I believe that's actually called back focus when the camera focuses to the rear (back) of your intended focus point. Sounds like you and I have the same problem and there's a possibility it may not be solved by use of the shim. I'll try to let you know tomorrow if using the shim made any improvements in my case.
With my Brightscreen (sans shim) and I focus on a tape at 45", the resulting file shows 44 3/4" sharp. Weather you refer to it as front focusing or back focusing is fine with me as long as you don't call it acceptable.
Well, looks like three out of four of us, the exception being John Luke, have a back focus problem with the Brightscreen.
The shim didn't come with instructions or written information of any kind. There was just the shim, placed in a plastic envelope, taped to a piece of folded cardboard and enclosed in a mailing envelope. But, based on what Jim Lakey (Brightscreen) has been quoted as saying, he thinks the focus problem is due to the screen carrier not pressing the focusing screen up tightly against the bottom of the prism. He has also been quoted as saying that the shim is to be placed between the screen carrier and the focusing screen in order to press the screen up tighter against the bottom of the prism. I can see how his "fix" might work for a front focus issue like John has but, if so, I don't see how it would be possible for it to also correct a back focus issue.
Anyway, this morning I tried installing the shim between the carrier and the focusing screen and that didn't change the focus point on my camera. It still back focused. I then tried placing the shim between the focusing screen and prism to see if that would move the focus point forward. It did but it actually over corrected the back focus into a front focus. The shim measures ~.010" thick so I've now replaced it with "painter's tape" that is ~.005" thick and will do some more testing to see if I can reach a happy compromise. If, like John, I can get mine to be within 1/4" over 45" I think I'll be ecstatic. Without shims or tape spacers it's off by 1 1/2" or so in three feet.
Maybe Jim Lakey will drop by this thread and correct any errors in what I've reported or clear up any misconceptions that I have about the problem.
(When I've used the term prism above I'm actually talking about the secondary screen that carries the auto focus points and spot meter circle.)
jjohnson wrote:
If, like John, I can get mine to be within 1/4" over 45" I think I'll be ecstatic.
Johnny
Johnny- I think you misunderstood. I am not happy focusing on 45" and then settling for 44 3/4 " on the file. The Canon screen gives me exactly 45" and I expect the Brightscreen, or any other focus screen replacement, to be exactly 45" as well. None of this 1/4" error stuff, or any error at all for that matter. With current shooting trends of using 85/1.2 lenses wide open, there is no DOF, so absolute perfection is necessary.
p.5 #10 · Microprism-only Brightscreen on Canon 5D
John, I'd like to apologize. After I posted the previous message I went back and read it and wished that I'd either left that comment out or clarified it. I understand your needs and totally agree with you. In my case I'm using manual focus for landscapes where I'm stopped down to f8 or smaller so I don't really have to have the type of precision that you require. It sure would be nice to get it pretty close though so that if I did decide to take a picture of a person with the lens wide open the eyes and not the ears would be in focus.
Thanks,
Johnny
Jan 24, 2006 at 01:09 PM
jjlphoto Offline • • • • • [X]
p.5 #11 · Microprism-only Brightscreen on Canon 5D
Jonny- no problem. However, if it is off at close-up, will if be even further off far away? Not sure, but part of my blood is German (hence all the exotics in my profile) and IMO, it is either correctly placed, or incorrectly placed. When I had my Arca Swiss and was doing 6x9 and 4x5 chrome, I botched a job shooting wide interriors and lost a client forever because my GG was not correctly placed, and my shots were bordering on being sort-of OK to mediocre, but not nearly as razor sharp as a 4x5 chrome should have been. After that experience, I made a decision to take lenses, film planes, and focusing to the über level.
p.5 #12 · Microprism-only Brightscreen on Canon 5D
John; if it is off a smidgen at close range it is off by more further down the twists of the barrel out towards infinity. In my case focus ended up half an inch off (behind) and at three meters it was off by maybe eight to ten inches (on the Elmarit 90) and that is a lot. I focused on the eyes and focus was well behind the ears...
p.5 #15 · Microprism-only Brightscreen on Canon 5D
I just tried out the shim, and it does seem to be a nice improvement. I'm still seeing a touch of back-focusing on some shots, but I think it's now a matter of focusing error (these were handheld with very thin DOF, so there's a margin of error as my body moves around while pressing the shutter).
I'm going to do some more controlled tripod testing tomorrow, but so far, I'm encouraged.
Edited by Mike Hatam on Mar 05, 2006 at 01:48 PM GMT
p.5 #18 · Microprism-only Brightscreen on Canon 5D
I received my microprism only screen for my 5D earlier this month. In fact it shipped within hours of placing the order .
While I haven't had time to do any serious shooting with it I did do some focus testing using a custom focus chart after reading this thread. While I want to repeat my tests I had some perplexing findings including both front and back focusing.
What I want to say here is I flagged the issue with Brightscreen yesterday and within minutes had a response from Jim Lakey himself telling me all international customers were shipped shims already and they were on top of it. Irrespective of what the final answer, or answers are, to the issue I for one am very impressed with the prompt and professional response that Brightscreen has demonstrated in relation to all of this.
p.5 #19 · Microprism-only Brightscreen on Canon 5D
JameelH wrote:
So how does the shim install? I got mine today as well.
It didn't come with any instructions, but I used the "tweezers" to lay the shim into the carrier, and then laid the BrightScreen on top of it, then closed the carrier.
I'm looking forward to trying some more tests today to see if it is consistently working.
Edited by Mike Hatam on Mar 05, 2006 at 01:49 PM GMT
p.5 #20 · Microprism-only Brightscreen on Canon 5D
I received the Brightscreen # 5 Large Split Image diagonal focus screen with Microprism a couple of days ago for my 5D. It came with the shim already in the box. I just now installed and tested it.
I was excited with the AF results, I thought I was all set. It lined up well and I only had a couple of new dust specs. Then I did a MF test and now my head hurts.
This is a problem with my 5D not the Brightscreen, right? Do I just need to reset my diopter?
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