JohnJ
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Digital Resolution Vs. Slide

Thu Oct 14, 2004 12:31 am

I've noticed a lot of vintage pictures being uploaded here on a.net and started to go through some of my old prints to see if I have anything older that might be of interest. I tried scanning a few 4x6 prints, and they really look horrible compared to the images my Digital Rebel produces. Just curious to see if anyone has any idea what the comparitive resolution of a print would be. I've heard, for example, that a 64 ISO slide is the rough equivalent of 40 megapixels. Is the resolution of a 6-megapixel image better or worse than 100 ISO print film?
 
ckw
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RE: Digital Resolution Vs. Slide

Thu Oct 14, 2004 1:33 am

I've heard, for example, that a 64 ISO slide is the rough equivalent of 40 megapixels.

I think not, given that medium format digital backs run at 22mp. What you might have heard is that a scanned K64 slide can produce a 40megabyte file, which may be true ... however, much of the data will in fact be noise.

And not all pixels are equal - 8mp on a digicam looks very different to 8mp of a 20D.

There is an interesting article on Luminous Landscape which tries to mathematically quantify image quality.

According to this, if 35mm Fuji Provia 100F is given a score of 1.00 for quality, a Canon D60 (6mp) scores .78 and a Canon 1Ds (11mp) 1.03.

Newer cameras are not included, but since the calculation incorporates the signal to noise ratio, which is improving with every DSLR release, on this basis it is probably safe to say that current models are very close to or exceed what is arguably one of the best 35mm slide films. They would easily exceed print film.

BUT (big but) this is only one way of assessing quality, albeit a fairly objective one. Digital and film are different, and respond in different ways to light. While it is possible to measure resolution, it is less easy to measure the ability to hold subtle colour transitions or shadow/highlight detail.

For example, DSLRs are quite poor at highlight detail as the cutoff is very abrupt between pure white and discernable detail. Negative film on the other hand has a very long and gentle fall off. At the other end of the scale, most people are astonished by the amount of shadow detail DSLRs can capture.

For all practical purposes, in any measurable sense it has been generally agreed that current DSLRs have met and exceeded 35mm film. Indeed, the question now is do the latest products from Canon and Nikon present a credible challange to medium format film! But because film has a look which can't quite be replicated digitally, there will always be some who prefer film.

Cheers,

Colin



Colin K. Work, Pixstel
 
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RE: Digital Resolution Vs. Slide

Thu Oct 14, 2004 1:58 am

That is a question where the answer is difficult because there are a lot of variables involved to really complicate the matter. Generally speaking, yes, you will end up with far more megapixels than any digital camera output currently. However, that doesn't mean the quality of what you see in print or on screen will be better, especially when viewing compressed JPG's on this site. When your original scanned image is 200meg in file size and you're posting a downsized 200K (200,000) file sized JPG to this site, you have thrown out 90% of your pixels so what good did all those pixels do you? All those thrown out pixels are great for printing in large sizes and for commercial printing purposes but for display on the net they're worthless.

One variable to consider is the resolution of your scanner. If you scan the negative you will end up with far more megapixels at 5400ppi than 2900ppi or 4000ppi. A scan of the negative will equate to something similar to a slide scan in terms of resolution. You will get more detail at higher ppi's but that can be bad detail like noise along with good detail.

If you scan the print, the resolution also depends on the size of the print you are scanning. Obviously a flatbed scan of an 8x10" print will yield far more megapixels at the same dpi/ppi than a scan of a 3x5" print. However, scanning a print is scanning a reproduction from the negative and you are at the mercy of whoever made the print from the negative in the first place.

I haven't scanned recently but I have a scanned (5400ppi) image from a Kodachrome slide of the Aeca Carga CL-44 belly landing that I recently posted and it is 7065 pixels wide by 4702 high - those two numbers multiplied come out to 33.2 megapixels. However I can't remember if I cropped it first or not.

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Photo © Don Boyd


Don
Photographing aircraft since the Earth was flat and on Airliners.net since #338

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