Archived posting to the Leica Users Group, 2004/02/16
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Feli di Giorgio wrote: >On Mon, 2004-02-16 at 03:00, frank.dernie@btinternet.com wrote: > > >>The problem is, Jim, that we do not need film dimension to match film quality, >> digital sensors about 1/4 the size of Minox film match 35mm P&S quality. We >> only need a full size sensor to utilise the full angle of view of our existing >> 35mm design lenses >> >> It is a mistake to assume that an optimal 35 mm lens (e.g. "M" series) will be optimal for a smaller imaging area. Consider state of the art LF lenses e.g. the Schneider Symmar XL 150mm f5.6 which covers 8x10" -- I don't see people clamoring to use this in 35mm -- indeed it would be a rather mediocre lens even though its sweet central spot covers > the 35mm film area. The issue is that optimizing a lens for 8x10 is much different than optimizing a lens for 35mm or a smaller area. >Frank > >That may be true, but I really detest the increase in DOF you get with >the 1.5 multiplication factor. > > > I believe the actual DOF is related to the F stop and focal length -- you mean the *apparent* increase in DOF. The way to correct for that would be to use a wider F stop. Indeed for a lens optimized to a smaller imaging area you should be able to use a wider maximal F stop. All else being equal such a lens should also have a *high* resolving power in lpmm. The question has arisen as to how many pixels are needed to approximate 35mm film. Assume 75 lpmm that would be approximately 2000 lines x 2000lines or at least 2 pixels/line = 16 megapixels. Now if your lens is not good, or your focus is off or your hands shake etc, you need less pixels -- and also remember that there isn't necessarily a 1:2 correlation between lines and pixels (for example it is widely acknowledged that a digital CD recording at 44 khz cannot truly reproduce an analog recording even though the human ear cannot hear > 20 khz) i.e. you probably should have more pixels to capture all the information. In any case this is a back of the napkin approximation that tells us that a good 5 megapixel sensor is not enough to equal good 35mm film given a high quality lens. The 75 LPmm is for Provia F 100 slide film -- for Tech Pan and on a tripod, you are going to need many more megapixels. Jonathan - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html