Moiré patterns possible on film scan?

[CookingWithGas]

I got a new scanner (Epson Perfection 4490 Photo) that will scan negatives, up to 4800 optical. On some negatives I see a subtle pattern that appears to be a Moiré pattern, possibly interference between some pattern in the grain, or more likely some other characteristic of the film, and the scanning pattern. Am I seeing things, or is this really possible? The pattern, when it appears, is skinny. I don't have time to post an example at the moment but may try to do so if asked.

BTW the biggest problem I am having scanning negatives is keeping them scrupiously dust-free. Any help on that would be appreciated. I want to clean them well but am afraid to use anything that could scratch them.

[Padeye]

Yes, it's possible to get moiré patterns in a scan of material that has a regular pattern. Also likely are Newton rings caused by placing film against a glass carrier.

[CookingWithGas]

Originally Posted by Padeye

. . .Newton rings. . .

I had not heard of Newton rings until I saw your post. I looked it up and decided this is most likely. The film is held in place with a plastic frame. The frame holds the film above the glass so that it is allowed to curve concavely (the film seems to have a natural curve, don't know why). The pattern is always lengthwise along the film and near the center of the picture, in areas where the color is fairly uniform, so I would say this is what's going on.

CalMeacham, you out there?

[Quercus]

I know next to nothing about graphics, scanning and other such things. But since I did stay in a Holiday Inn last night, I'll go ahead an mention that if you're looking at the scanned output as .JPG or other compressed files, then there could be artifacts introduced by the compression process.

[pulykamell]

Originally Posted by CookingWithGas

BTW the biggest problem I am having scanning negatives is keeping them scrupiously dust-free. Any help on that would be appreciated. I want to clean them well but am afraid to use anything that could scratch them.

Do you have the Digital ICE option turned on? That should eliminate all the dust and scratches for you, as long as you're scanning from film.

[CalMeacham]

CalMeacham, you out there?

Hmm? What? Who's waking me up?



Moire patterns in scans? -- I get them, especially when I'm scannming lithographed works with those little dots. Sometim,es they're really annoying. when I can, I scan actual photographs, which don't have regular pixel patterns.


I haven't seen "Newton's rings" in any of my scans -- even those using transparencies. Newton's rings are interference phenomena, easily visible in monochromatic light. You'd think that scanners, which seem to use sets of three different color LEDs to create "white" light, would tend to wash out such coherent interference effects. But i suppose if you're really unlucky you'll get Newton's rings of slightly different size with color separation. As I say, I haven't seen this myself.

[CookingWithGas]

Originally Posted by pulykamell

Do you have the Digital ICE option turned on? That should eliminate all the dust and scratches for you, as long as you're scanning from film.

Haven't tried it yet, though I have a hard time believing that it is as good as if you got the dust cleaned off to begin with.

[pulykamell]

Originally Posted by CookingWithGas

Haven't tried it yet, though I have a hard time believing that it is as good as if you got the dust cleaned off to begin with.

No, but it is rather remarkable. Before I scan--well, when I used to scan at least--I'd just use a couple blasts of Dust-Off, turn on the ICE, and never have to use the cloning tool or Dust & Scratches in Photoshop again. Seriously, it's a miracle worker...at least as it's implemented in the Nikon CoolScan series of film scanners.

[pulykamell]

Here is a before and after example of Digital ICE.

I also just noticed that your might not know what Dust-Off is. Basically, it's a can of compressed air. You can find it or a similar product in most office supply or computer stores.

[Padeye]

Originally Posted by CookingWithGas

(the film seems to have a natural curve, don't know why).

Film is coated on one site with a photosensitive emulsion which absorbs water during processing. The emulsion shrinks on drying causing the curl you see. Look at the surface of the film at a shallow enough angle that light reflects off it. You'll see one side has a image texture and the other side is completely smooth.

[CookingWithGas]

Here is an example of the phenomenon I'm talking about. This scan was at 2400 dpi IIRC.

Note the narrow band of elliptical distortion near the top, which was across the middle of the original (this is way cropped down).

www.seigle.net/distortion.jpg

[Padeye]

We have Newton rings