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Author Topic: Scanning color negatives - best resolution?  (Read 25164 times)

Bart_van_der_Wolf

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Re: Scanning color negatives - best resolution?
« Reply #20 on: January 04, 2015, 06:32:31 am »

I am scanning some 35mm Kodacolor negs using an Epson V750. Is 1200 DPI a good scanner setting to use? I am trying to balance detail extraction with scanning speed. ANy other tips would be welcome - I am totally new to this.

Hi Peter,

I've been scanning film for a long time, probably close to some 20 years, but less since my digital cameras files exceeded 16MP (roughly(!) matches 4000PPI). There is still a lot of misinformation going round about how scanners and film interact, but it is essentially quite simple once you understand the factors that play a role. Let's concentrate on resolution for now, since that's you main question.

First we need to establish what resolution the film actually caught. If detail isn't resolved on film, you won't resolve it in a scan. This immediately becomes a question that's hard to answer, because it depends on the interaction of the film (slow thin emulsion, low or high contrast), the camera optics (focus/diffraction/aberrations), and shooting conditions (tripod/handheld).

However, we can empirically demonstrate that it is possible to extract more detail from film with scanning densities of upto 6000-8000 PPI, presuming that such level of detail was captured on film to begin with. That does also help to reduce grain-aliasing, but that's a somewhat different subject.

Now, the difference between, say, 4000 PPI and higher sampling densities is in the range of diminishing returns but it still helps. However, this also presumes a scanner is being used that is capable of delivering high MTF at those resolutions, and this is where things change quite a bit with the Epson V7xx and V8xx scanners when compared to dedicated filmscanners or especially drumscanners.

One major obstacle is getting the film in the plane of best focus, which includes height of film in the film holder, and film warp or even movement during the scan due to heat build-up. Then there is the quality of the scanner optics (lens/prism/mirrors whatever is included), and the relatively poor protection against veiling glare (also because a large area of the film is exposed while only a small strip is being scanned).

Another obstacle is that it's hard to predict how the MTF of the film image and the MTF of the scanner (assuming perfect focus and a clean lens) will combine into a system MTF.

One possible way of knowing the practical upper limit on scanning PPI with a particular scanner is by determining the limiting resolution of the scanner itself, and then assume a perfect film image (which it isn't). There is an ISO 16067-2 procedure that describes the best way of doing that, and it involves scanning a slanted edge which then allows to produce an MTF curve which should tell us where we can draw the line on practical resolution limits.

The 1951 USAF resolution test chart which was mentioned, is designed to be used with analog recording systems (i.e. film), and is not that well suited for discrete sampling systems such as line scanners and digital camera sensors. It's main issues are that the contrast is unreasonably high (lower contrast will already be unresolved when higher contrast can still be resolved) and, more importantly, the bar patterns may either align with the sensel pitch of not. That makes it sensitive to positioning which can result in up to a factor of 2 resolution difference between tests.

Another and much more practical method, if one can still produce a film image with the equipment and film/processing, is by scanning a (sinusoidal) grating at various rotations as captured on the actual film. My free radial grating (Star) test target is very well suited for such tests. It allows to visually compare performance, and it allows to quite accurately quantify the limiting system resolution of capture+scanning operations, all the way up to the Nyquist frequency of the scanner/digitizer.

Here is an example of a test I did on a much older target design with a dedicated film scanner. Those old webpages were not maintained/expanded when a newer model scanner was introduced, since I received fewer questions that needed to be specifically answered for that first generation 5400 scanner. However, it still shows that the scan resolution effectively increased with scanning/sampling density on those dedicated film scanners for home use. BTW, an effective system resolution of 76.1 lp/mm on that SE5400 resembles a direct digital camera capture with an approx. 6.4 micron sensel pitch.

Concluding (I'm jumping ahead to it a bit), we can assume that there is a benefit to scan at at least 2400 PPI on the Epson scanners, with diminishing returns for resolution all the way to 6400 PPI. It will help to reduce grain-aliasing a bit at the higher scan densities, but the limited scan resolution (and relatively diffuse lighting) already have a 'positive' effect on the visibility of noise.

Personally, I would scan at full resolution capability, and down-sample for storage. The amount of down-sampling will determine the level of loss we can tolerate for the intended use.

Cheers,
Bart
« Last Edit: January 04, 2015, 06:35:17 am by BartvanderWolf »
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Oldfox

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Re: Scanning color negatives - best resolution?
« Reply #21 on: January 04, 2015, 11:34:14 am »

Peter, I have Epson V750. About a year ago I scanned about 3000 color negs. Here is what I found out.

- Most important: don't start a big scanning task until you feel comfortable about what you are doing. Do tests with several settings including resolution and focus (and software).

- At first I also thought that scanning ONCE is the right way to go (as Alan suggested). After doing my tests I decided that scanning TWICE is my way.

- FIRST, I scanned all the 3000 negs. Epsonscan. Batch mode. Resolution of 1200 PPI. JPEG.  For me, I found this to be good balance of resolution ('quality'), scanning time and disk space.

- SECOND, I picked 'the best' (not many, I'm afraid…).  EpsonScan or Vuescan. Resolution of 2400 or 4800. TIFF 16bit. This took more time and a LOT more disk space, but there wasn't very many 'the best' images to scan. I plan NOT to do a THIRD scan. Hopefully my SECOND scan is good enough.

- I also tried SilverFast and found it, to put it politely 'not for me'. Btw, if anyone can produce a successful batch scan with SilverFast in 30 minutes, I'll buy him/her a lunch. Vuescan is very good in individual scans, but not in batch scanning. I found EpsonScan the most intuitive of the three.

My two cents

/fox
« Last Edit: January 04, 2015, 12:25:56 pm by Oldfox »
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Mark D Segal

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Re: Scanning color negatives - best resolution?
« Reply #22 on: January 04, 2015, 01:53:51 pm »


- I also tried SilverFast and found it, to put it politely 'not for me'. Btw, if anyone can produce a successful batch scan with SilverFast in 30 minutes, I'll buy him/her a lunch. Vuescan is very good in individual scans, but not in batch scanning. I found EpsonScan the most intuitive of the three.

/fox


Which version of SilverFast did you try? Was it 6.6 or 8?
Do you live in Toronto and have a Visa or Mastercard for the lunch? You'll need it.  :-)
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Mark D Segal (formerly MarkDS)
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Oldfox

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Re: Scanning color negatives - best resolution?
« Reply #23 on: January 04, 2015, 02:17:25 pm »

It was 6.6. The comments about v8 that I have read are more or less the same as 6.6.

I live in Europe ;). And what I really meant in my offer: "in the first 30 minutes with SF". But for you Mark, I'll buy you a lunch anytime, if we meet (I'll choose the place).
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Mark D Segal

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Re: Scanning color negatives - best resolution?
« Reply #24 on: January 04, 2015, 02:29:49 pm »

You should do more than read comments. nothing replaces trying it - the demo of version 8 is free. You'll be reading more about batch scanning anon, so stay tuned. I think you are wise to hedge your bets by controlling the lunch venue  :-)
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Mark D Segal (formerly MarkDS)
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Mark D Segal

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Re: Scanning color negatives - best resolution?
« Reply #25 on: January 04, 2015, 02:32:37 pm »

Ah yes, you said "anytime" for lunch - that's gracious of you. Much appreciated. :-)
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Mark D Segal (formerly MarkDS)
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Bart_van_der_Wolf

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Re: Scanning color negatives - best resolution?
« Reply #26 on: January 05, 2015, 09:41:01 am »

Another and much more practical method, if one can still produce a film image with the equipment and film/processing, is by scanning a (sinusoidal) grating at various rotations as captured on the actual film. My free radial grating (Star) test target is very well suited for such tests. It allows to visually compare performance, and it allows to quite accurately quantify the limiting system resolution of capture+scanning operations, all the way up to the Nyquist frequency of the scanner/digitizer.

