90cron backfocusing (and noct)

[carstenw]

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Lars, he did say backfocus, not focus shift, with the 90 Cron, and I do believe that he knows the difference.

 

Additionally, the camera is off way more often than most lenses. The occasional lens has trouble though, so it should be carefully determined where the problem is.

 

I understand not that Zeiss plays games with where the set the focus, but in the design, where they compromise wide open sharpness in order to gain a more consistent focus distance through the aperture range. They also aim for high contrast, which would counter-effect the appearance of softness, and would explain a few observations made in the past, by Sean among other people. Anyway, Zeiss lenses are very good, so we are likely talking small amounts here.

 

The young man from the age of logic.

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[lars_bergquist]

Lars, he did say backfocus, not focus shift, with the 90 Cron, and I do believe that he knows the difference.

 

Additionally, the camera is off way more often than most lenses. The occasional lens has trouble though, so it should be carefully determined where the problem is.

 

I understand not that Zeiss plays games with where the set the focus, but in the design, where they compromise wide open sharpness in order to gain a more consistent focus distance through the aperture range. They also aim for high contrast, which would counter-effect the appearance of softness, and would explain a few observations made in the past, by Sean among other people. Anyway, Zeiss lenses are very good, so we are likely talking small amounts here.

 

The young man from the age of logic.

 

I agree that the amount must be small! And I do agree that the setting of the focus I was referring to is probably done at the design stage, not during assembly. And yes, the *mechanical* focus calibration of the lenses (as distinct from optical focus shift) is remarkably rugged, so that purely mechanical misalignment is more likely in the camera. I do think however that most of the problems that have aroused posters here have been due to focus shift – but that very often they have called it 'backfocus'.

 

The old man from the Age of Perverse Logic