07-11-2011, 10:50 PM
[quote name='Martin_MM' timestamp='1310421010' post='9928']
That´s what I don´t really get: "..and the closer distance to the sensor". Closer distance to the sensor would change the focal length of the lens, wouldn´t it???¨
Otherwise, thanks a lot for explanation.
[/quote]
No, it would not. If you do not change the lens construction, moving a lens closer to the sensor just means you can not focus anymore, or just very far away. But the focal length stays the same. Just like when you put a lens further away from the sensor... put an extension tube between lens and camera. You will lose ability to focus far away, and can focus much close by. But the focal length does not change.
Lenses usually have the focal point inside the lens system. With Leica M/LSM lenses, where the lens sits 28mm from the sensor/film, this will be the case except with the widest angle lenses. With SLRs, where lenses sit 40mm or more from the sensor/film, you have to design lenses that have their focal point in front of the lens system (in front seen from the sensor side), and I believe this is called retro-focus. If I am saying it wrong, I am sure Wim will correct this.
The design of lenses for M with its 28mm flange distance and SLR lenses will be different, just due to the flange distance difference alone. For Leica M/LSM you will not see lenses longer than 135mm. The longer lenses will need an extension tube, the size of the Leica R mirror box I believe.
Anyway, key point is that the lens design will be different for M and SLR. Especially in the shorter focal lengths the differences in design will be big.
When you look at longer focal lengths, the designs start to be similar.
Leica 90mm f2 Summicron:
http://www.camerarepair.com/Summicron-90-2-2nd-Puts-edited-Arcsoft-A368-W150-H-S.png
Nikon Nikkor-Q 85mm f1.8:
http://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/6070nikkor/telephoto/85mm18.pdf
That´s what I don´t really get: "..and the closer distance to the sensor". Closer distance to the sensor would change the focal length of the lens, wouldn´t it???¨
Otherwise, thanks a lot for explanation.
[/quote]
No, it would not. If you do not change the lens construction, moving a lens closer to the sensor just means you can not focus anymore, or just very far away. But the focal length stays the same. Just like when you put a lens further away from the sensor... put an extension tube between lens and camera. You will lose ability to focus far away, and can focus much close by. But the focal length does not change.
Lenses usually have the focal point inside the lens system. With Leica M/LSM lenses, where the lens sits 28mm from the sensor/film, this will be the case except with the widest angle lenses. With SLRs, where lenses sit 40mm or more from the sensor/film, you have to design lenses that have their focal point in front of the lens system (in front seen from the sensor side), and I believe this is called retro-focus. If I am saying it wrong, I am sure Wim will correct this.
The design of lenses for M with its 28mm flange distance and SLR lenses will be different, just due to the flange distance difference alone. For Leica M/LSM you will not see lenses longer than 135mm. The longer lenses will need an extension tube, the size of the Leica R mirror box I believe.
Anyway, key point is that the lens design will be different for M and SLR. Especially in the shorter focal lengths the differences in design will be big.
When you look at longer focal lengths, the designs start to be similar.
Leica 90mm f2 Summicron:
http://www.camerarepair.com/Summicron-90-2-2nd-Puts-edited-Arcsoft-A368-W150-H-S.png
Nikon Nikkor-Q 85mm f1.8:
http://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/6070nikkor/telephoto/85mm18.pdf