Here is an example of a test I did on a much older target design with a dedicated film scanner. Those old webpages were not maintained/expanded when a newer model scanner was introduced, since I received fewer questions that needed to be specifically answered for that first generation 5400 scanner. However, it still shows that the scan resolution effectively increased with scanning/sampling density on those dedicated film scanners for home use. BTW, an effective system resolution of 76.1 lp/mm on that SE5400 resembles a direct digital camera capture with an approx. 6.4 micron sensel pitch.

Further on this note, I think I've found the original (Provia III RDP) film frame from July, 2002. I've scanned it with my Epson V700 using VueScan Pro as scanning software. I've been using VueScan since 2001 (paid once, free upgrades since then), and it still allows me to scan with all scanners I've owned since then. VueScan offers scanning resolutions for the Epson V700 of 6400 / 3200 / 1600 / 800 / 400 / 200 / 100 PPI, and its IR cleaning works fine with the V700.

I've scanned the film strip in the standard Epson 35mm filmstrip holder. If I were seriously contemplating scanning 35mm film with that scanner, I would get a better film holder, but I have access to a dedicated film scanner that delivers higher resolution scans, so I only use the Epson for scanning larger film sizes.

As it turns out, my scanner produces a difference between horizontal resolution (the 'Fast scan' direction), and vertical resolution (the 'Slow scan' direction), the latter being overall some 25% higher. The chart's stars are scanned perfectly symmetrical, so the resolution difference is not due to differences in scanning density, but rather optical, or due to the lack of film flatness.

The first challenge would be to find the best height for the filmstrip holder, in order to achieve the best focus. However, that would take too much time, so I'll just settle for the results with one setting. That will already answer part of the original question, namely; does it matters much at which resolution one does the scanning?

Well, as I said earlier, a higher resolution can be achieved with higher PPI settings, although the gain gets more gradual as one reaches the maximum settings. Since we already know that with a dedicated scanner we can achieve something approaching an 80 cycles/mm resolution, how does the V700 do with the default filmholder settings?

@  200 PPI   4.0 x   3.9 cycles/mm (H x V), this would be almost adequate for contact print resolution.
@  400 PPI   8.4 x   8.4 cycles/mm (H x V), a 35mm frame becomes  567 x  378 pixels, with excellent contact print quality.
@  800 PPI 15.6 x 18.0 cycles/mm (H x V), a 35mm frame becomes 1134 x  756 px, with some 3.6x magnification potential.
@1600 PPI 37.6 x 41.0 cycles/mm (H x V), a 35mm frame becomes 2268 x 1511 px, with some 8.2x magnification potential.
@3200 PPI 44.0 x 50.1 cycles/mm (H x V), a 35mm frame becomes 4535 x 3024 px, with some 10x magnification potential.
@6400 PPI 47.5 x 60.2 cycles/mm (H x V), a 35mm frame becomes 9071 x 6047 px, with some 12x magnification potential.
The mentioned magnification potential assumes something like up to 5 cy/mm in final output, which can be viewed from short distances.

As can be seen, somewhere between 1600 and 3200 PP, the proportional increase of resolution with scanning density starts to drop off, but resolution keeps increasing with scanning density, because there is more resolution in the film than the scanner resolves. Therefore increasing the scanning density will produce more resolution, although the available resolution potential is not reached. Maybe this can be improved a bit by optimizing the filmholder and its positioning, and there will of course also be some variance between actual scanner copies, some are likely better than others.

How this turns out for the newer V8xx line remains to be seen, but a test with a star chart is usually quite revealing (and quantifiable in a useful way).

Cheers,
Bart

P.S. This also suggests that the 6400 PPI scan can be downsampled to some 75% of the scanned dimensions in pixels, where it will have a similar resolution as a dedicated film scanner can achieve directly. It should be possible to go even a bit further down without significant losses, but more acceptable storage requirements.
« Last Edit: January 05, 2015, 09:58:51 am by BartvanderWolf »
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Alan Klein

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Re: Scanning color negatives - best resolution?
« Reply #27 on: January 05, 2015, 09:55:24 am »

Epson scan on my V600 has settings from 50 to 12800.  I won;t list them all but there there are these settings in the middle 1200, 2400, 3200, 4800, 6400.  I use 2400 with my V600   Does the Epson scan on the V700 allow the same settings?

