Aliasing and scanning resolutions

[Ed Richards]

A bit more detail for a 300 DPI output file,

You scan at 600 x size of the long dimension of the print in inches / long dimension of the negative. For a 4x5 negative and an 8x10 prints, this would be:

600 x 10 /5 = 1200 DPI

[John_4185]

This would be consistent with the observation that good consumer scanners can produce 16x20 prints from negatives that are indistinguishable from drum scans, because a good consumer scanner can probably produce a good 2000/2200 DPI scan from a negative by scanning at 4800 DPI and downsampling. (DMAX and noise become a big problem for chromes.)

What good 'consumer' scanner does 4800 "DPI"? Or do you mean SPI or PPI? In any event, please share the brand name and model so I can try the thing.

When did downsampling by 50% become nondestructive?

[Brian Ellis]

"Some people prefer to downrez using "bicubic" sampling, others with "bicubic sharper."

There was a lengthy discussion about this in one of the digital groups recently. The conclusion there seemed to be that "bicubic smoother" was the best way to go.

[Ed Richards]

> What good 'consumer' scanner does 4800 "DPI"?

Canon 9950, Epson 4990. If you mean, do they really do 4800 - no. But averaged out and processed, they probably do a good 2200.

> When did downsampling by 50% become nondestructive?

When you have oversampled so all you are losing is the noise.

> The conclusion there seemed to be that "bicubic smoother" was the best way to go.

Bet that was with digital or 35mm, but I could be wrong. At these scan levels grain is not an issue in large format.

[David Luttmann]

Brian,

While generally accepted that bicubic smoother is better for uprezzing, some people prefer it to the somewhat edge enhancing effects of the bicubic sharper routine. I'd say that as long as you're downsampling something to no less than 1/2 its original size, you may find the sharper routine to work better. I find the undue softness introduced by the smoother routine to be less pleasing to the eye.

Once again, this is really up to personal choice rather than a right or wrong. Whatever gives you the print you want is what I would suggest.

What I find works well in my workflow is to do a USM on the RAW file, and then downsample using the sharper routine.

[sanking]

OK, I have been following this thread closely and have a question. I would like to archive all of my 12X20 negatives in digital file. I have a Microtek 9800XL, which will scan at a maximum optical resolution of 1600dpi. Based on what has been said so far I have come to the following conclusion. To optiize the quality of my scans I should scan the negatives at 1600 dpi and hen down sample to 800 dpi using bicubic sharper. Does this sound reasonable?

[David Luttmann]

Sandy,

As it is so dependant on the quality of the original scan (low quality scanners benefit from downsampling more than higher quality scanners), you really need to see what provides a better output at your desired print size by testing various routines. If the idea is to just print at 12x20 or 24x40, than there may not be much of a benefit to downsampling.....the aliasing effects would be buried in the output and probably not create any undesirable effects. 12x20 though is outside my scanning realm. I've only had 4x5 & 8x10 chromes scanned....but mainly 4x5.

The main idea to downsampling is that if the scanner really doesn't have a true 1600 dpi ability, all you'd be throwing away is aliasing garbage and noise.

Also, you may try downsampling less. Make your 1600 dpi scan a 1000 or 1200 dpi scan & see how that works. You may need to do no downsampling at all. Sometimes it makes no difference at all whether left raw or downsampled.

As usual, a lot of experimentation is needed with any scanner as they're all different.

[Ed Richards]

> The main idea to downsampling is that if the scanner really doesn't have a true 1600 dpi ability, all you'd be throwing away is aliasing garbage and noise.

Absolutely. Consumer scanners do not get anywhere near 4800 DPI, so the overscanning and downsampling is all about getting rid of noise and artifacts. Are these color or B&W? If they are color, you probably will want to downsample a bit just so they are small enough to edit in Photoshop. Since color has lower resolution than B&W it will not matter, and since your negatives are HUGE, none of this will matter until you get to REALLY big prints.

[chris jordan]

Hi guys, I think you might be looking at the problem from the wrong end, so here's my brain dump about scanning.

If you make your scans based on the print size you want, you take the risk that your desired print size and resolution might increase sometime later, and then you will have to rescan and re-image the photograph. Instead, start off by making one "legacy" scan that captures all of the image detail that is on the film. Do your Photoshop imaging work on that scan, and save that as your "master" file. Then resize it for different sized prints. If you eventually want to make a big print, or a new print process is developed that prints at 2000 dpi or whatever, you can still use that scan. Scanning quality is high enough right now that a good scan will capture everything on the film, so unless your master file gets corrupted and the scan is lost, you will never need to rescan even if someone makes a "better" scanner in the future.

So then the question is, what dpi do I need to scan to capture everything on the film? That has been researched extensively by lots of very smart people and here is the result, assuming you are using the finest films available: For 35mm scan at 4000 dpi; for medium format scan at 3000 dpi; for 4x5 and 8x10 scan at 2100 dpi. That scanning resolution will capture all of the image detail that your lens could focus on the film. That resolution won't capture all of the film grain, which is superfluous information that only complicates the imaging and printing process so you don't want it in your scan anyway.

This means that the scanners that claim to scan at 4800 dpi are doing something that you will never need. Some drum scanners can scan at 10,000 dpi, which is also total overkill for any kind of film in the world. The thing that matters is each scanner's ability to capture sharp image details at the resolutions listed above. In other words, a 2000 dpi drum scan will be far sharper and better than a 2000 dpi scan using a cheap flatbed scanner, despite the fact that the flatbed scanner can supposedly scan at 4800 dpi. Wet-mounting also makes a difference in the quality of the scan and in the quality of the final print, so a wet-mounted drum scan at 2000 dpi will be sharper, smoother and cleaner than a non-wet-mounted scan made by the best Epson flatbed at 2000 dpi.

And by the way, when ressing up or down, DON'T use bicubic "sharper"; it is highly destructive to your scan, and doesn't accomplish anything that you couldn't do with USM.

[John_4185]

If you make your scans based on the print size you want, you take the risk that your desired print size and resolution might increase sometime later, and then you will have to rescan and re-image the photograph. Instead, start off by making one "legacy" scan that captures all of the image detail that is on the film.

That had better be drum-scan because flatbeds will probably get better.

The rule-of-thumb in computing is that the rule will change.