So you've decided to take the plunge on a super-wide lens. But you've done enough reading to know that not all of these lenses are created equal. Some provide sharp clear images and others distort reality more than a congressman. 

How do you know what you're getting? You can read reviews and see what they say about the lens if you're looking at new lenses. But those reviews sometimes leave a bit to be desired, and they're not even an option if you're looking at a used lens. The good news is that there's a simple way to get a feeling for how much a lens distorts. It's called the brick test - and it will take longer to read this article than to do the test.

The type of distortion that you're most likely to see in a wide-angle lens is rectilinear distortion. Rectilinear distortion simply means that straight lines aren't recorded as straight lines by the lens but rather they curve across the image. The most obvious, and intentional, example of rectilinear distortion is a fisheye lens and in fact rectilinear distortion is sometimes called fisheye distortion or fisheye effect. Images taken with a fisheye lens appear to bulge out towards the viewer. But non-fisheye lenses aren't supposed to do this. They should render the image that appears flat and normal - as you would see it with your eye. For lens designers this gets harder as the focal length of the lens gets wider.

Now there are two ways you can go about testing a lens. The first is to take it into the lab, set it up on a test bench loaded with expensive equipment, and measure distortion in increments smaller than a human hair (people love to debate these measurements in online news groups). But I'm going to suggest a more practical method for measuring what you can actually see. No test bench will be required. After all if you can only see the difference in a test chart it really doesn't make any difference anyway (I can already hear my mail box filling up).

What you'll need is a tripod and a brick wall. I like to use a cable release just to make things a little easier but the self-timer on your camera will do as well.

Setting up
Your eyebrows probably went up when I said brick wall. But if you think about it the easiest way to spot this kind of distortion is to take a picture of a grid - and a brick wall is a nice handy grid that most of us can find easily.

Start by checking the minimum focusing distance for the lens and set the tripod up a little further away from the wall than the minimum focusing distance. If the minimum focusing distance isn't stamped on the lens and you can't get the spec somewhere just experiment to see how far back you have to get before the lens will lock clear focus.

Get it square and level. The quick and dirty way is to look through the viewfinder and line up the edge of the viewfinder with the mortar lines in the brick. If the lines diverge in any direction make slight adjustments until they line up in the corners. I've seen some lenses that fisheye so badly that you can only line up the corners - some old Vivitars and Samyangs for example. If you want to be more accurate you can buy a bubble level to install on your hot shoe. If you want to be really thorough you can bring a mason's level and check to make sure the wall is level (just kidding). Don't get carried away! Remember this is the non-scientific test.

If it's a fixed focal length lens I'll fire off one shot wide open (usually f2.8 or larger), one shot at f5.6, and one shot each at f8, f16, f22, up to the smallest aperture. Those of you really wanting to be thorough can take one shot at each aperture setting. If testing a zoom lens I'll do this at intervals through the zoom range. For example when I did this test for the Sigma 17-35mm lens in this issue I shot sets at 17mm, 20mm, 24mm, 28mm and 35mm, shooting max, middle, and minimum apertures at each focal length (see below).

Interpreting the Results 
Get a full frame 8 inch print made from the negatives. I'll start with one print of the widest angle at f2.8. If you're going to see distortion it's most likely at the widest angle. Lay a ruler across the mortar lines in the brick starting at the edges and working toward the center. They should be dead straight. If the lines curve than you have distortion.

The digital version is to get the lab to do full frame scans. Open the scans in your image editing software and use the line tool to strike a straight line overtop of every couple of mortar lines starting at the edges and working toward center. If the lines are curving you'll see it immediately.

In both cases I'm stressing full frame. Often prints or scans from you lab will cut off part of the frame. You want to make sure you're looking at the whole image.

Also take a look at sharpness. Brick is a highly textured surface. On those frames that you shot wide open take a close look at the sharpness in the center of the image and compare it to the sharpness in the corners. In many lenses there is some softening in the corners. This is not necessarily horrible - but it is something you'll need to keep in mind when composing a scene. By f8 you shouldn't see and loss of sharpness. 

Light fall off, a condition where the corners are darker than the center of the image is far more serious. If you can see noticeable light fall off you might want to consider a different lens. It will stand out like a sore thumb, especially in continuous tone areas such as blue skies.

Practical vs. Scientific 
There are those of you who live and die for specs, charts and graphs. They are probably horrified at how unscientific this approach is. But remember it's not meant to be scientific. It's a quick, dirty, and pragmatic way to evaluate a lens in the real world. As I said above - if you can't see distortion in an 8x12 print of a pattern that will make that distortion jump out then you'll never see it in a normal image.

Looking at the Results for the Sigma 17-35mm
The Sigma distorts noticeably at 17mm and this was obvious in some of our slides (which is why we decided to use it as an example). More worrisome is the fact that light falloff at 17mm and f2.8 is very noticable. Click on the images below to enlarge them. The effect of the light falloff is exagerated by resizing the image. At f8 at 17mm light falloff is significantly reduced but you can see a difference in exposure across the frame.

By 20mm light falloff has become a non-issue and the amount of fisheye distortion has been reduced to the point where you'll never see it in most images. By 35mm there is virtually no distortion and almost no difference in exposure across the frame. 

Click on the image to enlarge and see the results
17mm / f2.8	17mm / f8	20mm / f8	24mm / f8	28mm / f8	35mm / f8

All of these test images were shot under uniform light 
in open shade conditions using a Nikon F100 
on a Bogen tripod using a cable release. 

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