Bart_van_der_Wolf

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Re: Scanning color negatives - best resolution?
« Reply #28 on: January 05, 2015, 10:04:59 am »

Epson scan on my V600 has settings from 50 to 12800.  I won;t list them all but there there are these settings in the middle 1200, 2400, 3200, 4800, 6400.  I use 2400 with my V600   Does the Epson scan on the V700 allow the same settings?

Hi Alan,

I don't know, I only have VueScan installed on my computer, maybe someone else knows. Maybe (probably) some of these intermediate resolutions are resampled from actual samples. One would need to examine the real resolution as measured with e.g. a star chart image on film, to see if these produce real resolution, or resampled resolution differences.

Cheers,
Bart
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Alan Klein

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Re: Scanning color negatives - best resolution?
« Reply #29 on: January 05, 2015, 10:10:58 am »

What do you mean re-sampled resolutions.  Does the V700 do that? 

Mark D Segal

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Re: Scanning color negatives - best resolution?
« Reply #30 on: January 05, 2015, 10:12:14 am »

Scanners will scan at their own set of discrete intermediate resolution steps up to the maximum resolution of the CCD as reported pursuant to ISO 14473. The steps Bart cites are determined by the scanner and not the scanning application. The scanning application works with them. If you specify a resolution setting in the software that lies between two such settings, the software will resample to cohere with either the step above or the step below in providing you with the resolution you have selected.  
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Mark D Segal (formerly MarkDS)
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Mark D Segal

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Re: Scanning color negatives - best resolution?
« Reply #31 on: January 05, 2015, 10:27:02 am »

So to Alan's question - no. The scanner doesn't do the resampling, the software does. So, for example, let us say the two discrete steps the scanner provides are 3200 and 6400, but you want 4800. In the software you select 4800. The software knows for that scanner there is no such discrete setting, so if it's smart software, it will scan at 6400 and then downsample its 6400 result result to the 4800 you requested.

But you know, while it's good to understand what's going on under the hood (and sometimes necessary for diagnostic purposes), the most important thing is to actually see what difference this or that setting makes to the printed output. That is really the bottom line. Often times imagined differences inferred from theory just don't show up in comparative prints - depending. That is why, going back to the post that started this thread, I would always recommend empirical testing starting with real photos typical of the media one starts with and ending with the output for which the scan will typically be used. This is real painless (except for some time, paper and ink) and you know for sure what you're about.
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Bart_van_der_Wolf

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Re: Scanning color negatives - best resolution?
« Reply #32 on: January 05, 2015, 10:57:15 am »

What do you mean re-sampled resolutions.  Does the V700 do that?

According to Ed Hamrick, the author of Vuescan and who reverse engineered the command set of all scanners to allow driving them with optimal settings, the V600,  the V700, and the V800 series have an optical resolution of 6400 PPI maximum. That means that only integer fractions of that can be exact samples, and other intermediate resolutions are an interpolated, version of the native sensor resolution.

So, 6400, 3200 (skip 1 or bin 2pixels), 2133 (skip 2 or bin 3 pixels), etc. would be integer fractions, with 6400, 3200, 1600, 800, etc. being exact halves (skipping to or binning of double numbers of pixels). In the fast scan direction (along the linear scanlines) there is not much one can do to change resolution other than skip or bin pixels and remain exact, but that doesn't influence scan speed. In the slow scan direction the stepping motor can skip line positions (within the accuracy of the stepping motor interval), and that would affect overall scan speed.

Since the data interface is the slowest factor, one can usually get an idea about skipped (slow scan direction) lines from the scan speed. If the scan speed changes, the resolutions in between will be mathematically interpolated. If the scan speed remains the same, then different resolutions will be interpolated. The most accurate way of determining what is going on, is to measure the actual captured resolution.

Cheers,
Bart
« Last Edit: January 06, 2015, 03:45:30 am by BartvanderWolf »
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Bart_van_der_Wolf

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Re: Scanning color negatives - best resolution?
« Reply #33 on: January 05, 2015, 11:12:44 am »

So to Alan's question - no. The scanner doesn't do the resampling, the software does.

