Finding Your Camera Lens Sweet Spotby Doug Niedermiller, dougniedermillerphotography.com
January 8th 2010
So you are looking for a new camera lens or you went out and bought a brand new D-SLR digital camera with the kit lens. Now you want to produce 11 x 14 or 16 x20 or larger sharp prints. You will need a good tripod and know your camera lens sweet spot. The lens sweet spot is the aperture or F-stop setting which produces sharpest image possible.
The lens sweet spot is determined by which F-stop or aperture to get the sharpest image. All lenses have a sweet spot. What we are going to determine is what aperture will produce the sharpest image. A simple rule of thumb is to take the widest aperture and stop it down 2 full f stops or 2 full aperture values ( see chart below). Let’s say you have a lens with a maximum F-stop of F4. That would make the sweet spot of your lens about F8. The problem is this may not be the exact sweet spot for your particular lens. The only way to really know is to test the lens for yourself or you may find it in a lens review article in one of the many photography magazines. The problem is that your lens may not have been tested or at least may not have identified the sweet spot of your lens.
Your lens may have other F-stops not listed on this chart above.
The chart above gives the corresponding aperture value to F-stop. As each aperture value increases it cuts the light in half. As each aperture values decreases it doubles the light.
So let’s test the lens. The first thing I did was find a test pattern chart to test the lens. I found one at http://www.graphics.cornell.edu/~westin/misc/ISO_12233-reschart.pdf You can print it right from the site or downloaded it. Once downloaded you can print the test chart in the best quality your printer will allow you to print. This is what you will need to run this test: your camera and lens; a sturdy tripod; a remote cable release; a board to mount the test chart on; the test chart.
Step 1. Attach the chart on the board with tape so that the chart will not move if there is any wind.
Step 2. Find a bright spot outside and set the board with the chart in the sun.
Step 3. Install your camera on a tripod.
Step 4. Select the camera to aperture priority.
Step 5. Focus your camera on the chart.
Test Chart Location Below
Next, we’ll start with full open. Let’s say at f5.6 and shoot your first picture. Next we’ll shoot a photo at each f-stop, making note of each photo’s f-stop so that when you import them you will know which F-stop corresponds to each photograph. Be sure to use your remote shutter release cable and don’t move the camera through the whole process. Please note that any camera shake or movement will cause an inaccurate test.
After you finish, take your memory card and import into your favorite photo editing software. (I use Adobe Lightroom 2.) Open the photos in your editor and set the magnification to one to one. Use a side by side compare function if available. Now, carefully inspect and compare each photograph to see which one is the sharpest. Once you have determined which photo is the sharpest you have found your lens F-stop sweet spot.
If you are using a zoom lens you may want to repeat these steps for several focal lengths. Let’s say you’re using a 70 to 200 mm lens. First use 70mm then 130mm then 200mm. This will let you find the best F-stop for the sharpest picture at all F-stops.
The chart below shows the tests results I found with the lenses I have.
One thing that I found when performing these tests is the better the lens quality the harder it is to find the exact sweet spot. These lenses generally had between two and three F-stops with the best sharpness. With some less expensive lens I found it easy to find a single F-stop sweet spot. But it is possible with higher end consumer lenses with ED glass (extra low distortion glass) you can get very sharp images.
Now you know what your lens sweet spot is. Even if you have a less expensive lens, by using this knowledge, you will be able to get very sharp photographs with most lenses. But, we do live in the real world. Sometimes, because of low light conditions, or the need to adjust the depth of field, we may have to adjust our F-stop to our needs. So, if the situation is right to use the sweet spot of your lens, you will find you will get the sharpest pictures possible with your lens.
I was not happy with the test results on the Nikon TC-20E II 2X Teleconverter. I have read other reviews of the Nikon brand Teleconverter that come to same conclusion. They also revealed that the Nikon TC-17E II 1.7x and Nikon TC-14E II 1.4x both had superior performance over the TC-20E II 2X Teleconverter.
If you are in the market for a new lens or camera, buy the best lens you can afford. In my opinion, the lens is more important than the camera. Consider this the lens has a life cycle of about 10 or more years and because the camera technology changes so fast, the camera’s life cycle is only 2 to 5 years.
So do your homework before you buy any camera, lens or teleconverter.
Please share your questions or comments below.
Happy sharp shooting.
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