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01-08-2011, 07:18 PM   #1
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Scanning film - dpi vs MP?

I have just got a Canon 9000F scanner. It can scan film at up to 9600dpi (optical resolution) which if I am correct works out at about 120MP for a 35mm film image. Is that correct?

As happens I have found that I cannot get any more detail out of a scan above 3200dpi (which gives 12MP images). Larger scans just take longer and use up more computer memory. Even the 12MP scans do not produce any where near as much detail as my K10's 10MP ones. What's other people's experience with scanning film?


Last edited by Spock; 01-08-2011 at 07:26 PM.
01-08-2011, 07:39 PM   #2
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Combination of limitation of the film, the scanner, and even the original camera/lens combo. Apples and oranges, almost. Obviously your scanner won't do anything above 3200dpi, but even below that you could get sharper scans from better equipment. That said, there's very little detail to be gained from film above 6000dpi (true 6000dpi, not consumer scanner 6000dpi). One thing to consider is what iso film you are comparing to what iso digital.
01-08-2011, 08:20 PM   #3
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I won't argue directly against anything said by @kxr4trids, but this is how I understand it:
  • The resolution specs for most scanners are grossly over stated. Your scanner has been tested against standardized targets to yield a real world resolution of about 1700 dpi max (LINK).
  • Real world resolution and the number of megapixels are not directly related
  • The limitation on real world resolution is based on the poor optical systems used on most scanners. The sensors and stepping motors might be capable of a large number of samples (more dpi), but they are sampling junk. Sort of like putting the lens from a Holga on a K5.
  • The number of pixels in an image (Megapixels) is just that. Pixels on the x-axis * pixels on the y-axis = number of pixels. This can be estimated by the scan resolution set in software. However, a large number of bad pixels is not the same as better resolution. What you mostly get are huge files that take a long time to create.
  • Currently available photographic film coupled with available cameras and optics is capable of resolving better than 100 lp/mm. This is not easy to attain, but is well documented and referenced in other threads here and on the Web in general. That being said, conventional wisdom assumes a more lax standard in that a good 35mm negative is considered to have about 20 megapixel of usable data (translation...4000 dpi is plenty good enough).
What does this mean in practical terms? In general, it means that there is little to be accomplished by wasting time and disk space scanning at resolutions much higher than 1700 dpi with your scanner. Some people have reported better results from certain scanners by scanning high and down-sampling, but the real world limit of 1700 dpi is pretty much the ceiling with your device. To do better, you have to significantly up the ante price-wise.

Don't be too sad though. Your scanner is not alone. My Epson V700 claims 4800x9600 dpi (the 4800 is the number that counts), but only delivers about 2400 dpi even when properly dialed in. The same dpi inflation is also true of most dedicated film scanners priced under $1000. The good news is that 1700 dpi is more than adequate for onscreen display and for printing to moderate dimensions from a 35mm negative. (Monitor pitch is less than 100 dpi and printing is nominally about 300 dpi for most ink jet systems.) Things are even better for medium format. I usually scan at less than 1000 dpi for 6x7cm negatives.


Steve

Last edited by stevebrot; 01-08-2011 at 08:43 PM.
01-08-2011, 08:44 PM   #4
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Steve explains it well - but just to repeat: multiply the two side dimensions of your bitmap and you get 'megapixels' - it works exactly the same way with the bitmaps made by your digital camera.

You notice that with the digital camera, the same bitmap size translates into different file sizes - depending on the jpeg compression used (let's just keep it simple and stay with jpeg) and the amount of detail in the scene... a narrow focus shot with lots of bokeh may be sharp where the focus is, but the file size will be smaller than the same scene shot stopped down...

The scanner's doing the same thing with the film. However, past the resolution ability of scanner + film + lens + scene, you don't really get more info, just a bigger file. Think of it like you're blowing up something that takes 10 pixels to draw up to 20 pixels - it's still the same information, you're just using more pixels (and file space) to describe the same thing.

With a 4490 / V500 class I find 1200 or 2400 dpi scans are about optimal...

Consider too that a 10MP digital camera probably costs more - and contains more manufacturer's value in design and build - than a $200 flat bed scanner that's adapted for film. The volume and the market is with the digital camera.

01-08-2011, 09:02 PM   #5
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Thanks for the info. I expected more from my scanner which is Canon's top of the range flatbed scanner and cost $349.

It claims 9600x9600 dpi optical resolution but as I've said, scans above 3200dpi apear to be pointless. That said, 3200dpi is definitely better than 2400dpi (which shows pixelation when blown up and compared to a 3200dpi scan of the same image).

I'll stick with 3200dpi for film and slide scans - this gives 12MP images of about 2.7MB file size. This is comparable to my K10's image file sizes - but sadly not when it comes to sharpness and detail.
01-09-2011, 05:41 AM   #6
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QuoteOriginally posted by Spock Quote
This is comparable to my K10's image file sizes - but sadly not when it comes to sharpness and detail.
This is where post scan sharpening comes in - there are many ways to do it, a lot of these are explained right here (see the last couple of pages of 'post your medium format pictures' thread for example)... The trick is to 'develop' the local contrast without introducing too many artifacts; this is possible and usually necessary. (I believe the digital camera is doing something like that 'in the box', calling it anti-aliasing etc rather than sharpening)

Here's a tutorial that came out of the medium format pictures thread:
https://www.pentaxforums.com/forums/digital-processing-software-printing/1285...ml#post1332826

I'm actually a bit jealous that you get a benefit of 32000 dpi
01-09-2011, 08:21 AM   #7
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QuoteOriginally posted by Nesster Quote
I'm actually a bit jealous that you get a benefit of 32000 dpi
32000?! lol!

