rscheffler
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 35 Cron v1 (8 element) replica Leica lens in the works... | |
Hi all, I received my copy of the replica on Monday and have had mixed results with it. In a nutshell, my copy shows very strong field curvature as it's stopped down. Combined with focus shift that isn't covered by depth of field in 24MP M240 images, my rangefinder focused images shot at f/4-8 are soft in the central third of the image. However, rangefinder focus at f/2 is good and shows slight signs of back focus at f/2.8. It's really at f/4 and f/5.6 where the problem is most noticeable. At f/11 depth of field results in consistent across-frame sharpness on 24MP. I found it's necessary to use live view for best central sharpness, however this shifts the entire wavy plane of focus forward and degrades mid zone and peripheral image sharpness of scenes where across-frame sharpness in a relatively flat plane is desired.
I made tests over the past couple days to better learn the lens's characteristics and how to work with, or around, the strong field curvature and focus shift. But I don't know how representative my copy is given I only have one, which is a recent production version with the new equally spaced half-stop aperture click-stops. According to Kevin, the RFF member coordinating overseas distribution of the lens for the manufacturer, he was told the optics remain the same as the prototypes and early production runs, including use of flint glass, as found in the original Cron 8E. However, others in the RFF thread have reported production tweaks, so it's unclear what, if anything may have changed during the course of production.
From my point of view, my decision to be waitlisted (for exactly one year) was based on the sample images posted in the RFF thread during its development, as well as by some early recipients. From my recollection, those images did not show the degree of field curvature seen in my copy. Ashwin Rao was an early recipient and he posted a fairly comprehensive set of images to his Flickr account, including a full aperture sequence of a man standing at a dock with a comparison set shot on the original Leica Cron 8E. These images, both from the replica and original, show no signs of central image sharpness loss, which is strongly contrary to the results from my lens. His images were also done with an M9M and therefore rangefinder focused.
I've been in touch with Kevin and hopefully we can figure out whether my lens is an anomaly and/or whether it can be adjusted to perform closer to my expectations (which would be for depth of field to adequately cover focus shift and field curvature at the center of the frame for sharp 24MP images by rangefinder focusing).
The field curvature becomes very wavy in a "W" shape. According to the MTF and field curvature documentation provided by LLL with the lens, some degree of wavy field curvature is inherent in the design. However I find that wide open across-frame sharpness is pretty good, considering all the SA and that I can focus and recompose wide open images quite easily without extra focus tweaks. So this aspect of the lens lives up to expectations with interesting character/rendering apparently close to the original.
I should also note that I have printed a few of my digital images at 10" long, typical of an 8x analog enlargement. At this image size, the difference between centrally soft RF focused images and centrally sharp live view focused images require fairly close inspection to notice. A photographer's eye will see it, but probably not the average person. So it's perhaps arguable that this ~60 year old design, including my copy, is within performance expectations of its era.
For anyone interested, some of my test images are available to download at full resolution in this Google Drive folder.
Images are watermarked to indicate whether the lens was rangefinder focused or live view focused and the taking aperture.
For rangefinder focus, it means focus was set once and not changed as the lens was stopped down or the scene recomposed.
For live view focus, each image was refocused at the taking aperture to optimize focus at the center of the frame.
The first 9 images have Find Edges applied in Photoshop to give the 'focus peaking' effect to highlight the plane of best sharpness and illustrate field curvature characteristics as the lens is stopped down. This sequence is in half stops from wide open to f/8. The object in the center of the image is a small can I used to more easily focus the rangefinder and to give a reference point for focus shift as the lens is stopped down.
The next set is a sign in a park and includes both a Find Edges version and normal version. In this set, note how focus extends behind the sign in the center of the image and the text on the sign loses critical sharpness. In the live view focused set the text remains sharp but the plane of focus in the outer 2/3 of the image is pulled significantly forward.
The following set is a row of houses opposite the park field. Again, both a Find Edges version and normal version are provided. Note how sharpness placement changes along the row of houses depending on whether the lens was rangefinder or live view focused.
The next set of the large white house/mansion is basically a repeat of the row of houses test, but includes the full aperture range. You'll note that f/11 is the point where across-frame sharpness is best.
Then a set of a tree flanked by two benches. This set was only rangefinder focused. You'll note that as the lens is stopped down, the tree goes out of focus and the small white cabin and area around it far in the background becomes sharp. Meanwhile the benches stay in focus. This IMO is a prime example of the "W" field curvature/focus shift exhibited by my copy.
The next set of a small building placed in the left third of the image: RF focus was the front of the building and the scene recomposed. As the lens is stopped down, you'll note the building remains consistently sharp, though the trees in the center of the image lose sharpness until recovered by depth of field. In this case, with the primary point of interest off center, the loss of central sharpness is less disturbing to my eyes.
Kitchen Garden sign set is similar to the earlier sign set, though includes f/11 and f/16.
The wooden fence set is similar to the earlier tree flanked by benches. Note how the fence goes out of focus in the center as the focus shifts to the background. And when live view focused, how the outer thirds of the image become very soft because placement of best focus in the center has pulled the peripheral field curvature forward of the fence.
Finally a few of a tolerant spouse to add a human element to the tests. The first two at a farther distance were both at f/4. The first was rangefinder focused and the second was live view focused.
The head & shoulder distance set includes rangefinder focused from f/2-5.6 and live view focused at f/4 and f/5.6 to highlight the sharpness difference at the two aperture settings I find most affected by the combined field curvature and focus shift of my copy. This would also be a more typical use case for me where normally I'd be shooting hand-held via the rangefinder (though these were locked down on a tripod for the purpose of the test). Were I desiring the reduced background blur of f/4 or f/5.6, it would be very difficult for me to nail sharp 'on the fly' results via the rangefinder with a centrally placed subject.
I recommend downloading the images and browsing them with your preferred image viewing/editing app rather than viewing in Google Drive to more easily flip between and compare images.
Ron
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