In general that's correct, although there are sometimes instructions in the scanner's firmware that allow to bin pixels and average them, before sending them to the interface with the computer. That could be interpreted as interpolation, albeit with a very coarse approach (although it also reduces noise). Epson is also known for using staggered line sensors in some of their models, where sensel lines are offset by half a sensel position, and 6 line sensors (2x Red, 2x Green, 2x Blue) are used to capture the 3 color bands. That also can be used for achieving some of the intermediate resolutions.

The question however, I agree, is mostly academic. It really only matters what comes out of the scanner, and that is measurable. And if it's significant enough for the output size and viewing distance, it may be visible.

Cheers,
Bart
« Last Edit: January 05, 2015, 01:59:25 pm by BartvanderWolf »
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Alan Klein

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Re: Scanning color negatives - best resolution?
« Reply #34 on: January 05, 2015, 12:47:27 pm »

Bart How do you measure the actual interpolated resolution?
How would I use my 8 1/2" x 11" printer to see what is the actual resolution that's the best to scan with assuming that I would want to enlarge later with an outside print service?  Should I crop a portion of the picture?

Bart_van_der_Wolf

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Re: Scanning color negatives - best resolution?
« Reply #35 on: January 05, 2015, 01:54:07 pm »

Bart How do you measure the actual interpolated resolution?

Alan, the IMHO easiest way is to use a Star target. One very well focused shot on low ISO film of a printed target can serve as a master file for any type of scanning situation. If you want a very accurate result, you should use the same film and lens as you want to answer the question for (because some lenses are more blurry than others), but otherwise use the best. Shoot the target (printed at 600 or 720PPI) from a distance of between 25 and 50x focal length (50x is better for low ISO film). Exact distance is not important, the blur diameter will always be the same if the lens is reasonably well corrected for the shooting distance.

The diameter of the target's central blurred region (usually circular or elliptical) of the scan in pixels, will allow to directly quantify the actual resolution in cycles/mm. The formula to use for scanning scenarios is (depending on the number of cycles of the target itself):
Cycles/mm = 144 / pi / (pixels / scanPPI * 25.4) ,
or pre-calculated and simplified for a 144 cycle target that becomes = (1.80459 * scanPPI) / pixels .

Quote
How would I use my 8 1/2" x 11" printer to see what is the actual resolution that's the best to scan with assuming that I would want to enlarge later with an outside print service?  Should I crop a portion of the picture?

You would typically aim at achieving a resolution on output of 5 - 8 cycles/mm at reading distance of, say, 12 inches. At twice that distance you only require half the resolution, etc. So you don't really need to print if you know the resolution of the scan, and the magnification factor for a certain output size.

If you want to print an uncropped 35mm frame scan, 24mm x 36mm = 0.945 x 1.417 inch, it would require an 8-9x magnification to reach 8 1/2" x 11" depending on the crop due to aspect ratio. So you'd want to start with a scanned resolution of 5 cy/mm x 9 = 45 cy/mm for uncompromised print quality (72 cy/mm if you want 8 cy/mm in output after 9x magnification). Letting the print software interpolate to the native resolution of the printer will allow to output sharpen at the final size before sending the file to a print service, or you can let them handle it if they know what they are doing ...

As long as the scan has the required resolution for the output size and viewing distance, the printing become easy. Larger output sizes require higher scan resolution, unless the viewing distance increases proportionally.

Cheers,
Bart
« Last Edit: January 05, 2015, 01:58:57 pm by BartvanderWolf »
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Phil Indeblanc

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Re: Scanning color negatives - best resolution?
« Reply #36 on: January 05, 2015, 03:17:59 pm »

I know this maybe different than the OP tools at hand, and it looks like his question was answered with a cement truck of possible answers. (If it helps the OP, Knowing intention of the image does set you up for efficient workflow).

But what about photographing the slides? Using nonAA  30- 40mpixel db?

