Image resolution and max print size

I have a Canon 20D. The maximum area resolution that the camera can produce in an image is 3504 x 2236 (approximately 8.2 million pixels) in either Large Fine JPG or in RAW.

I also have an Epson Stylus R1800. Epson recommends an image resolution of 200 - 300 dpi in its "Complete Guide to Digital Printing". According to its "Guide", one can determine the maximum print size possible from a digital image without any computer interpolation created by selecting the "Resample Image" box in their print driver if the area resolution of an image is known. For example, 200 dpi multiplied by each side of an 11" x 17' print would require an area resolution of 2200 x 3400 (determined by multiplying each side of the 11" x 17" print by the number of dots per inch - in this case, by 200).

The "Magic Lantern Guide" for the 20D claims that with the 20D, one can make great prints at 16" x 20" and more, but "only when the image is shot at 8.2 megapixels."

Even the user's guide for the 20D states that one can make A3 prints (which are approximately 11" x 17") or larger when the image is produced in either Large Fine JPG or in RAW.

I'm having a little trouble with the math. If I follow Epson's rule of thumb (and I assume that they know what they're talking about) and use an image resolution no smaller than 200 dpi, and I try to make a 13" x 19" print (which the Stylus R1800 is capable of printing), wouldn't I need an area resolution of 2600 x 3800 (200 x 13 and 200 x 19)? Since this is in excess of the area resolution of an image that the 20D can create (3504 x 2236), how can I make this print without asking the computer to do some image interpolation? If I do the math on the 16" x 20" image (if I had a printer that could print that sheet size), it's even worse: 3200 x 4000.

One thing that I haven't considered, nor have I even played with at all, is the RAW setting on the 20D. While the 20D's user's guide indicates that the number of pixels for a Large Fine JPG is the same as that of a RAW file (3504 x 2236, or approximately 8.2 million), the user's guide also indicates that the image file size (apparently different than the number of pixels) for a Large Fine JPG is approximately 3.6 MB, but is approximately 8.7 MB for a RAW file - quite a bit larger.

So, how does one print at 13" x 19" with an image taken from the 20D?

Can I only make the 13" x 19" print or large if the image was captured in the RAW format?

Does one have to print with an image resolution smaller than 200 dpi (meaning that the rule of thumb in Epson's "Complete Guide to Digital Printing" can be stretched and still get great results)? The size claim that the Magic Lantern Guide makes ("...great prints at 16" x 20" and more), or even that of the 20D's user's guide ("A3 prints or larger"), seem to be so in excess of the print size when considering only the area resolution that the camera is capable of and the image resolution that Epson recommends that there must be something that I'm missing.

Can anybody help me?

Thanks a lot (and sorry for all of the details).

I apologize. I didn't read your whole post. But I read enough to understand what you're asking. Really, the numbers are just superfluous details. The bottom line is, interpolation is fine. I have sold cropped, 4-megapixel images, blown up to 6 feet. If it looks good, it looks good. It's that simple. Generally, I try to print from images that are about 250 dpi at print size. I think 200 dpi is probably fine, too. If I need to interpolate to get the desired resolution, then that's what I do. Digital photography has created an unhealthy obsession with numbers. People have begun to think that there are real rules about what is right and wrong when it comes to resolution. Ignore that kind of talk.

Use the camera you have, Interpolate - > Print - > Enjoy

Thanks for the info Photo-John. I too am a brand new 20D user (about 4 months now) and am starting to look into printing very large prints... 20"x30" and larger of stuff like this:

http://img.photobucket.com/albums/v5...ksideCloud.jpg

I'm looking for tips on commercial printing and using Photoshop to enlarge pictures.

1) How do I get the best results in Photoshop for enlargements? Is it as simple as going to "Image Size" and putting in 20x30 at 200dpi and then letting PS work its magic? Is there a method to get better results?

2) About the printing... I've looked at mpix and I love their metallic paper, but they don't go as large as I'm going to eventually want. I have a local place that I think will do just about any size I want, but I don't think they have metallic paper. They do offer something they call cibachrome which I think is a different name they use for ilfochrome. Can anyone make some recomendations to help ease the commercial printing process?

I'm from the Nikon world but pictures are pictures and from what I have tried and read there are a few ways to enlarge photo's with minimal loss. The two that seem to work the best are the stair stepping method in Photoshop or one of the commercial programs specifically made for enlargement. The Photoshop method is to use their standard enlarging method but to do it in 10% steps up to the size you desire. Doing it in 10% steps is a slow process but seems to give the best results and is close in quality with the commercial programs. The commercial program I have is Genuine Fractals which from what I've read and in personal use gives great results and is fairly fast to use. With the photoshop method I suppose you could create an action which would speed up the process too.

Thanks JCphoto1. I found the tutorial at Rocklobster about the 10%. It works great!

I just resized the image in Photoshop to 12"x18" and 300dpi, as the printing company wanted. The result is excellent.
I didn't try the 10% step solution, just a resize in one operation worked fine.

I'm sure I can easily get larger prints, but 12x18 was only £2.20 per print regardless of quantity - a ludicrously low price for a decent size print.

I use step interpolation to resize images - smaller. But for enlargement, I've found just wacking it to the final desired size works best. I think that the stepping might even introduce artifacting. At one time I did some testing, although I admit I dont exactly remember the results. I just remember that I like one Photoshop bicubic enlargment.

One trick I do use is not enlarging to the full output resolution. I other words, if the printing device prints at 300 dpi, I'll only interpolate to 250 dpi, at the final print size. That allows the print device to do some hardware interpolation to reach the final print size. And hardware interpolation is always better than software interpolation.

There are lots of methods for enlarging. But I like straight Photoshop bicubic interpolation, with a little hardware interpolation to smooth things out. In other words, if the print device has supports a resolution of 300 dpi, I'll interpolate to 250 dpi at my final print size, and then let the printer do the rest. I think that delivers a much smoother print.

As for the best print, without a doubt, hands down, optical digital output like the LightJet is the best way to go. And get your prints made on Fuji Crystal Archive paper. It is the current standard for longevity and "gallery quality" prints. I get my prints made at Calypso Imaging.