Up until a few years ago, if you purchased a quality lens you could be sure that with proper care it would continue to perform well even as you upgraded your body in the future. After all, bodies decay and glass lasts. However, with the sudden influx of high-resolution cameras and the seeming resurgence of the megapixel war, some are asking: “Can lenses keep up?”
When the Canon 5DS and 5DS R were released, Canon also released a list of recommended lenses “for getting the best from” the two camera bodies. This caused a minor uproar in the photography community, particularly since some notable Canon lenses weren’t on the list. The implication was, of course, that the lenses could not resolve 50.6 megapixels within the 35mm full-frame format. The camera had apparently outgrown the lens. This was potentially huge: as technology progresses, bodies are rendered obsolete and the cost of upgrading is a generally accepted part of the profession. However, a good lens is just that: a good lens. Never before had the proposition been introduced that along with the body, our lenses must be upgraded as well. Some argued that indeed, the ultra-high resolutions were simply too dense for some lenses. Others argued that it was a marketing ploy to encourage photographers to invest in even more high-end glass.
One Statistic to Rule Them All
Back in 2012, DxoMark introduced the perceptual megapixel, or P-Mpix for short. Meant to obviate the need for the MTF chart, it distilled the sharpness of a camera/lens combination down to one number: the number of megapixels you were effectively getting from the camera and lens. It essentially measures the maximum resolving capability of a camera/lens combination. For example, my 5D Mark III has a 22.3-megapixel sensor, but when paired with the EF 100mm f/2.8L Macro, the combination has an 18 P-Mpix rating, meaning somewhere in the system, I'm losing 4 megapixels to imperfections. DxO claims this measurement “bypasses the problems inherent to MTF,” namely readability, use of understandable units (which I would argue falls under the former category), and a canonical correspondence with human vision.
As an aside, I take anything that comes from DxOMark with a healthy grain of salt. Here’s the problem: they refuse to publish detailed methodology. There’s a reason science is an open community: science is hard… like, really hard. That’s why we publish, critique, evaluate, question, test, test, test, retest, and replicate results before they are accepted as fact. The “because I said so” method simply would not fly and it’s why I highly suspect I see sparse mention of DxOMark when I search peer-reviewed academic journals, despite the scientific importance and industrial reach of the study of modern optics. I own and use both the 5D Mark III and the EF 100mm f/2.8L Macro, and honestly I have a hard time believing I’m “losing” 4 megapixels with that combination. Alas, without some hard equations and procedures to look at, I can’t tell you much more. I’m not saying DxOMark is wrong; I’m saying that without more information, we can’t decide if they’re right or wrong and unfortunately, that undermines their relevance in my humble opinion. Talking about “resolution” in photography is a deceivingly complex proposition and if I don’t even know the working definition we’re using and how it was arrived at, I can’t hope to have a fruitful conversation.
For example, consider this skeletal outline of how a medicinal study might operate: construct a hypothesis, devise a method that fairly tests the hypothesis, conduct the experiment with many, many repetitions to detect outliers and unforeseen phenomena, analyze the data using appropriately chosen statistical methods that both illuminate patterns and minimize bias, publicly publish both the methodology and the data in a peer-reviewed context, receive feedback, repeat and refine dozens of times, and finally, if your hypothesis has been verified many times by multiple, independent resources, it is promoted from hypothesis to fact. This is how science works. If I'm presented with data with a scientific air to it, I expect to see this sort of stringency in its genesis.
The Camera or the Lens?
Anyway, back to lenses. The question of what out-resolves what is a complicated one; it depends on pixel pitch (which in turn creates a transitive dependence on sensor size), control of various aberrations, the limits of human perception, the ever-present and increasingly important (as megapixel counts skyrocket) phenomenon of diffraction, and various other factors. Suffice to say that smaller, sub-full-frame sensors may be at or past the line with some combinations. That is to say, put a lower quality EF-S lens on a Canon 7D Mark II and you may well be getting less megapixels than you paid for. This is because pixels tend to be packed more densely on smaller sensors (and thus, their poorer low light performance), meaning that a lens must be able to resolve finer detail to avoid essentially smearing information across the pixels.
On the other hand, full-frame users should be thinking more about diffraction limits and control of aberrations. Part of why full-frame glass is so much more expensive than its crop counterparts is not only because there is physically more glass, but controlling aberrations becomes more difficult as the image circle grows larger. As I discussed in my article on the 5DS and diffraction, as sensor resolution goes up, the aperture at which diffraction effectively becomes a legitimate concern drops (becomes wider). The 5DS pushed the threshold of this into legitimate working photographer apertures. As we see megapixel counts continue to rise, photographers will have to consider how this affects them, particularly those who work at smaller apertures and require high levels of detail (landscape and macro photographers, take note).
There’s also an added complication independent of the physics: we don’t normally see lower level glass paired with higher level bodies. This in itself has normally kept a certain level of separation, but now, as megapixel counts approach unforeseen levels, that gap is being closed. And so, we have to ask: if you buy a ultra-high-resolution camera, will even your good glass be rendered obsolete? Well, I can’t answer that for two reasons: first, it depends on your definition of “obsolete.” If you have a 50 megapixel sensor, are you willing to take a hit of 5 megapixels before you call a lens archaic? How about 10 megapixels? That’s up to you. Second, at this point in time, we don’t have data obtained from a publicly available, verified method to make that call. My suspicions say that indeed, the threshold of degradation may be starting to cross into lower level professional lens territory, but they're just that: suspicions.
Nevertheless, I leave it up to you now. Do you own a Canon 5DS, a Sony a7R II, or a Nikon D810, and have you noticed this phenomenon? What sort of hit in resolution would you be willing to take? Or is this all marketing to push newer lenses? Or do you want to see more transparency of technical data in photography before you decide? Let us know your thoughts in the comments.