Topic: Homebrew focus screen for Canon XT

Some of you may have seen my lens project.  I've been searching and reading about the focus screen options for the new Canons,  and suddenly realized---  I HAVE THREE OLD CHINONS  lying about that I paid about 1 US buck apiece.  I  found a few articles on the ol www, and off we go. 

This set of destructions  seemed clear enough:

http://www.katzeyeoptics.com/ccp51/medi … en_20D.pdf

I COULD NOT figure out how to get the old screen out of the Chinon without totally destroying the thing, but oh well.....

I finally took guts in hand, and tore down the Canon.  After some fiddling, and looking at the factory screen parts, I discovered that.....

The prism side of the screen is inlayed, such that the plane that is the focus screen is the same plane as the prism side of the original shim, that is, the frosted side is inlayed into that shim, therefore....

If I cut the new screen to lay in instead of the shim,  I won't need it, and the screen should work.

Then, I got lucky.  The narrow dimension of the "to be" screen turned out to be just slightly smaller than the large dimension of the old shim, therefore I did not have to cut that dimension.

So,  I marked the edges off carefully, and made two cuts   off each end of the long dimension.   The picture shows the cardboard pattern--same size as the MINOR dimensions of the shim, to avoid fiddling with the keys and cutouts.

After some fiddling, I could NOT get a clear picture--but I could see that the focus circle agreed with the focus confirm light.   I finally tried the eyepiece diopter, and AWAYYYYYYY we go!!!

http://i13.tinypic.com/4qgjpyw.jpg

http://i11.tinypic.com/6c8h3x5.jpg

Last edited by 440roadrunner (2007-07-31 06:23:24)

Re: Homebrew focus screen for Canon XT

GREAT! You Sir, are indeed a "tinkerer". wink Can't wait to hear how it worked out for you. smile
-Bruce

I put that adapter in a safe place, never to be seen again.

Re: Homebrew focus screen for Canon XT

Well, it hasn't worked out yet.  Short story,  I began to suspect a focus calibration problem in the camera.  I did some testing,  later pulled out the homebrew screen and reinslalled the original, only to discover the same thing.

The focus point is some 3/4" short at some 6ft with a 200mm,  and some 5/8" or more at a lesser distance with a 50-55mm or the kit lens.     I MAY have to send the camera back.   It is possible that this has been my problem all along.


Followup--it turns out that the focus screen has nothing to do with the AF system at all.   On a lark, while trying to determine if the camera really IS off,   I caught myself with the screen   OUT OF THE CAMERA.   I snapped a few shots, and the AF works  just like always.    what this means, is, that since the AF agrees with the homebrew focus screen,  it REALLY IS the camera that is out of adjustment.

More testing will follow

Last edited by 440roadrunner (2007-08-02 02:00:15)

Re: Homebrew focus screen for Canon XT

Follow some more:    Just received an EC-b screen which I bought via Amazon for 40.00US including shipping.   I did the same to it as the Chinon screen, IE   cut the ends off the long dimension in order to match the NARROW dimension of the original.   The EC-b screen, as arrived,  was just shy  of being the same dimension in the narrow width, as the original was in length,  so I only made two cuts.

It seems brighter than the Chinon screen.   Today's tests have been encouraging, but it still apears that this camera wants to front-focus.

I used a small box, moved 1/4"  at a time, roughly 8ft away, using my raft 'o 200mm, including

the modified Canon FD 200mm F2.8

Takumar 200mm F4

Tamron  "Adapt-a-matic"  200mm

Tamron (early model) SP 70-210 F3.5 zoom

All lenses except the zoom showed about 3/4"  front focus!!!!

Pictures taken with the Nikkor 300mm, at nearly 50ft seemed very sharp.   

So far, I like this new screen better than the old Chinon

It seems brighter, but has no prism circle.   It has the split screen, and works OK in conjunction with my AF confirm chips.

I hope to do more testing to determine the truth of the front focus abnormality.