Moire-like patterns appear. what is it?

[roman_iazovitch]

hi all. just got an Epson 4490. as i am trying to scan a few 6x6 delta400, i get concentric spots of something that

looks like a moire pattern. as i remember reading about this, i cannot recall what it was about. help, anyone? how to

beat this?

some info: i scan, by laying the negatives straight onto the glass emulsion down. the focus of the resulting image is

as good as it gets. the negatives are reasonably flat. no uneven spaces between them and the glass. i go with 2400

dpi. but there are those curves, that look like the height-lines on a map. it really distracts the eye...<div></div>

[lex_jenkins]

Newton's rings: http://en.wikipedia.org/wiki/Newton's_rings

 

ANR (anti-Newton ring) glass can help, but I'm not sure whether it's readily available for your scanner. Sometimes ANR glass can be adapted to different purposes.

[bobcossar]

These are Newtons rings.....you can purchase anti-newton ring glass and fix it in that way. Basically it's caused by the glass being TOO smooth and interference patterens showing up as a result...Cheers, Bob

[roman_iazovitch]

ooo! thanks, people! is there anyone familiar with the physics behind this? if i bring a negative a little higher above the glass, would it eliminate the thing without bying one more thing for the collection? how is this scanner supposed to be for negative scanning if it ruins every negative? there must be some other way around this besides throwing money, dont you think?

[roman_iazovitch]

strike the former. i just read the article Lex provided (thanks!).

i dont know about a magical "ANR" glass, but if i got it right, a simple glass above the negative which flatens it considerably should work just fine, doesn't it?

[larrydressler]

I use the negative holder and I never get that problem sometimes I have to put the negative in the holder backwards and reverse it in PS but the only stuff I put directly on the glass is my 16mm.

[ole_hjalmar_kristensen]

Use the negative holder to get the negative off the glass. Or use a wet mount kit, which can be a bit messy.

[chris_waller]

Newton's Rings are the result of half-wave interference. The dark fringes are where the waves of light are 180 degrees out of phase and cancel out. Anti-Newton glass has an etched surface which is microscopically uneven and scatters the light, thus preventing the interference.

[Mike Gammill]

Or think of it as alternating bands of constructive and destructive interference. You can demonstrate this by holding your thumb and index finger close to an eye and gradually bring them together just short of contact. Have a light source behind them (fluorescent works best) and if you're careful you will see dark bands. You should send that scan to a sciece teacher to share with students. Teachers always like to have multiple examples of physics phenomena.

[larrydressler]

But to avoid them just don't let the negative or positive touch the glass I have seen them with glossy paper on a Scan. first time I ever got one was with a glass negative carrier using some Kodalith film in 4x5. Anti newten glass was something I thought made apples go back into a tree ..... LOL we live we learn I am so glad that scanners returned this thought.

 

AN glass though is much cheaper than it used to be otherwise it would have been used in the days of curley film I remember how curley some films were in those days...

 

Larry

[roman_iazovitch]

thanks people. i know the physics now. Larry, using the epson's holder is not an option for me as i am after the optimal sharpness. so, i have to figure out how to bring the negative higher above the glass for just a fraction of milimeter. that's a tough one : ) how do you do that?

[menashe_soffer]

this only happens if the negative curl.

for some of the worst negative I scanned, I cut 2 pieces of carton and placed them below the negative, above and below the frame.

not very convenient and you can't scan a complete row in one shot but it flattens the negative to an extent that prevents the rings.

[roman_iazovitch]

well, what i do is place the epson's 6 by 9 frame on top of the negative, which allready appears as flat to the eye as it can get... but apparently, it is not sufficient : (

[the world in black white]

Roman: Why do you think you won't get "optimal sharpness" using the the negative carrier that came with your scanner? I often use a 4490 scanner with the associated carrier and am almost always quite pleased with the results. Admittedly I do sometimes sharpen the scan in Photoshop, but that is seldom. (I use Delta 100, FP-4, or Acros 100 developed in 510-Pyro most of the time, so I don't have any inherent grain to contend with.)

[john_shriver]

To get the most resolution out of your Epson scanner, see http://www.betterscanning.com.

 

I have their large-format adapter for my Epson V750, I'm very happy with it indeed.

[patrick_gainer]

Try 1200 dpi and see what happens. 2400 dpi is probably about twice the reso;ution of the camera lens. Scanning is a sampling process. The patterns you see are interference patterns. There is probably a microscopic lump of something between film and glass in the middle of the pattern.

 

In a sampling process, there is a phenomenon known as "foldback" which causes a sinusoidal waveform of frequency (in this case, spacial frequency, but it also happens with sound) beyond the resolution of the system to be seen or heard at some multiple of its wavelength. Sometimes, in trying to get the highest resolution, one only succeeds in distorting the wave form. In the audio world, it is better to use an amplifier that has a bandwith greater than human hearing than to try to emphasize the highs of a cheaper amplifier.