Well, I don't want to interpret the resolution, but just the 2 methods. I was thinking of getting a slide attachment for my setup, or making one. I know keeping things flat and a few things that will be needed to take in consideration. I guess my Q is more on the ability of the 2 techs on the quality they can produce.

This might help in some efficiency depending on workflow as well? Any thoughts?
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Mark D Segal

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Re: Scanning color negatives - best resolution?
« Reply #37 on: January 05, 2015, 03:33:24 pm »

I know this maybe different than the OP tools at hand, and it looks like his question was answered with a cement truck of possible answers. (If it helps the OP, Knowing intention of the image does set you up for efficient workflow).

But what about photographing the slides? Using nonAA  30- 40mpixel db?

Well, I don't want to interpret the resolution, but just the 2 methods. I was thinking of getting a slide attachment for my setup, or making one. I know keeping things flat and a few things that will be needed to take in consideration. I guess my Q is more on the ability of the 2 techs on the quality they can produce.

This might help in some efficiency depending on workflow as well? Any thoughts?

Yes, there's lots of thoughts : http://www.luminous-landscape.com/essays/scannerless_digital_capture_and_processing_of_negative_film_photographs.shtml

I'll add to that: there is no digital back included in this. I have one, but the cost of a Phase One macro lens for doing this work would be about 4000 dollars and I'm not sure it can fill the frame without tele-extenders. This is something I was intending to look into but the cost is "a bit" of a disincentive. If you're going to do this, you want the very best lens quality your camera can accommodate, because the ultimate constraint on the quality of results will be the lens. Other issues such as flatness, alignment, lighting etc. are much more easily dealt-with. Another really interesting inference from the work we did is that a very high quality film scanner of yester-year will give moderately priced digital camera set-ups a good run for the money.
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Bart_van_der_Wolf

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Re: Scanning color negatives - best resolution?
« Reply #38 on: January 06, 2015, 08:37:09 am »

As can be seen, somewhere between 1600 and 3200 PP, the proportional increase of resolution with scanning density starts to drop off, but resolution keeps increasing with scanning density, because there is more resolution in the film than the scanner resolves. Therefore increasing the scanning density will produce more resolution, although the available resolution potential is not reached. Maybe this can be improved a bit by optimizing the filmholder and its positioning, and there will of course also be some variance between actual scanner copies, some are likely better than others.

In order to verify my findings, I also scanned another test film shot from April 2002, on Ektachrome Elite (EB), also with a star target. The shot was made with another lens (and possibly aperture), and the combination of lens/focus/film produces a lower overall resolution scan. However, since I've not optimized scanner focus and film flatness for 35mm scans, I'd rather not focus on the absolute resolution limit for now.

Instead, it is IMHO more interesting to see (attachments) that a similar relative resolution pattern emerges with increasing sampling density. Up to 1600 PPI the resolution increases proportionally with sampling density. Double the PPI, and resolution also approx. doubles. Then between 1600 and 3200 the curve flattens and only smaller, sub-proportional, increases of resolution can be achieved by denser sampling, although resolution does keep increasing.

Cheers,
Bart

P.S. It also looks like it that intermediate Custom PPI settings in VueScan, produce even more moderate increases (probably due to smooth resampling), so it would seem to be better to scan higher, and down-sample.

P.P.S. I've also added a scan resolution test overview for Sensia (RA), which essentially shows the same pattern.
« Last Edit: January 06, 2015, 10:52:31 am by BartvanderWolf »
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Re: Scanning color negatives - best resolution?
« Reply #39 on: January 06, 2015, 05:14:22 pm »

the cost of a Phase One macro lens for doing this work would be about 4000 dollars and I'm not sure it can fill the frame without tele-extenders.

Just to add a bit to the topic drift, you could use a bellows plus something like the Rodenstock APO Rodagon D 75mm F4 which is optimized for 1:1 and covers 6cm x 6cm. You could find a clean copy of the lens for $300-350.  Admittedly with an MDB you would not be working at its optimal reproduction ratio...  You'd also need to drop a couple hundred on the bellows and rig up an adapter (or else press a good view camera into service).
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