Archived posting to the Leica Users Group, 2008/03/19

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Subject: [Leica] Canon DSLR R lens use/exposure inconsistencies
From: at (Douglas Sharp)
Date: Wed Mar 19 01:42:42 2008
References: <> <0JXY003FYE359X20@l-daemon> <>

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.


simon jessurun wrote:
> Thx for the tip.
> simon
> On Wed, Mar 19, 2008 at 3:13 AM, Ted Grant <> 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 for more information
> _______________________________________________
> Leica Users Group.
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Replies: Reply from red735i at (Frank Filippone) ([Leica] Canon DSLR R lens use/exposure inconsistencies)
Reply from len-1 at (Leonard Taupier) ([Leica] Canon DSLR R lens use/exposure inconsistencies)
In reply to: Message from grduprey at ( ([Leica] Canon DSLR R lens use)
Message from tedgrant at (Ted Grant) ([Leica] Canon DSLR R lens use)
Message from simon.apekop at (simon jessurun) ([Leica] Canon DSLR R lens use)