Acetate Mask on Focusing Screen

[matthew_emond]

<p>I wanted to create an acetate mask to show cropping lines for a 4x5" aspect ratio and place it on my 500CM's focusing screen. How do I figure out exactly how to measure it out so that that lines are accurate? Won't placing anything on my focusing screen alter what I'm looking at through the viewfinder where focusing precision will be slightly altered?</p>

[vick_vickery]

<p>1.8" x 2.25"</p>

[matthew_emond]

<p>Can I ask how you came up with that number? Teach me to fish :)</p>

[paul_hagan]

<p>Hello,<br>

Place the clear acetate screen on top of the screen and all the focussing is done on the screen below - so no effect on focus accuracy. Measure actual screen size (the ground glass portion). Then multiply by 4/5. This will give you the side of the rectangle which is equivalent to 4". This can then be placed anywhere on your acetate the long side is equvalent to 5" Suggest placing it centrally will give you more flexibility. Draw one up on a word document then print out on a clear acetate slide using a laser printer. You can then cut from the slide using a craft knife. Do a few while you are about it as you may as well fill up the acetate before printing<br>

Paul</p>

[vick_vickery]

<p>Paul has it right! 4/5 = 0.80; 0.80 x 2.25 = 1.8 ... easy as that! :-)</p>