Strange color cast area on MF digital back

Discussion in 'Medium Format' started by tom_higgins|3, Feb 23, 2010.

  1. Hi, would appreciate any help or info anyone has. following two shots made with a hasselblad 903cw and cfv 16mp digital back (just purchased)- both exhibit a quite definite purple patch in the mid to lower left half of the frame. Nothing similar when using same back with 503 and lenses. lighting was high contrast and the temp was a few degrees below zero. Anyone seen anything like this before?
     
  2. Color Cast is common in medium format digital when using wide angle lenses. There should be either a specific setting or a the ability to make a custom one in the Hasselblad software to correct it. I am not sure what they call it, in Phase One it is color cast correction. It has something to do with the light hitting the sensor sites at an angle instead of more straight on. It is even more pronounced with with very wide view camera lenses.
     
  3. Thanks for reply David. There is an "SWC" setting on the back menu, which I had selected- and I haven't really gotten too far into the bundled "flexcolor"software. I've heard of color cast issues with MF backs, but normally in regards to the whole frame being affected. Do you know, can they appear as "patches" of color, like in this case?
     
  4. The magenta effect is patchy, as in your example, and can be worse in high-contrast situations near the edge of the sensor. There is a little parallax between the Bayer filter and the sensor, which causes this problem. You don't see it with Distagon lenses because inverted telephoto lenses have a negative element which increases the clearance to make room for the mirror and reduces the angle of incidence at the image plane. An SWC has a true wide angle lens and no mirror, so the rear element is very close to the film plane.
     
  5. Since the temperature was below zero; is there a chance on had a 1/2; 1/4; 1/10; 1/20 wave of dew on the sensor?
     
  6. You may want to experiment with taking flat field images with your SWC at various apertures to determine if the colour cast is a product of the lens/back combination or due to the nature of the subject. If you notice colour patches in the flat fields, you can use those to process them out of your images. For background see image processing of astronomical images for a detailed explanation of flat field images.
     
  7. One can also get a ghost image off the real element of some lenses and get some weird color effects too.
    A non retrofocal lens has a steeper angle of light hitting the sensor than a retrofocus lens. Sometimes the new or upgraded backs have a better coating to address these issues. one gets a ghost image

    What lens was used?

    one has polarized light; a wideangle lens; maybe an older sensor.

    Run some tests and control the variables
     
  8. Thanks for the help Lads, appreciate it. I'm pretty sure now it's a feature of the lens/back combination, especially shot into bright light. Later yesterday I took some frames pointed directly at bright clouds and got the same patches. Shots taken indoors or away from the sun look fine. I can work around when it happens, it's too nice a camera to let go. Thanks again, Tom.
     
  9. From Hasselblad web site about the CFV 39 digital back (I recall was the same about the CFV 16)
    Camera support: Hasselblad V System cameras manufactured since 1957. (Not recommended for critical work together with SWC models and ArcBody due to optical incompatibility.
    http://www.hasselbladusa.com/products/digital-backs/cfv-39.aspx
     
  10. From the CFV-39 manual (page 48):
    "PLEASE NOTE: Due to the mechanical design of the SWC, pressing the exposure release button too slowly may result in faulty capture with a magenta cast. Either press the button much more distinctly or alternatively change the setting from SWC to Flash sync and use the flash sync cable to connect the lens to the CFV."
    - Leigh
     

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