[Leica] Edge effects

[Frank Filippone]

For those interested in geeky things..... and why a SA 21 is lousy on a 
Sony A7 and the WATE is great on the same Sony body.....

On 5/25/2020 10:29 PM, tk breeding wrote:
> Hi,  so the reason still isn’t clear;  I get edge effects in the digital Ms, likely the Sony as well for the SA 21/4 which practically kisses the film/sensor, but for the WATE to be ok and the 28 chron not is perplexing….  Tx,  Jim

The SA 21  ( Super Angulon) is a very deep lens/  It protrudes into the 
body a long way. When the light rays go from the lens to the sensor 
CORNERS, the angle those rays make with the sensor are quite obtuse... 
)( I want to say NOT PERPENDICULAR).  The rays in the center of the 
frame are quite perpendicular to the sensor.

Any Integrated Circuit ( which is what a digital sensor is...) is very 
efficient at using perpendicular rays.  Very bad at obtuse angles.  ( 
the physics of this will be too detailed for this discussion).

The WATE is designed as a retrofocus lens.... Theses were popularized 
with the need to miss the mirror in SLR cameras popular starting 
starting in the 40's   In simplistic language, the retrofocus lens 
design allows the lens to sit further away from the "film", allowing the 
operation of a mirror....  With this design came the ( 1990's) advantage 
of putting the rays (more) perpendicular to the sensor.....

I digress for a second.... Why do I say WA lenses?  Because telephoto 
lenses, in the physics of the design, are by their very nature, sitting 
further away from the sensor..... and the rays are significantly more 
perpendicular to the sensor.....   The whole issue is about the WA 
lenses.....

And THAT is why a SA works lousy and a WATE works much better on digital 
sensors......  In very simplified terms, the SA 21 is not a retrofocus 
design.  The WATE is a retrofocus design.

The 28 Summicron was designed at the time of the M8.  It was probably 
the last design from the pre-digital M time. The M8 is a smaller 
sensor... I think it is a half frame size... and therefore, to reach the 
corners of the image, the rays are MORE PERPENDICULAR than for a full 
frame sensor....  I am not sure Leica understood all the problems of 
digital sensors....   By note, it is the LAST lens introduced that was 
lousy on a digital sensor..... The smaller M8 sensor size accommodated 
the 28 Cron well.  The larger sensor M9, not so well.  It is all about 
the design decisions that were made when the lens was developed.

If you want to take a piece of paper and draw out the simplified 
lens-sensor relationship, this all becomes a bit easier to see....


Retrofocus design:

Sensor                                               Lens

[

[                                                       ()

[

[


SA Design

Sensor    Lens

[

[                 ()

[

[


Connect lines from sensor to lens.... and you will see the problem....  
These are the light rays paths.... Perpendicular to the sensor is good,  
More angle to the sensor is bad....
Leica put in a special optical component over the cover glass of a M 
sensor... this component has small prosmatic lenses that bend the rays 
more perpendicular at the last monment .... immproving the 
perpendicularity of the rays..... No one but Leica has this design.... 
.  No one else needed it... the rest of the market is DSLR, which uses 
retrofocus lenses designed 5-10-25-50 years ago.....   Mirrorless lenses 
are being redesigned into shorter physical sizes.... but again, all the 
older lenes were retrofocus... and they work.  Leica designed the M 
lenses NOT to be retrofocus.... which complicates the design and 
inntroduces aberrations.... . and in the film days, there was no 
resason. Retrofocus was not required.  With digital sensors, that has 
changed.
.

Frank Filippone BMWRed735i at gmail.com