Twain 266dpi = Photoshop 266 ppi

T
Posted By
Tommo
Feb 10, 2004
Views
679
Replies
12
Status
Closed
Can anyone explain that when I scan an image in Twain, at 266 "dpi"(as Twain calls it), it is a Photoshop 266 "ppi" image(pixels per inch).

Whatever I scan it at, say 600dpi, that is 600ppi in Photoshop image size.

What is the difference?
The scanner presumably scans in lines? or points?

Anyway, when I then go to print, the Epson printer is in dpi, and I usually print at 720dpi, on photo paper, with good results.

What is the relationship between Twain dpi, P.S. ppi, and ink jet printer dpi?

If I change my image in Photoshop to, say 100 ppi, then the printed image is no good obviously, so changing the ppi changes the print quality, so what is the optimum(smallest file, best results) ppi for printing to an ink-jet printer?

Signed….Confused.:-(

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CW
Colin_Walls
Feb 10, 2004
You are far from alone in your confusion!

The scanner actually scans in dots. Each one acquires a [24 bit maybe] number for the colour it is seeing. This becomes a pixel in the image file. So, in this case, DPI=PPI. OK so far?

Your printer, on the other hand, only has around six colours, which it can print at a resolution of 720, 1440, 2880, or something like that, DPI. It does not have the 4 million colours needed to represent all the colours that each pixel needs. So it prints a pattern of dots for each pixel. So, in this case, there is no direct/predicatable relationship between PPI and DPI.

It is usually said that optimum PPI for printing on an inkjet is 300PPI. Typically you can go down to 225 or even 200 and get reasonable results. It depends upon the image. I did one recently at <100PPI, which was fine, but that was a slightly abstract image. Going much above 400PPI is unlikely to be beneficial. Actually, some printers start producing quite bad results if you send them too high a resolution. They try to use too few dots for each pixel and screw up.

Am I helping or confusing you further?
J
JasonSmith
Feb 10, 2004
Actually, the scanner scans samples per inch – it should be SPI. When the file is digitized, it is PPI, and printers print dots, therefore DPI.
T
Tommo
Feb 10, 2004
No you are very clear.
Thankyou so much !

I am printing this for reading later.
Thanks again

Signed….Unconfused:-)
CK
Christine_Krof_Shock
Feb 10, 2004
Oh great…add ANOTHER term (spi)to the dpi-ppi wars!!!!
CW
Colin_Walls
Feb 10, 2004
Christine

Although Jason is quite correct – the sensor in a scanner samples at the specified resolution – he is being a little pedantic. The difference is unimportant. Unlike PPI and DPI, where the difference is critical.
J
JasonSmith
Feb 10, 2004
Colin, I was just trying to give another example of how far off the mark the people at Epson are for even calling it DPI.

Thankfully both InDesign and Illustrator now export/save as/rasterize using ‘PPI’ instead of ‘DPI’….the DPI-PPI wars have been won.
CW
Colin_Walls
Feb 10, 2004
the DPI-PPI wars have been won

You are right. I can still remember when I was totally confused by it all. Hence my sympathy and, otherwise uncharacteristic, patience in trying to explain it.
MM
Mick_Murphy
Feb 10, 2004
I don’t really think Jason is being pedantic here. It is the incorrect use of terminology which causes all this confusion. It is much simpler to think of dpi only as printer dots. The resolution of the input scan is much better referred to as ppi rather than dpi, as samples per inch is exactly equivalent to pixels per inch and has no meaning until it is viewed in an image editor anyway. But I think this will continue to cause confusion for many years to come.
CW
Colin_Walls
Feb 10, 2004
Mick

On reflection, I agree. However, since [some] scanner manufacturers do use the term DPI, it’s worth pointing out that, in this context it is equivalent, for practical purposes, to PPI.
J
JasonSmith
Feb 10, 2004
Hey, I would be more than ecstatic even if the scanner manufacturers refer to it as PPI.

DPI is just plain wrong.
MM
Mick_Murphy
Feb 10, 2004
I think we all agree here and this thread wouldn’t exist if Epson and co quit using dpi in their scanner software. The film scanners I’ve used don’t seem to suffer this problem. Like Colin and most other people, I had to get my head around this once upon a time and it just annoys me that it continues.
J
johnkissane3
Feb 11, 2004
And will continue, forever.

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