Megapixels vs. Print Sizes

In this post I will touch on a very controversial subject: how many megapixels do you really need to make large prints?

I recall from my film days that with 35mm film we could make very nice 8″ x 10″ prints and good 11″  x 14″. In some cases, if the negative was perfect, we could make a half-decent 16″ x 20″ but that was generally too big and too costly anyway. The pros shooting the most common medium format, which was 645 or 6cm x 4.5cm, they could make really nice 11″ x 14″ prints and good 16″ x 20″.

In today’s digital age, most people only publish their photos on the Internet and view them on screens. For this you only need 5-6 Megapixels, maybe 8 Megapixels if you happen to own a 4K monitor. Some say to maximize quality you should use a file that has twice the resolution of the display in order to reduce artifacts, so that would mean you need a 16 Megapixel image file for best-quality viewing at full-size on a 4K TV, but in my experience the improvement is marginal at best.

When it comes to making prints, things are a little more complicated. You have to take into account acuity and viewing distance. The longer the viewing distance, the lower the required resolution. That’s why a highway billboard that is always viewed from 200′ away or more can be printed as low as 10 or 20 DPI (the minimum DPI from 20′ away is 15!). A couple of years ago I printed a 12′ by 30′ trade-fair booth backwall from a D700’s 12 Megapixel file and it looked great from 20′ away (I don’t recall the actual DPI but it had to be low!) The generally accepted rule-of-thumb is that the proper viewing distance for a print is 1.5x to 2x the size of the diagonal dimension of the image in order for your eyes to properly appreciate the whole frame. This means the required DPI actually goes down as print sizes go up due to the larger viewing distance (otherwise digital cinema would not be possible!) Of course this breaks down a bit for small prints as you’re not going to bring them right up to your nose and for very large prints as the room dimensions and layout may prohibit you from moving further away. Here is a chart where I have calculated the “recommended” minimum DPI* at 1x, 1.5x and 2x the diagonal. I think the 1.5x recommendation is a really good starting point but if you are really demanding you can use the 1x column to be safe.

DPI_per_viewing_DistanceInterestingly for high-quality magazine or book printing on glossy paper publishers normally require 300 DPI, which is pretty much the golden standard, although most of the time they actually print at 150 DPI. You can see from this chart that 300 DPI is just above the recommended DPI for an 8″ x 10″ viewed from a few inches away so it’s actually overkill.

For some reason, many photographers think that 300 DPI is necessary for ALL printing as they like to “pixel-peep” them right at nose-length distance but I think this is absurd. The reality is that 240 DPI is nearly impossible to tell apart from 300 DPI from a few inches away even under a loupe! If you don’t believe this, remember that Apple’s famous “retina displays”, being predicated on the maximum resolution that the human retina can discern at laptop or iPad viewing distance, is “only” 220 PPI. This means that from 18″ away 220 DPI is plenty. 180 DPI is actually more than enough from a two to three feet away and is a good resolution for most of our printing needs. A professional printmaker told me that 135 DPI works great for wall-mounted large prints (24″ x 36″ and larger) and this chart confirms it. The same guy tells me that for large publicity posters and murals 60-120 DPI is usually good enough. This all assumes you are printing on glossy paper; matte paper doesn’t require quite as much resolution and canvas requires even less as the texture blurs the dots.

Using the information above, I calculated what maximum print sizes can reasonably be achieved with a 16 and a 20 Megapixel m4/3 camera (4:3 aspect ratio) and with a 24 Megapixel APS-C camera (3:2 aspect ratio). In blue I’ve highlighted the 300 DPI “golden standard” print quality. In green is the 180-220 DPI which should be plenty good enough for most of us for medium-sized wall-mounted prints. In yellow is the 135-150 DPI which my printmaker friend feels is adequate for good quality large prints that will be mounted on a wall. In red is 60 DPI, still an acceptable resolution for wall murals.

MaxPrintSizes

You can see that 16 Megapixel is just a tiny bit short of a magazine double-spread at 300 DPI but is plenty for a single page. A 20 Megapixel sensor does allow you to achieve a full 11″ x 17″ magazine spread at 300 DPI. However, the 16 Megapixel sensor still allows you to make a really nice 24″ x 36″ or even a 4′ x 6′ wall mural. In other words, 16 Megapixel cameras have already surpassed 35mm film and medium-format in achievable print sizes. You might notice that the difference between the max print sizes between a 16 and a 24 Megapixel camera is not that large. This is because the increase is in two dimensions (width and height) so you actually need to quadruple the megapixels to “double” the print size from 8″ x 10″ to 16″ x 20″ (a 16″ x 20″ print has four times the surface area of an 8″ x 10″ print).

