inksandpaper
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 Re: FM Review: Voigtlander 35mm f/2 APO-Lanthar | |
tsdevine wrote:
I'm not going to profess I know what I'm doing, but that has never stopped me before. Not sure I framed these 100% perfectly, but i think it's close enougn. First is the CV 35/2 APO @ f/2, the second set is the Sigma 65/2 @ f/2. Both on my a7R III. I did allow the vignetting profile to trigger for the Sigma, but no distortion correction. For the CV shot there is no correction of either.
The second shot in each set is using the Find Edges filter.
CV 35
Simga 65
These look good to me....
-Tim
tsdevine wrote:
PM'd you with some RAW files from my CV 35/2 APO and Sigma 65/2. A little quick and dirty as I had limited time to fit this in to a crazy day I have. I hope they are useful. Feel free to analyze and post.
-Tim
inksandpaper wrote:
I usually don't notice Fred posting this information, and virtually all review sites cannot be trusted for their vignetting data since they measure it from OOC JPEGs with contrast curves rather than from linear raw data. Also, no one posts information about the "inner corners" of the lenses, which are important where they overlap if doing stitching for astrophotography. For a single row stitch, I overlap frames by 50%, and the long edge of the sensor is orientated perpendicular to the long edge of the final stitched image. If doing a multi-row stitch, I overlap the frames 50% both on the long edge and short edge. Here are the three sets of vignetting data for those interested. I'm not allowed to post images so the table format is a little wonky, my apologies.
Voigtlander 35mm F2 APO-Lanthar, 17 Apr 2021
Extreme Corner Single row Corner Multi-row Corner
Aperture Falloff EV Falloff EV Falloff EV
2 -2.45 -1.94 -0.78
2.2 -2.18 -1.69 -0.54
2.5 -1.89 -1.39 -0.33
2.8 -1.61 -1.15 -0.26
3.2 -1.42 -0.98 -0.26
3.5 -1.24 -0.84 -0.26
4 -1.12 -0.81 -0.25
4.5 -1.06 -0.80 -0.25
5 -1.05 -0.81 -0.25
5.6 -1.05 -0.81 -0.25
6.3 -1.06 -0.81 -0.26
7.1 -1.05 -0.81 -0.25
8 -1.07 -0.81 -0.26
9 -1.07 -0.81 -0.26
10 -1.05 -0.81 -0.25
11 -1.06 -0.80 -0.25
13 -1.06 -0.81 -0.25
14 -1.06 -0.80 -0.26
16 -1.09 -0.83 -0.25
And some notes on the capture process: camera on tripod, pitched roughly 30 degrees up towards heavily overcast sky. Raw uncompressed. ISO 100. Single-shot mode. IBIS off. LENR off. Lens Comp, all off. Lens focused at infinity in Live View using manual focus on distant subject. Using my sanded-to-matte translucent white acrylic "diffuser" filter, 4mm thick. Ensure shutter speed slow enough that falloff pattern is centered in frame. EFCS off, full mechanical shutter used.
Readouts taken from RawTherapee 'save for reference' with no white balance exported TIFFs, linear raw values from G channel. Used Photoshop colour picker 11x11 sampling, 4 sample points averaged for corners, used ruler guides to ensure exact sampling spot across frames
Extreme Corner read out at 50 pixels from image edge.
Single-row stitch corners 50% overlap coordinates in sensor dimensions, origin is top left image corner, are (0,6)(36,6)(0,18)(36,18) in millimetres
Multi-row stitch corners 50% overlap coordinates in sensor dimensions, origin is top left image corner, are (9,6)(27,6)(9,18)(27,18) in millimetres
Congratulations Tim!! Your CV 35/2 is very flat in its field curvature. There is a very slight amount of swing which falls slightly nearer on the right image edge vs the left image edge, so if one intends to shim it to perfection. My estimate is that you are standing exactly at the edge of the running track? So four lanes plus two feet, let's say 18 feet to the centre of the image. Focus at 19 feet on the left edge, and ~17 feet on the right edge. That puts the left side of your lens ~26 microns too close to the sensor.
But most importantly, this is the first confirmation I have that very flat curvature is possible of this species, which is radically different from the two copies that I tried.
It does appear that the left edge is quite a bit less crispy than the right edge, and I'm not sure if it's due to the light (unlikely?) or actual texture on the running track? It would be unfortunate if this copy was slightly decentered 
Your Sigma 65mm does have slight field curvature, bending very slightly nearer off-axis in a gentle parabola (no weird M or W shaped messes). It is also slightly swung, focusing slightly nearer on the right side again than the left. If you are standing the same distance away, that puts the left edge at 20.5 feet and the right edge at 20 feet, meaning the left side of your lens is ~17 microns too close to the sensor.
The field curvature is about half a foot's worth, so about 17 microns, if my distance estimate is correct, or maybe closer to about 15 microns worth, which is sufficient when combined with the swing to make a field of stars/PSFs at infinity to not be perfectly in focus on the left and right edges of the frame when shooting wide open or even stopped down to f/2.8, if the lens was not shimmed straight.
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