For new and aspiring photographers, resolution is simultaneously the most easily understood and most misunderstood aspect of a camera. Most people know that it has something to do with megapixels — a number that’s clearly stated in the specs and on the box — and that more megapixels equals more resolution.
This isn’t false, but it’s not the full story.
What really is image resolution, how much do you need, and how should you invest your budget to get more of it? I reached out to Roger Cicala, founder of online photography gear rental service Lensrentals, to get the answers. Cicala is known for his extensive lens tests and gear teardowns, providing a level of insight that goes far beyond manufacturer specs and puts lesser camera geeks (like yours truly) to shame.
A sensor is less than the sum of its pixels
A sensor will never achieve the full potential of its stated pixel count for various reasons.
“I think megapixels are sort of ‘the most resolution you could possibly get,’” explained Cicala. “You never get all of it, of course. The Bayer array and anti-aliasing filters, if present, reduce resolution somewhat.”
The Bayer array is the RGB filter that’s responsible for turning your camera’s sensor into one that can actually see color. That’s right. An imaging sensor by itself can see only black-and-white.
The Bayer array puts a red, green, or blue filter over every individual pixel. The camera knows which pixels are assigned which color and uses an algorithm to “de-mosaic” the sensor into a full-color image, giving an RGB value to every pixel.
The problem with this approach is that color is added at the expense of spatial resolution. You might have 24 million pixels total, but you only have 12 million green ones, 6 million red ones, and 6 million blue ones. The de-mosaic process works pretty well, but obviously there are some losses compared to what would be possible if full RGB data was captured for every single pixel.
The anti-aliasing filter (also called an optical low-pass filter) is a bit different. Located in front of the sensor, it purposefully blurs small details in order to prevent moiré. Moiré is what happens when you try to photograph a very fine pattern, like those in some textiles, without enough pixels to accurately reproduce the pattern. This often shows up as a jagged, rainbow-like secondary pattern.
But many manufacturers have chosen to go without anti-aliasing filters today. As pixel counts increase and pixel size decreases, the chance of moiré goes down. You’re unlikely to run into Moiré on the 47-megapixel Panasonic Lumix S1R, for example, but I can personally vouch for it still being a problem on the 24MP Lumix S1.
The competing goals of increasing resolution and suppressing moiré have led to some novel innovations over the years. From Fujifilm’s X-Trans sensors, to the Sigma Foveon X3 chip in cameras like SD Quattro H, to Leica Monochrom cameras that give up color altogether, manufacturers have fought this battle with varying degrees of success.
But even with the best possible sensor, there’s something else that will prevent it from ever achieving its full potential.
Lenses have resolution, too
The lens in front of your camera is as important to your final image resolution as the sensor, but no lens is perfect. As Cicala put it, with any lens, “the image sent to the sensor has less resolution than the object does in real life. Basically, if you have 48 megapixels of ‘possible’ resolution, what you actually get as the image is made is less than 48 megapixels.”
In other words, no matter how good your sensor, it doesn’t have a chance at being as good as it can be. The light it receives is already tarnished.
“It’s comfortable to think things like, ‘If I have a lens that resolves 48 megapixels and my sensor resolves 48 megapixels, I get 48 megapixels.’ The reality is that each step in making the image is a resolution loss.”
It’s fine to use pixels as an indicator of resolution of a camera sensor, but lenses, of course, don’t have pixels. Instead, lens tests will look at how many line pairs per millimeter (lp/mm) a lens can reproduce, and plot this information in a modular transform function (MTF) chart. It’s beyond the scope of this article to delve into all the details, but basically, an MTF chart illustrates how the sharpness of a lens changes from its center to its edge.
However, even if you take the time to look up MTF charts (most manufacturers publish them), you’re not really getting the full story of a lens’ resolution. This is because a manufacturer chooses the resolution at which to test the lens, and different manufacturers may test different resolutions.
“For a very high-resolution camera, what we consider fine detail would be 50 or even 80 lp/mm,” Cicala said. “Unfortunately, the manufacturers usually only give MTF at 10 lp/mm or 30 lp/mm.”
Why would a manufacturer use a lower-resolution test? Remember, MTF shows resolution both in the center and the edges of a lens, so even if a lens could resolve 80 lp/mm in the center, a manufacturer might be wary to show that if its edge performance doesn’t keep up. With “edge-to-edge” sharpness being a selling point of many lenses, a lower-resolution MTF will look more impressive on first blush.
Upgrading your resolution
Cicala cautions that we shouldn’t think of the lens as being a limiting factor in a camera system. Even a bad lens will perform better on a good sensor, so you will gain resolution by upgrading to a higher-megapixel camera even if you don’t upgrade your lenses.
Still, he also recommends that replacing the weakest link in a system is the best way to improve resolution — with one caveat. If you upgrade a lens, you only upgrade that one lens; if you upgrade a camera, you essentially upgrade all of your lenses.
However, be aware that what sounds like a huge upgrade in megapixels is not actually as big as you may think. “The improved resolution is the square root of the increase in megapixels,” Cicala explained. This means if you go from a 24MP sensor to a 48MP sensor, you’re not doubling resolution, but rather getting about a 1.4x increase. In other words, you have to quadruple the pixel count to double the resolution.
Especially for photographers who already have a camera in a 36 to 45MP range, this is why looking into a new lens may be a better use of your time than drooling over the 61MP Sony A7R IV camera.
“For the vast majority of photographers I talk with, the biggest bang for the buck improvement in images is to put the zoom down and get a couple of decent f/1.8 primes,” Cicala said. “The price you pay, of course, is one of much less convenience; sometimes the need to zoom far outweighs the benefit of a sharper image.”
How much resolution is enough?
There’s no one-size-fits-all answer to this question, but one way to think about it is to consider what your final output will be. Any modern camera has more than enough resolution for web and social media. Instagram, the most popular photo-sharing app, only displays photos at about 1 megapixel, for example.
For prints, you can do some math to help you out. Three hundred pixels per inch (PPI) is considered the gold standard of print resolution. If you want to print sharp 8 x 12-inch prints, you’ll need a resolution of 2,400 x 3,600 pixels, or about 8.6MP.
The industry seems to have settled on 24 megapixels as the standard for beginner-to-enthusiast-level cameras (and even some professional models). And for most people, a 24MP sensor, when combined with a good lens, will yield plenty of resolution, certainly enough for all common print sizes. There are some cases for needing more, though, from making very large prints to simply wanting the freedom to crop.
And remember, even with the best lens, your sensor can’t actually deliver that maximum resolution. You may want to keep some overhead in mind.
Fortunately, there’s a simple fact of human behavior and physics that can help you out when it comes to prints. The larger the print, the farther away people tend to stand when looking at it, and the less per-inch detail they will see. This means you can likely get away with less than 300 ppi in certain situations.
There are two key takeaways here. One is that a camera’s resolution is a combination of lens and sensor, and while neither is ever perfect, nor is one necessarily a limiting factor. Upgrading either will make the other look better. Obviously, having the best of both worlds is what you want, but you don’t need to upgrade everything at the same time to notice a benefit.
The other takeaway is, simply, that lens shopping is hard. If you really are concerned about maximizing resolution, you’ll need to do more than look at manufacturer-provided MTF charts, or at least pay attention to which resolution was tested. To truly know how a lens will perform as part of your overall system, you’ll have to try it on your camera — this is why renting is a great idea.
While resolution is important for a number of reasons, it alone isn’t what makes a good image. There are numerous other factors, both objective and subjective, that contribute to image quality, not the least of which is your own creative vision.