How I Test a Newly Acquired Lens

Now? Scrap Iron!  Colorado State Railroad Museum, Golden, Colorado (click to enlarge)

When I purchase a lens, whether new or used, I like to take it through a couple of tests to find out if the lens will meet my needs and provide the kind of performance I would expect from it.  The tests are practical ones and not meant to take the place of a highly sophisticated optical bench or other computerized testing.  My goal is to find out if a specific lens works and the images made with it please me.  Only me.  I'm not really concerned if someone on the Internet doesn't like it.

After a very close outside and inside visual inspection, here are the two tests I conduct.   Test 1 gives me a handle on whether or not the lens' has any mechanical/optical issues.  Test 2 is for determining if the lens produces aesthetically pleasing images.

I developed these two tests just for me to ensure that lenses I buy are working correctly.  The first test will
test for correct and consistent autofocusing, correct aperture blade closure, consistency in exposures with different aperture settings, de-centering of an element and distortion.

Test 1

1.  Find a brick wall of sufficient size; it should by nature be perfectly vertical and the bricks aligned perfectly      horizontal.  If it is not, find another wall and don't hire the brick masons who built that one!  Depending          upon which focal length you are testing, a small wall or a large wall will suffice.
2.  Go out on a sunny morning or afternoon with the sun at an angle to the wall so the bricks throw shadows.
3.  Set up your tripod at a medium distance depending upon the focal length of the lens; closer for wide angle      and longer for telephoto; for example for a normal lens about 30 ft.  I recommend testing a zoom lens at        several focal lengths so you will have to move the tripod and re-level it.
4.  The camera must be level, square to the wall and plumb, the object is to get the sensor plane as close to        parallel to the wall as possible. UPDATE:  Don't forget to turn off any stabilization, whether in the camera      or lens.  Leaving image stabilization on while the camera is locked down can cause blur.
5.  Set you camera to aperture priority, the lowest ISO, 2 second timed shutter delay, mirror lockup if it has       it, single autofocus and lens wide open
6.  Take an exposure and check your histogram to ensure your exposure shows detail in the bricks and              mortar.  I tend to overexpose slightly so details are clearly visible.
7.  Manually de-focus the lens, then give it a half-press to autofocus on the wall.  Make an exposure.                  Repeat the process twice more of manually de-focusing the lens, half-press to autofocus, then make the        exposure.
8.  Stop the lens down for each f/stop repeating the process.  I make three exposure for each aperture                setting looking for consistency.
9.  If you are testing a zoom lens, pick at least three focal lengths to test.  That should suffice.
10.That is it.  It shouldn't take you more than 15 minutes.
11.Take your images back to Lightroom or another image editor and look at them very closely, at 1:1.  If          you are doing things right, you will spend more time evaluating the images than out making them.  Looking      closely will tell you what you need to know from a mechanical/optical/focusing point of view.

Also, if you are testing a lens that is a specialty lens, like a tilt/shift, or do a lot of close in work, such as flower photography or portraits, you may want to change your distances to test the camera focused at a closer distance, or on a flat detailed surface for macro work.  I am mainly a landscape, nature and travel photographer and my subjects are usually a good distance from me so that is why, for my tests, I chose a moderate distance.  I will also make some images at infinity or close to it to check focus at a very far distance.  However, at great distances, depth of field usually will negate focusing errors so I try to use a closer distance to eliminate the depth of field issue.

From this you should be able to see a) whether or not your lens is autofocusing accurately and consistently, b) how much distortion there is as the horizontal/vertical aspect of the bricks will tell the tale, c) if your aperture/shutter combinations are consistent at various exposure combinations and, d) if your lens is focusing properly across the entire frame.  This last item is critical.  Read on.

I recently had a lens that I tested and, after careful examination at 1:1 in Lightroom's Develop Module (don't check focus in Lightroom's Library Module as I have read and heard that the accuracy of its display of an image's actual sharpness is suspect), I found that the lower half of the frame was in very sharp focus, but as one looked higher in the frame, the focus became softer.  The upper corners and upper center at the top of the frame only reached close to being in sharp focus at f/11while the rest of the frame was in sharp focus wide open. Of course the center is almost always sharper than the edges, but you could tell it was in focus.  I believe this was a case of a lens element being slightly de-centered.  If I am wrong, one of you readers/optical experts please advise as I am always anxious to learn as well as make sure everything in the blog is correct.

Test 2

Take the lens out on a variety of days and make images with it.  At this point you will know that anything unsatisfactory is not a mechanical or lens/camera combination issue.

A)  You can check for chromatic aberrations by photographing dark green leaves against a bright white sky.
      Make a series of images of the leaves/sky with in the middle, edges and corners of the frame and at               different apertures to get a feel for what kind and how bad any chromatic aberrations are.  See if you can       easily correct it in Lightroom, Adobe Camera Raw or another image editor.
B)  Make images with the lens wide open and look at the bokeh.  Do you like how it is rendered? Close the       lens down fully and see how much diffraction degrades the image at what apertures it becomes                     unacceptable.  Also, if it is a zoom lens, do all these tests at various focal lengths.
C)  Make images on sunny days, cloudy days, early morning and late evening.  Make images of brightly               colored objects and very subtle color as well to see how the lens renders color.  Some lenses render             color warmer or cooler than others.
D)  Make images with strong backlight or with the sun or another point light source in or just out of the frame       to check how the lens handles flare.
E)  Make some images of moving objects to see how fast it focuses with your camera.  Everything from              people walking to cars driving toward you and at an angle to you.  Now, if it doesn't hit perfectly on all          images, it just may be your camera's capabilities and not a problem with the lens.  But some lenses do            better than others and you need to know how this specific lens performs.
F)  Finally, either make a few enlargements or send off and have a few made to see how the lens really               performs, not just what you see on a computer screen.  There are big differences in what you see on a           computer screen to what you see in an actual print.

I could go on, but the point being use the lens in a variety of ways to then judge how you like its performance.  If you like the images it renders, keep it.  If you don't send it back.  It is really your judgement and vision that is important to meet, not some reviewer, a testing lab somewhere or what another photographer might say.  You have to be the one who is happy with the images it makes.

I think if you do all this, you can make an informed decision about whether or not the lens is functioning properly and whether or not you like the images it produces and, ultimately, whether or not you will keep it.

These types of practical tests don't take a long time but provide you with valuable information.  I recommend you try it sometime.

Thanks for looking.

Dennis Mook

Many of my images can be found at www.dennismook.com.  Please pay it a visit.  I add new images regularly.  Thank you.


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