Re: filmscanner vs hi-res flatbed
- From: Kennedy McEwen <rkm@xxxxxxxxxxxxxxxxxx>
- Date: Tue, 5 Jul 2005 03:40:43 +0100
In article <1120517620.bd0de17c85219ba42ce256f5ab901dc1@teranews>, Polar Light <tpt12345@xxxxxxxxxxxxx> writes
At a photography equipment shop, the assistant recommended one of the 'photo' flatbed scanners (Epson & Cannon) instead of a dedicated 35mm film scanner. They range in resolution from 2400 to 4800 dpi, i.e. similar to the dedicated scanners, at lower prices, plus they also scan prints. One of them is A3+, another takes film up to 120. Is there a catch?
Yes, the catch is that resolution is not the only measure of scanner performance. Another measure of performance is MTF, or modulation transfer function. This is just a measure of how much contrast the scanner can reproduce at different resolutions. It is just the scanner's equivalent of the frequency response plot that you used to see on the back of quality audio cassette sleeves. Assessing scanner performance requires consideration of resolution and MTF together, as well as other parameters such as density and dynamic ranges.
Like most technologies, there is a "god law" that sampled data cannot unambiguously reproduce signals with more than half the sampling resolution. So if you have a scanner that is, say, 4000ppi then it cannot reproduce signals which are more than 2000cy/in, or 2000lp/in. In other words, you need at least two pixels per cycle - crudely one for the positive half and one for the negative half. If you sample signals which have more resolution then this results in aliasing, or beat patterns. I am sure you have seen these before, if not in scanned images then on TV with presenters wearing fine patterned jackets or fine striped shirts. In traditional sampled systems, such as digital audio, the signal is deliberately filtered to less than this limit before being sampled so as to avoid audio beats and aliases.
This is effectively what most flatbed designs try to achieve, by using a staggered CCD array. This is a linear CCD where each colour is actually produced from two separate lines of sensors, offset from each other by half a pixel pitch. We can go into the technical details of how this works if you like, but the key point is that this approach produces an MTF which is zero exactly at half the sampling resolution and pretty small above that. The consequence is that the scanner can reproduce all of the information presented to it without aliasing, right up to the maximum resolution quoted.
However, in the audio the signal is electronic and it can be restricted to the unambiguous band by steep cut-off electronic filters. In optics there is no practical equivalent. Consequently, by ensuring that no contrast is present in the sampled scanner image at the limit where ambiguity begins, there must also be a reduced contrast in the region where the scanner image can be reproduced unambiguously. The consequence of this reduced contrast, or low MTF, is that the image looks soft - even though it technically has the full resolution quoted, the MTF at the limit of that resolution is zero and it is also low for quite a way up to that limit.
In a dedicated desktop film scanner the emphasis in the design is to get the maximum MTF right up to the limit of resolution even if some significant MTF is present beyond that limit. Consequently the scanned images have more contrast in the detail and look sharper - even though the limit of resolution may technically be the same as the flatbed, there is essentially no MTF reduction below that limit.
You might ask why these scanner types differ, and the answer is the usual combination of compromise and cost. In a flatbed scanner, the device is not just used for film, and some of the media may have very specific spatial frequencies that extend over large areas of the subject and these will alias into particularly obvious and objectionable patterns. Printer dot patterns are just one example. So the flatbed scanner design is generally optimised to eliminate these effects by ensuring that it is physically impossible to create aliasing at the limit of resolution. If no aliasing is present at the maximum resolution then it can safely be downsampled by algorithms that minimise its introduction. Such algorithms all contain the digital equivalent of the sharp analogue cut-off filter used in the audio signal.
On the other hand, the finest detail that is usually presented to the dedicated film scanner is the film grain, which is a random range of spatial frequencies, few of which correlate between samples. Consequently, any aliasing that occurs in the dedicated film scanner is usually only an amplification of the random grain structure, there is no beat pattern or typical alias image. The original grain may be finer than a pixel, but it is all reproduced as coarse as a pixel but with the original contrast. In some cases, with very high resolution film and camera lenses some image aliasing can occur, although it is generally quite small with current 4000 and 5400ppi scanners.
Ideally, dedicated film scanners should use the staggered CCD approach as well to ensure that this simply does not happen and to eliminate grain aliasing entirely - but there is a cost. That would mean that your film scanner would have 4x as much data to produce and a more expensive CCD - for a very limited set of images from cameras and film which are capable of producing the problem. Well, one day it might happen, but it isn't there yet.
If you have your images scanned with a proper drum scanner then the operator will have a control called the aperture (or similar) which basically adjusts the sensor pixel size independently of the resolution of the scan - effectively adjusting the amount of overlap between pixels somewhere between that of the desktop film scanner (zero) and the flatbed (up to 50%). This allows the drum scanner to obtain the best tradeoff between MTF (ie. apparent image sharpness) and aliasing for each image and film type. Unfortunately, this is not a facility that is available with any current dedicated desktop scanners.
So the bottom line is that a flatbed may have the same resolution on paper as a film scanner, but the image from the dedicated film scanner will be considerably sharper, at the expense of some visually amplified grain and possible image aliasing if your film, lens and camera technique are absolutely tip top.
Is the quality comparable to that of dedicated filmscanners?
No. They are different trade-offs.
If you restrict your output to A4, then the best of the current crop of flatbeds scanning at 4800ppi will do an adequate job, even with reasonable cropping. You simply won't see the reduced MTF and the elimination of aliasing will produce nice smooth images. However, like most things, once you get used to printing at that size you will want a little bit more, or there might be situations where a bit more cropping is necessary and then the MTF limitations of your flatbed will start to be objectionable. Ultimately, you want the MTF of the film to be the limit on your print size, not the MTF of the tool that you use to digitise those film images. It is essentially the same argument for choosing the best lenses for your camera.I have a large amount of 35mm slides plus a small number of negs to scan & I'd like to be able to print them on special A4 photo papers. Are flatbeds really a good alternative?
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
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