TIFF File Resolution Question

[laawaaris]

I have a TIFF file which is 15 MB in size and has a scale of 1950x2700. The print size for this file when I open it up in photoshop (Edit-> Image Size) is approximately 6 inches by 9 inches.

The file has a resolution of 300 DPI.

I need to to crop this photo so that it can be printed at 18"x24".

When I use the crop tool and constrain the Width and Height to those dimensions, the resulting file has a shocking resolution of 105 DPI.

I was not expecting such a loss in resolution.

Why does this occur? Does it have something to do with the CROP tool or with the fact that I am using a TIFF file?

[srwatters]

It has to do with neither. It's simple math.

If you take your image size in the long dimension (2700 pixels) and divide by the 24" size you want you get 112.5ppi. Doing the same in the short dimension (1950/18) yields 108.333ppi. If you want greater resolution you will have to interpolate the pixels. You are only starting with a 6MP image and that is why you have a low ppi on the up-sized image.

[laawaaris]

thanks scott

how do i interpolate the pixels? do i just change the Resample Image option to Bicubic Sharper before I hit "OK"?

[srwatters]

Here is what I would do:

- Use the crop tool set to the ratio you want (3in x 4in) but do NOT set the resolution
- Once you're happy with the crop, then choose image->size
- With the constrain proportions and resample image boxes selected, change the document size parameters to what you need (I'm not sure you NEED 300dpi for a 18x24" image, but that's another issue - viewing distance)
- select Bi-cubic smoother and hit OK

Your resulting image should be 5400x7200 (18x24 @300dpi)