Too cheap to buy Air Force slide, can I get scan height without it?

[steven_clark]

<p>I want to find out where the focal plane of my Epson 4990 is but I'm just too cheap to buy an Air-Force pattern test slide from Silverfast. If my eventual plan is to buy a betterscanning holder it nearly doubles my cost, and I'd at some point have to dismount it to test the film holders anyway as I don't really mount slides. Also it's 35mm and I'd kinda like medium format.</p>

<p>Just how impractical is it to judge changes in sharpness by a picture instead of a target like this. Can I create a picture that works better for it with a test chart?</p>

[Ed_Ingold]

<p>Get a good machinists scale (e.g., Starrett, Brown&Sharpe or Mitutoyo). Rest one end on the glass and place a cylinder of known diameter at one of the inch markings past half way, then scan it. It's easy to pick the center of the sharpest focus from the engraved scale, and calculate the height above the bed by ratios (or trigonometry, which actually reduces to ratios).</p>

[peterbcarter]

<p>You are overthinking it by a long shot.</p>

<p>Set your scanning to the highest resolution you can bare, but just select a smaller area. Turn off any scanning enhancements like sharpening and ice. We are just looking for sharpness and if there are any dust or scratches, these will be good areas to scan; they are likely to be sharper than the film you use.</p>

<p> 1) Put a film strip in your adjustable holder, adjusted to be as close to the surface as possible. Scan it.</p>

<p> 2) Adjust the holder for a little upwards and scan it. Compare the two images.</p>

<p> 3) Look at the *grain* or dust marks on the film.<br>

<br>

4) fuzzy: Adjust the the holder up a bit more. Scan it. Compare the last scan with this one. Any sharper? </p>

<p>5) No: repeat step 4</p>

<p>6) yes: Note the adjustment. Repeat step 4 until it goes fuzzy again</p>

<p>7) fuzzy again: You now have the focal range. It's from where it first became sharp to where it is now. Best adjust it for the middle.</p>

[steven_clark]

<p>That's the thing, I don't have an adjustable holder yet. I don't/cant know whether to buy a normal or thin version. I was planning to shim with post-its for testing to see if it got less sharp or more sharp. More sharp means I get the normal adjustable holder.</p>

<p>Also isn't the center of a focal range 1/3rd in?</p>

[custom film holders for fl]

<p>An air force target is not needed. The optics on these scanners don't require that level of precision. Usually, a street scene with lots of detail (e.g. building details, signs, etc.) is fine for judging the image. Alternatively, scratching a hatch/grid pattern using a sewing needle across the dark part of a film leader works very well too.</p>

[thirteenthumbs]

<p>Search engine usaf 1951 and download one of the photos that result or one of the pdf files with the target then print it on your photo printer.</p>