Resolution Questions

Sorry if the topic of ppi is a bit repetitive guys, but looking around through a few threads still didnt quite answer my questions.

I know that a higher resolution means more pixels per square inch (or cm), and I know that you cant just get a 72ppi image and change it to 300ppi and expect it to look beautiful at large sizes...so heres my questions.

1) What is the resolution of an image before touching it in PS?
Say for example i take a photo with my Nikon D80 at 3800x2600 pixels.
I right click the file and "open in photoshop.
What resolution (in terms of ppi) is the image?

2) What would make this resolution change?
Say for example my camera's photos are in 300ppi resolution, i open that in photoshop and manipulate it a bit. When I open the "image size box" and I change it to 600ppi, why does my image lose size? Is it downsizing the image for optimal quality?

3) When do I need to resample?
Say Ive changed my 300ppi image to a 600ppi image and it reduces the size of the image. Is resampling done to increase the image back to its original dimensions?

4) My printer says that I should be printing at 600ppi, and I want to do this, but it lags my computer alot since the image is about 2 metres long by 1 metre high. If my image is that big do I need to have 600ppi resolution? Or can I get away with 300ppi in this case?

Thanks for taking the time to read and respond guys

As you say, there have been some excellent threads explaining resolution issues. I don't want to re-hash those, but I'll try to answer your specific questions as much as I understand them! I'm guessing that you're mainly concerned with Image resolution, which is what determines print quality.

1. When you work with an image on screen in PS, the image will be whatever resolution your camera (or scanner) captures it at. Obviously, the more pixels the better, but whatever you do in PS will look the same at 72 ppi and 300 ppi on screen. The resolution only becomes important when it's time to print.


2. Your image 'loses size' if you un-check the 'Re-sample Image' check box in the 'Image Size' dialog. This is because when you don't re-sample, you are telling PS to preserve every pixel in your image.

If you have an image measuring 3800 pixels wide at 300 ppi, the image will be 12.66 inches wide. (3800/300) If you change the resolution to 600 ppi, PS crams the same number of pixels into a smaller space, making the image 6.33 inches wide (3800/600).


3. That's right, you re-sample when you want to change the resolution but don't want to change the dimensions of the image. As a rule of thumb, it's usually ok to re-sample down in size, but you lose quality when you re-sample up. In the example you give, if you doubled the ppi, but kept the same image width and height, this would be a case of re-sampling up in size.


4. I cannot think of a good reason to print at 600 ppi unless your image consists entirely of fine line art. Photographic images are usually fine at 300 ppi. I'm not surprised your computer lags, the file size must be huge! Surely an image 2m x 1m is not meant to be viewed extremely closely and therefore could manage at a lesser resolution?

Well I hope I have helped a bit and not confused you more with these answers! It's not an easy thing to grasp, and I've just discovered it's not an easy thing to explain to somebody else!

Thankyou for answering my queries promptly and straight forwardly.

My printer advised me that to get optimum quality I should try and print my images at 600ppi. He said that 300ppi was that lowest resolution he would reccomend.

Im getting a test print today of a bit of my 2x1 canvas at 300ppi. Hopefully it will look fine, though I upsampled my images on the canvas instead of lowering the resolution to obtain a larger image, so Im not expecting it to look very nice.

Hopefully its adequate, though I wouldnt mind starting again if it isnt.
Thanks again