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Max Field

Digiprime Adapter Bloom Mystery

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I saw that my new Pocket 6K camera would crop the sensor to a 2.7k slice, making it small enough to cooperate with the image circle of my Zeiss Digiprimes.

I put a 14mm on via a B4 to EF adapter (no glass element between), backfocus adjuster set to 0, and with the lens at T2.8 everything seemed pretty nice from what I could see.

b5ee38af07db93fc8ada50827cb7e05f.jpg

 

HOWEVER when I slipped the aperture all the way to T1.6, the optics had a very sudden dropoff and went wild with bloom on the highlights:

97b5df3845ab10d98ad9b56ac5c67e25.jpg

 

Any lens experts (maybe @Dom Jaeger ?) have any insight on what's going on? Flange distance? Coating? Poor adapter?

Thanks for anyone's help on this mystery.

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Those lenses were designed for 3 chip cameras with a beamsplitter. If you use them on a single sensor camera without using a proper (compensating glass) adapter they’ll exhibit those kinds of aberrations unless stopped down. 

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12 hours ago, Dom Jaeger said:

Those lenses were designed for 3 chip cameras with a beamsplitter. If you use them on a single sensor camera without using a proper (compensating glass) adapter they’ll exhibit those kinds of aberrations unless stopped down. 

That makes sense, but the next thing which I'm confused of is why the image doesn't chromatically aberrate with little RGB slices as opposed to this incredibly soft bloom. When a lens messes up from adaption I guess I'm just used to it being weird at all settings instead of making a couple stops off limits.

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3 hours ago, Max Field said:

That makes sense, but the next thing which I'm confused of is why the image doesn't chromatically aberrate with little RGB slices as opposed to this incredibly soft bloom. When a lens messes up from adaption I guess I'm just used to it being weird at all settings instead of making a couple stops off limits.

I've seen this myself with B4 zooms and the results were exactly what you're getting here. My knowledge of optics is not sufficient to explain why it happens but it's well-known.

Get an adaptor with correcting optics in it. Get a good one, because the Digiprimes are lovely.

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In converging rays, a prism will introduce spherical aberration and astigmatism as well.The larger your aperture is, the wider is the cone of rays pointing to the pixel and the steeper is the angle they converge at. Since spherical grows rapidly with aperture (not linearly, but rather a bit faster than (aperture diameter)^3), visually it kind of unexpectedly kicks in after being very mild in the lower range of apertures. 

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The issue is the prism rather than the chromatic separation that the prism facilitates.

It’s similar to the issue with the reflex prism in Bolexes, where special RX lenses were made to compensate for the optical aberrations introduced by the block of glass in the lens light path. Since compensating for aberrations in effect introduces aberrations in the opposite direction, if you use these adapted lenses without a prism or a correcting adapter it results in similar issues to those being compensated for.

As Michael mentioned, spherical aberration and astigmatism are the common issues (which is what you’re seeing with that blooming and softness), but as you stop down a lens the cone angles reduce and the aberrations diminish. It tends to be more of a problem with shorter focal lengths, though this is a correlation associated with the exit pupil depth rather than the focal length itself. In essence, when you look at the back of a lens through the rear element, the further away the iris appears, the less affected the lens will be by a block of glass in the light path. The worst affected lenses will have the iris apparently sitting right behind the rear element.

 

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