I think the notion of FF = heavier lens may not be true

Started Nov 8, 2013 | Discussions thread
bosjohn Veteran Member • Posts: 4,890
Re: I think the notion of FF = heavier lens may not be true

designing lenses is a very complicated process with many many compromises. Of these compromises one in particular is pertinent to this discussion which is the distance between the lens flange and the sensor surface or focal plane. this distance in most traditional dslr cameras is determined by the mirror box and the movement of the mirror. no lens can protrude into the camera far enough to inter fear with the mirror. Because the mirror box has to be big enough to house the proper size mirror to re direct the image to the pentiprism or pentimirror. this means that in order for the lens designers to make wide angle lenses they have to resort to a design that allows for a wide angle coverage but whose lens flange to sensor plane is the same as a norma or telephoto lens. This then means a design referred to as the retrofocus lens. I am no expert on how it works but I do know that it will allow a very wide angle lens to still have the same lens flange to sensor plane distance, even though you would expect shorter lenses to have a shorter lens flange to sensor plane distance. In order to achieve fast and wide lenses for the traditional dslr this all means bigger heaver lenses.

now days as in film days there are choices for ff cameras which are not single lens reflexes of the traditional mirror housing so there are much fewer constraints on lens design. Take for example the Leica range finder, the lenses for this full frame camera are about a quarter to a third smaller than their dslr brothern because this camera has no mirror box to limit not only the lens flange to sensor plane distance but to also allow rear elements to protrude almost to the sensor plane itself/

this of course causes its own set of problem as anyone who has followed Leicas agonizingly slow entrance into digital m cameras. The problem being that these lenses with an already shorter lens flange to film plane distance and even more pronounced in very wide angle straight focus design lenses which protrude way back into the camera the angle the light from the exit pupil which striks the sensor becomes more than forty five degrees causing sever light fall off and sharpness problems. offset sensor sites and software have basically over come the problem for leica I am not sure how sony does it.

despite the much bigger lenses and bodies the dslr lenses are much easier on the sensor design as the exit pupil for all lenses wide and up have a very comfortable angle with the sensor/

so basically thats why dslr lenses tend to be so much bigger the evf lenses for the most part

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bosjohn aka John Shick bosjohn@yahoo.com

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