How to Test Your Lens

So, you just bought a shiny, new, and maybe expensive, lens for your camera, and being the savvy consumer, you did your homework. You pored over customer reviews on the B&H Photo website, read online reviews splattered all over the Internet, grabbed a copy of every photo magazine that reviewed the lens, bookmarked dozens of websites, and now have the lens's MTF curve charts burned into your retinas.

Now, your lens is here and it is time to go out shooting. Honestly, not a problem. Your lens should be as good as it can be and, as photographers, we want to take photos. So, stop reading and go out and make some photographs!

Still here? OK, let me tell you why you might want to test that lens yourself. The main reason is that even with today's precision computer-controlled manufacturing techniques, there are variations in each and every lens that rolls off an assembly line. This means that some examples will be better than others. Some lenses will be outstanding and others will make you wonder if all of those glowing reviews you read online were completely bogus.

I once purchased a new 40mm lens that was advertised and reviewed to be, based on all reviews, incredibly sharp. The Internet is full of tests that show this is a superb lens. I spent some good money on this lens and hurried home to try it out. However, me being me, I took the new lens out for a shoot with my tried-and-true 50mm f/1.8 lens in the bag. I took the same photos with the two lenses, albeit at the slightly different focal lengths, and hurried home. After analyzing the images, I was left to speculate that, either the reviews are full of lies about this 40mm lens, that I got a bad example of this lens, that my 50mm f/1.8 is the most awesome lens ever made, or a combination of those possibilities.

"I like to test my lenses to determine where in their aperture range and focal range (if a zoom) they are performing at their best."

Had I not had a lens to which I could compare it, I might have been happy with the performance of the 40mm and not known the difference. But, because I did a quick test of my new lens, comparing it to a lens that I was familiar with, I was able to determine that it should be returned to the store and my money saved for something else.

Knowing that no two lenses are the same, as far as performance, why else would we want to test a lens? For most of my photography, I want to maximize the sharpness of an image. In order to do that, I like to test my lenses to determine where in their aperture range and focal range (if a zoom) they are performing at their best. A lens's sharpness and degree of vignetting will change based on the aperture selected, as well as its focal length, if it is a zoom.

Before We Begin

The goal of this article is to give you some pointers on how you can do some basic lens testing at home. To do that, I will share with you techniques I have used to test my lenses. There are two parts to my lens-test goal:

  1. evaluating the lens's performance,
  2. getting the testing out of the way so that I can go out and make photos.

There are a lot of people out there who go absolutely nuts with lens testing. So, if, after you read this, you find yourself building a dust-free clean room in your home, walking around in a lab coat, and trying to figure out ways to shoot laser beams through your lenses onto a computerized target sensor for dispersion analysis, you may want to consider a new career with some of the more well known lens-testing websites and magazines.

Also, as anyone with an Internet search engine can attest, there are many different opinions and techniques regarding how to test lenses. I generally test my lenses for sharpness. However, you can test for vignetting, symmetry, distortion, focus, and other factors in the comfort of your own home without fancy gear. So, feel free to share your ideas and methods at the end of this article, but please know that what I am sharing here is, simply, my personal technique and I am not suggesting it is the only way, or best way, to test a lens.

Mental Prep

Before we get into the gear needed, you should, if you have it, turn on the part of your brain that may have been lying dormant since grade school science classes. The testing of your lens is an experiment and you need to have a basic scientific mindset to make sure you do not have to go back and repeat the tests over and over again. What do I mean? You need a plan in place before you get started. Have paper and a pencil handy to take notes. Photography can be done "from the hip," but good lens testing needs a methodical approach. Don't worry, we aren't doing crazy stuff here, just know that a bit of pre-organization and a good plan will make it all go smoother.

Also note that there will not be a "control" for this experiment. If we all owned an optically perfect lens with which to test all others against, we would not have to test our lenses, right? So, your lenses, with their inherent flaws, will either be tested against themselves or other flawed lenses.

Gear Prep

The most crucial piece of gear you will need for a lens test is a tripod. If you do not have a tripod, you will not be able to perform an accurate lens test. Period. No one that I know can handhold a camera completely steadily. Going back to the scientific process mentioned above, we need to eliminate as many variables as possible when performing our tests. Movement is something we need to eliminate in both our camera and target. I highly recommend using a remote shutter release (cable, electronic, or wireless), and if using an SLR camera, the mirror lock-up mode to reduce vibrations.

The Targets

I have been told that a famous photographer once said, "I do not photograph test targets." Well, I do. But, I am not famous. Yet.

There are lots of things you can photograph to get a solid lens test. You can do it indoors or outdoors. You can create your own target, buy a target, or print one from the Internet. You can use what is around you. The possibilities are endless, as are the opinions about what works best.

Full-resolution image available by clicking on the illustration above.

Test targets are versatile and can be used for a variety of tests. Some, like the ISO 12233 chart and USAF 1951 Resolution Test Chart, will come up on most web searches. Personally, I located the highest-resolution chart I could download and then I had it printed on photo paper at a big box store at 20 x 30" and dry-mounted on foam core. I was tempted to mat and frame it for permanent display, but all of the wall space in my home was already taken up by photos.

