Help Interpreting Scanner Performance

[Michael Heald]

Hello! Not having a test target, I decided to use a 4x5 astrophoto I took of the Sagitarius star cloud using TMax 400 last year and see the closest stars that the scanner could separate. I scanned a one inch square section of the original at 2400 spi, 48 to 24 bit in the scanner. I enlarged the image to 400% and cropped a small section and split the RGB layers.
Given all the variables of this image -problems in the field, lack of extremely fine focus, and that most people can split lines better they than can split stars, this image probably represents the worst case senario of how the scanner can perform.
To my eye, the three images suggest that the red layer is the sharpest.
The two stars in the the mid-field at the left are separated by 5 to 6 pixels. If I understand the math correctly, (2400 pixels per inch)/(6pixels per star pair)/(25.4mm per inch)/(2lines per star pair) = 8 lines pairs per mm.
In other words, it seems to me that, purely from a sharpness aspect, the scanner should give results that print up to 16x20 since this had a 4x enlargement, and most people consider 5 lines pairs per mm good resolution. Since there are obviously other variables involved that will degrade the image, I would think that the scans should produce excellent prints with my 8x10 R220 and MIS inks.
Am I interpreting this correctly, since I am using spot pairs (star images) rather than line pairs? Best regards.

Mike

[Michael Heald]

Hello! Looking the rest of the negative scan, I think I can separate stars whose centers are separated by 3 to 4 pixels. Best regards.

Mike

[Ken Lee]

"In other words, it seems to me that, purely from a sharpness aspect, the scanner should give results that print up to 16x20..."

If your intended use is astronomical images, then you should make a real print at that size, and see if it looks as good as theory would suggest. If you don't have a printer which will output that size, then you can make an 8x10 from the center, and another from the edges.

If you have more terrestrial subjects in mind, then real test prints will also set your mind at rest - since there is more to these things than just lines, line pairs, and dots.

Ultimately, you are testing your whole workflow, of which the scanner is only one link in the chain.

[JW Dewdney]

... out of curiosity Michael - which image is the one inch square cutout from? The big one, I hope??

[Michael Heald]

Hello! The big one is the one inch cut-out. Best regards.

Mike

[JW Dewdney]

Hello! The big one is the one inch cut-out. Best regards.

Mike

One would certainly hope! (trying not to make assumptions)

How do you deal with film flatness during LF astro exposures? Also curious what mount you're using and how you preven wind from catching bellows (if any).

[Michael Heald]

Hello! I was in the Australian Outback. There was no wind, and since I piggybacked on my 8 inch LX200, the setup was severely under-mounted. I used a normal holder for the 4x5 Tmax 400. I believe for this exposure, I used a home-made box with a f2.5 aero-ektar stopped down to f5.6, though it might have been the Speed Graphic I had at the time. Anyway, I was lucky with the exposure and there was some star trailing at the corners. Best regards.

Mike

[Jack Flesher]

Hi Mike:

I think your image will print very well at 16x20 (at least for an astrophoto). You could proabably take it up to 24x32 if viewing distance was not overly close. Your red channel is slightly better than your green and sharpened up reasonably well for print -- at least IMO... See atatched R channel sharpened version below, I used USM of 150, 2, 2.

FWIW, a really strong loupe, like 10x, and confirming your film is at the exact same plan as your GG may be of bennefit for more perfect focus -- especially with point-source light.

I'll share a tip I learned from an old LF shooter for holding film more flat and helping to keep it in registration for longer exposures and odd camera angles. He used a small dot of beeswax to the center of the film plate in the holder. Beeswax is tacky enough to stick the emulsion base to the holder, yet any residue cleans off easily with negative cleaner after processing. Just wear cotton gloves to avoid finger-prints on the emulsion when you press the center of the sheet to it.

Cheers,