Scanned image size

[justthings]

<p>I'm looking into having negatives scanned, and I have been running into descriptions that tell me the size of the scanned image in terms megabytes. While generally I know that bigger is more, this doesn't really help me know just how big in pixel dimensions an image scanned by this service is and thus don't know how big a print I might be able to make from it. I've also seen file size used to describe scanned image sizes in internet forums. This seems like a sort of useless metric outside of a discussion about storage concerns and makes me wonder why is scanned image size reported this way? Here is an example from one company offering scanning services:</p>

<blockquote>

<p>provides the highest resolution, suitable for large format prints (approximately 50MB).</p>

</blockquote>

[travismcgee]

<p>I agree, that sounds like a pretty useless metric. They're probably assuming their customers don't understand the relationship between resolution and print size.</p>

<p>The last time I looked at sites like this, however, I found plenty that actually did tell you the resolution they would scan at and had different prices depending on the resolution you wanted. You might want to keep looking. Good luck!</p>

[mark_sirota1]

<p>Divide the number of megabytes by three -- that'll give you the number of megapixels.</p>

<p>It's an unfortunate result of Photoshop expressing "image size" in bytes. The "megabytes" they're quoting is the size of the image data, assuming an RGB file with 8 bits per channel. It's effectively the size of the image data in an uncompressed 8-bit RGB TIFF.</p>

[justthings]

<p>Thanks Mark - and knowing the megapixels I can determine the pixel dimensions. In this case that would be roughly a 16-17 megapixel image scanned from a 6x7 negative.</p>

[dave_redmann]

<p>Mark has given you basically correct info, but his "assuming" is a real issue. You really need to check whether the scan will be delivered 16 bits per channel or only 8. Lower-end scans are usually 8, but most labs doing better-quality scans of 6x7 frames will at least offer 16, and getting 16 can be a significant advantage if you want to change the color or density much. If it's 16, you divide megabytes by 6 (instead of 3) to get the <em>approximate</em> number of pixels.* Note that any semi-standard JPEG will be 8 bits per channel, so if the files will be delivered as JPEG's, divide by 3; note also that any JPEG will involve compression probably not much less than about 2.5:1, and often 10:1 or more, so the actual file size would be much smaller in terms of bytes, but not in terms of pixels.</p>

<p>So for exactly 50 MB, <em>if 8 bits per channel</em>: 50 * 2^20 / 3 = 17,476,267 pixels (approx.). If the 6x7 frame is exactly 56 x 69.5 mm (common) and the scan gets exactly the frame (no more, no less--NOT common), then: SQRT(17,476,267 * 56 / 69.5) = 3753 pixels (approx.) on the short side, and therefore 4657 pixels on the long side. <em>If it's 16 bits per channel</em>, you have only half as many pixels, therefore SQRT(2) as many on each side, or about 2653 x 3293 pixels.</p>

<p>*By longstanding convention based on the way computers address memory, 1 MB = 1 * 2^20 bytes = 1,048,576 bytes; but people almost invariably mean 1 MP = 1,000,000 pixels, so there's roughly a 5% discrepancy.</p>

 

[mark_sirota1]

<p>Dave is 100% right on all of that, I was trying to keep it simple. :-)</p>

[david_henderson]

<p>You're right, people do tend to size scans in MB and there are reasons for it. Perhaps the most significant of those reasons is that people print at different resolutions- depending on the size of a print, or the machine they're printing on. So for example for a smallish print- say 20" x 16" or less , made on a LightJet machine I'd probably make a print file at 300ppi. If on the other hand I was using an inkjet printer I might choose 360 ppi. And then if I were intending to make a large print of say 30" x 20" I might choose to make the print file 200ppi because larger prints tend to be viewed from further away. And if that large print was going to be viewed from especially long distances, even for its size, I might set the print file to be 150ppi. </p>

<p>So the fact is that for a scan of a given size in MB - even if you know the bits per channel -as Dave Redmann discusses above- you can make prints of all sorts of sizes and there's no fixed relationship between the size of a scan in MB and the size of print you can produce from it.</p>

<p>Your question is a bit like wondering why they sell milk in pints when that doesn't tell you how many mugs of coffee you're going to get out of it, but the world still turns because people intuitively understand that its going to depend how much they put in each mug, and how big the mugs are, what proportion take black and so on. And thew answer to your dilemma here is a bit like knowing that you have to make 300 mugs of coffee and wondering how much milk to buy.</p>

<p>And a lot of people do learn to work out what they can do with a scan of a certain size. For example a 50MB 8bit scan will make a 300 ppi print file of approximately (just a little smaller than) 16" x 12". But it will produce a lot of differently sized printfiles too. </p>

<p>And then there are other complexities created by the fact that you can "upres" - artificially increase the overall size of your scan in Photoshop or similar. Believe me, buying scans by the MB or milk by the pint is not the most complex of the issues that you face here. </p>

