on 7 May 2018)
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 typically included in these measures.
|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
sharp and 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.
|Polygonal bokeh can be distracting. Shot at f/2.|
|Sunstars! Shot at f/8 on a tripod.|
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, because of its flaws.
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
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 always 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.
*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....
In Hindsight - Updates from the future
I will write more about this in a future 'blog post, but my beloved Nikon 50mm f/1.4 AF-D has been replaced. After spending a good deal of time with manual focus lenses, I came to realize that much of the "softness" of my f/1.4 was really just missed focus. I tried focusing it manually for a while*** and noticed that my photos were coming out sharper and cleaner than before while still maintaining much of the character of how it rendered. However, one of those manual focus lenses that I spent so much time with is the Nikon 50mm f/1.2 AI-s. This lens is a work of art. It is about the same size as the AF-D, although it is considerably heavier since it's all metal and glass, and it is a much stronger performer optically, both in terms of sharpness and character-generating flaws at faster apertures. I couldn't justify keeping two lenses that were so similar...so I broke down and sold the AF-D. I'm still glad that I owned one, it definitely taught me a lot.
I've also learned more about lenses in the intervening time since I originally posted this, more nuances that aren't easily quantified in single numbers in lens reviews. But adding them here would make this post unbearably long, so I'll save them for another time.
*** Or using auto focus in live view to eliminate any "fine tuning" errors....