Can someone explain lens cast by aperture to me

Started 3 months ago | Discussions thread
ProfHankD
ProfHankD Veteran Member • Posts: 7,512
Re: Can someone explain lens cast by aperture to me
2

fferreres wrote:

This is something I've seen Hank and other discuss. I considered an oddity and made some guesses, things that would be subtle, and hardly distinguishable. Then one day, this...same scene, same time, same light, same angle, same lens....same ... white balance:

Stopped down

Wide

First off, the stopped down one generally has the colors closer to right... although many of us might actually prefer the warmer rendering, it's basically missing blue. You'll also note that contrast tends to increase when you stop down.

Index of refraction of most materials is a function of the wavelength of the light. Blue rays are deflected more than green or red ones, so getting the red and green marginal rays to the point of focus tends to imply that the blue ones miss their target. I like to think of it as some rays being sent outside the diameter of the glass, and thus being lost rather than collected back in by subsequent element of the lens. It's not uncommon to see a blue fringe around the image circle of a lens, and this might be why. Of course, some of the misdirected blue might also end-up as blue haze in the image.

Stopping down also cuts out the green and red marginal rays, so things get better balanced.

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