Pixels for print

Dan7312

With the new iPhone there has been a lot of talk about the "Retina Display", meaning a display with pixels so small the human eye cannot see the individual pixels so it really does look like "ink on fine paper". Here is one of the tech blogs about it http://prometheus.med.utah.edu/~bwjones/2010/06/apple-retina-display/


I guess I'm really a pixel peeper at heart but I know that what you see at 1-1 on the screen does not necessarily tell you how sharp a print will be. So I decided to see if I could apply the info about the retina display and do some typical pixel peeper number crunching with Excel about what it takes to make a sharp print... and yes in the distant past I listened to my hi-fi on an oscilloscope . I also know the photog's regularly make spectacular large prints out of seeming too few pixels. I also know that some of the prints I have done seem sharper than my pixel peeping eyes say they should be.

The upshot of all this is that you never need more that 6 megapixels regardless of the size of the print, unless you are a pixel peeper:ivar

I put 2 pieces of info together to do this:

1. Resolution of the human eye is 1 arcminute (1/60 of a degee)

2. Prints are most comfortably viewed at a distance equal to the length of their diagonal.

Of course there are differences between how pixels are presented on an iPhone and the kind of mushed together dots you get with an ink jet printer and you don't view billboards at the length of their diagonal, but it seems like they should approximate one another. Any hints or comments on this being a reasonable way to figure this out would be appreciated.

Putting these two things together I found that regardless of the size of the print in order to get retina display sharpness you never need more than 6 mega-pixels. The actual spreadsheet is here http://danal-examples.s3.amazonaws.com/PixelSpacing.xls if you want to poke at it and maybe show me what I might have done wrong.

And here are the results for some print sizes. Dimensions are in inches.

The actual calculation is to figure out how big 1 arcminute is at the viewing distance and then divide that into the width or length of the print to see how many pixels you need.

adbsgicom

Disclaimer: I am not an optical scientist, but I have worked in high-end graphics display systems for a while. The eye is also very sensitive to anomalies, where I wonder if you printed a 40x60 with the amazing low resolution if the results, even viewed at 70" away would really match looking at the 4x6 7" away. Seems that there would be some significant errors that your eye would notice the anomalies.

Certainly the overall concept works. It is the idea behind those mosaics made of of little images. From an appropriate distance, you will see the little image just as one value and the whole image takes shape.

It is very interesting the counter-intuitive aspect that larger prints require less pixels....

Thanks for doing the math!

craig_d

That prints are most comfortably viewed at such a distance does not mean they always will be, nor that people won't come closer to look at the details.