OK so let's add an image inverting lens, and there you have it. While we're at it, how about some circuitry to convert the manual aperture position to an electronic signal, and all M and K lenses can be made fully compatible! I'd buy one if I had a *ist-D (one day!). Nick -----Original Message----- From: Scott Nelson [mailto:[EMAIL PROTECTED] Sent: Sat 13/12/2003 02:08 To: [EMAIL PROTECTED] Cc: 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