<p>The formula to calculate file size requirement for a print of known size and a desired resolution of 300ppi in the print file is (fingers crossed) </p>

<p>(Length in inches X300) x (height in inches x 300) x 3 = print file size at 8bit in<strong> bytes</strong></p>

<p>so</p>

<p>(16x300) x (12 x 300) x3= 51 840 000 bytes = 50 MB approx.</p>

<p>There are a few approximations here but they are not the kind of approximations that are going to hurt you.</p>

<p> </p>

[justthings]

<p>Again, thanks - the key piece of information I was missing is how to translate from something I consider to be volume (megabytes) to something I consider to be dimensional (pixel dimensions) and thus appear to be an apples to oranges comparison. I didn't understand the relationship, so thanks for taking the time and explaining that.</p>

[david_henderson]

<p>One of the other things you need to understand about scans is that not all megapixels are equal depending on what equipment is used to scan and who's using it. I'd much rather make a 16" x 12" print from a 50MB film scan (from say a Coolscan or an Imacon) than a 50MB scan from a consumer flatbed. The bigger the scan and the more likely you are to upres , the more true that becomes. </p>

[justthings]

<p>Oh, I understand - this is why i'm hoping to find a local company who can do this for reasonable cost - i'm not sure how much of my time and money i want to invest in scanning. I don't have a big back log of frames to digitize, and I've only just recently started shooting MF again so we'll see where that goes. Again, thanks for the information.</p>

[User_503771]

<p>One thing to consider is that in the early days of scanning, just about the only high quality scans available were drum scanners. It was -- and still is -- typical of drum scanner operators to present their wares in terms of file size, rather than resolution.</p>

<p>File size will not only reflect potential resolution, as noted above, but also how much time the scan will tie up the machine.</p>

<p>With a call or email to the vendor, this should be easy to sort out.</p>

[robert_k1]

<blockquote>

<p>Not sure which resolution will best meet your needs? Call Jim and he'll help you decide.</p>

</blockquote>

<p>Jim happens to be a PN member with a scanning service. Lots of helpful info at his site (a very L-O-N-G page, somewhat cluttered, but all there). He seems to know what he's doing and willing to work with his customers.</p>

<p>http://www.saugus.net/Photos/scanning.shtml</p>

<p>Disclaimer: I don't know Jim, nor have I used his service.</p>

[steve_fay]

<p>I too find the file-size descriptions less helpful than I want. Some companies who do scanning only refer to the dpi of their scans on their websites. It is important to realize that this is not dots-per-inch of the negative's size, but is related to inches of the output size the scanner for. 300dpi at 5x7 and 300dpi at 11x17 would be very different megapixel amounts, but not all of the scanning operations who advertize their dpi mention the output size their dpi figures apply to. Why do photographers want to translate the resolution back into megapixels? In my case, I am just want to know how this scanning resolution might relate to what I might have gotten with a good digital camera. It would be nice if all of the scanning services provided a comparable amount of detail about what they provide. Unfortunately, each time I look at one of their websites, I find myself setting up a different set of algebra equations to figure out how their product compares to others.</p>

[steven_clark]

Actually it's more like having a legitimate complaint that they're selling you milk by ounces of fat.

[mattb1]

Well, actually, for me I want the most out of a scan. So, what I really want from a scanning service is the best scan they

can make. That will different for different scanners, ask for more will not get you any more detail. So, talk to them and

find out what they would do to get the most out of the film.

 

It really is not a simple subject. I have seen sites quote MBs, MPs, dpi and ppi. A lot of times dpi and ppi are used to

describ the same thing, usually you can get what they are talking about from the context. If talking about scanning and

they say dpi without saying the print size they are talking about the ppi of the scan.

 

It use to be that the print resolution was kind of important, but any more using a good print driver or RIP will take care of

the resolution for you. Just send what you have and tell it the print size you want.

 

I would suggest sending a test scan to the service and see what they can do.

[lenny_eiger]

<p>I scan professionally. I sue a top-of-the-line, Aztek Premier 8000 ppi drum scanner. I refuse to quote prices in megabytes. I don't do a lousy scan for one price, a junky one for a little more and if you want a good scan, well ---- you have to pay a lot.<br>

I figure that if someone wants to make an 11x14 today, that tomorrow they might want to make a larger one and they should have to pay for a scan all over again. Further the scan should do its best to "archive" the image so that if the film were lost, or it faded, that as much of the image as possible is retained.<br>

For these reasons, I charge by the size of the original negative or chrome. I do 35mm and med format at 8000 ppi, 4x5 at 4000 ppi and 8x10 at 2666. Med format and above are in the neighborhood of 2-3 Gigs of data, which is enough to do almost anything.</p>

<p>Finally, scanning is about the operator. it should be someone who can listen to you and set up the scan based upon what you are trying to accomplish.<br>

Lenny<br></p>

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