35mm focus screen mask question

Hi all

I was wondering if anyone could shed any light on this, I would be very grateful.

I often like to shoot 35mm film with different aspect ratios in my mind, such as very wide, or square "1:1".
Rather than just cropping after the fact though, I would prefer to have the actual aspect ratio in the viewfinder itself at the time of composition.

Short of buying an XPan (I wish) or going up to 120, I was wondering if I could achieve this by "masking out" a focusing screen in say a Nikon F100 (or other 35mm film camera with changeable focus screens.

I have seen some companies that do custom focus screen engraving, but I think these are just like custom grid "lines" and not fully masked off edges which is what I am looking for.

Would there be anything stopping me from removing the focusing screen and blanking out parts of it using a black permanent marker, and then re fitting it?. I realise that if I was using the camera meter it would have to be spot or centre weighted enough so as to avoid the reading being skewed by large areas of black, and I also realise that the actual exposed image would be "full frame" and show no such crop.

Has anyone ever tried it or do you think it would be viable?

Thank you for reading, any info would be much appreciated
Rick

I advice only to do anything with the screen itself if you are shure what youre are doing. A ground glass (when from plastic) is utmost sensitive.

What you can do, when the screen is exchangable, is trying to insert a thin foil marked with that frame on top of the screen. Again be careful not to scratch the screen by movin the foil over it.
You also must be sure that the screen will be in its original position.

if the screen is removable... why not draw lines for the desired format on the top shiny side of the screen?

Nikon "E" for the f100 has compositional aids:

Consider using opaque tape to mask off the focusing screen. Be careful to not have any tape where it might keep the focusing screen from properly seating, which could throw off focusing.

Olympus OM cameras have removable/interchangeable screens. I think you would tire of looking through a finder that you customized with a magic marker.

Agx has a good idea, to use foil. Jim Jones has a good idea to use an opaque tape. I might suggest using "opaque" which is a thick fluid paint-like substance.

Lithographers tape might work.

never mind.. i see the op wants to black out the frame.

if drawing lines id use an ultra fine tip marker.

http://www.sharpie.com/en-US/ultra-fine-point-permanent-marker-sp-00067--1

The OP is seeking "fully masked off edges", not grid lines or composition aids.

Many thanks for all your replies. I think I will buy a spare focus screen and give some opaque tape a try. I seem to recall using that stuff years ago when working in the newspapers. Thanks again to you all, much appreciated.

Rick