As many of you may have noticed I keep carrying two lenses with me most of the time. These two lenses have never disappointed me and they are a good choice at every opportunity. Of course, I’m talking about the SIGMA 50mm F1.4 DG HSM | Art and the SIGMA 24-105mm F4 DG OS HSM | Art. And for this lens review I decided to put the fairly reliable SIGMA 50mm F1.4 DG HSM | Art to the acid test. Therefore I’ve written two informative reviews. For all those among you who are technically experienced and who want to keep informed about all aspects of their lenses I can only recommend reading these reviews. I’ll briefly explain them and tell how well my lens fared.
TESTING THE AUTOFOCUS:
A very essential and meaningful test relates to the autofocus. Many lenses have minor or major errors with their autofocus right from the beginning. These errors can be deviations to the back or to the front, which is called back or front focus. On many cameras you can manually adjust the autofocus in the menu to a certain degree.
The autofocus can certainly be tested for every picture you take. You can check whether the focus is all right or too much to the front or to the back. But of course there’s also a more exact and standardized way of testing this. The company LensAlign produces equipment that facilitates these kinds of tests. For autofocus tests you shoot with an open aperture in order to check out the result in a much better way. The open aperture makes the level of sharpness smaller and allows you to define the focus point in a better way.
The autofocus test should be done under moderate and natural light conditions. For this kind of test there shouldn’t be any bright sun light or dark rooms. The camera is placed on a tripod and the testing equipment on another tripod. With a rangefinder you can measure the distance between both tripods and adjust them according to the description given by LensAlign. With a 50mm lens on a full- format camera that’s 1.2 meters. With a 85mm 1.8 lens for instance that’s 2.1 meters. The important thing about this test is placing your camera and the testing panel (which is located in the center of the slightly tilted scale) parallel to each other. This is the case when the red dot behind the panel becomes visible. According to the alignment it is covered with a magnet so it won’t distract you. Afterwards you shoot with a wide open aperture. You should take several (about three) photos in order to make the result more visible and minimize errors. I also took some sample pictures with the smallest aperture (f2). In these photos, however, the level of sharpness was too big to recognize the result.
When you take a closer look you’ll see that the 8 in the front is located slightly further inside the focusing plane than the 8 behind. The 50mm F1.4 DG HSM | Art combined with the Canon EOS 5D MarkIII which I tested has a slight front focus. You can notice this because the frontal 8 and 16 are a bit more focused than the numbers behind.
The feature “lens correction“ allows you to change details about the autofocus and thus eliminate minor front and back focus deviations. After making these configurations you repeat the test until the sharpness level is exactly in the center of the measuring field.
Vignetting counts among the seven imaging errors in photography. Vignetting means a decrease of luminance on the margins of a picture. That is, a dark frame around the image (and in the corners in particular). This error mostly occurs when shooting with an open aperture. You can also notice that the error occurs more often when using wide-angle lenses than normal focal lengths or tele-zoom lenses. You can correct this imaging error digitally or even with some configurations on your camera. By dimming vignetting can be reduced or even eliminated. The higher the quality of your lens the lower is the risk of this imaging error to occur. Many cameras, including my Canon EOS 5D MarkIII, have a configuration that automatically brightens vignetting. For this test, however, I deactivated this feature in order to not distort the result and to interpret it correctly.
For the vignetting test you should shoot under normal lighting conditions and with a wide open aperture right into the shining spot of the overhead projector. You repeat this with different apertures such as the critical aperture, f8 and f16. The pictures are then converted into TIFFs by using Lightroom. Afterwards you use the software program MAtLAB Handle Graphic. This program primarily analyzes the luminance values of the picture taken, which makes the number of color channels redundant. Then the pictures with different apertures are transferred to the software one by one to be evaluated. Each picture comes with two diagrams.
The result proves that the SIGMA 50mm 1.4 DG HSM Art has some minor vignetting at a level that can be corrected very easily. Thanks to the vignetting correction feature in my camera I could amend quite a lot while the rest can be corrected very well with an image editing program like Lightroom. Both with an open and a critical aperture the results of the tests are all right.
These tests proved to me once again that my SIGMA 50mm 1.4 DG HSM Art rightfully counts among my all-time favorite lenses. In my opinion you can learn a great deal by doing the autofocus test. You should do this test regularly in order to get the best out of your lenses. You don’t even need expensive equipment as this test can be done, for instance, with a ruler or a self-made bar.