Can someone explain lens cast by aperture to me

Started 4 months ago | Discussions thread
ProfHankD Veteran Member • Posts: 7,522
Re: Can someone explain lens cast by aperture to me

Alan WF wrote:

Conversely, if the lenses suffers wavelength-dependent vignetting at the iris, I’d expect the color shift to be independent of the spatial scale of the feature.

Any idea which is it?

I think it is most often wavelength-dependent vignetting, as you called it -- and nearly always the blue is much weaker wide open and reaches normal levels when stopped down. The interesting thing is that it is very common, but usually not very obvious.

My theory on that is along these lines: Color film would clearly shows these color casts, but it was rare that people would shoot anywhere near wide open. Wide open was thought of (and heavily marketed) as primarily about giving a brighter OVF for composition and focusing. Even so, the color casts from most lenses are subtle enough to be difficult to notice unless you have side-by-side shots in the same lighting (perhaps even on the same roll of film, because color did vary a bit with processing). Most lens testing was monochromatic; color tints were not really something people checked for.

Now, however, it's really common to try to shoot images wide open. Perhaps because small-sensor cameras have everything in focus, bokeh are much more appreciated than they were decades ago. Digital cameras also have enough light sensitivity and adjustability to make shooting with a range of aperture settings in the same lighting possible. However, digital cameras don't usually get treated as having a fixed color balance. Thus, even heavy tints tend to disappear as "gray world" auto white balance corrects for them. The result is that color tints turn into color noise biases... again hiding the tint. Incidentally, most cameras significantly alter saturation as a function of ISO, essentially muting colors at high ISO as a side-effect of noise reduction, and most people don't even seem to notice that....

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