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Focus Accuracy: Focusing Screens for f/1.4 Lenses in Canon, Nikon

This discussion applies to the Canon 5D Mark III and Nikon D800/D800E.

Andrew R writes:

I’ve seen your comments about the new Zeiss 55/1.4.

Would it be advisable to have a better (dedicated) focusing screen for manual focus, and if so, how could be done? Katz Eye and have some, but none specifically designed for D800 or D4.

DIGLLOYD: The issue is that the as-shipped (and only) focusing screens for these cameras limit subject separation to f/2.5 or so (and perhaps not even that), making precise manual focus by eye an error-prone exercise. There are certain “custom” focusing screen solutions, but none of these are palatable for general use, at least not for me, and maybe none at all for Nikon D800.

Generally speaking, the latest focusing screens as-shipped are a tradeoff for screen brightness versus in/out-of-focus “pop”, but it also involves new “overlay” features (e.g., gridlines). Since many of my favorite lenses are f/1.4 or f/2, the tradeoff is a poor one for me (brightness is hardly a concern).

Especially with an ultra high-performance lens, tiny focus errors become obvious as a loss of contrast or outright blur of fine details. This is mitigated in part by the high contrast which helps differentiate in-focus from out-of-focus areas, but this is not sufficient for razor-sharp focus at f/1.4 because the peak sharpness differentiation is killed by the focusing screen. In short, “Close only counts in horseshoes”.

With lower performance “classical” lenses, the wide-open smearing (hazy low contrast) masks small focus errors since focus is effectively smeared over a deeper zone by various optical aberrations (spherical aberration and longitudinal chromatic aberration in particular). The Nikon 50mm f/1.2 AI-S or the Zeiss 50mm f/1.4 Planar and 85/1.4 Planar fall into this camp, but most f/1.4 lenses qualify.

Focus accuracy is the single biggest impediment to using manual focus lenses today, and why an EVF and/or rangefinder can still be preferable for those shooting ad-hoc (without a tripod or time to use magnified Live View). Neither Nikon nor Canon has done the obvious: supply an EVF for use with their DSLRs which would mitigate the issue, since the sensor image sees whatever aperture is in use. With suitable aids (focus peaking), this could be a valuable addition to a DSLR.

Shooting alternative

Zacuto Z-Finder

The last paragraph leads us to one workaround: use of the Live View feature handheld. This is not as fast or convenient as an optical viewfinder, but it can be pinpoint-accurate.

You’ll need the Zacuto Z-Finder loupe with the stick-on mounting frame so that the loupe can be snapped onto the back of the camera (the regular cine-friendly mount interferes with use of my favorite L-bracket).

Thus configured, the camera can be pressed to the forehead/eye, and one has a beautiful live image image by engaging Live View. Configuring the DSLR to zoom in with the press of a button allows pinpoint focus. Another press reverts to the entire frame, snap the picture.

This approach works well, though it is not for capturing the precise moment. I have seen one elderly client use this approach with success in the field (for him the optical viewfinder was hopeless due to diopter issues). And I’ve used this technique myself ad-hoc and even without the loupe at times.

The trend

The effort to design and offer manual-focus-friendly groundglass screens and cameras has apparently fallen by the historical wayside at Nikon and Canon, due in part to designing in “overlay” focusing screen features, such as rule-of-thirds grids, illuminated focusing points, etc. Camera design is also kept cheaper and simpler by not offering alternate focusing screens, which also means that third-party screens cannot be easily developed or installed.

A professional-grade camera body with these features makes sense (the still-missing Nikon D4x and Canon 1Ds Mark IV). Yet a new pro body from either company seems far off, and perhaps might never appear (unprofitable).

Jeroen B writes:

I have a BrightScreen (sadly no longer available due to death of owner) in my D800e, which makes a big difference in terms of focussing accuracy and ~ speed.

The overlay and AF-point illumination still work (for example shooting in 4x5 mode) so overlay is NOT part of the screen itself, they're incorporated in or below the prism. Nikon really should bring out other screens for the D800.

DIGLLOYD: Good to know, but this option now sounds like a dead-end.
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