DPI and upsizing

[Geoff]

Hi all,
I'm hoping to be printing my first panorama soon.
Getting it ready for print, I notice that it's currently at 240pixels.
Am I able to up that to 300DPI without any harm? I notice that the lab are happy with anything more than 240DPI but I noticed that when I changed the image from 240 to 300 it (naturally) increased the image size. I hope my question makes sense.

[Matt. K]

Geoff
How big are you printing?The truth is that there's almost no visible difference from printing at 180/220 or 300 DPI! I went to a large format Canon printing workshop and they were printing at 120 DPI! I'd leave them at 220 and relax.

[ATJ]

Increasing the resolution should decrease the size of the printed image, if you change the resolution without changing the number of pixels in the image.

For example, if you have an image that is 3000x2000 at 240 DPI it would print at 12.5xx8.33". At 300 DPI, it should only be 10x6.67".

I don't know about Photoshop, but PSP 9 lets me change the resolution from the default of 200 DPI (D70) to 300 DPI while maintaining the width and height in pixels.

[Geoff]

Hmmm...I'm sure that when I increased the DPI the image size also increased but I will take on board what's been suggested and play around a bit more. Cheers ppls.

[ATJ]

Hmmm...I'm sure that when I increased the DPI the image size also increased but I will take on board what's been suggested and play around a bit more.

It would have if you "told it" to maintain the print size.

i.e. to print a 10x8 at 300 DPI requires 3000x2400 pixels, but the same size output only requires 2400x1920 pixels at 240 DPI.

[Matt. K]

Geoff
Here is how PS handles an upres....from the menu 'IMAGE SIZE' if you change your resolution from say 220 dpi to 300 dpi and check the 'RESAMPLE' box then PS will not change the physical size of your image. It will figure out how to maintain that size whilst it fiddles with the number of pixels in the image. If you leave "RESAMPLE' unchecked then it will make the physical size of your print...smaller in this case or larger if you downres. Be aware that if the software is resampling the image it is interpolating pixels and there may be a very slight loss of quality. Many folk are confused with DPI in relation to image size so here's an attempt to clarify the picture.
A digital image has 2 sizes and they are not related in any way. There is the pixel size...IE This image has 3 x 3 pixels so it has 9 pixels in total. It is a 9 pixel image. But...We can print our 9 pixel image 2 x 2 inches, 21 x 21 inches, 45 x 45 inches....in fact, any damned size we please! However, the bigger the size then the bigger each pixel gets. When you alter the size of an image in PS then you have the option of retaining the current number of pixels and simply make the pic bigger or smaller...or....increase or decrease the number of pixels without changing the images size or increase or decrease the number of pixels and change the image size. I hope I got this the right way around...I've had a big day.

[Geoff]

Thanks Matt (and ATJ) I comprehend