I took some photos that I want to send to print.I open them in Krita and the resolution in image properties is 72-I want to change it to 300 but without affecting the image size.Is there a way to do it? and does it good practice to change the resolution without changing size?

Hi,

alinsh wrote:I want to change it to 300 but without affecting the image size.Is there a way to do it?

For what I know, Krita by default will "resample" pixels to keep the print size (mm, cm, inches...) which otherwise would be smaller (this goes for dimensions below 10000 px).
"Adjust print size separately" option won't resample pixels but will shrink the print size instead.

so you are saying that it's impossible to change image resolution without changing it's size in krita?

The thing is, what do you mean with "size"? -- width * height in pixels or in physical units like centimeters?

alinsh wrote:so you are saying that it's impossible to change image resolution without changing it's size in krita?

If I want to rise the pixels per inch without changing the physical size, then I need to interpolate the pixels (default scaling behavior in Krita).
That's because an image has a fixed amount of pixels, so if I fill an inch with 72 or 300 pixels I will get different lengths.

by size I mean physical size

What is the image size in pixels from your camera? I'd assume it was quite high.

When you send a digital image for printing, do you tell the printer what size image you want (e.g. 6" x 4" or A4, etc) or does the printer take information from the EXIF metadata of the image file and print at a size corresponding to that? I suspect that you tell the printer what physical size you want. It may be that you are using an advanced professional printing company that takes their printing instructions from image file metadata, in which case you should ask for their advice.

If you tell the printing company what print size you want, it's up to you to adjust the image resolution (size in pixels) to give a printed 300 ppi at the size of print which you tell the printer to make. This is a fairly simple calculation. For a 6" x 4" print at 300 ppi, you need an image size of 1800 x 1200 pixels. Whether or not this print would look any better or any different from the same size print at 100 ppi would depend on the quality and resolution of the original image. If it looks really good filling your computer screen then it will probably look good on an A4 print.

It's also up to you to decide if your image is of good enough quality and resolution to have it printed at large print sizes. Using scaling with 'smoothed' interpolation (such as bicubic, sinc, etc) can give good results when increasing the size and resolution of image but this is not guarenteed and it depends on the details of the original image and how far you push the scaling.

The ppi figure you get from Image Properties could depend on two factors. The first would be EXIF metadata from the original image file and hence the camera. This would be quite meaningless since a camera takes pictures, it doesn't print them. There may be a strange assumption, built in to the camera, about intended print size.

The second factor affecting the ppi would be Krita's assumptions about printed image size. These are obvious when you create a new image and you specify the physical size and ppi or you use an existing template. For an image that has been imported, these come from the image EXIF metadata. These physical size and ppi resolution figures are really just a guide/advice for intentions to print. The important thing is the image size in pixels and your printing size instructions to the printing company.