For what it's worth my scanner claims 19200dpi through the software (and 9600x9600dpi optical) but seeing as anything above 3200dpi does nothing other than increase file size, I'd say it was only a marketing exercise by Canon.
01-09-2011, 08:33 PM   #8
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Spock,

You have received some great advice from the guys here, it lines up with my experience in scanning so far (after probably 8 months of doing it and about 1500 images).
I might just add that a very important factor in your resultant IQ is how you treat the negatives. Like how flat do they sit in the holder? If the holder height is adjustable, what is the optimum height? I have noticed large differences in sharpness between curled and flat negatives.
And of course, learn the best sharpening method - don't be afraid to add more than you might usually do with digital, depending on the circumstance.

cheers

01-10-2011, 01:19 AM   #9
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QuoteOriginally posted by goddo31 Quote
Spock,

You have received some great advice from the guys here, it lines up with my experience in scanning so far (after probably 8 months of doing it and about 1500 images).....And of course, learn the best sharpening method - don't be afraid to add more than you might usually do with digital, depending on the circumstance.

cheers
Yes, some good advice. Thanks. I can see how careful sharpening could improve image quality of scans. I will definitely endeavour to learn and hone my image sharpening skills

QuoteOriginally posted by stevebrot Quote
What does this mean in practical terms? In general, it means that there is little to be accomplished by wasting time and disk space scanning at resolutions much higher than 1700 dpi with your scanner.
Yes, I can see that 1700 real dpi is about the best it can do. However, in that link they said:
QuoteQuote:
In order to achieve an effective value of 1700 ppi, the pattern should be scanned with 4800 ppi. The scan of a 35-mm negative or a 35-mm slide with 4800 ppi provides an image file with 32 megapixels. But from these 32 million pixels only 4 million is actually real information contained in the scan; the remaining pixels are double and multiple.
Having tried scanning at various resolutions I am inclined to think that 4800dpi doesn't give any worthwhile advantage over 3200dpi - at least for for a Kodak Ektachrome slide anyway (iso 200 I think).

For some 1960s B&W negatives and some 1970s 126 colour film negatives I scanned, I found 2400dpi was more than enough.

Last edited by Spock; 01-10-2011 at 01:28 AM.
01-12-2011, 07:44 AM   #10
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cheaper scanners crank up the DPI to offset inferior scanning technology

you always want to scan at the scanners native DPI for best results

stevebrot is correct on all points

bottom line is, with cheaper scanners, you have to settle with very large file sizes to achieve good quality. More expensive scanners will give you better quality in a smaller file size.
01-14-2011, 01:04 PM   #11
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QuoteOriginally posted by Spock Quote
scans above 3200dpi apear to be pointless. That said, 3200dpi is definitely better than 2400dpi (which shows pixelation when blown up and compared to a 3200dpi scan of the same image).
Just how much are you blowing the image up, and what format are you saving in? TIFF I hope.

TIFFs from my scanner at 4,000dpi (it's a Nikon Coolscan V) saced as TIF come in at 59MB. If I scanned at 2,000dpi they would be 1/4 the number of pixels so would come in at 15MB as a TIF.

BTW These are 8-bit TIFS. If I have what I think might be a fantastic shot that needs some work I'll rescan and save as a 16-bit TIF and then pass it over to Photoshop.

And I've been very carefuly about how I name and organise both the scans and the negs. I can go straight to the negative for any scan from the file name.
01-14-2011, 06:59 PM   #12
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JPEG. I'll have to try scanning in TIFF and see if I can get a bit more detail out of important shots.
01-15-2011, 08:22 AM   #13
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On my Nikon Coolscan 9000, I can see some benefit from scanning at 4,000 dpi and saving as a TIFF. Dedicated film scanners with good autofocus systems can use the extra resolution a bit better. In the end, you are talking about how sharply you can resolve grain and how much benefit you get from that resolution. For K25, old Agfapan 25 and other fine emulsions, I can use much more of the resolution, and hard drive space is pretty cheap.

However, I have not found many scans where going to 16 bit depth did much to offset the huge size increase. Here, I suspect that negatives with more shadow detail buried in the dark spaces might be better subjects to try than Kodachromes.
01-20-2011, 11:29 AM   #14
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QuoteOriginally posted by GeneV Quote
...

However, I have not found many scans where going to 16 bit depth did much to offset the huge size increase. Here, I suspect that negatives with more shadow detail buried in the dark spaces might be better subjects to try than Kodachromes.
16 bit is not something you will see on your 8-bit monitor. Some high end monitors offer 10 bit, but still, what you see on screen is only a very very small portion of what's in the file. The reason you'd scan at 16 bit is because even small tonal edits in Photoshop or equivalent will start to destroy the very small amount of tones an 8 bit file can hold. You can see this in the histogram. Add some curves to a smooth 8 bit file, all of a sudden the histogram will show gaps and spikes which are posterization and complete lack of tone. You'll see differences there sooner than you notice the posterization on screen. Skies, smooth skin, and other areas of delicate tonality are what gets hurt. A detail rich negative will hide the posterization but a portrait won't.
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