realVivek
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 p.20 #9 · Official: Voigtlander 35mm f/2 APO-Lanthar announced! | |
Hi, Thanks.
What Tim found and provided the solution with (using an UV cut filter) is quite right. Also, one of his observations that it depends on the conditions is also correct. Please do not take this the wrong way (I know Singaporeans are very sensitive) but there may not be sufficient UV in your air due to ground pollution whereas where Tim lives perhaps has very clean air, especially during the COVID-19 times. Paris (France) at ground level there isn’t much UV.
The UV/IR cut stacks in FF Sony cameras have both absorptive and refractive filters.
Take a look at this experiment. Shots of the filter stacks in UV and Visible light. No UV gets through even at angles. BTW, although Cosina have not published it, the exit pupil in AL 50/2 is likely very large.
 Filter-stack by Vivek Iyer, on Flickr
Visible: Sony A7 (full spectrum), UV/IR cut filter on UV-Planar 60/4
UV: Sony A7 (full spectrum), Baader U 2” bandpass filter (320-390nm) on UV Planar 60/4
inksandpaper wrote:
Thanks for the information @realVivek@! Do you happen to know if the UV cut filter for this Sony you measured is based on absorptive or interference coatings?
BTW I have not noticed any magenta shift off-axis on any of my lenses, no matter how I looked, with or without diffuser (similar to expodisc) filter, with or without UV filter in front of the lens, in all kinds of low and high UV sky exposures, even when deliberately testing and trying to look for it, with or without camera "Lens Comp. Shading" on and off, with the same lens that @tsdevine@ uses. But I can easily notice it in his samples and the new Cosina samples of the new 35mm. This is quite disturbing.
I am trying to understand how it might work. If the UV filter on the sensor is the interference type, the effectiveness is usually tuned to be maximal when the incident ray angle is orthogonal to the filter surface, and the effect is diminished as the ray angle becomes more oblique, which it will off axis, when the exit pupil is nearer to the focal plane, such as with wide angles of non-telecentric design. The BSI sensors of newer Sonys are greatly resistant to lens/sensor cast from oblique exit ray angles but it does not help when the sensor filter stack is transmitting light of a different colour. As the incident angle to the filter becomes more oblique, interference coatings become less effective and the reflected light colour shifts towards the longer wavelengths, i.e. reflected UV becomes reflecting more blue and more cyan light, and thus the transmitted light, having been filtered of that colour, becomes shifted to the opposite colour, which is more reddish or magenta. That's the magenta shift that has been noted.
I suspect this is the true nature of what is happening, though I've not yet seen any other explanation like mine. This is predicated of course, on the assumption that the UV filter is indeed using interference-based coatings. I happen to know, with good authority, that the IR filters (there are two), on the other hand, are using both absorptive and interference-based coatings. The interference-based IR filter should shift to longer wavelengths of reflected light also as the ray angle gets more oblique, which means more red (and far IR) light gets reflected, and thus more blue light is transmitted. Hot filters typically have a bluish reflection tint on the external surface, and reddish reflection tint on the inside surface, I gather. I don't use these filters and have only researched them online.
The effects seem to mostly offset each other in my tests, since I see that the colour distribution from on to off-axis is mostly very neutral, and if anything, the shift has been 1 or two points towards blue/cyan, rather than red/magenta. Interference coatings are difficult to apply consistently, and in all likelihood, the effectiveness of each do not balance each other out perfectly for some batches of filters. It may be that some batches of cameras are more susceptible to colour shift as a result. I honestly don't know if that's really the case. Others more intelligent than I might be able to offer a better guess....Show more →
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