Epson V700 vs 4990

[Leonard Evens]

Ben,

I'm sorry if I made it too complicated. In effect, 'dpi' for a scanner comes down the same thing as 'spi' or 'ppi' . Any distinctions are too subtle to be concerned about in practice. If you want to call it 'dpi', do so. The reason for making a distinction is that 'dpi' is also used in descrbing printer resolution, where it means something very different.

An interesting question, which others might comment on is just what happens when you ask your scanner to return pixels at less than the specified optical resolution. Since you end up with fewer pixels in each line than there are sensors, there has to be some averaging done. Just where is this done: the scanner firmware/hardware or the scanning software? My conjecture is that the hardware will return half multiples of the optical resolution. So a 4800 spi scanner can return 4800, 2400, 1200, etc pixels per inch, and to return something intermediate, the software writer, e.g. Silverfast, has to do some further programming. I can pretty much see what the software would have to do, but I am not clear what the hardware/firmware combination in the scanner would do. For example, apparently the Epson V700 has a multiple lens system and does different things if you scan at 6400 than if you scan at 3200.

[bmarcin]

Thanks, Leonard. This thread raises an interesting question. I had always been told that I really didn't need to scan any of my medium or large-format chromes at greater than 300 dpi because a printer won't "see" or "process" anything beyond 300 dpi; in other words, I wouldn't achieve any significant detail/resolution beyond 300 dpi that would be worth the larger file sizes. So, I've always scanned my chromes using the DPI setting of 300 using my SilverFast driver. I have some very nice color 30x30" prints made from an Epson 9600 printer (not by me) hanging on the walls. However, on this thread there's a lot of discussion of using very high spi (or dpi) settings: 2400, 3200, etc. Am I being mislead about not going beyond 300 dpi on my scans or am I misunderstanding something completely and describing a totally different situation?

Thanks,

Ben

[Ron Marshall]

You should scan such that at your intended print size you will have 300 dpi (or 360 dpi for Epson printers).

For example: You want to make a 16x20 from a 4x5 negative. That is a four times linear enlargement, so you scan at four times 300 (or 360) that is 1200 dpi.

[Doug Fisher]

Ben -

As mentioned previously, the www.scantips.com website by Wayne Fulton has great tutorials which will help you get a grasp on the difference between scanning ppi/dpi and printing ppi.

Doug
---
www.BetterScanning.com

[Henry Ambrose]

Ben,

Watch your settings in Silverfast. There is a thing they call "quality factor" (I think its called) that has to do with offset printing line screens. Set this to "1" to effectively disable it. (1 x any other settings= any other settings)

I second www.scantips.com as a good resource.

A handy way to keep this straight is to consider your print size and the resolution needed by the printer you are using. A 10 inch wide print from a printer that works best when fed at 360ppi, (spi) requires 3600 pixels to print that 10 inch wide print. (360X10=3600) You'd want to scan so your file contained those 3600 pixels (and a little more so you can crop). If you were using 2 inch wide film you'd need to scan it at 1800 ppi (2x1800=3600).

So when wondering about how big to make the scan, think about how big you might want to print your picture. Then figure backwards from there to arrive at the scanning resolution.

Thats the simplest way to work with scanning I've found.

[Leonard Evens]

Ben,

I agree with what the others have said about the relation between print size and resolution of the image. But let me expand.

The image stored in your computer just has pixels, e.g., 8000 x 10,000. You don't get pixels per inch until you specify a size in inches. For scanning it would be the size of the scanned area. For printing, this would be the size of the print. To go from one to the other involves an enlargement factor.

There isn't a direct relation between pixels stored in your computer and dots of ink deposited by your printer. Each dot is a single color while each pixel consists of three numbers, one for each color R,G, or B. The number of different colors potentially stored in a single pixel is quite large and it is simulated by the printer by a collection of dots of different colors. The number of different colored dots can vary from 4 to seven or more.

But the printer, before figuring out how to distribute dots, converts the pixel array it is sent into its native resolution. You may have better results if you send it a simple multiple of its native resolution, i.e., 360 or 720 for an Epson printer. That is because your photoeditor, e.g. Photoshop, can probably do a better job of converting to the desired resolution.

On the other hand, when you scan, the scanner hardware will collect a certain number of samples per inch. To change to the appropriate resolution to send the printer, you either have to combine pixels or interpolate additional pixels. This can be done by the scanning software, but it is probably better to do it with your photoeditor. So most people recommend scanning at the highest feasible scanning resolution and resizing in the photoeditor.

Generally you want to resize to a smaller number of pixels. Increasing the number of pixels doesn't add any new information to the image because it is done by interpolating new pixels each of which is based on its surrounding pixels.

You should also keep in mind the difference between avoiding pixellation and preserving detail. If you send your printer too few pixels, you will be able to actually see the pixels if you look closely enough. Even if you can't see the pixels, they may still have subtle effects if too large. Usually it is recommended that you send a printer at least 240 ppi, but many people are more comfortable with 300 or even more as a lower limit. This is different from the issue of the printer's native resolution.

But pixellation is not simply related to resolution of detail in the print. 300 ppi, corresponds to about 5 lp/mm and a typical person may be able to resolve between 5 and 10 lp/mm in a print at 10 inches. So if you expect people to view your prints that close, you should not go much below that. But whether or not the image you send to the printer has that detail in the first place is another matter. Suppose the combination of lens, film, and scanner yielded a real resolution of 40 lp/mm (or about 2000 ppi). That would be at the size of the frame, about 95 x 120 mm for 4 x 5 . If you change the size of the final image, you have to divide by the enlargement factor. If you expect critical people to view your prints from 10 inches, you would not be able to enlarge by more than a factor of 4. If you anticipate less critical viewers standing further back, you could enlarge considerably more.

Just how the inherent resolution of the image combines with the pixellation is something I've never seen discussed, but you can't expect to do better than either factor by itself would predict. Most likely, the combined effect is even less than that.

[bmarcin]

Well, thanks very much for the education on dpi, spi and ppi. It seems like I misunderstood the business about scaling to size and, whenever I'd set my dpi in Silverfast to 300 dpi, the overall dpi would adjust for the target size. That answers my basic question here - plus I got a lot of other useful info on this thread. Thank you!

Ben