Raw file size

[framed]

I recently started shooting in RAW format (and am now wondering why I haven't always done so!) My question is this - the files are 350 dpi but the actual document size in inches (13.5 X9) is smaller than the jpeg files (66 x44)I have been working with in the past, will this affect the ability to enlarge the images say to large wall portrait sizes? Why are they so much smaller in inches? Is this due to how I am converting and saving them?

[dcclark]

Image files have no inherent DPI -- it's just a number stored in the image which acts as a "hint" to image editors. It doesn't matter whether the file is RAW, JPEG, or what. So, you can safely ignore the whole DPI problem.

You can print any image you like at poster size -- most photo processors only ask for your image file, and your desired print size, and they'll MAKE them fit that size. Again, the DPI "hint" in the file doesn't mean much.

The main useful numbers for determining whether an image will look decent at large print sizes is the number of pixels per side. You can divide (# of pixels vertically) by (desired image height in inches) to get an estimate for what your real DPI will be. Use this only as a guide. At large sizes, 150 or eve 100 DPI is perfectly fine, since you'll be viewing from several feet away.

I've printed many 6 megapixel and 10 megapixel images at 24x36 sizes, and they look great. Don't worry about it.

[Edsport]

Im new to this printing also. A question for someone who may know. Why print 100 or 150 dpi and not 300 dpi with large prints?

[dcclark]

Your image only has a fixed number of pixels. The number of pixels and your desired image size together determine the DPI. Or, if you choose the DPI, then you are limited in terms of what image sizes are possible.

Here's an example: suppose you have a 2000x3000 pixel image. If you want an 8x12 inch print, then there will be 2000/8 = 250 dots per inch along the short side, and 3000/12 = 250 dots per inch along the long side. Notice how the number of pixels, and the print dimensions (in inches) completely determined what the DPI would be.

Now, suppose you want a 300 DPI image (in both directions). Then your can get at most 2000/300 = 6.67 inches by 3000/300 = 10 inches.

Doing things like "rescaling" (or "resampling") the image in Photoshop just creates new pixels which have been interpolated (read: invented) from the existing image data. So, it's effectively pointless and does nothing to increase your real image quality.

I don't know why everyone is obsessed with 300 dpi. When you print a 3x4 foot poster, nobody's going to look at it with a jeweler's loupe -- you'll be looking from quite a ways away, and it will look great even at 120 dpi. I know this from experience printing my own poster-sized prints!