Resolution - how much is enough?

[ChuckT]

In Understanding Resolution

"Inkjets
Most photographers do their printing these days with a desktop inkjet printer and the Epson Photo printers are the most popular so I'll use them by way of example. These printers, such as the models 870/1270/2000P are (somewhat misleadingly) listed as 1440 dpi printers. This means that they are capable of laying down that many dots per inch. But, to create a colour image they need to use 6 different inks, so any particular pixel reproduced on a print will be composed of some dithered composite of coloured dots using some or all of these inks. That's why you need more dots from your printer than you have pixels in your image.

If you divide 1440 by 6 you end up with 240. This is the true minimum resolution needed to get a high quality photo-realistic prints from a 1440 dpi Epson printer. Many user, myself included, believe that a 360 ppi output file can produce a somewhat better print. If my original scan is big enough to allow this I'll do so but I don't bother ressing up a file to more than 240 ppi when making large prints."

Ok so for a very large print, 40x60x240, that's a 395.4 MB file.

Is that correct?
Is that practical?
How long would that take to print?

cvt

[Gemmtech]

In Understanding Resolution

"Inkjets
Most photographers do their printing these days with a desktop inkjet printer and the Epson Photo printers are the most popular so I'll use them by way of example. These printers, such as the models 870/1270/2000P are (somewhat misleadingly) listed as 1440 dpi printers. This means that they are capable of laying down that many dots per inch. But, to create a colour image they need to use 6 different inks, so any particular pixel reproduced on a print will be composed of some dithered composite of coloured dots using some or all of these inks. That's why you need more dots from your printer than you have pixels in your image.

If you divide 1440 by 6 you end up with 240. This is the true minimum resolution needed to get a high quality photo-realistic prints from a 1440 dpi Epson printer. Many user, myself included, believe that a 360 ppi output file can produce a somewhat better print. If my original scan is big enough to allow this I'll do so but I don't bother ressing up a file to more than 240 ppi when making large prints."

Ok so for a very large print, 40x60x240, that's a 395.4 MB file.

Is that correct?
Is that practical?
How long would that take to print?

cvt

Sounds easy doesn't it?  Actually it's not accurate to state that there's 240 dots of each color in each DPI, IOW it can be any combination.   And for a very large print 180ppi will do the job.

[JeffKohn]

In Understanding Resolution

"Inkjets
Most photographers do their printing these days with a desktop inkjet printer and the Epson Photo printers are the most popular so I'll use them by way of example. These printers, such as the models 870/1270/2000P are (somewhat misleadingly) listed as 1440 dpi printers. This means that they are capable of laying down that many dots per inch. But, to create a colour image they need to use 6 different inks, so any particular pixel reproduced on a print will be composed of some dithered composite of coloured dots using some or all of these inks. That's why you need more dots from your printer than you have pixels in your image.

If you divide 1440 by 6 you end up with 240. This is the true minimum resolution needed to get a high quality photo-realistic prints from a 1440 dpi Epson printer. Many user, myself included, believe that a 360 ppi output file can produce a somewhat better print. If my original scan is big enough to allow this I'll do so but I don't bother ressing up a file to more than 240 ppi when making large prints."

Ok so for a very large print, 40x60x240, that's a 395.4 MB file.

Is that correct?
Is that practical?
How long would that take to print?

cvt

This relationship between pixels in your image and the physical DPI of the printer just doesn't exist in the way described above. That description is pretty much nonsense, especially since a lot of the newer printers are capable of variable dot sizes and/or 2880 dpi.  Most of the high-end printers also have more than 6 inks now.

240ppi of native image resolution (eg no interpolation) will indeed give you excellent results on today's inkjets. I don't think anybody would be able to differentiate 240ppi versus 360ppi except maybe with a loupe. And you can get by with quite a bit less than 240 and still make very good prints, although exactly where the cutoff is will vary depending on who you talk to and what type of image you're printing.

[Ray]

It's interesting that Qimage has a default setting of 180 dpi for poster size prints, although the option is available to increase this to 360 dpi. For 'normal' size prints, the default is 360 dpi.

I see no disadvantage in using 360 dpi for any size print, but maybe there are some. Since Qimage does all the interpolation automatically, it's not an issue for me, except I would expect to see a hint of jaggies on a large poster at 180 dpi, if I pressed my nose against the print.

[JeffKohn]

It's interesting that Qimage has a default setting of 180 dpi for poster size prints, although the option is available to increase this to 360 dpi. For 'normal' size prints, the default is 360 dpi.

I see no disadvantage in using 360 dpi for any size print, but maybe there are some. Since Qimage does all the interpolation automatically, it's not an issue for me, except I would expect to see a hint of jaggies on a large poster at 180 dpi, if I pressed my nose against the print.

QImage queries the print driver for its 'native' resolution and uses that for normal prints (and 1/2 for poster prints). The exact number you get will depend on the printer you're using.

But I was talking about how much native resolution you need, eg before any interpolation takes place.

Whether it makes sense to interpolate to the print driver's default resolution as QImage does is really a separate issue. IMHO it can't hurt, although interpolating to 720ppi or 600ppi as QImage does with some desktop printers is pretty stupid IMHO has you're not going to be able to tell any difference from if you'd used 360/300. All that does is just increase print spooling times.

[Ernst Dinkla]

It's interesting that Qimage has a default setting of 180 dpi for poster size prints, although the option is available to increase this to 360 dpi. For 'normal' size prints, the default is 360 dpi.

I see no disadvantage in using 360 dpi for any size print, but maybe there are some. Since Qimage does all the interpolation automatically, it's not an issue for me, except I would expect to see a hint of jaggies on a large poster at 180 dpi, if I pressed my nose against the print.

To avoid a misunderstanding, Qimage uses PPI for the input resolution and you are free in the quality choices from 75 to 300 PPI for posters on for example the Z3200. That doesn't mean the printer will print at 75 DPI. The print resolution in DPI (300, 600, 1200) + print quality (weaving strokes, uni or bidirectional etc) is set in the printer driver, the driver will compute what it likes for input resolution based on that and Qimage will pick up that input information and use that as a maximum for the extrapolation, again in PPI. If you do not want to use the upsampling of Qimage and prefer to use that of the driver you can switch the extrapolation off in Qimage. If you want Qimage to do less than the max you keep the extrapolation level at low and it will do the job at say 150 PPI  send it to driver and the driver will do the last 150>300 PPI input resolution upsampling if it expects 300 PPI input. So you are free to select a lower resolution in PPI than the driver requests. If the paper coating quality doesn't bring better image quality with a 600 PPI input than with 300 PPI input then it is up to you to set the driver print resolution and quality at a a level that asks for 300 PPI input. Both the Z3100-Z3200 and Qimage give ample information on what input resolution is asked in the different settings. Qimage has some test images on its site to test what the papers you use can deliver in image quality.


met vriendelijke groeten, Ernst Dinkla

Try: http://groups.yahoo.com/group/Wide_Inkjet_Printers/