If we look at it another way, let’s say you have a 16 Mpix m4/3 camera and you are printing a 3′ x 4′ poster at 90 DPI (as per the chart) and you would like to significantly improve the perceived sharpness. How many DPIs do you need to go up to for the improvement to be significant? According to human vision experts, all else being equal you need to double the DPI to achieve a significant change in perceived sharpness so you would need to go to 180 DPI in this case. But, this doubling is both in the horizontal and the vertical dimensions, so you need 4 times the Megapixels to achieve this doubling in DPI. In other words, if you have a 16 Megapixel camera and you want to make a significant improvement in the quality of your prints at a given size, you would need to go to a 64 Megapixel camera! Moving to a 24 Megapixel camera will have a marginal impact and you can see this if you compare images between a Fujifilm camera like the X-T1 with the X-TRANS II sensor (16 Mpix) and the X-Pro2 which has the new X-Trans III sensor (24 Mpix). There is a subtle improvement in perceived sharpness and detail, but it isn’t nearly as much as you would think with a 50% step-up in sensor resolution. I bet if you placed a 16″ x 20″ print from each of these cameras side by side you couldn’t tell them apart. Moving to 36 Megapixels will have a more noticeable impact on the quality of your prints but still not as much as you would be led to believe.

If you recall that 220 DPI is the maximum discernible resolution to the human eye from 18″ away, then if you are already making 16″ x 20″ prints at 180 DPI with your 16 Megapixel camera, moving beyond about 24 Megapixels will not yield any noticeable improvement in print quality either as you will just be going beyond the 220 DPI limit.

Canon-inkjet-printer

So, if you’re only publishing your pictures on the web an 8 to 12 Mpix camera will be more than enough. Even if you are interested in doing prints, most people really don’t need more than a 16 Megapixel camera with which you can make a top notch 10″ x 16″ (at glossy magazine quality 300 DPI) or a really really good 16″ x 20″ (at 200-240 DPI). If it will be mounted on a wall and viewed from a few feet away you can easily put out a 30″ x 40″ or even 45″ x 45″ that most people will think looks fantastic; only pixel-peepers (dot-peepers?) will be able to tell the slightly lower resolution. Of course the image files have to be properly prepared for printing. They need to have good contrast, have the sharpness adjusted for printing and they should be upsized to match the actual DPI used by the printer (usually 300 or 360 DPI) in order to avoid leaving the resizing to the printer’s software.

To keep things in perspective, lots of pros are still using their 12 Megapixel Nikon D3s. Remember that the extraordinarily sharp 70″ 4K TV that had you drooling at the local electronics store is only 8 Megapixels (and about 65 PPI)! Today’s top of the line Nikon pro DSLRs are still only 16 Megapixels (D4) and the recently released $6500 Nikon D5 monster is “only” 20.8 Megapixels. Canon’s recently announced new pro camera the $6,000 1D X Mk II is also “only” 20.2 Megapixels. So why do so many amateurs think they need 36 or 50 Megapixels again? 

Interestingly, the highest megapixel counts are found in Canon’s and Nikon’s prosumer cameras (5D and D800 series), not in their top-of-the-line pro DSLRs. As far as I’m concerned, unless you’re a landscape photographer who makes big prints (and you would know already) or you work for the fashion industry which requires 300 DPI for everything, then you really don’t need more than 20 or 24 Megapixels. If you’re into stock photography, there might be some advantage in producing higher-res files as well.

From my point of view, 16 Megapixel is really enough for the vast majority of amateur photographers. I still find the quality of images from my 12 Megapixel Nikon D700 to be excellent. If you really want to maximize quality or plan to make lots of very large prints, then I think 20-24 Megapixels is the sweet spot. Unless you are a pro with some very specific needs, anything above that will actually become detrimental to your photography. Why? First the camera will be a lot more expensive. Second, it will show the deficiencies in your glass and will cause you to want to upgrade your lenses to the latest pro models which are expensive, large and heavy. Lenses from just a few years ago are often proving to be inadequate with these huge pixel densities. Third, it will greatly increase the need for memory cards and will increase file transfer times and file storage space requirements. Fourth, you will probably need to upgrade your computer in order to be able to process these huge files in a reasonable time and no matter what, you will still waste a lot more time processing your images.

Please do share your thoughts and experiences on this somehow touchy subject!



*
PPI refers to pixels per inch, which is image resolution (the resolution of your image file) and is also how you describe a screen’s resolution. It is usually an information tag that is embedded inside an image file to tell the viewer at what size (in inches) the file is intended to be viewed.

DPI refers to dots per inch, which is the resolution at which a print is actually made.

Note that printer manufacturers use confusing terminology when they say, for example, that their printers print at 2880 DPI x 1440 DPI (H & V). But this refers to droplets per inch, as modern printers dither several micro-droplets of various colours to create one information dot of a specific mixed colour. Most printers actually print at 300 DPI (Canon) or 360 DPI (Epson) and you should always rescale your files to that resolution using high-quality software or a RIP (Raster Image Processor) to avoid letting the printer do the resizing which can have side-effects.
When I talk about using  135 DPI or 180 DPI or 240 DPI, I’m referring to the actual amount of information in the image file, but you should always rescale it to match your printer’s native resolution. Rescaling it sort of fills in the gaps so that you still do not see pixels at higher print sizes but it doesn’t really add any more information to the image.

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