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What print resolution works for what viewing distance?
Guide to print and image resolution and minimum viewing distances for prints
Is there a 'standard' resolution (300dpi?) to print at?
This short guide explains some of the choices you can make, so you can decide what is important for printing your own photographs.
Print Resolution and viewing distance
We often get asked why we don't print at 300dpi or some other so called 'standard resolution'.
A lot depends on the size of print and how it is meant to be viewed.
Many of Keith's large landscape photographs are printed at relatively low print resolutions.
The information and links to additional resources below should help you choose an appropriate resolution for your photographic prints.
Printing at higher resolutions won't cause any harm, apart from needing larger files and perhaps longer printing times.
If you look at the specifications for your inkjet printer, will see all kinds of large numbers. People often assume that the higher the printer resolution (higher Dots per Inch - dpi) they use, the better their prints will look.
There is however an important difference between the resolution of the ink dots (assuming you are using some sort of ink-jet printer) and the resolution of the pixels of your original photograph, as it comes from your digital camera or film scan.
Let's start with the ink dots on the page.
The image shows a much magnified scan of a graduated series of vertical bars, printed on my HP K80 office printer. You can see the individual patches of coloured ink that go to make up a print from a typical ink jet printer.
Since a patch of any desired colour will be made up of dots of individual different coloured inks, the smallest distinct element of your picture will need quite a few ink dots to make it up.
So for example, if your image file that you want to print, has 300 pixels per inch, each of those pixels needs to be composed of many more ink droplets. In general this means that for a particular image resolution, your printer resolution needs to be quite a bit higher.
The printer driver software that you use, takes care of this conversion and in general you needn't worry about it. The driver software often effectively limits your maximum image resolution anyway, so for my own Epson Stylus Pro 9600 large format printer, it's not worth sending images of higher resolution than 360 pixels per inch. This figure is probably 720 ppi for smaller desktop printers like my Epson Stylus 1160 or Stylus Photo 1290.
For my own large prints on the SP 9600 I'll usually print at 1440 dpi.
It's best to try and print at least three times the image ppi.
The human visual system has a good, but limited resolution. This means that beyond a certain level of detail there is nothing much to be gained by ever higher resolution images.
The point is that this trade-off occurs depends on how far your eyes are away from the print.
I'm going to assume perfect 20/20 vision for the purposes of this discussion, so in reality many people won't see an improvement beyond even lower resolution figures.
Also, don't take all these figures as exact values to be followed either - use them to help decide what works best for you.
The -maximum- resolving power of the eye is about 60 lines per degree. That means that in bright light you should be able to resolve 60 black/white line pairs over an angular distance of a degree.
Note - If you are not familiar with thinking of things in degrees, remember that the diameter of the full moon is about half a degree
The table below (originally from the DigitalWorkflow Yahoo group) gives an idea of the maximum resolution at various distances. Remember that this is for very high contrast at optimal lighting - not your print of a photo hanging on the wall. These values could easily be halved for 'average' or poor viewing conditions.
The formula is ppi = 1/((distance x 0.000291) / 2)
The sensitivity of our visual systems to differing angular resolutions also varies with colour. Our eyes are more sensitive to larger scale variations in colour than in brightness. For black and white images, the eye performs a degree of spatial filtering which peaks at around 7 cycles per degree (more info).
One subtle effect of this, is that if you are looking for faint camera sensor dust specks to clone out of a sky on a monochrome image, they will be more obvious at some image magnifications than others. It also affects how much 'sharpening halos' will be visible on a print -- see my review of Nik Sharpener Pro for more info on this. - Updated in 2009 for Nik Sharpener Pro V3
At a recent trade show, where some of my black and white prints were on display, someone (you know the sort ;-) came up, pressed his nose close and declared that my prints were "over sharpened"
Fine, they were -- for a 6 inch viewing distance!
As I pointed out, I don't tend to do 26"x17" prints for people to view from less than a foot away. It's why billboard posters can make do with 10ppi.
So for prints bigger than A2, I'll typically use 240ppi, and for the two big prints below (across from the balcony) I used 180 ppi.
That doesn't mean that if the camera file had sufficient detail to print at higher ppi, I wouldn't use it, it's just that if I'm resizing an image to make a big print, then there is no need to take the interpolation too far.
One other thing to note from the table above is that if you are making small prints (say 6"x4"), and your camera image size supports it, you really might get better prints at an image resolution of 720ppi and printer resolution of 2880dpi (or higher) - try your images at their highest (not resampled) resolution and see if you can notice any difference...
What about the 'best' viewing distance for a particular size print?
At the simplest, it's about what focal length you took the photo with and how much you have magnified the image (multiply one by the other to get the correct perspective view)
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Lets take a 35mm full frame sensor (or bit of film) as an example.
In landscape mode, the frame is 24mm high.
Now 20 inches is perhaps a bit close for that size of print, so we are going to see the perspective looking not quite right from a further, more reasonable viewing distance.
What if I'd used a 200mm lens for the same size print?
The magnification is still 21.2 times, but the 'correct' distance is now 21.2x200mm = 4.24 metres or about 14 feet !
So much for the The Proper Viewing Distance or "Ortho-Stereoscopic Position" :-(
What this does do, is explain why a lot of wide-angle shots look slightly distorted and telephoto lenses appear to foreshorten perspective (they don't)
What do I use? ...well I just take one to one and a half times the length of the diagonal, which for a 17x26 print is about 30-45"
It seems to work, and give results that a lot of other people like.
For an A3+ (13"x19") print, we get a 23" diagonal and a typical 2-3 feet viewing distance. Looking at the table above I'd probably print at a minimum of 280ppi
For my Canon 1Ds the actual camera images would print at about 14"x9" at 280ppi, so I need to do a bit of resizing. If we take a possible new 23MP version of the 1Ds (Yes, I would like one!), we could print a 13"x19" print at 315ppi, which is more than enough resolution for printing at any size without resizing.
It helps to understand some of the info I've included here, but when it comes down to it, they are your prints, and you should go with what you are happy with. Make your own choices and don't let the print sharpness police intimidate you :-)
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The views in this article represent those of Keith Cooper.
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