Scanning 35mm Negatives -- What Resolution & File Size Should I Set?

[Trevor]

I am used to 2 MB image file sizes with my digital camera and that seems fine for printing snapshots.

Scanning my old 35mm negatives, I can set :

Default resolution of 300 ppi (dpi) and get a file size of 1.7 MB.

or, Resolution of 600 ppi and get a file size of 3.7 MB.

The scanner goes up to 4800 ppi but that would take an age and is overkill.

Which resolution / file-size would you recommend for archiving? Thank you.

[Guest Reimar]

I think it's depends on the quality of your pictures. I would try different resolutions from a same photo and print them to see the difference. Then choose what you think is the best for you.

Cheers.

[Crushdepth]

For high quality print production you should aim for around 300dpi resolution *at the intended printing size*. So if you scan at 300dpi then you can only print the photo clearly at the same size as the negative. If you wanted to print it double the size of the negative, then you would need to scan at 600dpi, 4x is 1,200dpi, 8x 2,400dpi etc etc.

So if you want to print out at a 'normal' kind of photo size, you'd probably want a minimum of 1,200dpi.

[Trevor]

I think it depends on the quality of your pictures. I would try different resolutions from the same photo and print them to see the difference. Then choose what you think is best for you.

Cheers.

Yes!

There is a way to solve every problem but it is sometimes difficult to find. Keep on trying, and at the end of the day, you may you get what you need! - Reimar (English syntax improved by Trevor).

You can't always get what you want but, if you try - sometimes - you get what you need. - Rolling Stones

Edited April 9, 2008 by Trevor

[Trevor]

The resolution limit was effectively 600 ppi/dpi. At 1200 ppi the scans were almost totally bleached-out. I don't know why this should be.

Comparing 6" x 4" prints at 300 and 600 ppi there was little difference but I could just tell the 600 was a little sharper. So 600 ppi for me it will be (4.3 MB file size) ... over twice the pixellation of my digital camera on max, so should be more than enough.

[nikster]

Couldn't you just give it to a photo shop and have them do it? I was planning to do that with a bunch of negatives... can't be bothered to do it myself, too much work...

[autonomous_unit]

To compare resolutions, here is a rough estimation based on the 24x36mm film frame size: a 5 megapixel equivalent shot would require a scan of about 1900 dpi (approx 2740x1830 image), while an 8 megapixel shot would require a scan of about 2400 dpi (approx 3460x2310 image). Beyond that, you're likely just adding more blurry bits unless you have specialty films and a real film scanner. Even these resolutions may be overkill, but some people would say scan at the highest your scanner and printing system can handle, to get the most possible image content from film to paper.

You might want to look at a site like photo.net for a lot of technical advice on scanning film. Only read further for more scanning/imaging geekery... it's a complicated topic and requires a bit of skill and experimentation to get good results.

But since we're in a Thailand forum, I've just been looking into this recently myself to try to make the best use of the Epson flatbed scanner I bought here (since I wasn't going to pay the exorbitant Bangkok prices for a dedicated film scanner). First, it is generally assumed that 35mm color negatives have somewhere between 1000-4000 ppi of resolution in them. (Range both for different film brands/types and also differing opinions on this topic!) Then, your scanner will have some practical upper limit, i.e. scanning at higher resolutions will just give a more detailed image of its blurry optics...

Under a microscope, color film looks sort of like a dithered print by an inkjet printer. Rather than continuous colors, it is just little flecks of different colors. If you can get a really high quality "raw" scan, you will actually see this grainy mess. A lot of post-processing is required to make the picture look appealing again, and even then it will look more noisy than a digital camera photo, because the grain is less uniform in color distribution and sensitivity versus a modern digital sensor. Because of all this post-processing, the quality of resulting images depends on the scanning software utility as well as the scanning hardware. I saw many recommendations online for the "VueScan" utility which you can download for free to try, and which costs a modest $40 US if you want to keep it and use it to its full potential.

On my flatbed scanner, I get a better scan at 3200x4800 dpi (it doesn't support 3200x3200 for some reason) than I do at 2400x2400 dpi. This is getting to the point of diminishing returns on my typical ISO 100 and 200 color film negatives, but the higher scan resolution avoids an aliasing problem where the grain of the film starts to create funny moire patterns. So I scan at higher resolution and then scale down the image again, in order to get a smoother anti-aliased image.

For special photos I want to enhance with much more effort, I find that the following rough process gives the best results for some of my grainy old negatives: scan "raw" at 3200x4800 dpi for 48-bit color; apply a gaussian blur of radius 3 or 5 or even 7 (depends on image) to smooth together the grains; adjust the white-point and black-point levels since the histogram is bunched together in the 16-bit range; maybe adjust the levels curve away from its default linear shape to change shadow versus mid-tone versus highlights; re-scale the image (to make it smaller and give the aspect-ratio I want, to correct for the unequal scan resolutions in horizontal and vertical dimensions); apply an unsharp mask to clear up some of the fuzziness (this will also re-create some of the graininess); finally convert to 24-bit color (normal 8-bits per channel) and save as JPEG.

A caution: the above manual process creates HUGE intermediate image sizes, e.g. a scan is about 100MB and the image editor seems to hog even more system RAM while working on them. The final resulting JPEG is in the 1-2 MB range for a 5-8 megapixel image size.

[Crushdepth]

The resolution limit was effectively 600 ppi/dpi. At 1200 ppi the scans were almost totally bleached-out. I don't know why this should be.

A lot of desktop scanners 'overstate' their maximum resolution by a large margin. If you want a really high-res scan it is best to use a specialised negative/slide scanner, will give much better results. Probably you can get this done at a shop somewhere.

[BEENTHEREDONETHAT]

Keep in mind that at times you may want to blow up part of a photo for a close up etc. so to do a 4x6 from just a portion of a shot could be the equivilent of a 8x10 or more. So if you plan to do much cropping you may want to keep more ppi.