More About Lenses

I've been trying to come up with a generalized way to think about lens quality and how that relates to the increasing pixel counts, as that question keeps coming up over and over again as I try to explain why I think certain lenses are more "optimal" for the D850 than others. 

First, let me repeat what I've written for decades: more sampling is always better. If you take a picture with a compromised lens like the 18-200mm DX on a low pixel count camera and then repeat that with a high pixel count camera, you're better off with the high pixel count camera. The difference is that problems that might have been masked with the low pixel count now become more obvious. The lens looked "good" at 6mp, but it looks only "fair" at 20/24mp.

Overall, there are two primary things that I see over and over as pixel density increases. 

First up is absolute resolution. Most lenses have enough absolute resolution in the center stopped down a stop or two to look really good on a D850. The difference between a lens with more absolute resolving power and one with minimal resolving power is a bit like looking at a D800 versus a D800E image: acuity appears to be higher when you pixel peep with the D800E image because there's nothing blurring edges/detail (e.g. AA filter of the D800). 

The problem is that many people mistake absolute resolution (acuity) with visual sharpness. What happens is that they crank up the values on their raw converter (or in camera Picture Control) and they get what they perceive to be nice sharp, defined edges. The problem is that those edges are polluted with contrast adjustments. 

Those of us with trained eyes can usually immediately see the difference between innate sharpness (great lens) and contrived sharpness (contrast changes added on edges after the fact). It is visually different. And it has to do with how detail/contrast is rendered across all pixels as opposed to some at edge boundaries. 

A really great lens doesn't need a lot of sharpening in post. A really poor lens may need a great deal of sharpening in post, and the end result just doesn't look the same.

The second aspect I see is related to what I call smearing or smudging. You've probably seen this when you pixel peeped in corners. What the heck is going on there? The detail just disappears into ugly blur. And often blur with abnormal characteristics.

With lenses that have astigmatisms and coma in the corners, here's a test you can do to see the problem more clearly: use a projected laser dot, or set a small LED to resolve at distance and then shoot those in those corners. These light sources should be points (or maybe small circles if you have megapixels galore). But they're often not. Instead, they spread and smear and even change shape from round to oblong or worse. 

So what happens when something that should be a point blurs into a stretched oblong object? Well, if you don't have a lot of megapixels, you don't really detect that blur very well. Indeed, because of the way demosaic routines do their work by looking at a range of pixels, sometimes that plays into things, too, and you just get some pixel blurring. 

But at 45.4mp FX? Yes, we're going to actually see the multi-pixel blurs/smears/smudges in the corners if they're present, and you're going to see their shape and size. You don't see that on a 12mp FX camera. 

So, when I say that a lens is D850-worthy, I'm suggesting that these two problems are not present, ignorable, or minimal. Particularly compared to other optics you could choose of the same focal length(s), where those problems will be present, un-ignorable, and in many cases spreading over multiple pixels. 

I'll be the first one to admit that my lens list is subjective. That's because so many factors enter into the analysis (e.g. different subjects, different subject distances, different apertures, different focal lengths on zooms). What you tolerate in terms of blurring and smearing may not be what I'll tolerate. 

At some point you have to be critical of why you're buying a high pixel density body with lots of pixels, though. What do you really gain by riding the Pixel Express? Well, not a lot if you try to use compromised lenses. That makes the other aspects of the D850 more important to add value—and they may for you—than the pixel count. 

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