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Zeiss ZE 85mm f/1.4 Planar for Canon EOS focus problem

Please see my initial comments on the Zeiss ZE 85mm f/1.4 Planar. See also Zeiss ZF Lenses.

Update: several readers have written to help me understand how autofocus systems work, and I’ve now gotten similar feedback from Zeiss. It’s all a little confusing but I think I’ve sorted it out well enough to explain it.

The central AF sensor “sees” a narrow angle corresponding to what is seen by the sensor when the lens is stopped down to f/5.6. Thus, even though spherical aberration affects the entire frame, the off-axis light rays are not seen by the AF sensor.

The net result is that the “green dot” indicates focus for f/5.6; shooting wide open then naturally induces a focus shift towards the front (front-focus). This is exactly consistent with what is observed when focusing using f/1.4 with Live View; focus shifts to the back.

The Zeiss factory previously indicated to me that the ZF/ZE 85/1.4 Planar has the least focus shift of any Canon/Nikon/Leica 85/90mm lens near infinity (actual quote is in Zeiss ZF Lenses). Yet I don’t recall a focusing error with those lenses— something to verify.

Bottom line: the logical advice (not yet verified) is that if you’re making the exposure at f/1.4 or f/2.8, focus by eye. If you're shooting at f/4 - f/5.6, use the AF assist. This is all theory, I haven’t verified it yet. Just realize that the ZE 85/1.4 Planar is a very sharp lens, but accurate focusing is a challenge.

--- end update ---

Canon EOS cameras have a “green dot” which I’ll call Focus Assist. When the green dot lights up, the image is (allegedly) in focus. This works only with “chipped” lenses eg Canon EF lenses or lenses like the Zeiss ZE line, which have the required electronics (it ought to work with any lens, but Canon has chosen to disable the capability, unlike Nikon).

Shown below is the full frame snapshot demonstrating a problem that occurs constantly with my evaluation copy of the ZE 85/1.4 Planar: focus confirmation is way off, so far off that f/16 won’t rescue it. It’s badly front-focused; it is never a backfocus issue. In this example, the seagull in the foreground is sharp in the mis-focused image, instead of the distant tower!

I’ve compared it to the Canon EF 85/1.2L II and 85/1.8 lenses switched to manual focus, as well as multiple 50mm lenses (including the Sigma 50/1.4). The problem is unique to the Zeiss offering. Note that this has nothing to do with focus shift; the lens was shot wide open.

No word yet from Zeiss on whether they’ll be able to send me a 2nd copy for evaluation. The problem is so acute that my assumption is that it’s a problem with my particular sample. If that’s not the case, it means that focus will have to be by eye or Live View, which is the way I shoot anyway. Still, Focus Assist can come in handy.

The test frame, focused on the distant tower

Shown below is an actual pixels crop at f/1.4 with focus racked to the infinity stop. The image looks sharp in the viewfinder, too.

The test frame, focused on the distant tower

This is a second frame, but this time with focus Focus Assist (green dot). The viewfinder image is blurry when the Canon 1Ds Mark III gives focus the green light. Houston, we have a problem! This is just one example, I’ve obtained similar results over and over.

The test frame, focused on the distant tower
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