07-04-2014, 07:47 PM
Hi,
here's my personal experience with the new Sigma 50mm f1.4 ART lens which is hyped all over the place. I tested with a Canon full frame body and mainly wide open (f1.4).
To summarize:
Here are some image details of a simple "lawn sprinkler" test. The drops from the sprinkler act as many little almost point-like light sources. The lawn sprinkler curtain of drops was shot from the side. The focus was set so that the drops in the center of the image (region 1) are about in focus. The drops on the left (region 2) are in front of the focus and and the drops on the right (region3) are behind the focus. The photo was made with the aperture fully open (f1.4) on a full frame body.
Below are an overwiew of the image and three close ups into interesting regions.
Region 2 (left side of image)
In region 2 on the left side of the image, I observed very weird diamond-shaped bokeh. The drops in this region are in front of the focus and therefore are expected to generate out-of-focus blur (bokeh). The effect increases with field size (== distance from center) and the rotation of the diamond-shapes obviously depends of the azimuthal position of the field point "the diamonds rotate with the field".
I would have expected circular out-of-focus blur with part of the out-of-focus disc beeing cut away because of vignetting.
The effect of the diamond-like bokeh doesn't seem to be present on the right side of the image (region 3) where the drops are behind the focus.
The diamonds don't look coma-shaped as from decentered or tilted lenses (and in this case the effect should be field independant anyway). My gut feeling is more that this could be something like field-dependant astigmatism but in this case, the field-dependency should be linear across the field (beeing zero in the center) and the diamonds should appear also on the right side of the image (with an astigmatism of opposite sign).
Any better interpretation of the diamond shape?
Center (region 1):
The drops are about focused in the center of the image. There are magenta and greenish drops. The only interpretation I can imagine is that the greenish drops are in front and the magenta drops behind the focus (or vice versa) and the color comes from strong longitudinal color such that in one focus position, the green is already defocused (while the magenta beeing still sharp) and in the other case magenta is defocused while green is still sharp.
The significance of this effect makes me thinking that this lens has A LOT of longitudinal color (or whatever generates this color split).
Any better interpretation of this effect?
Right (region 3):
This is the region where the out-of-focus blur comes at least closest to what I would have expected. The out-of-focus blobs are systematically colored (center magenta, edge green) which could come from a mixture of longitudinal color, spherical aberration and spherochromatizm.
My conclusions:
Image regions in detail:
here's my personal experience with the new Sigma 50mm f1.4 ART lens which is hyped all over the place. I tested with a Canon full frame body and mainly wide open (f1.4).
To summarize:
- The autofocus was correct in less than 50% of the shots, even for still objects. If shooting the same still object several times, some of the shots are sharp, some aren't. This was true for objects at any distances.
- The AF stopped working completely once -- the AF drive just didn't move at all any more. Turning off and on the camera made it running again.
- The image quality was not what I had expected from all the reviews to say it politely, see below in more detail.
Here are some image details of a simple "lawn sprinkler" test. The drops from the sprinkler act as many little almost point-like light sources. The lawn sprinkler curtain of drops was shot from the side. The focus was set so that the drops in the center of the image (region 1) are about in focus. The drops on the left (region 2) are in front of the focus and and the drops on the right (region3) are behind the focus. The photo was made with the aperture fully open (f1.4) on a full frame body.
Below are an overwiew of the image and three close ups into interesting regions.
Region 2 (left side of image)
In region 2 on the left side of the image, I observed very weird diamond-shaped bokeh. The drops in this region are in front of the focus and therefore are expected to generate out-of-focus blur (bokeh). The effect increases with field size (== distance from center) and the rotation of the diamond-shapes obviously depends of the azimuthal position of the field point "the diamonds rotate with the field".
I would have expected circular out-of-focus blur with part of the out-of-focus disc beeing cut away because of vignetting.
The effect of the diamond-like bokeh doesn't seem to be present on the right side of the image (region 3) where the drops are behind the focus.
The diamonds don't look coma-shaped as from decentered or tilted lenses (and in this case the effect should be field independant anyway). My gut feeling is more that this could be something like field-dependant astigmatism but in this case, the field-dependency should be linear across the field (beeing zero in the center) and the diamonds should appear also on the right side of the image (with an astigmatism of opposite sign).
Any better interpretation of the diamond shape?
Center (region 1):
The drops are about focused in the center of the image. There are magenta and greenish drops. The only interpretation I can imagine is that the greenish drops are in front and the magenta drops behind the focus (or vice versa) and the color comes from strong longitudinal color such that in one focus position, the green is already defocused (while the magenta beeing still sharp) and in the other case magenta is defocused while green is still sharp.
The significance of this effect makes me thinking that this lens has A LOT of longitudinal color (or whatever generates this color split).
Any better interpretation of this effect?
Right (region 3):
This is the region where the out-of-focus blur comes at least closest to what I would have expected. The out-of-focus blobs are systematically colored (center magenta, edge green) which could come from a mixture of longitudinal color, spherical aberration and spherochromatizm.
My conclusions:
- I didn't observe unexpected results like this (both image quality and AF) with any other lens I had before (eg. Canon and Zeiss). This clearly doesn't have any statistical significance, as I only had a few lenses and just might have been unlucky with this sigma lens. It makes me wonder though, if sigma sacrifices quality control in favour for beeing competitive. I had a Zeiss Otus for loan some time ago and it delivered exceptional image quality. Again -- to be fair, this is just a comparison of two samples but in this case Zeiss delivered high-end as-built image quality and sigma didn't. To be even more fair, I've to say that with the Zeiss lens not only 50% (like for the sigma) but rather something like 90% of my shots were defocused because my kids move faster than my manual focus capabilities ;-)
- All the single sample reviews are of limited worth because they don't tell anything about the distribution of the as-built image quality but only give results from a single sample. If the sample is provided by the manufacturer, there's an additional risk that the manufacturer cherry-picks the sample to get an excellent review.
- unfortunately many reviews don't evaluate AF capabilities but these might be important for the real world usability
Image regions in detail: