# 720 scan of an 8mm frame equals what dpi?

scan resolution

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### #1 Greg Miller

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Posted 31 October 2013 - 01:02 PM

Could someone please confirm my math and if I'm wrong show me where I'm wrong. I'm having this debate with someone.

My argument is that a 720P scan of a regular 8 frame is approximately a 5200 dpi scan.  A regular 8 frame is .137 inches high.  So...if there is 720 dots per .137 inches then that is equal to about 5200 dots per full inch making that a scan resolution of 5200 DPI on a regular 8 frame.  This would make scanning at 720P of regular or super 8 film and upscaling to 1080P virtually indistinguishable from scanning directly at 1080P from such a tiny frame.

Thanks

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### #2 Zac Fettig

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Posted 31 October 2013 - 01:37 PM

Your math is right (I got 5290 dpi and 6490 dpi with 1080). It would be distinguishable by someone with sharp eyes, but the results would probably be good enough for most applications.

Edited by Zac Fettig, 31 October 2013 - 01:37 PM.

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### #3 Greg Miller

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Posted 31 October 2013 - 04:40 PM

Thanks Zac...just need confirmation so I knew I wasn't nutz.

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### #4 Will Montgomery

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Posted 05 November 2013 - 05:11 PM

It would be noticeable on an A-B test on a large monitor but only to trained eyes. The 5200 dots per inch doesn't really enter in to my thinking; it's simply up-resing from 1280x720 to 1920x1080 and if that would be noticeable.

The answer would depend more on the sharpness of the lens and focus as well.

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### #5 Alan Rencher

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Posted 05 November 2013 - 06:53 PM

DPI is a print standard, not a scan standard. If you are scanning an 8mm frame at 720 pixels, then it is 720 pixels wide.

Your PPI (pixels per inch) might vary dependent on the display density, but that does not affect your scan. For example: A 1080 scan is always going to be 1080 pixels across; if you are watching it on a Nexus 5 phone, the PPI will be 445, but the resolution of the image will still be 1080.

Hope this helps.

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### #6 Carl Looper

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Posted 09 December 2013 - 06:16 PM

Could someone please confirm my math and if I'm wrong show me where I'm wrong. I'm having this debate with someone.

My argument is that a 720P scan of a regular 8 frame is approximately a 5200 dpi scan.  A regular 8 frame is .137 inches high.  So...if there is 720 dots per .137 inches then that is equal to about 5200 dots per full inch making that a scan resolution of 5200 DPI on a regular 8 frame.  This would make scanning at 720P of regular or super 8 film and upscaling to 1080P virtually indistinguishable from scanning directly at 1080P from such a tiny frame.

Thanks

The math might be right but the logic is not.

So yes:

720 pixels/0.137 inches = 5255 pixels/inch

But then:

1080pixels/ 0.137 inches = 7883 pixels/inch

So in what way would it follow from the maths that scanning at 5255 pixels/inch (and upscaling) would be virtually indistinguishable from scanning at 7883pixels/inch.

In fact the contrary conclusion occurs: scanning at 5255p/in is not virtually indistinguishable from scanning at 7883/in, (and upscaling doesn't change anything as there is no new information being added to the mix). The numbers are significantly different.

Note that mathematically you could have also just ignored DPI

The DPI ratio of 5255:7883 is exactly the same ratio as the pixel ratio: 720:1080

So whatever difference you find (or not) between the numbers 5255 and 7883 you would also find (or not) the same difference using the numbers: 720 and 1080

The issue of distinguishability (or not) requires some further considerations

Carl

Edited by Carl Looper, 09 December 2013 - 06:18 PM.

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