[roman_iazovitch]

<p>Patrick, you are right ofcourse. I did not tell you, I am an electronics engineer. So, I know, what is sampling and familliar with aliasing and interference issues. But i don't think going to 1200dpi will help it. Maybe I'll not see those rings, but the scan will suffer even more. On the contrary, it is better to higher the sampling resolution and then downsize it using "bicubic" in photoeditor. I believe, it is in parallel with using a wider bandwidth amplifier if we go with your example.<br>

Donald, I tried that. And was very disappointed with the result. IMHO, you absolutely have to put the negative to the focal plane of the scanner optical system if you want a scan to be in focus. So you put it onto the glass just like you put a sheet of paper during reflective scanning. I scanned it like that and it was much better. Try it. The only issue with it are those Newtonian rings brought by the negative being too close to the very flat glass. Also, when you scan a big format sheets, you do not notice the lack of sharpness as you do with 6x6 or 135 wich is even worse.<br>

John, thanks for the link. Maybe eventually I'll go for it. But only if I am unable to find another way around.<br>

I think I'll try putting another glass on top of the negative. When I find one, that is...</p>

[helenbach]

<p>Newton's Rings are not created by the sampling frequency of the scanner, so as you say they will not be affected very much by changing the sampling frequency - unless it reduced so much that they are not resolved of course (neither the ring frequency nor the wavelength of light are close to the scanning frequency in the image you show).<br>

Remember that the concept of 'flat' is fairly severe if you want to get flat enough to avoid rings: the film-glass separation alters by 1/4 of a wavelength between the centre of a dark band to the centre of a light band. The usual answer is to roughen that interface - by using AN glass, or putting the emulsion side to the glass.Trying to flatten the film between two smooth glass plates simply results in two film-glass interfaces - one of which is the smooth base to the smooth glass.</p>

<p>Best,<br>

Helen<br>

PS 2400 ppi seems a bit low to be twice the resolution of a camera lens. 2400 ppi is similar to 50 lppmm. Half that is only 25 lppmm.</p>

[lex_jenkins]

<p>Occasionally I'd get Newton's rings even with ANR glass on my Durst M605 enlarger, and only with medium format TMX.  To relieve the pressure slightly without sacrificing too much flatness, I'd use a bit of masking tape between the split halves of the Durst easel-type negative carrier.  Always did the trick.  You might be able to try a similar trick with a scanner.  The house painter's blue masking tape is good stuff, peels up cleanly with no residue.</p>

[roman_iazovitch]

<p>Helen, yes, the sampling frequency is not involved here. I will conduct an experiment while laying the negative emulsion down and up, than see what happens. Hmm, i thought here it was facing down, but now i am not shure.<br />Lex, i'll try that too. Thanks!</p>

[gt1]

<p>"IMHO, you absolutely have to put the negative to the focal plane of the scanner optical system if you want a scan to be in focus."</p>

<p>And the film holder puts the film at the focal plane for scanning transparencies, right?  At least on an Epson scanner whether you are using a holder or not tells the scanner whether to focus on the glass or a couple of millimeters higher.</p>

[roman_iazovitch]

<p>Hm, Greg, how does the scanner know if i am using a holder? And why do i get better focus when i put the negative on the glass w/o the holder?</p>

[gt1]

<p>The holder covers up certain lights depending on the film holder used.  The scanner then knows where to focus.  At least that's the way it works on a V700, and I would think for the 4490.  I wouldn't think a scanner manufacturer would sell something that purposely doesn't focus on the target.</p>

<p>On the V700 the full area 8x10 template is used for on glass focusing, and the various holders are for  focusing above the glass.</p>

<p>I don't know why you don't get good focus with the holder.</p>

[patrick_gainer]

<p>Yes, you are right. Sampling frequency should be at least twice the bandwidth of the waveform being sampled in order to prevent foldback. If there is any such effect, I think it will be in the images of the grains,<br>

Such patterns as you see are used by lensmakers to determine how well a lens surface matches a test glass. In that case, you hope for nice circular patterns. If the distance between the negative surface and the scanner surface is either everywhere zero or everywhere greater than a wavelength of the light the scanner sees, you should not get the patterns. Most scanners have some depth of field. I have some ancient glass plate negatives made by my grandfather that I can scan either face down or face up with no noticable difference in resolution. Photographic prints are, of course, contact printed with emulsion sides together, and these plates make very sharp contact prints, so I know the negatives themselves are very sharp.</p>

[roman_iazovitch]

<p>Greg, when i use those 6x6 holders that came along with the scanner (5.3x5.3 actually), the bundled software crops the image to 5.2x5.2 which is frustrating and not acceptable for me. also in the software menus i just choose wether i want to scan 6x6 or 6x9 or other sizes, so i am pretty sure the scanner doesn't guess the holder size or type, but the program chooses the crop area according to the supposed holder placement and the size chosen by you in the menu. so, i use vuescan software, that overrides all that and gives me the whole transparent scan area for manual cropping. it only prooves my previous words. then the focusing issue... why bother with most intricate mechanism, that is failure-prone, if you can make the scanner focus only on the glass surface and then bring all the subjects into that plane? or, as Patrick said, go for a wider depth of field. and i am sure, that's what they do, especially considering 4490's and v700's modest price. but that depth apparently isn't sufficient for that 3mm high 6x6 holder.<br />you can experiment with this. i've read that v700 is built over 4490 with some modifications. but not such drastical.</p>