Expert: Ran - 1/8/2010

Question
Hi Ran - All the best to you and yours in the New Year and hope you enjoyed your holidays!

My question this time is more to prove I am more-or-less right about something (unless I'm not that is...)
A friend took a picture with an Iphone at 72dpi and wholly believes that it can be "photoshop'd" to a higher dpi and still retain high quality. to my knowledge, to make the picture larger for print beyond size 4"x6" you would have to resample the image and adjust the pixel width/height or increase the dpi. He said something about "interpolation" in photoshop...
Now I don't know really what "interpolation" is in photoshop 'cause I use Corel which may have a different word for the same function - but I told him that you can't really make the picture "bigger" because you can't really add details that were not recorded in the original photo- you can make it smaller, ,but not bigger at least not without loss of picture quality (grainyness and pixelation, blur, etc).  I told him if he made it much bigger than say 5"x7" at 72 dpi it will look grainy.  Is there something I don't know?  Can it in fact be done to resample a picture taken at low dpi and still have a quality picture in the same realm of say one taken at 600 dpi?
Also he argued that "no one uses corel - that it is obsolete - they went bankrupt"....yadda yadda yadda.  I tried to tell him that was very false and that Corelgraphics suite is very much preferred in industries such as sign cutting, engraving, fashion and printing - but he is pretty stubborn....

Answer
Hi Alice - thank you!

You can't add quality, period. People who think you can have usually been watching too much CSI!

Interpolation is the process of adding new values in between two known values. So, if you had an image only 2 pixels by one pixel, where the left pixel was black and the right pixel was white, and then you resized it to 4x2 using interpolation, you would end up with black on the left and white on the right as before, but dark and light grey colours in the new pixels between. Photoshop offers the choice of "Nearest Neighbour", "Bilinear" and "Bicubic" interpolation in the Image Size dialog. These are different mathematical interpolation algorithms and Bicubic is the smoothest (by being the most complex and therefore slowest) one. Whichever is used, it has the effect of smoothing things out, commonly called blur as you quite rightly said.

The crummy camera on an iphone still has either 2 or 3.2 million pixels which should be ample quality for a reasonable enlargement providing you're not too fussy, but the quality of the optics will give distortion which will probably be noticeable when you view the thing printed out - such as curved walls on buildings etc.

If I was to produce such a print, I would increase the image size and resolution to 300dpi and then possibly fiddle with the Unsharp Mask to increase the sharpness if it looked too blurry. 300dpi is *enough* even if your inkjet can do 1200dpi, because human eyes cannot see things so small.

CorelDraw does not seem to interpolate at all when it resamples, although I've not delved into it particularly, because Photoshop is a much sharper tool for image work.

To finally get around to answering your second question, whether you can turn a low-res image into a high-res one. Well, yes you can, but it will inevitably be more blurry up close than a native 600dpi picture (otherwise Canon and Nikon would be out of business).

As for your friend's response, I believe he is misinformed on all counts. I suggest you buy him a few beers then roll him into a river!