Limitations of Scanner + Taking Lens

[rdenney]

... This would be equivalent to a 100% MTF (well a not so rigorous interpretation I think) but maybe useful.

If the density of your test target varies according to a sine wave, then this would indeed be 100%.

But this is really rather demanding of the system. The critical spatial frequency at 100% MTF is perhaps not that interesting a value--no lenses not even the really good ones can achieve that. I would suggest setting two values--50% MTF and 10% MTF as the critical values for "contrast" and "resolution". The spatial frequency at 50% is the smallest spatial frequency that will show good edge detail and contrast, while the other finds the minimum discernable detail.

If the 50% value aligns with about 4 or 5 line pairs/mm in a print, the print will look about as sharp as can be perceived by a viewer with an unaided eye. The scanner would need to produce 50% MTF at 16-20 lpm to support a 4x enlargement (4x5 to 16x20), or twice that to support an 8x enlargement. Your tests show that the scanner is actually much better than that, which is rather comforting.

At this moment, I'm looking at a 4x5 negative I just scanned on my V750, using Vuescan. The picture was made using FP4 processed in HC-110 dilution B. I struggled to get the scanner to see all of the density range on the negative, with the clearest parts (deepest shadows) blocking up very slightly. With a different developer, I might easily have reduced that density range to solve that problem (I can't measure density--I don't have a densitometer).

The lens I used to make the picture was an Ilex Paragon 8-1/2", and I was seeking selective focus so I used f/5.6. Not much could be expected of that old tessar design at that aperture. I'm seeing an edge spread over about 3 or 4 pixels in the focus plane (which is pretty thin indeed--I had to really hunt for it), which, at 2400 pixels/inch, would correspond to a lens performance of perhaps 12 or 15 lpm at high MTF. Probably about right for that lens used at a far larger aperture than it was designed for. It is not in any way limited by the scan, however. A 48x microscope view of the negative reveals no more detail. I have no problem with the softness of this lens in this application. It has a really old-fashioned look that complements the subject.

I can see where there was a speck of lint on the film when I made the exposure, and one assumes this is as sharp as is possible. It's edge is 2 pixels wide at very high MTF, and that corresponds about 24 lpm--probably the capability of the scanner at an MTF that makes a good appearance of contrast.

The experience and the numbers are consistent, it seems to me.

Rick "who absolutely wants the scanner to reproduce a fuzzy lens faithfully" Denney

[Nathan Potter]

Your figure of 2300 spi is the same result that photo-i obtained. They show the actual scan of the resolution target.

To get that 2300 spi, are you scanning at higher resolution ? wet-mounting ? In my (less rigorous) tests with an Epson 4990, I found that I once I got past 1600 spi, I got the same resolution no matter how high I set the scanner. But I just tested dry scans using a BetterScanning holder.

With the V700, I can't see any difference once I scan higher than the 2400 spi setting.

Ken, I was scanning at 2400 spi. Chrome emulsion down and not wet scanned thus the image goes only through the Epson platen. I would assume that Epson has thought a bit about the effect of the platen and possibly done some sort of compensation to obtain the performance we see.

I think, roughly speaking, you are on track with your observation of 1600 spi although my initial observations show some marginal gains in resolution above that but one really needs to specify a contrast when speaking about this. Certainly comments about characterizing a V750 using MTF sort of data tells a much better story about performance.

Actually I was initially interested in determining the size of a capture pixel in order to understand what the machine was picking off my films. That is to say - what is the physical size of a capture pixel in RGB referenced to the film surface. It appears that 1 mil (25 um) is a reasonable number. BTW I should repeat this for using only the green channel.

Tests done some months ago using a Toppan resolution mask with scans at 1200, 2400 and 4800 spi showed no significant gain in resolution from 2400 to 4800 spi. 1200 to 2400 showed a bit but I didn't tabulate that that rigorously. In time I'll try to do that.

Nate Potter, Austin TX.

[Nathan Potter]

If the density of your test target varies according to a sine wave, then this would indeed be 100%.

But this is really rather demanding of the system. The critical spatial frequency at 100% MTF is perhaps not that interesting a value--no lenses not even the really good ones can achieve that. I would suggest setting two values--50% MTF and 10% MTF as the critical values for "contrast" and "resolution". The spatial frequency at 50% is the smallest spatial frequency that will show good edge detail and contrast, while the other finds the minimum discernable detail.

