How Many Megapixels Do I Need?
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It’s fair to say that at one time or another, we’ve all been captured in the megapixel race, and the typical consensus is that more is always better. This may be true, but it should then beg the question, does need equal want?
To answer this question, we need to think about our personal desired outcomes, and for simplicities sake, we will split those outcomes into only two segments. The first, and certainly most common, is where a screen representation is our desired outcome. This means an image that is going to be viewed on a display of some kind, be that a computer display, a tablet, a smartphone or even a television. The second, considerably less common is where the outcome is a physical print. Before leaping into this, let’s all remember that megapixels have nothing to do with sharpness, Internet wisdom (an oxymoron if ever was) notwithstanding.
Screen Outcome
Let’s use a high resolution display as our maximum target. Anything that works nicely on a high res display is going to work nicely on a lower resolution display. Let’s do a bit of math here. We will use as our high end example, Apple’s 5K Retina display as found in the 27” iMac. This display has a pixel layout of 5120 pixels by 2880 pixels. This gives us an aspect ratio of 1.78 : 1 . If we use Pythagoras as our math model we find that our display is approximately 23.55” wide by 13.23” tall which rounding to whole numbers gives us a resolution of 218 pixels per inch.
This means that no matter what we feed the display, the most that it can display is 218 pixels per inch.
Now let’s look at a popular image sensor such as is found on Canon’s R5 mirrorless. This sensor is defined as 36mm wide by 24mm tall. It’s actually a tiny bit smaller, but we will use the published numbers. The sensor is defined as having a sensor resolution of 8216 by 5477 pixels, although the actual deliverable is 8192 by 5464 pixels. We will use the capability rather than the deliverable for our calculation because the numbers look better. This means that the sensor is delivering (rounded up) 5797 pixels per inch. This sensor is marketed as having a megapixel rating of 45 megapixels.
We can see immediately that the sensor at 45MP delivers far more resolution than can ever be displayed on a 27” diagonal iMac 5K Retina display.
But what happens if we have a lower megapixel sensor?
Let’s consider the Olympus OM-D E-M1 Mark III. This camera has a sensor size of 17.4mm x 13mm in a 4:3 aspect ratio. This ratio means non equidistant spacing in the sensor grid of the pixels so we will use the long side pixel count for our calculations. The sensor is documented as being 5184 x 3888 pixels, so delivering on the long side 3551 pixels per inch. Even the short side delivers 1990 pixels per inch. In both scenarios, the resolution far exceeds the maximum resolution of the iMac 5K Retina Display. Thus even a 20MP sensor delivers image quality that you cannot see on a top end high resolution computer display.
So for the sake of argument, what is the minimum megapixel count that a 5K iMac Retina can fully render? We already know this, it’s in the specs 5192 x 2880 pixels which by simple math is 14.95 megapixels. Thus you can conclude by scientific process that a high end, high resolution display maxes out at 14.95 megapixels and anything more than that cannot be seen.
Now I know that there are folks who swear that they can see a difference between a 20MP image and a 50MP image on their display. Not in resolution they cannot. That’s a JEDI mind trick. You cannot see the difference, no matter what you believe.
Print Outcome
When we print, we think in terms of dots per inch in a printer and in a print file. The two are not the same thing at all. When constructing a print file, that is not a JPEG, meaning save as TIFF or print direct from your application without compressing to JPEG as an intermediary step, different makers propose different dpi ratings. Let’s assume a normal wall hanging print, and also assume a dpi construction of 300dpi at the file level.
What this can tell us is the maximum native print size that a sensor can handle and it is very simple. Divide the pixel width on the long side by 300 and that is your native print maximum. In our earlier Canon R5 example that would suggest a native maximum print size before the need for some kind of expansion of 18.25 by 27.4 inches which is a pretty nice print size. Even our “small” Olympus sensor has a native print size of 13 by 17.25 inches, substantially larger than most people ever print. So do I need a ton of megapixels to print? Probably not. Using tools like Perfect Resize or more recently Topaz Gigapixel AI, I have made 24x36 prints for clients from 6MP iPhone images and the client has been thrilled. Certainly some publishers demand higher resolution, but I am inclined to wonder what the heck for.
Thus if you are printing 4 inch, 5 inch, 8 inch or 11 inch (short side) prints, you have more than enough native resolution on the sensor to deliver a 300dpi print file.
Now let’s take into account the printer itself. Printers are also rated in dots per inch capability and have powerful CPUs that do only one thing which is fill the space between the informational dots. They use these processors to interpolate smaller dots that use tiny amounts of ink and create transitions between individual print file dots that are different.
Let’s use an Epson Surecolor P600 as an example. This is a high quality inkjet printer built for photographic prints. It has all the good stuff, lots of ink tanks, great paper handling, support for roll paper and other things that printers find important. It’s default maximum print size is 13 inches by 19 inches which accommodates the 1.5:1 aspect ratio of the classic 35mm sensor. However on perusal of the documentation we discover that at maximum print quality, it will print 5760 x 1440 dots per inch.
Wait, what?
We only provided it a 300dpi file, how can it do that? That is what the CPU is doing in the printer. It is interpolating to create a higher resolution print image. You can of course turn down the print resolution for proof prints, but the capability of this moderate cost printer is outstanding. Even the low end XP-15000 can do this high level of resolution, although less efficiently and with less photo orientation. In Canon’s 13” capable line we finde the IMAGEprograf 300 which by similar process can deliver a maximum dpi on paper of 4800 x 2400 dpi.
Summary
The point of this article is to encourage the smart photographer. If a 20MP sensor can delivery more than the best display can show, and deliver a native print size, before the printer gets involved at all, what is the rationale for chasing more megapixels. For lots of us, the answer may be no practical reason at all.
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I'm Ross Chevalier, thanks for reading, watching and listening and until next time, peace.
