Amazing D90 discovery wait till you hear this!

Started Jan 25, 2010 | Discussions thread
kenw
Veteran MemberPosts: 4,168Gear list
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FYI, purple fringe not microlenses or bloom
In reply to toomanycanons, Jan 27, 2010

Just to heat up an already hot thread I thought I might point out that as far as I know there has never been any evidence shown that microlenses or digital sensor bloom has anything to do with purple fringing. Every example I've ever seen and every opinion I've heard from people who actually understand optics has been that longitudinal chromatic aberration (sometimes called axial color) adequately explains purple fringe.

Sensor bloom is a ridiculous explanation as it would not be symmetric but instead strongly proportional to the direction of readout in a CCD - an effect never shown in any example on any camera. Also, it wouldn't be purple if this was the case!

It is certainly possible that microlenses may be involved to some degree, but again no demonstration has ever been shown that I'm aware of that wouldn't be better explained by plain old longitudinal CA. An excellent smoking gun for it being the microlenses as opposed to longitudinal CA would be an example in which the "purple fringe" did not improve with reduced aperture - particularly on a WA lens.

Also, I might point out that the DPR glossary definition that references the microlenses is without basis, evidence, over even a basic demonstration of knowledge of aberrations. It states that microlenses must be involved because the effect is uniform over the sensor unlike CA. Well, actually, only lateral CA varies with position. Longitudinal CA typically doesn't, it is usually the same across the field and is only improved by smaller apertures.

So anywho, what the OP is seeing here, which is the same thing every user of a camera sees when they see "purple fringing" is actually longitudinal chromatic aberration. And, YES, the degree of overexposure will make a dramatic difference in the results - so OP, in your retests use manual exposure mode! And also, very small changes in focus will make a huge difference in the degree of longitudinal CA.

For probably the best and most accessible description of types of CA see here:

http://toothwalker.org/optics/chromatic.html

And now back to your regularly scheduled flame war
--
Ken W

Rebel XT, XTi, Pany G1, LX3, FZ28, Fuji F30, and a lot of 35mm and 4x5 sitting in the closet...

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