If the 50% value aligns with about 4 or 5 line pairs/mm in a print, the print will look about as sharp as can be perceived by a viewer with an unaided eye. The scanner would need to produce 50% MTF at 16-20 lpm to support a 4x enlargement (4x5 to 16x20), or twice that to support an 8x enlargement. Your tests show that the scanner is actually much better than that, which is rather comforting.

At this moment, I'm looking at a 4x5 negative I just scanned on my V750, using Vuescan. The picture was made using FP4 processed in HC-110 dilution B. I struggled to get the scanner to see all of the density range on the negative, with the clearest parts (deepest shadows) blocking up very slightly. With a different developer, I might easily have reduced that density range to solve that problem (I can't measure density--I don't have a densitometer).

The lens I used to make the picture was an Ilex Paragon 8-1/2", and I was seeking selective focus so I used f/5.6. Not much could be expected of that old tessar design at that aperture. I'm seeing an edge spread over about 3 or 4 pixels in the focus plane (which is pretty thin indeed--I had to really hunt for it), which, at 2400 pixels/inch, would correspond to a lens performance of perhaps 12 or 15 lpm at high MTF. Probably about right for that lens used at a far larger aperture than it was designed for. It is not in any way limited by the scan, however. A 48x microscope view of the negative reveals no more detail. I have no problem with the softness of this lens in this application. It has a really old-fashioned look that complements the subject.

I can see where there was a speck of lint on the film when I made the exposure, and one assumes this is as sharp as is possible. It's edge is 2 pixels wide at very high MTF, and that corresponds about 24 lpm--probably the capability of the scanner at an MTF that makes a good appearance of contrast.

The experience and the numbers are consistent, it seems to me.

Rick "who absolutely wants the scanner to reproduce a fuzzy lens faithfully" Denney

Rick, yes the better way to go about this type of characterization would be to pick a few spacial frequencies in the resolution values of interest say 100%, 75%, 50% and 10% then locate the minimum linewidth pairs which show those contrast values. What I'm not so sure about is the validity of picking the density values directly off the enlarged pixels on the computer screen. This technique is quite fundamentally different than using a scanning microdensitometer like I am familiar with. I guess I'll start thinking about this but would be very interested to know of any standards that would apply, say ASTM, even NBS, etc. that would refer to such a technique.

Lint I assume would be of some sort of hairlike origin perhaps 2 to 6 mils wide so easily imaged by a nominally 25 um spot on an Epson 750.

The masks I use are step function with line edges of TRO around or less than 100nm (0.1 um). The best of these are from Canon Lithography and are chrome on quartz. Pricey new, around $4000. Ain't surplus auctions great.

Nate Potter, Austin TX.

[rdenney]

Rick, yes the better way to go about this type of characterization would be to pick a few spacial frequencies in the resolution values of interest say 100%, 75%, 50% and 10% then locate the minimum linewidth pairs which show those contrast values. What I'm not so sure about is the validity of picking the density values directly off the enlarged pixels on the computer screen.

Anything I did using the computer screen was intended as a practical guesstimate--not intended to develop a model of performance but rather to tell me some parameters that might limit what I do. Also, it provides a reality check on the theoretical calculations.

I have done somewhat more rigorous lens testing using Norman Koren's test charts. These produce a sine wave output at a larger size, and then when photographed have a frequency high enough to be interesting. The value in his chart is that he includes a comparison strip. So, on a transparency, I can compare the blacks and whites at the test frequency with blacks and whites in a similar strip at low enough frequency to be near 100% MTF. This is imprecise but I suspect reasonably accurate for practitioners. I made the comparison using a microscope, not a computer screen. Of course, I was testing lenses not scanners.

I remember reading somewhere that the MTF 50% resolution of Velvia is 37 lpm. I have not tested it and don't know, but if it's true, that does give some context to the discussion.

Rick "wishing for time to play with this more" Denney

[Joerg Krusche]

ken,

could not find the data on photo-i which show resolution data based on test target you referred to .. on which page of the report ?

thank you !

joerg

[Ken Lee]

Sorry Joerg - You are right !

The article is on ScanDig, not photo-i.

I corrected my earlier post.

[Joerg Krusche]

Ken,

thank you for the information .. very informative .. soft scan .. quite a bit of sharpening may be needed unless you like the soft image,

joerg