Lens Sharpness or: How I learned To Stop Worrying And Love The Lens

There are a lot of ways to quantify the performance of a lens.

One standard method is the Modulation Transfer Function, or MTF.  Without getting into the technical details, MTF charts can be used to represent a lens's optical performance: a horizontal plot means that a lens will produce an image equally sharp at all points across an image plane, whereas one that drops off will be less sharp away from the image center*.

Another handy resource to use are websites that do standardized testing of lens performance, one of which is DXOMark. Here you can get plots of lens performance at various apertures, including sharpness, transmission, vignetting, etc. You can even get everything rolled up into a single score for the lens. Handy, right?

Yes...to a point. Something I've run across is photographers judging lenses as inferior simply because their MTF plots aren't good enough or their DXOMark scores aren't high enough. While I'm all for using these resources to learn about my gear and its limitations, they are just that: resources. They aren't meant to be end-all-be-all judgements on the superiority of a lens because there is so much more to consider when choosing a lens than pure optical performance in a technical sense. I'd like to take a moment and call out a few factors that aren't included in these measures.

Bokeh

Bokeh! Shot wide open at f/1.4.

If you're trying to isolate your subject using a narrow depth of field, you likely want those areas to be as out of focus as possible. The pleasing quality of these out of focus areas is called "bokeh". If you're using a more advanced lens that is tack-sharp, corner-to-corner you can achieve pleasing bokeh, but it's also possible that any out of focus highlights will render so sharply as to be distracting. However, if you're using  a simpler lens design that is a bit softer away from the image center, you may have a better chance of getting pleasing bokeh because those highlights will be softer. This is why some portrait photographers still love simpler lens designs. Conversely, this simpler lens may not be an obvious choice for shooting landscapes or architecture, where your subject is likely to be taking up the entire frame and you want it all to be in focus. And the thing is, this is all subjective, and it isn't necessarily constant either. A lens can render amazing bokeh in certain circumstances and be decidedly "meh" in others. And what one photographer sees as great bokeh could be not what another one is looking for. Like all art, it's largely subjective.

Aperture

Polygonal bokeh can be distracting. Shot at f/2.

Part of what determines the "feel" of bokeh is the configuration of a lens's aperture. The rounder the aperture is at a given setting, the "softer" the bokeh will generally be. As a result of this, many lens manufacturers have lately been shifting more and more towards using rounded aperture blades. This is great for bokeh, but the price you pay is often the inability to produce sunstars**. Sunstars are those brilliant, pointy stars that get produced by bright points of light on longer exposures when shooting through a small, polygonal aperture. One of the reason I love older Nikon lenses is for their odd number of straight aperture blades that produce amazing sunstars when stopped down. The tradeoff is that the bokeh on these lenses can be somewhat harsher when shooting at faster apertures because highlights render as polygons instead of circles.

Sunstars! Shot at f/8 on a tripod.

I am a total sucker for sunstars, the donut-shaped bokeh of catadioptric lenses, or any other kinds of optical oddities that lenses produce that you can't just "add in post".  They can be distracting in certain circumstances obviously, but when used right, they add a touch of magic.


Bulk

Lens design has come a long way since the first SLRs revolutionized photography. Computers have allowed for more complicated and precise optical designs, more advanced optical coatings help ensure superior transmission and reduce aberrations, and autofocus systems have gotten faster and more accurate. Well, guess what? All of this engineering means that lenses have generally gotten bigger and heavier too. As someone who enjoys shooting small, fast primes, the idea of toting around some of the newer, heavier offerings for most work isn't really appealing to me in spite of their superior optical performance. Why? Because a photo that you missed because you were too tired or sore from lugging around a heavy kit is always less sharp than one you actually took. On the other hand, if I can limit my kit to just one or two large, heavy lenses and still get the job done without limiting my creativity, that works too. It's a balance, and striking the right balance depends on what you, the artist, needs.

A Lens Is A Tool

My favorite lens. Not in spite of its "flaws", but because of them.

What does this all mean? It means that a lens, just like a camera, is a tool, that different tools have different strengths and weaknesses, and that judging a lens by measuring it along only one axis of performance will likely limit your creativity. My Nikon 50mm f/1.4 AF-D is my favorite lens. It is tiny, is fantastically sharp when stopped down, and produces amazing sunstars, but faster than f/2.8, it gets...quirky. Bokeh is super-soft wide open at f/1.4, but has a weird harshness from f/1.6 down that goes away by f/2.8 due to the straight aperture blades. And faster than f/2, getting your subject sharp is hard because your depth of field is paper-thin, and chromatic aberration tends to soften even things that are in-focus. It's MTF charts reflect this quirky behavior, and at first glance it seems decidedly "meh".

Does that make it a lesser lens than more modern 50mm lenses with "better" MTF plots at faster apertures and higher DXOMark scores? No. It only makes it a different lens. I would consider a newer lens with rounded aperture blades for a studio portrait shoot, or a more rugged lens with better weather-sealing for use on the water, but for candid or street photography, where small size is a virtue, or landscape shots, where stopped-down performance reigns and sunstars can add a nice creative flare, it is absolutely perfect.

Photography isn't just about producing the sharpest possible image. It's about translating your artistic vision into an image using the proper tools.

Know your tools, and choose them wisely.

Epilogue

I have become a huge fan of "try before you buy". I've been renting 70-200 f/2.8s for racing shoots for about a year now, and I realized that I was wasting a huge opportunity by always getting the same lens. If I really wanted to know my tools, I should be trying different ones. So, as much as I love the Nikon 70-200 f/2.8 VR2, I branched out and started trying other offerings, and I've learned a lot. I also have a few other rentals coming up for other occasions. Stay tuned for my write-up of these experiences!

*Assuming your MTF plot is vs frame position. Other fields outside of photography will often plot them vs frequency.

** Some manufacturers have managed to produce apertures that are round when wide open for good bokeh but polygonal when stopped down for good sunstars. I try to find these whenever I can....