Why do film scanners offer better results than flatbeds at the same resolution?

[marc_schmidtmayer]

Hi all, sorry about my question but I have not found a 'real' answer

on the matter ...

 

Myself, I have an Epson 3200 flatbed scanner. But when I ask

something about scanning 35mm film, the answer I get is that I should

use a 'dedicated' film scanner ... and not a flatbed. Regulary,

people suggest the Minolta Dual Scan III because of its price ...

 

But when I read the specifications of both scanners, I see that the

Epson has got an optical resolution of 3200 dpi and the

Minolta 'only' 2820 dpi ... and a dpi is a dpi ...

 

So, where do I go wrong ? What should I now about this matter ?

What's the difference between scanning on 2820 dpi on a flatbed- and

a filmscanner ?

 

Maybe for you (experienced) people it's crystal clear ... but for

a 'beginner' it's not ...

 

Thanks, Marc.

[kelly_flanigan1]

A flatbed at 3200 dpi/ppi is not a sharp as a dedicated film scanner at 3200 dpi/ppi. Film scanners have a lens that focuses on the film. Flatbeds have a scan bar; that has alot less sharpness.

[jay_de_fehr]

Hi Marc. The short answer is that there is more to a scan than its dpi. The quality of the optics,the maximum density that the scanner can read, film registration, scanner software, etc. all contribute to the resulting scan's quality. As with most things versatility represents compromise. A dedicated film scanner does one thing, and so it is optimized for that one thing. I hope that helps.

[robert_martin5]

Dpi is determined by the number of elements in the image sensor in one direction and the stepping motor steps per inch in the other direction. The Epson 3200 generates 3200 dpi, but the lens system in the flatbed scanner is fixed focus and does not have adequate resolution to support 3200 dpi. Basically you get a fuzzy image that is digitized at 3200 dpi. A film scanner that has a better lens that focuses on the film will give you a sharper image even though it may not have a dpi rating as high as the Epson 3200. I found that 35mm scans at 2820 dpi on a dedicated film scanner had considerably more detail than scans of the same image on the Epson 3200 at 3200 dpi. Medium format scans at 1128 dpi on a dedicated film scanner were better than scans of the same image on the Epson 3200 at 3200 dpi.

[leonard_evens]

The optical scanning resolution tells you how many samples are collected. It is often called dpi for 'dots per inch' but it should really be called ppi for 'pixels per inch'. But it doesn't tell you how much information actually gets to the sensors which produce the pixels. That is usually measured in lp/mm or 'line pairs per mm'. You've probably seen bar patterns used to measure resolution of lenses. These come in patterns of dark bars separated by white spaces of equal width. Each pair of a bar and the space next to it (say on the right) is called a line pair. From digital processing theory, you can resolve in lp/mm at best half of what you sample in pixels per mm. 3200 ppi amounts to 3200/25.4 or about 126 pixles per mm. The best you could expect is actual resolution would be about 63 lp/mm. However, no scanner has perfect optics, so the actual resolution is somewhat less than the theoretical resolution. For a high quality scanner it is closer to the maximum than for a lower quality one. Also, if the total number of pixels scanned is smaller because the format is smaller, it is easier to make sure the optics are better. The Epson is designed to scan up to letter size sources, while dedicated 35 mm scanners are designed just to scan something 24 mm wide. If you look at scanner prices, you see that the price goes up rather steeply as the size of the area scanned goes up, while the quality of the scan is preserved. Compare for exmaple the Nikon 4000ED and the 8000ED.

 

From actual tests, most of us have found that the Epson 3200 can deliver at best 28-30 lp/mm when used with typical cameras and lenses. The actual resolution of the scanner is probably somewhat higher, say 34 lp/mm, but when different components are combined, the resolution is always less than the lowest resolution component. It is often assumed that the human eye can resolve about 5 lp/mm when viewing a print at 10-12 inches. That means the Epson can provide images at 28-30 lp/mm which can be enlarged at most 5-6 times to still produce a a print (or other image) which will be viewed close up. For 35 mm, this means you can produce at best a good 5 x 7 print. It can be enlarged more, if the enlarged image will be viewed proportionately further away. Some people think the 5 lp/mm is too low, and claim they can resolve close to 10 lp/mm close-up. Such people would be even more demanding of the scanner.

[jerry_freeman1]

It actually shoud be called SPI, scans per inch. A scanner neither scans dots or

pixels...jf

[jim mucklin]

Marc Start with this <a href="http://www.scantips.com/">site<a/>. It really give a lot of info. There is a lot of info in a negative, so the higher you can scan the more detail you can get to a print. If you scan a print on a flatbed your limited to what you have to work on. Scanning for prints in one thing and scanning for the net(video, monitor) is something else. Good luck the best help you will get will be here on this site.

