Megapixels vs. Max print out size?

[LisaMarie]

Ok so i understand that for determining a max photo print size for the Mark II (or any digital camera) you must mulitply its megapixels hight and width dimensions.
Question is though, if you were to do that by printing out a picture at a photography lab's quality of 300 pixels per inch, the mark II would would only make a nice high quality picture at about 13" X 10"....But i have seen really nice blown up pictures at larger sizes at what seems to be a failry decent image quality!? How is this done? Is there a way to do this without greatly sacrificing image quality? Also since i am still indecisive over which DSLR to get (the Mark II or the 50D) could somebody please compare image quality with these cameras when there pictures are blown up to larger sizes, Or even better there max size that each can optain when there pictures are blown up to still decent quality?
I am hoping to be eventually able to create poster sizes of my pictures if possible, and i am still wondering which camera is the better choice for this?

 

[Big Mike]

You are right that the 'standard' is 300 pixels per inch of print. So for an 8x10 print, you would want 2400x3000 images...about 7.2 mega pixels.

However, it's certainly possible to make much larger prints with less resolution than 300 PPI. Firstly, you can use software like Photoshop to interpolate the image and increase the number of pixels. There is always a loss of quality when you add pixels...but you can probably double the number, or more, if the image is good.
Secondly, you don't really need 300 PPI...especially on larger prints. Many people say that you can't really tell the difference between 300 PPI and 240 PPI...and for large prints, you could go down to 150 or even 100 PPI.
Remember that big prints aren't meant to be viewed with your nose pressed up against it. A poster sized print is meant to be viewed from several feet away

Now, of course, when you enlarge an image, you enlarge the good and the bad...so any faults will be more apparent. So you want to make sure that the image is as sharp as possible. This means things like using a tripod, with mirror lock up and a remote...or at least using a fast enough shutter speed to ensure sharpness. Also, the lens quality and even the aperture used should come into the equation.

I've seen poster sized prints from 6MP DSLR cameras and they looked just fine,

 

[Phranquey]

I have access to a large color plotter at work, and have printed 12.2 MP up to 30"x20" (143dpi), and they looked great. If you stick your nose in 5" away, yeah, you are going to see a little pixelation, but prints that large are meant to be viewed from a distance, not like a 4"x6" or 8"x10".

 

[tsaraleksi]

Lisa, at 300dpi the 5D2's native output size is about 18x12. However, given that as the size goes up the DPI demands go down, you can see that at 150, which would make a very acceptable print, the size is 37x25.

 

[Dwig]

The bottom line is that it is all about viewing distance.

The general 300ppi rule is a good rule of thumb for prints that will be viewed at a normal reading distance (arm's lenght) or closer. Lower effective ppi's can be quite satisfactory when the viewing distance is greater.

I recently did a 60x40" composited print that included a roughly 30"x40" portion from a slightly cropped image taken with an 8mp camera. After cropping to shape and a little compositional adjustment the image worked out to something in the 80ppi range. I upsampled it to what would work out to about 120ppi to include in the composite image. The resulting print, including the composited text, is displayed as a sign for a podiatrist here in Key West. It looks great from a car driving by and still rather decent for someone walking by on foot (~10' viewing distance). True, its a bit soft when you put your nose up to it, but no normal viewer ever would.

Upsampling is a valueable tool when making such large prints. It can't add any real detail, but it can keep the image from pixelating. For my sign work, I hold to a 100ppi minimum and prefer 150-240ppi. For my conventional photographic prints I keep to 300ppi when printing anything on my own printer (13x19" HP B9180), upsampling when necessary, though I don't know that I can tell the difference between 300ppi and 240ppi.

Judicious sharpening is also valuable. You should never sharpen to the point that it is detectable in the print at the expected viewing distance. For gallery class photographic work, consider "expected viewing distance" to be "nose rubbing the print".

 

[dcclark]

There's some very good advice here. There is no single number which will always be the "best" quality in any given print situation. Don't let the DPI values get you down -- if you have a good, well-composed, strong photo, it will vastly overshadow most print quality "issues".

 

[epp_b]

Generally, 300 DPI is the standard for perfect print quality, which means that a 6MP image will give you fantastic 8x10's, even when you're looking too closely. The shorter side needs to be interpolated slightly on a 6MP image, but 8x10 prints from my D40 still quite good.

There is an inverse relationship between print resolution and viewing distance. It's not zero sum, but the bigger the print, the further you need to be looking at it for your eyes to encapsulate the entire composition. Only when the curves on the "viewing distance / print resolution" chart intersect do you need to start concerning yourself with pixelation.