Masking off a Mamiya 645AFD Focus Screen

[jmboss]

I'm hoping that someone here on the LL forum has encountered this same issue:

At this time, I have an Aptus 17 that I use with a Mamiya 645AFDII along with the "special" Leaf etched Mamiya Focusing Screen for this camera-digital back combination.

On several occasions, I have found that my "minds' eye" ignores the etched border for the cropped coverage of the Aptus 17 model, and I end up capturing less of the scene than I envisioned at the time of the exposure.

And, in lower light situations, especially with the smaller maximum aperature wide angle and fisheye lenses, I find it very difficult to even see the etched line at all!

If I mask off or blacken out the border area of the focusing screen (probably with a Sharpie or Magic Marker pen), this would solve my problem, but I suspect that all subsequent exposures determined by the camera metering system would now be affected by my alteration.

Before further "customizing" an already expensive "custom made" focusing screen, and assuming that I will never use a film back or a larger digital sensor back with this screen installed in the camera, I have the following questions:

Will the change in the camera's meter readings just result in a more "center weighted" exposure?

Is this change in exposure easily adjusted for by using the Mamiya camera's Exposure Compensation control dial?

Or, will all future exposure readings using the camera's metering system just be constantly out of whack every time as a result of my screen alteration?

Or, just maybe, there will be absolutely NO affect on the camera's metering at all?

Thanks for any and all feedback.

Joe

[snickgrr]

I would think you could try it with a Sharpie and see if there's a difference.  Denatured alcohol takes the sharpie ink right off and I would think would not effect the screen.  Or I recently used some black tape on my screen in an experiment of sorts and after just pulled it off without an issue.

[Mike W]

Maybe if you give the crop-area a transparant color? I've used a transparant orange mask with a Hasselblad V and DB....worked just fine.

[jmboss]

Will the change in the camera's meter readings just result in a more "center weighted" exposure?

Is this change in exposure easily adjusted for by using the Mamiya camera's Exposure Compensation control dial?

Or, will all future exposure readings using the camera's metering system just be constantly out of whack every time as a result of my screen alteration?

Or, just maybe, there will be absolutely NO affect on the camera's metering at all?
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I was hoping that some of the more technically savvy or more experimental forum members out there could answer the above questions about the possible exposure issues created when masking off the focusing screen on this camera.

Thank you.

Joe

[snickgrr]

Well, I did a little test for you.
Masked off the screen right outside the crop lines and shot the same scene under Program mode.  Once using the Average/spot auto switching exposure metering mode (A-S)
Once using the Center average exposure metering mode (A)
and then twice again without the mask.
I see no real difference.

I used black photo masking tape.

This is with an A75, how that relates to the 17 I don't know.  I do know it's no big thing to pull out the screen and test it yourself if you have a spare 15 minutes.

[jmboss]

snickgrr,

Thank you for testing and providing your masking results.

Though I appreciate all the replies on this thread (the non destructive orange mask idea is intriguing), this response was even more than what I was hoping for.

So far, it looks as though there is NO effective change in the camera's exposure readings, as long as the focusing mask matches the cropped or FOV "format" of the particular digital back in use at the time.

This is great info.

BTW, as to not effect the critical focus of the camera, I assume that you placed the black masking tape on the underside of the focusing screen?

Thank you again for posting your test.

Joe