M-Lens coating, -75mm

[wbesz]

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Has any one noticed how the coating on lenses differ so much?

 

My 75/2.00 Apo-Summicron-M asph is very heavily coated compared to all of my other lenses (deep blue in this case).

 

The Summilux-M 50/1.4 asph is basically green, but not a deep colour.

 

In every case other than the 75/2.0, I can see clearly through the lens at some particular angle.

 

Not being too serious, only curious.

[adan]

I was just reading in (I think) Osterloh's "History of Leica M" that the coatings are applied to lens elements in different orders, depending on the refractive index of the coatings and the glass. Whichever one is applied last determines the apparent reflected color, so that's why some lenses are blue or purple and others are yellow or green.

 

Incidentally - the color is not actually the color of the coating (obviously - or one's pictures would have purple or green or whatever tints).

 

Coatings reduce reflected light (and thus flare) through destructive interference. The thickness of the coating is so shallow that it is less than one light wavelength. Thus light reflecting off the coating and light reflecting off the glass below it end up out out-of-phase (the valleys of the light waves reflecting off the coating line up with the peaks of the light waves reflecting off the glass, canceling them out and leaving no waves - and thus no light).

 

The apparent "color" of the coating is that range of wavelengths that are not totally destroyed and leak through as reflections - 5-10% of the original amount.

 

Don't ask me where the energy in the light goes when the waves cancel out and it's "destoyed" - I'm not that good a physicist (random heat?)

 

Iridescent animal colors (bird feathers, butterfly wings, bug eyes) are often "colored" through the same process - the actual cells coating the wings/eyes are colorless, but so thin that they also cause destructive interference of certain wavelengths, leaving the others untouched to reflect as the apparent color.

[wbesz]

Hmmm, ..I only mentioned this out of interest because the newer Apo-Summicron 75 asph has a strong color compared to the other lenses, including my R-series.

 

The real reason I looked is because I was conducting some focussing tests after the recent service and return of my M7, and I needed peace of mind

about focussing accuracy (at shallow depths of field).

 

In any case I found that this was all OK with all of my lenses except the 75/2 which was short by about 5-8mm. Somewhat annoying and I need to have this lens checked out.

 

Although not originally planned, I will post the results.

[wbesz]

[ATTACH]5793[/ATTACH] Complete frame

 

[ATTACH]5794[/ATTACH] Small section

[albertwang]

D'accord, cool... What is the 90mm f2.8 to demonstrate?

[ulso_tsang]

I have a 50 lux ASPH in Black and a 50lux ASPH LHSA version in silver. They are the same lenses but still have different color coating.

[wbesz]

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"What is the 90mm f2.8 to demonstrate?"

 

Albert, I placed the 90 lens in position as an easy reminder for which lens I had in my camera at the time, ..so when the 90 was used, the 75 was in view.

 

In any case, the M7 checked out flawlessly, and I'll have to thank the Leica guys at Solms for a great job!