Sat Feb 17, 2018 6:40 pm
QUOTE: "Thanks for your input."
You are welcome. I hope I have not misled you in any way.
QUOTE: "I've been doing some more testing today and I think this has more to do with the lenses. I tested the lenses (TLS rehoused Cooke Speed Pancros) on a different Ursa Mini Pro and the effect is the same."
Those old Cooke ( Taylor Taylor and Hobson ) Speed Panchro Series 2 lenses by today's standards are more flarey and the coatings may be by now uneven in reflection suppression. The URSA Mini Pro as far as I know has two ND filter disks behind an IR filter. Even with no ND selected, to ensure focal consistency there will still be clear planar elements in the disks.
Therefore maybe a total of five planar surfaces exists, capable of creating several generations of internal reflections versus one for the Alexa Mini which as far as I know, may have only a IR/anti-alias/contrast filter in the optical path between lens and sensor.
QUOTE: "I then tested the camera without a lens but with an ND in place to control the exposure. There was no dramatic colour variation. Saying that I have an Alexa Mini kit and this doesn't happen on that camera with these lenses. Could the sensor be more sensitive to these colour variations/light refractions?"
I cannot answer this question except by wild guesswork. The colour defects in your sample images appear to be across the span of the URSA MP sensor. There can be colour defects towards the edges or corners of some sensors with some wide-angle lenses and IR filter combinations but these generally appear as a symmetrical pattern.
I understand that the green channel in the URSA MP sometimes may need boosting in colour grading.If the green channel needs bringing up, then a green colour defect in the image will itself become aggravated.
Another aspect to the old Speed Panchros is that their sweet spot seems to be in the ballpark of f5.6-f11. Indoors you may well have had to open them up to fully wide. Compared to modern lenses, small optical aberrations ( maybe astigmatism ) I might expect from older lenses.
They may even have manually-figured or manually-finished elements in them, They may be provoked by wide-open or near wide-open aperture into yielding sharpness variations and related chroma variations in areas of the image.
I am sorry I cannot be any more help but hopefully my comments may prompt the right answers by people more knowing than I.
Hopefully a lens tech like Matthew Duclos might chip in and put right any wrong assumptions I have made here.