Jonas B wrote:
Maybe somebody can explain a little about this? Is it about bad quality control or a bad quality policy? Is it about bad karma or are all the 35i lenses like this? Why at one side only - is that a result from a polishing machine in need of some adjustment? (...)
lora_to wrote:
Some thoughts - it doesn't explain why the other manufacturers lenses look good and the Sigma's are all tilted on one side, but the consistent tilt is more likely a problem with the setup in my opinion than a consistent error in all the Sigma lenses.
I agree, and that's the puzzling thing. Or one of them.
lora_to wrote:
How do you align the camera with the shelf to make sure it is perpendicular? Small deviations from perfectly square can have a big effect on sharpness due to depth-of-field, especially so close to the target. That is why labs have a permanent setup and most hobbyists check centering at infinity. If you have a permanent setup, did you maybe accidentally shift it after testing the other lenses?
Aligning (the same target and the same method for all the lenses in the graph):
I adjust a tripod so the center of the lens mount is at the same height above the floor as the shelf is. I place the tripod at approx. the correct distance.
Correct distance, to me, is when the USAF patches, all the patches and nothing besides the patches is framed (border to border of course). With a 35mm lens that would be around 0.85-0.9 meters. This is followed by measuring the distance from the central electric contact in the hotshoe to each end of the shelf. Patch No. 4 is in the middle of the target and the optical axis is very very close to perpendicular to the shelf (and patch No.4). The camera and shelf forms an isoceles triangle (I think that is right?).
Here is what it looks like:
Seven small patches, crops, from an USAF chart (year 1951 tech.), evenly spaced along the side of an IKEA shelf.
This is all simple and pretty easy as the resulting test shots usually are are for me only, eye-balled and easy to judge just by reviewing the images in the camera. In this case I measured carefully as I knew I would have to look at the images again after having a first glance at the first series.
lora_to wrote:
It's also possible your camera's mount has some tilt, you could try to focus the lens and then rotate the lens as if you were to take it off the camera while making sure it stays on the camera (that'd disconnect the electronic contacts). If it's the lens, then the region of softness should move, otherwise it's the camera lens-mount or camera/shelf alignment.
Hmm. Then it would be a special Samyang/Sony 35GM/Pentax FA31..../xxx tilt. I used the same method for the RX1 for a comparison. It worked as expected.
This is the first time I got a result looking this way. Please let me know if my description is faulty - English is not my first language.
lora_to wrote:
Generally though, the Sigma 35/2 is probably not the lens for close-focus sharpness - they made different trade-offs here. The upcoming Sigma 35/1.4 DN will probably do better.
But is it reasonable to believe the trade-off is the smearing and ugliness at a part of the right side of the image?
I don't know... I can't explain the situation really. Would it be reasonable to believe a grinding machine at Sigma making an aspherical piece of glass is in need of some adjustment? But surely Sigma should have noticed? Then what?
In any case the lens is a no-go for some of my needs, no matter how nice it is.
EDIT 1: Here used to be a close-up of one of the patches. Removed as somebody looked at it and thought it was good. It was. But from near MFD and not with the Sigma. (April 20)
EDIT 2: My findings about these three copies of the Sigma 35/2.0 DG DN seem to be of very low interest. Interesting! (April 20)
Jonas B wrote:
Maybe somebody can explain a little about this? Is it about bad quality control or a bad quality policy? Is it about bad karma or are all the 35i lenses like this? Why at one side only - is that a result from a polishing machine in need of some adjustment? (...)
lora_to wrote:
Some thoughts - it doesn't explain why the other manufacturers lenses look good and the Sigma's are all tilted on one side, but the consistent tilt is more likely a problem with the setup in my opinion than a consistent error in all the Sigma lenses.
I agree, and that's the puzzling thing. Or one of them.
lora_to wrote:
How do you align the camera with the shelf to make sure it is perpendicular? Small deviations from perfectly square can have a big effect on sharpness due to depth-of-field, especially so close to the target. That is why labs have a permanent setup and most hobbyists check centering at infinity. If you have a permanent setup, did you maybe accidentally shift it after testing the other lenses?
Aligning (the same target and the same method for all the lenses in the graph):
I adjust a tripod so the center of the lens mount is at the same height above the floor as the shelf is. I place the tripod at approx. the correct distance.
Correct distance, to me, is when the USAF patches, all the patches and nothing besides the patches is framed (border to border of course). With a 35mm lens that would be around 0.85-0.9 meters. This is followed by measuring the distance from the central electric contact in the hotshoe to each end of the shelf. Patch No. 4 is in the middle of the target and the optical axis is very very close to perpendicular to the shelf (and patch No.4). The camera and shelf forms an isoceles triangle (I think that is right?).
Here is what it looks like:
Seven small patches, crops, from an USAF chart (year 1951 tech.), evenly spaced along the side of an IKEA shelf.
This is all simple and pretty easy as the resulting test shots usually are are for me only, eye-balled and easy to judge just by reviewing the images in the camera. In this case I measured carefully as I knew I would have to look at the images again after having a first glance at the first series.
lora_to wrote:
It's also possible your camera's mount has some tilt, you could try to focus the lens and then rotate the lens as if you were to take it off the camera while making sure it stays on the camera (that'd disconnect the electronic contacts). If it's the lens, then the region of softness should move, otherwise it's the camera lens-mount or camera/shelf alignment.
Hmm. Then it would be a special Samyang/Sony 35GM/Pentax FA31..../xxx tilt. I used the same method for the RX1 for a comparison. It worked as expected.
This is the first time I got a result looking this way. Please let me know if my description is faulty - English is not my first language.
lora_to wrote:
Generally though, the Sigma 35/2 is probably not the lens for close-focus sharpness - they made different trade-offs here. The upcoming Sigma 35/1.4 DN will probably do better.
But is it reasonable to believe the trade-off is the smearing and ugliness at a part of the right side of the image?
I don't know... I can't explain the situation really. Would it be reasonable to believe a grinding machine at Sigma making an aspherical piece of glass is in need of some adjustment? But surely Sigma should have noticed? Then what?
In any case the lens is a no-go for some of my needs, no matter how nice it is.
Here is a close-up of patch No. 6:
Apr 19, 2021 at 01:11 PM
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