August 3, 2015 at 6:57 pm #9249
Original email from Artur:
first, let me say I really enjoy reading your lens reviews. I think they are as informative as they are entertaining. 🙂
Second, I have read your review of the Zeiss Loxia 35mm lens. I have been using this lens for a few weeks on a Sony A7R camera.
In your review you write that the Biogon design is known for its flat focus field. However, from my experience the Loxia shows some quite pronounced field curvature. I notice it most when taking, say, a photo of a landscape and focusing at something at medium distance (3 to 5 meters). In the center of the picture, the background (at infinity distance) is sharp(ish), whereas it quickly gets much softer towards the edges. Near the edges on the other hand, focus shifts more to the front. In some pictures the edges are very sharp, so I don’t think it is a problem with resolution but rather a field curvature issue. Did you notice anything of this kind when testing the lens?
I have discussed this in the dpreview forums, if you can find the time please take a look at my posts over there. I have included some sample pictures to illustrate the effect.
I first wanted to write this in the comments section of your review, but as the review was published half a year ago I was not sure if you still check the comments. That’s why I am writing you this way.
Best regards, Arthur”August 3, 2015 at 7:24 pm #9250
thank you for your comments and observation. When we speak about field curvature and lens design in general, we should consider few things in addition…
Most important is focal length. While 35mm is sometimes considered as a standard focal length, in reality it is quite wide and wide lenses have to deal with many aberrations. Like with most other optical designs, it is always question of priorities and compromises.
Biogon is indeed known for “rather” flat field curvature in comparison to Distagon e.g.,but it is even better known for low distortion. (Biogon was originally based on Biotar, which was another way to call Planar and Planar name was based on planography – prints from flat surface)
Distortion is usually one of the most problematic aberrations with wide angle lenses and Loxia 35/2 shows extremely good control over it. However, good control of the distortion needs some compromises and some of them are partially paid off in field curvature while other partially reflects in increased sagittal oblique spherical aberration (butterfly effect).
Thing is, that wide angle lenses which seems to perform better in the extreme corners then Biogon/Loxia, usually have so called mustache (irregular, complicated) distortion. This type of correction is today very popular (thanks to much more precise and cheaper production of aspherical elements), especially when combined with automatic software correction. In most situations, image looks sharp from edge to edge with such a lens. But looking more carefully for the critical distortion control (when shooting architecture e.g.), one might be rather disappointed.
Loxia/Biogon would perfectly sharpen the extreme corners from f/11 onward, but we are facing here problems with diffraction on modern sensors.
While I understand your concern, it won’t be easy to find perfectly “flat” 35mm lens yet. As you can see from my tests, only Sigma 35/1.4 Art will beat Loxia in that regard (showing indications of mustache distortion though) and I was quite impressed with new Zeiss 35/1.4 Distagon ZM on Leica M body, even if Distagon usually means quite heavy field curvature.
In my review Loxia shown better corner performance (by very slight margin) then Sony FE 35/2.8 from f/4 onward, and except Sigma and new Distagon ZM, neither of tested lenses did come even close.
ViktorAugust 3, 2015 at 8:43 pm #9251
thank you for the very interesting reply! I was previously unaware of how correcting one aberration of a lens negatively affects others.
You are right that distortion control of the Loxia is excellent, this is one reason why I chose it over the Sony FE 35/2.8, which I also considered. I should say that generally I am very happy with the Loxia, and the field curvature issue is certainly not a deal-breaker for me. Especially as the effect is only noticeable in some pictures and I think it is more or less pronounced depending on focus distance. I will examine this further and also try f/11 and smaller apertures.
Closely examining some full-resolution photos I found online taken with the Sony FE 35mm lens it indeed looks to me like the Sony shows a similar field curvature as the Loxia. I have to confess I have not read your review of the Distagon ZM, but will do so now. 🙂
Best regards, ArthurAugust 3, 2015 at 10:53 pm #9252
I learned my lesson with field curvature when I shot group of managers with Canon EF 24-70 at the wide end, focusing on the center. At f/16, I thought that I am safe from FC, but few pals who were forced to be on the frame edges, weren’t happy with their blurred and distorted versions… and thus I lost great client. (It was beer brewery, whic translates in many events with free beers, nice chicks, etc :-()
In some critical situations (it usually works for landscape), you can try focus stack (5 shots should be enough) for outstanding sharpness across the frame. In other situations, you might find focusing on the edges at f/8 and smaller sufficient for good results.
I don’t have Loxia any more, but I quite liked it when I has.
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