Archived posting to the Leica Users Group, 2008/03/19
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]This was sent to me by Bob Palmieri a while back (thanks Bob), he got it from someone at Canon. The EOS 20D focusing screen is optimized for superior brightness at moderate apertures from about f/3.5 and smaller, compared to conventional ground glass designs. This makes the viewfinder image brighter and easier to focus at those moderate apertures, but the trade-off is that it passes disproportionately more light to the metering system. When a Canon EF lens is mounted to an EOS camera, a variable exposure compensation factor (a program curve, not just a fixed compensation factor) for this phenomenon is fed through the system in order to provide correct metering for all apertures. However, when using a non-coupled manual diaphragm lens as you describe, no such communication takes place, so the responsibility for exposure compensation reverts to you. It's unnecessary to use an external meter. Instead, you can take a series of test shots at the working aperture(s) you plan to use, then analyze the test photos to determine the most desirable exposure compensation factor for each aperture. The 20D's auto exposure bracketing (AEB) function speeds up the process of taking the test photos, and you can use the Info palette in Photoshop to determine the most accurate exposure. If you can standardize on one particular aperture you plan to use (for maximum sharpness, desired depth of field, etc.), that will simplify the calibration process by eliminating the need for tests at other apertures. Cheers Douglas simon jessurun wrote: > Thx for the tip. > simon > > On Wed, Mar 19, 2008 at 3:13 AM, Ted Grant <tedgrant@shaw.ca> wrote: > > >> Gene offered: >> >> Subject:Canon DSLR R lens use >> >> >>>> Focussing with the 24 is not easy with the 20D, so I will order the >>>> >> focus >> confirmation adapter in the next day or so.<< >> >> Hi Gene, >> >> Here's a focus tip when using wide lenses and SLR bodies of any kind or >> match-up. >> >> Instead of starting to focus with the lens at infinity to focus closer. It >> works much easier to snap the lens around to it's closest point of focus, >> then camera to eye and come back to the point you wish to focus on. >> >> In this manner when you look in the viewfinder everything is completely >> out >> of focus blur. So as you turn the focus ring the eye picks the sharpest >> point much easier than going from a point of all in focus, as in at >> infinity, then trying to focus on your sharp point closer in. The focus >> confirmation works beautifully with Leica glass and the 20D without >> question. >> >> Trust me this focus trick works perfectly as I've used super wides of all >> sizes and makes on any number of odd ball SLR's and it works. >> >> Just easier with the confirmation adapter. >> >> ted >> >> >> _______________________________________________ >> Leica Users Group. >> See http://leica-users.org/mailman/listinfo/lug for more information >> >> > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information > >