Is a printed test target a perfect target? No. If you want to get crazy, you can spend a lot of money on laser-etched glass targets that are used for calibrating things that are way more precise than your camera and lenses. A printed test target will be your least expensive option.

Some folks will make a homemade diorama with different objects (wine bottles, color charts, tourist trinkets) to photograph in one scene. This is fun and works well for the test, but the problem is, unless you have very understanding roommates or the only key to your photo cave, you will have to put all the objects away when you are finished and you might not be able to recreate it perfectly in the future. The two-dimensional test target gets put away into the closet when I am finished.

Do you need a dedicated target to perform a lens test? No. You can simply go outside and point your camera at something in "real life."

I kind of cringe when I go to a camera/lens review test website and see photographs of trees as a test target. Why? Well, trees move when the wind blows. Remember what I said about eliminating variables? When I do a lens test outside, I look toward manmade structures (usually stuff that humans make has a ton of right angles and sharp edges) and I try to put lettering of some sort (street sign, license plates, storefront signage) in the center and corners of the image. Remember, you are testing your lens, not creating the greatest photograph on the planet, so don't worry about composition and all the other things that you might usually consider before taking a photo.

Also, you want the things you are going to study in the image to be as close to equidistant from you as possible. When shooting a target on a wall, level your camera and aim for a vertical wall. When outside, try to find a wall or scene in the distance that will have the center and edges of your frame about the same distance away.

I have found that a well-populated bookshelf is a great tool for lens testing. Remember what book you are targeting in the center of the frame and use the writing on the spines of the surrounding books to check your sharpness. Also, the shelves, if straight, can let you evaluate lens distortion.

Speaking of distortion, the "brick wall" is often mentioned in lens-testing circles. Not the worst idea, but I would hope to find one with some signage on it to give me some sharp edges and lines to look at in the images.

If you can do it, I recommend the test target for normal (around 50mm) and telephoto lenses (greater than 50mm) and a wall or bookshelf for wide-angle lenses.

The Plan

Many of us have a camera bag full of zoom lenses and prime lenses. If you have only one prime (fixed focal length lens) to test for sharpness, your mission is simple. If you have a quiver full of lenses and many of them are zooms, you will thank me later for that suggestion to get paper and pencil ready.

For a normal or telephoto prime lens, I simply set up the tripod and camera at the distance where the target fills the frame, and then I take photographs as I run through the aperture settings. When out doing non-lens-test photography, I adjust my aperture in full stops to simplify the exposure math for me. I do the same for lens testing. If you adjust your aperture in half stops or third stops, you will get two or three times the number of test images to sift through and that might be enough to make this process less fun.

For zoom lenses, I roll through each aperture setting (full stops again) at the focal lengths marked on the barrel, which usually correspond to popular prime lens focal lengths (85mm, 105mm, 135mm, etc). Of course, I will always test the zoom at the widest angle and the greatest, as well, even if they do not correspond to popular fixed focal lengths.

This is where a little planning helps. Say you are evaluating an 85mm f/1.8 prime lens, an 85mm f/1.4 prime lens, and a 70-200mm f/2.8 zoom lens. You can mount each 85mm lens and roll through the apertures, but, before you move your tripod you might want to grab the 70-200mm lens, set it to 85mm, and test at that focal length. Take notes. The image metadata will help remind you what you were doing later, but it is good to have a check-off sheet tracking your progress, so you do not waste time by repeating tests.

Also, if you are testing older, manual focus, pre-electronics lenses, your notes will help you when and if the metadata does not recognize the lens or aperture setting. When I test my manual focus lenses, I try to set up the test so that the targets fill the frame, while allowing me to focus the lens at infinity in an attempt to eliminate focus errors.

Afterward

Well, you just took several dozen pictures of a test target, bookshelf, wall, or city scene. Now you get to spend some time in front of the computer analyzing the data. I start by re-naming the files with the focal length of the actual shot, and the aperture plus the name of the lens. Example: 105mm f8 - Nikkor 70-200 f2.8 AF.jpg. Then, when finished, I can easily compare the shots from a certain lens or a certain focal length by opening just those images on my screen.

Still got your paper and pencil handy? Open a series of images (single lens or single focal length) and start comparing the visual sharpness of the center of the images and the corners. Take some notes. You can also evaluate the images for vignetting and symmetry, as well. For symmetry, verify that the lack of sharpness in the corners is the same for each side of the lens. For vignetting, you can push the contrast and levels sliders to see if the corners darken.

When I compile my notes, I type them up and send the file to myself in an email, saving the notes on my smartphone. Now, when I am out shooting, I can quickly remind myself that my 50mm f/1.8 is slightly sharper in the corners at f/5.6 than at f/8 and that when I go below f/2.8 I start to lose sharpness noticeably.

Mission accomplished. Let us know if you have different proven techniques or questions. Thanks for reading and good luck with your do-it-yourself lens tests!

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