internal lens reflections in Alexa XT, but not in 5D Mark III ML (crop)

[MartijnR]

Hi All,

I hope all is well. I have recently released a short with a lot of macro shots, reversing the Zeiss Otus lens (5x crop). This seems to work perfectly.

If I do the same trick with an Alexa XT (preparing for a feature) there seems to be some banding on both sides of the image:

password: wasp



Most likely this is due to internal reflections in the lens. However, since the sensor of the 5D is larger than the S35 sensor of the Alexa XT, I would expect this not to happen. Or am I missing how the 5x crop works. Is it not using the whole area of the sensor?

Here is the short using the 5D, without any banding:

Thanks for your help.

All the best,

Martijn

[tupp]

Not sure what you are seeing, but what I see isn't bit depth banding.  It would help if we could see the same setup and camera angle, both with and without the phenomenon.

If I do the same trick with an Alexa XT (preparing for a feature) there seems to be some banding on both sides of the image:

... but is it actually the same?  There seems to be only a single shot with a single camera in one video,  but the other video shows completely different shots with a variety of setups.

Were both camera shot under the exact same conditions (at the same time, with the same lighting, from the same camera angle)?

Was the reverse lens configuration "light tight" on both cameras, or could light leak in, around the back of the lens?

Did one setup use a matte box or lens hood (or a lenser flag) while the other setup didn't use a matte box or lens-hood?

You mentioned the possibility of internal reflections.  If the Alexa setup was light tight, could the lens be sending light off to the side internally, that reflects off of shiny parts of the lens adapter/mount?

[MartijnR]

Thanks a lot for your indepth thoughts, tupp. Much appreciated!

My description ‘banding’ was poorly used, since I indeed meant some internal reflections (also thought by ARRI). In almost all the tests I did with the Alexa, these darkened areas were almost always visible (sometimes a larger area when on 1 side, depending on the light source location). Not only with the 55mm that I used for the test and short, but also with the 28mm (100mm not so much). In all the shots I created with Magic Lantern, and the reversed 55mm, no banding was visible.

You are right about a better scientific approach, and have the same setup (I will try this when I have time for it). Because the difference seems so clear to me, I was hoping to get to know if the sensor of the 5D is not fully used in crop mode. Do you happen to know if this is the case?

I haven’t thought about your last suggestion. The Alexa is light sealed, but I have adapted it with a Leitax mount. Maybe that might be the problem..

Thanks again!