Enlarging problems

I send photos for printing to either our local Meijer, Wal-mart, or Walgreen when it is just for myself. When ordering, I sometimes go into their poster menu just to see how large a size I could print. They give you a warning when the selected size is too large due to resolution.

Many times the warning will say to not go larger than 16x20 or sometimes 20x24. I understand it when I have cropped the heck out of an image, but I thought with the resolution of these cameras the prints can go much larger.

Do I need to resize the image in PS first? I'm just not understanding this at all.

Many commercial print making establishments have a guideline that there needs to be a minimum of 300 dpi in order have good quality output which will not be disappointing to the person ordering the print. Take your pixel dimension in one direction (e.g. width) and divide by the number of inches of print size in the same direction; for example, 2592 pixels / 16" = 162 dpi...too few for 300 dpi minimum, so resizing to 185% would be necessary in order to have 4800 pixels across the 16" print dimension.
Not all commercial printers insist on 300 dpi, some do, some don't.

Thanks for the information Wilt. So, using your example, can I re-size in PS to 185% and not lose anything as far as resolution etc in the print?

Resizing to a larger size will add pixels by digitally "dithering" them -- spreading out detail over more pixels than your original. It is a normal part of printing, but the quality depends on the software doing it. It is frequently fine when done by the print provider (those who don't specifically require say 300 ppi or 360 ppi).

But, the advantage of doing it yourself is that you have both more control over the quality of your enlargement and also you can apply "output sharpening" that is "tuned in" to your actual print size.

So, there is some personal preference that can come into play as well as quality considerations that typically need to be adressed by contact with the print provider(s) and doing test runs trying different approaches.

DVW wrote in post #11996314
Thanks for the information Wilt. So, using your example, can I re-size in PS to 185% and not lose anything as far as resolution etc in the print?

Personally I rather choose nice round numbers when resizing...for example starting with 2592 pixels, 150% = 3888, and 200% is 5184, but 160% = 4147.2 or 170% = 4406.4 ! So run it thru a calculator first, to see if the product is a nice integer.

If you consider the ability of the human eye to resolve detail, for a 12x18 inch print viewed at 18 inches, you need about 240 pixels per inch. The figure of 300 pixels per inch was created years ago by someone who did not know much about human vision. To resize, you can use Photoshop with bicubic smoother to enlarge the image appropriately.
For larger prints, you still need about 240 ppi if you want to view them at 18 inches. However, if you want to view them at a greater distance, you can get away with fewer ppi. For example, if you want a 16x24 inch print and want to optimize it for viewing at 24", you can use as low as 180 ppi. Whether the photo houses will be happy with this, I don't know, as I do all my own printing.

To preview how a file will print when enlarged in Photoshop, simply go to the image tab, select "image size", change the "document size" to whatever size you'd like to print at, then click "actual pixels"
Generally, if it looks good at that view, it will print well.

Thanks everyone, I'm starting to understand this a little more. Will have to experiment with it some this weekend.

What a great forum. Everyone is so helpful and willing to share their knowledge.

JEC wrote in post #11997960
To preview how a file will print when enlarged in Photoshop, simply go to the image tab, select "image size", change the "document size" to whatever size you'd like to print at, then click "actual pixels"
Generally, if it looks good at that view, it will print well.

I dont see the option, "actual pixels" in CS5

look on the view menu or ctrl+1

Thanks

JEC wrote in post #11997960
To preview how a file will print when enlarged in Photoshop, simply go to the image tab, select "image size", change the "document size" to whatever size you'd like to print at, then click "actual pixels"
Generally, if it looks good at that view, it will print well.

This isn't clear. I may be missing something in what you are suggesting. From what I know viewing "Actual Pixels" will show the image at "100% Zoom", whereas viewing the "Print Size" will show the "approximate" print size (when the image is set to dimensions in inches). There can be a huge difference between the two unless the print resolution is about 100 ppi...

Or you may be getting at something I'm not aware of?