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 p.9 #19 · Voigtlander 35mm f/2 APO-Lanthar Review | |
Fred Miranda wrote:
I definitely trust your eyes! Thanks for sharing your work on lens and camera mount alignment. I do these reviews for fun and of course, if I get things wrong I will definitely correct it. This is a rolling review and I'm still learning about the lens.
This test was done with my first copy: (which was well-centered on my A7R IV).
https://www.fredmiranda.com/forum/topic/1695782/0#chapter3
..by the way my second copy was just as well centered but it had way better corner resolution.
This was done at infinity. Basically, it's a very easy test. I focus the subject at center and take a picture, focus at mid-field, take a picture and focus at the extreme corner and take another one. In post, I compared all three images at center to see if there is any discrepancy. The crops I posted were at 100% magnification. I will test my second copy and let you know if it has similar behavior.
PS: I am getting a third copy next week. Something I noticed is that my first copy could not match the corners of my second copy but my second copy was not quite as sharp 'at center' as my first (wide open). This is the type of variation probably caused by element spacing since both lenses are well-centered.
My Voigtlander 35/2 APO M-mount is similar in sharpness to my first copy at center and my second copy at the corner. So, I know what this lens is capable of....Show more →
Thank you Fred! Your reviews are invaluable and I'm sure many, many members of your forums will agree. I have referred to them many times over the years to make my lens purchasing decisions. You are a careful and experienced tester and it is deeply appreciated. I really want this V35 to be great. The V50 is, and the MTF charts suggest something amazing from the V35. All 6 copies of the V50 I've tried are, and they only varied most in swing and tilt. I was very surprised by how severe the V35's field curvature is. It didn't seem that severe when I eyeballed it at the store.
Are the crops of the centre of the V35 you posted from copy #1 or #2? If #2, are you saying this is less sharp than before? Could you show us the difference please?
Usually, when testing lenses for landscape use, I prefer not to refocus in the mid-field and corners to get a better idea of how field curvature will affect the final result, if there are any weak regions in my particular copy, and how much stopping down is necessary to overcome them, if any. Refocusing on the region of interest, which is what you've done, helps to eliminate subtle tilt and swing issues (good, if the lens isn't being adjusted) and curvature if any, but individuals testing their own lenses and wondering how best to overcome their flaws for their own shooting might want to consider this. Joseph Holmes comments on this and his recent testing of the GF lenses might be intersting to some.
Do you still have both copies of the lens with you? Do you mind doing a quick and simple field curvature test that's a little different from what you've done? If you look at my raw images here, the focus is placed in the middle of the width of a road, camera is positioned parallel to the strip of tarmac, road should be relatively flat, and there needs to be sufficient subject material in front of and behind the plane of focus. The plane of focus should be positioned in the middle of the frame, camera horizontal orientation. Do you mind providing the full resolution file from each copy of the V35 you have? Or at least the second one, if you no longer have the first?
The issue I see with your diagonal infinity test is that if there is field curvature that bends further away, you can't tell because there is nothing but sky beyond infinity. If it bends nearer, which is more often the case, you can see it.
I'm uploaded two raw files from my copy in the dropbox link, linked to in the post above, which shows something interesting. If the focus was best on-axis, the edges look very astigmatic but they are actually out of focus. If I compromise the focus, the edges and corners look OK, but the sharpness on-axis drops a little. It's not a big difference in central sharpness, but the edges improve massively. Look out for these filenames in the folder: _DSC3510_DSC3563 V35 F2 SN07120513 a7R II compromise focus for curvature.ARW and _DSC3563 V35 F2 SN07120513 a7R II.ARW.
A third copy! Oh my. The hunt for a good V35 is harder than I expected. In case anyone is wondering why I'm fussing over the V35's wide open performance, these are what the A-L lenses are known for. Also, I'm intending to use it for astrophotography at f/2.8, so the performance wide open must be stellar. It can be, like with the V50 and based on Cosina's published MTFs. It should be amazing. Even if there is field curvature, the MTF promises incredible resolution on and off-axis, which I'm not seeing with the two copies I've touched.
This is the type of variation probably caused by element spacing since both lenses are well-centered.
I'm not sure what you're seeing, but this is what element spacing errors cause, which may not be what you're seeing.
From Brandon Dube's comments on the Lensrentals blog:
The mechanism of misalignment in a lens determines how its MTF departs from the average or nominal value. A spacing error will cause a relatively uniform error across the field of view because it changes only spherical aberration and axial color, aberrations that are constant over the field. There is a balance between aberrations so it is not precisely uniform over the field, but for small changes the aberrations are reasonably orthogonal.
And more interesting comments by Brandon on curvature:
Surfaces are not equal for different purposes. If you want to image on a flat surface, you absolutely must have both positive and negative powered elements for the Petzval sum to be anywhere near zero. If you curve the image, you don’t need negative lenses anymore because the image plane can curve as the positive lenses want it to. This means you can delete the negative lenses, which probably injected aberrations into your design anyway.
Or, in the more general case, it gives you a knob that is essentially a 1:1 control over field curvature. Where you may have had 5, maybe 10 elements all fighting each other to get near zero field curvature otherwise.
This makes the curved image surface as valuable as several other elements, not just parameters, in the design.
The reason the image is curved is first and foremost because of "petzval curvature," which has a design type named after it, since that is the only primary aberration the design does not correct for. Petzval curvature comes simply from the positive focal length of the elements and is, as a result, not very sensitive to alignment.
Next from petzval you have "astigmatism," which affects the tangential ray fan three times more than the sagittal because of geometry. It's obvious, then, that the design should immediately manifest any perturbation of astigmatism three times more strongly in T than S. Then you have the designer's efforts, where they introduce "tangential astigmatism" or "sagittal astigmatism" to flatten the field. They need three times as much of this pill to bring T on top of S, and generally the more aberration you put in, the more sensitivity you get, too.
Astigmatism and field curvature are invariant with aperture. They are reduced in apparent magnitude when the aperture is closed, because the depth of field is increased. If a lens has focus shift from spherical aberration, the plane selected in the MTFvFvF plots may move to one that is more astigmatic (e.g. if the T and S fields move away at the edge, which is what astigmatism is).
The separation of the T and S lines in an MTF vs Field plot is not directly indicative of astigmatism; there can be several causes for that (astigmatism being one of them.)...Show more →
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