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Re: [OM] OM4T misfocus?

Subject: Re: [OM] OM4T misfocus?
From: Moose <olymoose@xxxxxxxxxxxxx>
Date: Mon, 03 Nov 2003 19:53:45 -0800
I make some assumptions here that there was enough light to focus well, shutter speeds were adequate to assure no movement blurring, etc. etc., i.e. that the problem in with the camera and not technique.

Possible causes fall into 2 categories:
1. Those you may have caused and can cure yourself.

a. Is the diopter adjustment correct? If not pushed in to lock, it can fairly easily be moved in handling, etc. While this will not, in itself cause misfocusing, it can make focusing hard enough that one messes it up.

b. Is the focusing screen fully and properly seated. If not, you have 2 problems. First, the distance from the lens mount to the screen is different than the distance to the film, so what is in focus on the screen is not in focus on the film. Second, the frame that holds the screen will someday pop loose, dropping the screen where it will scratch itself and/or the mirror and/or the rear lens element and may jam the lens so it won't dismount. (A topic recently discussed when such a thing happened.)

2. Those caused by physical damage of some sort to the camera which generally require professional assistance.

a. The pin against which the mirror rests can be bent, so the distances from mount to film and to screen are different. I discovered this on a body when trying out a new zoom lens. While it seemed to focus to infinity at 200mm, it definitely didn't at 28mm. First I panicked, then calmed down and tryed it on another body; everything was fine. The error in mirror alignment is a fixed amount which is a larger 0f short focal lengths than long ones, so it affects the shorter ones more.

b. Physical damage/distortion of the camera body which causes the distances from mount to film and to screen to differ. This seems unlikely unless the camera has suffered a fairly significant trauma.

You can test focus accuracy of the body/lens combo by setting the lens at infinity or a marked distance and taking fast shutter speed, tripod mounted shots. The relationship of those shots can be compared to shots carefully focused through the viewfinder under good lighting and viewing conditions. Use of a Varimagni finder at 2.5x will make this even more certain. Have you tested out your focusing ability on another body?

Moose

Richard F. Man wrote:

It appears that my OM-4T is misfocusing? I shot a whole roll for Halloween and the focus seems to be off. I am using a new 2-13 screen from John, and perhaps I need to thoroughly test it out, but what are the possible problems with SLR going misfocus?

My eyes are not the greatest, but the focusing on the Le$ca rangefinder is fine so I'm pretty sure something about the camera is not working quite right.





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