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! Shot wide open at f/1.4.
|Polygonal bokeh can be distracting. Shot at f/2.
|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.
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.
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.
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....