and the somewhat poor resolution of the Ricoh's VF-2 viewfinder in magnified mode. We've focused on the target to the left. If you look at the ruler to the right, focus is centered evenly on the +4 0 -4 digits.

If you're concerned by the amount of LoCA I can say that photographing targets with high contrast edges like this at close distances easily reveals this trait and others (such as SA) but real world images aren't likely to look quite so aberrated.

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Besides, as you can see from the photo above, I can remove nearly all traces of LoCA in the latest version of Lightroom 4 - it's one of the most powerful features of the recent upgrade and one of the reasons why I recommend the utility for professional users.

F/2.0

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Focus shift is immediately apparent at f/2.0, as the focus plane has shifted rearwards (exhibiting back-focus) with the depth of field over +12, 8, 4, 0 -4 (the +12 and -4 being equally unsharp)

F/2.8

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By f/2.8 the depth of field continues to expand but the red ±0 is now out of focus, with the focus plane being over +24 to +4 range and the sharpest point of focus being around +20 to +16

F/4

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At f/4 the focus shift continues rearward with the sharpest focus point being around +26 to +20, note the red colored rear target intersecting the large 4 is clearly in focus.

F/5.6

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At f/5.6 the depth of field is beginning to encompass the original focus target (it's still a bit soft however) but the point of sharpest focus remains to the rear at around +36 to +20.

F/8

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Much the same can be said at f/8, only now the depth of field now encompasses the original focus target (though the sharpest point of focus remains rearward at +36). And things don't change at f/11 or f/16 (there's little point showing those). Apart from the focus shift there's a lot to like about the Nokton, which we'll reveal in our forthcoming review so please check back for that.

The Voigtlander 35mm f/1.4 Nokton Classic MC can be purchased online in the UK at pro-dealer Robert White.

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