Kudos Scott, for a very logical and well thought out think-out (even if the middle bit was in a foreign language :)) I didn't think it would be that complicated, but coming from a very non-scientific background, you're much more likely to be on the right track. What I pictured in my mind was basically simple trapezoid (is that what it's called? like imagine a pyramid with it's head cut flat so that the flat top and the base have the same angles) prism type of element (as opposed to a conventional curved lens). I'm not even sure if it would bend light like that.. If it did though I reckon it'd have to be pretty thin to prevent loss of image quality? Anyway I'm thinking from a very abstract point of view, physics really isn't my forte.. But even if it's not feasible, in line with your logic and the upside down image in the viewfinder, I suppose that'd be saleable too, though maybe bundled with a refconverter type of device, attaching itself to the viewfinder and re-flipping the image.. What say you Scott?
Regards, Ryan ----- Original Message ----- From: "Scott Nelson" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Saturday, December 13, 2003 12:08 PM Subject: Re: Sensor size and convertor idea > On Fri, 2003-12-12 at 17:10, Ryan Lee wrote: > > Just an idea since there's at least one thread discussing lenses and > > multiplication factor. How feasible would it be theoretically for > > manufacturers to come up with a teleconverter like accessory to fit normal > > lens projections to APS size sensors? I hope it doesn't sound incredibly > > naive, but I just figured that in some cases people would be desperate > > enough for a focal length that they might not mind the 1 or 2 stop loss or > > the degradation. It doesn't sound terribly difficult to do or expensive > > either.. Why haven't they done it already (or have they..)? > > > > Rgds, > > Ryan > > I've thought about this before, but this little discussion is going to > get me thinking about it properly. My conclusion is that I don't think > this is possible without interfering with the SLR mirror. Here's my > logic: > > First consider a regular teleconverter. Just looking at one we can see > that the focal length is positive because of the way the elements are > curved. For this very crude calculation, we can approximate it as a > single thin lens, and use the thin lens equations > > 1/f = 1/o + 1/i > m = -i/o > > i = image distance > o = object distance > m = magnification > f = focal length > > for the regular teleconverter, o is negative, because the object for the > TC is the image from the regular lens, which is usually formed behind > the TC. > i must be positive, otherwise there is no real image > |i|>|o| because we want a magnification greater than 1 to expand the > size of the image. This works fine and the TC refocusses the new image > farther away, leaving room to stick a TC housing between the lens and > camera body. This also explains why 2x TCs are almost always shorter > than similarly designed 1.4x TCs > > correspondingly, m is positive, so we don't have an upside down image. > > ************************************** > For the so called reverse teleconverter, we would need to have |m|<1, > which means |o| > |i| and we have a problem. The wide angle converter > moves the real image closer to the back of the lens than the original > image, so there is no spaces for a converter. You could make this work > if you let o be positive, but then the image would be upside down in the > viewfinder. > > -Scott
