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24mm Lux w/ SL- purple center spot issue


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Hello everybody, I’m fairly new still to photography as a hobby, but am an avid Leica SL shooter. I switch between seven different Leica SL and M lenses on my SL. But I’m hoping perhaps SOMEONE can help me figure out why my SL/ 24mm Lux pics are getting purple spots in the center of the photographs. This doesn’t happen with any of my other lenses on the SL. Only with the 24mm LUX, and I actually don’t recall this being an issue when I first bought the 24mm. It seems to be something that only developed after about 4-6 months of owning the lens. Could it need a proper cleaning? It appears clean to the naked eye.

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Can you show the picture?

 

You don't say under what conditions you shoot Summilux 24mm and what other lens you use. 

 

It may be so called purple fringing, it appears with fast lenses when shot at maximum aperture with strong highlights in the picture.  If that is the case it is completely normal and it can be easily fixed in post processing software.

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I’ve had to compress this photo all the way down to 500kb to upload it, it you can still see the purple-ish blue-ish spot above the kayakers. I don’t believe this is vignetting.

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The apartere Settings are not extreme, not stopped all the way down or up, but basically anywhere between 4-8 it happens. I can control it somewhat by the direction of the camera, but it’s usually a very small window in which it does not appear. In. Ist directions it is visible.

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Here is another example. You can see the purplish spot in almost the exact center. I’ve cropped this pic to help get it down to 500kb, but the spot is always very close to the center.

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Some of these pics were taken at Glacier national park during the fires last month, so the sky is very smoky/hazy.  Here are some links to these pics in Dropbox, these are jpg samples, I'm on the road and don't have access at the moment to the DNG files.

 

https://www.dropbox.com/sh/rhao27xdodr93l5/AABATzRDcOGm-HuEWM_tw4hWa?dl=0

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Hiarocca,

 

My guess is that the purple spot is light entering through the lens, reflecting off the SL's sensor cover glass onto the rear lens element and back onto the sensor where it's recorded along with the rest of the picture.  It's coloured purple because it reflecting off the wrong side of the lens coating.  If the 24/2.4 Summilux's rear element is concave that would explain the small size of the spot because the concave shape will diminish the reflected size onto the sensor.

 

This is only a guess but it seems reasonable to me.

 

I haven't had the same with any of the M and R lenses I use with my SL.

 

Pete.

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Yep, 24 Summilux has a concave rear element. 75 Summicron can do the same thing, and sometimes the 28 Summicron (less tightly focused, since it is the second-to-last element in the 28 that is concave).

 

Whether it shows up in any given picture depends on the picture composition - it'll show more when the center of the picture is a dark tone (i.e dark green water or trees). One can shoot 1000 pictures of sand or sky or medium-gray stuff and not see it - but project purple onto the contrasting/complementary color of dark green and it stands out like a spotlight.

 

It'll also vary with the exact curvature or "focal length" of the concave surface, and how far that is from the sensor. The various 35 ASPH lenses also have a concave rear element, but don't seem as prone to the central bright spot effect.

 

The light gets the purple coloring from the lens coating - but not because it is "the wrong side" of the coating (no such thing). The coating is intended to (and does) eliminate reflections of all wavelengths except purple, as designed, but the curved shape of the glass itself focuses all the purple into one place, producing the purple dot.

 

It is just Leica's luck that, having discovered a really cool new tool for designing great lenses (a concave element on the back, dating to the 1990 35mm Aspherical), along comes digital with a nice shiny sensor to bounce the picture back to the lens, which then bounces it (refocused) back to the sensor again.

Edited by adan
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I referred to it as 'the wrong side of the coating' because I vaguely presumed the coating works in the same way as interference (dichroic) filters absorbing some visible wavelengths in the intended direction, which would reflect them in the other - similar to the way that UV/IR filters work with the M8. Mea culpa.

 

Pete.

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