Practical Limits of Enlargeability

I have been asked numerous times about how big a print can be made from a digital image. Up until recently, the answer was quite easy: one would need a 200 PPI or higher resolution image for matte papers and 210 PPI or higher for semigloss surfaces. Canvas is a much more forgiving medium, one could go much lower with careful processing. My biggest enlargement, from a 4.5 megapixel original, was printed on Hahnemühle FineArt Canvas in 36 x 100 cm size. This was shot with an 1D Mark II, so this is a 35x enlargement. The print was made at 90 PPI. Yes, this was a result of several hours of careful editing and a matching media choice.

This is a crop from a 8MP image. The biggest print that still looks great is 38×100 cm. But this is an exception in enlargeability, not the norm. Subject matter really helps here.

Of course high resolution is a must for hyper-realistic prints. Posters can be made at much lower resolutions. But I’m not interested in making posters at all. I even wrote an app (PrintCalc), that can calculate all this resolution requirement stuff for you.

To put it another way, digital prints were limited by the sensor’s resolution.

In these days, however, we face another limits. Diffraction, depth of field and lens quality. Let’s take a Canon 5D Mark III for example. The full frame sensor at 22MP starts to get diffraction limited below f/10. The 7D at 18MP is visibly diffraction limited at f/8. The problem is worsened if you want big prints. One often overlooked attribute of depth of field is that it gets shallower as you make bigger enlargements. But you can’t stop down to increase depth of field at your will, because diffraction kicks in. This might, or might not be a problem depending on subject matter.

For landscapes, diffraction puts an upper limit to practical enlargement ratio. You can only go larger if you use a bigger sensor. For other subjects, where you can shoot at wide apertures, this isn’t that big of a problem, so you are limited by the number of pixels. Speaking of the number of pixels: DxO’s new “perceptual megapixels” ranking is a good indicator what kind of resolution a lens can give you. You can increase sensor resolution, but the lens will still be a limiting factor. Think about this perceptual megapixel number as the one you can use as the basis of maximum print size calculations. Look at the best lest they tested to date: Canon’s EF 300mm f/2.8L IS II USM. It can fully utilize a 21MP sensor, but you’ll end up around 14MP on a 18MP APS-C sensor (the 7D).

So it’s easy to see that blindly increasing sensor resolution in any given format above lens capabilities and so much that is severely affects usable apertures is not the smartest thing.

What is that practical limit? I found that about 20 times enlargements (in linear dimensions) make great hyper-realistic prints, keeping ill-effects at a minimum. This translates to about 50 x 75 cm (20 x 30″) for full frame and 30 x 45 cm (12 x 18″) for APS-C. Regardless of megapixels. Of course you’ll need to hit the 200-210 PPI minimum, which is about 11-12 MP for APS-C and 23-24 MP for full frame. But increasing the sensor’s resolution beyond these points will only allow to reap the benefits of oversampling, and not really allow bigger prints at the same quality (not to mention increased storage requirements).

Want bigger? There’s only one way with today’s technology: increasing sensor surface. Basically we have to paths in pursuing bigger recording area:

  • Go medium format. Digital 645 will give you images that can be printed at 80 x 110 cm (30 x 40″) – from an 50 MP or larger file. This costs a lot.
  • Stitch several images together. I routinely use my TS-E 24mm lens to get images equivalent in size to a medium format sensor (36 x 48 mm). This is more work, but at the fraction of the cost of medium format.

20x enlargements are far better than you could achieve (in high quality) from film. This is actually at least one format size better (full frame 35mm beating 645). But remember that every technology has practical usage limits, and make them work for you – don’t blindly believe manufacturers’ marketing stuff.

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