[upscan]

Marc: Leonard Evens said it very well. As you see, a dpi is not a dpi. People that market flat bed scanners do not tell you that, because they would rather have you take in the hook and the sinker and buy the Epson flat bed they make and not a film scanner they don't. Several people in this forum often estimate the resolution of the E3200 as merely 1600 to 1800 dpi. I too think it is in that area. That means that on MF it is barely OK and on 35mm all you will get is a 5X7. Most people coming fresh into scanners only look at the claimed dpi and buy on that basis. As already stated in this thread, there is more to scanners than DPI. The richness of the tonality you get with top level scanners will not be there in the flat beds. It is no different than with camera optics. The image you get from a Leica or Zeiss lens will be different that what you get from a disposable camera. Look upon flat bed scanners as the disposable cameras equivalents, .......in spite of the price difference.... and the inflated dpi.

[leonard_evens]

Perhaps I should add that I use the Epson 3200 for 6 x 7 and 4 x 5, and I am happy with the results, both for resolution of fine detail and tonality. Of course, I would get better results from a Nikon 8000ED (for medium format) or an Imacon for 4 x 5, but these are priced at close to $3,000 for the Nikon and over $11,000 for the relevant Imacon.

Given its price, the Epson 3200 is a good choice for the formats I use it with, but not for 35 mm.

[kelly_flanigan1]

Ther term dpi; or dots per inch has been around before Photoshop; or even digital photography. We used it on specifications for scanner bar sensors in thermal fax machines two decades plus ago; in engineering documentation; testing; and even patent filings. We used dots per inch and lines per inch terms to mark the scanning resolutions. One of the terms is the actual sensor pitch; the other is the stepping; ie the paper travel direction. Many of these faxes are at a nominal 200 dpi as written by marketing crews; but was actually 1/8 mm; 8 dots per mm. This was actually 8 * 25.4= 203.2 dpi. Many photographic writters seem to be abit confused about dpi; and ignor earlier scanners specifications. Early literature in scanning does call each pixel a "dot"; this is mentioned in patent info decades ago. Today the common person gets printer dpi and scanner dpi confused; this is why the writers try to rewrite ancient scanner history; and call it ppi today.

[kelly_flanigan1]

Marc; flatbed scanners are not as good as dedicated film scanners; at a given "dpi". Here I have about 8 scanners; the film scanners are always better than the film scanners; at a given "dpi".

[upscan]

Kelly: In your last post I think you meant to say that flatbed scanners are not as sharp as dedicated film scanners at the same DPI. (rather than film vs film).

[qiang_lin]

Marc, there are several reasons.

<ol>

<li>Most flatbed scanners do NOT have the resolution as they claim.

<li>Most if not all flatbed scanners do NOT have the ability to adjust the focus. This is one of the resons why they can't achieve the resolution as I stated above.

<liThe sensor and the ADC may not have enough dynamic range to capture your images on the film. They are good engough for scanning prints, which, by the way, you don't need that much resolution (300 dpi/ppi should do the work).

<li>It makes engineering easier if you optimize for one particular target, i.e. scanning film in the case of film scanners. There are a lot of thing to be worried for the engineers designing large flatbed scanners so they have to compromise.

</ol>

[kelly_flanigan1]

Thanks Julio; for catching my error!<BR><BR>A flatbed at the "3200 dpi software setting" does output an actual 3200 pixels/inch digital file. But each pixel one sees in the flatbed scan doesnt track as well with the reality of ones negatives details. A 1/3200 inch object perfectly aligned to one's flatbed scanbar may trigger 3 or 4 pixels on the scanbar; and thus one gets a much lower resolving power image. Many flatbed can be scanned at 3200 dpi; and then the image down sampled to 1600 pixels/inch in Photoshop; and the image will appear to not loose much real detail. On can place a negative at an angle; so part of it hits the glass; and another part of it is several millameters high. Here the best focus position can be detected. In my tests flatbeds at the best focus position still have only 40 % to maybe 65 % best case resolving power of a real dedicated film scanner. Here I have 300, 600, 1200, 2400, and a 3200 "dpi" flatbeds; and 2720 and 4000 "dpi" film scanners. <BR><BR>Getting hung up on the "dpi" spec alone is abit dangerous. Flatbeds are of great value; when one learns their limits; many times they are "good enough" for many applications.