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Sigma 20mm f/1.4 DG HSM A1 MTF Curves

Published November 28, 2015

OK, here we go again, with what has become a regular routine. Sigma announced another new lens doing what nobody has been able to do before. My expert friends, technical people I ask questions of, all say, “I don’t know this time. I just don’t see how they can pull this one off. Not at that price point. When are you going to test it?”

And then I test it, and tell everyone Sigma has done it again. (That groan you just heard is the blog editor who HATES when I don’t build up suspense and make you read to the end.) And yes, I realize I’m starting to be called a Sigma Fanboy. But that’s kind of like shooting the messenger, isn’t it? I’m just putting the lenses on the machine and writing up the results.

Let’s face it, Sigma is just hitting one home run after another. I don’t really have any use for a 20mm f/1.4 lens myself and I’ll never buy one. But I’m a geek and there’s nothing cooler to me than a company who likes doing stuff nobody has ever done before, and then does it better than anyone believes possible. So, OK, maybe I am becoming a Fanboy.

Anyway, let me show you what Sigma has accomplished as far as making a f/1.4 lens wider than any other SLR manufacturer has been able to do. Before you minimize the difference between a 24mm f/1.4 and a 20mm f/1.4, let’s just mention that’s a really big difference. So my expectations going in were that the Sigma 20mm f/1.4 DG HSM A1, even if it was great, wouldn’t be quite as good as either the Canon or Nikon 24mm f/1.4 lenses.

MTF Curves

As always – we are showing the average of 10 copies of each lens, each copy shot at 4 rotations to give a complete cross-section of the field of view of the lens.

Roger Cicala, Olaf Optical Testing, 2015

 

Roger Cicala, Olaf Optical Testing, 2015

 

The bottom line here is pretty obvious. Despite being a wider field of view, the Sigma 20mm f/1.4 clearly has a higher resolution than either the Canon or Nikon 24mm f/1.4 lenses do in the center half of the image, and are at least the equal of the others in the outer half. That’s just flat amazing. Even more amazing to those of us who have in the past shot the old Sigma 20mm f/1.8 lens, which could be charitably described as ‘artistic’, but never described as sharp.

Of course we checked the variation between copies, too. Again, given the wider field of view I wasn’t going to be surprised at a lot of variation between copies. We always expect wider lenses to vary more than standard range.

 

Roger Cicala, Olaf Optical Testing, 2015

 

Roger Cicala, Olaf Optical Testing, 2015

 

The variance plots for the Sigma look decent, certainly as good as the Nikon although perhaps not quite as consistent as the Canon. Still, I think it’s an excellent performance for a lens that’s significantly wider than the two we are comparing it to. Not to mention significantly less expensive.

Summary

With a wide-angle, wide-aperture prime lens, MTF is probably not the main consideration in whether you buy the lens or not. Theses lenses are used for different things by different photographers and bokeh, handling, vignetting, and dozens of other things I don’t test for will make a bigger difference in whether you like the lens, rather than simply how sharp it is.

Everyone’s first comment seems to be, “Well, I don’t need a 20mm f/1.4 lens”. I said that myself. But then I realized, well, I’ve never had the opportunity to use a sharp 20mm f/1.4 lens before, because there’s never been one . Will I like it? I have no idea. But I think I’ll at least check it out, it might be fun. Especially at this price point.

But the geek in me, at least, is totally impressed. Wider, faster, sharper, cheaper. What’s not to be impressed by?

 

Roger Cicala

Lensrentals.com

November, 2015

 

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

Posted in New Items
  • Radek

    I really would like to see comparison to Nikon’s 20f1.8 lens. Considering Nikon is smaller and 1/3 of weight while loosing less than a stop of light … and Nikon is cheaper … it might be better option IF performance is there for Nikon.

    I am looking into adding 20mm lens and crop to get to 24mm 🙂

    Cheers, Radek

  • MR

    Roger, thank you for the great info, as always. Do you know how the coma is on this lens? Thinking for night shooting and the Canon 24 f1.4 which isn’t great due to coma issues. Thank you for any insight.

  • Mark Turner

    Roger, great information on a neat lens, like everyone else I like what Sigma is doing lately. Thanks for publishing all of this data over the years.

    Brandon, that was a pretty clear explanation of the distortions we see in the corners, thanks for that. It jives with both my 30 year old memory of my optics course in college, as well as with my experiments last week with my various 40 year old manual focus lenses just looking at the corners trying to image an LED 40 feet down the hall in live view. In the corners, wide open, the old zuiko 50f1.4 and canon fd135f2.5 gave a range of best focus, none of which was very good, and in that range of best focus the LED image distorted between vaguely radial to vaguely tangential shapes. No straight line streaks or X’s in these old lenses until you stop down a bit I’m afraid, but a single stop makes a huge difference with those lenses. jtra’s idea of sweeping focus and stacking then with a median filter may be interesting to try with my 50f1.4, but focus breathing may be a problem. At the time I was mostly looking for differences in the corners, looking for signs of tilt/centering issues.

    Thanks again, Mark

  • If I get my mits on this one, it will be used for pano astro work. From what I have seen, any coma exists only in the extreme corners, and therefore make a lick of difference to me – I think I have found a perfect astro landscape lens!

  • JohnL

    BTW have you ever noticed a lens performance improvement after a short period of use? I’m not convinced (at all), but the “running in” concept is interesting:
    http://cameraergonomics.blogspot.de/2015/12/does-your-new-lens-need-running-in.html

  • NancyP

    I suspect that there will be NO FREE LUNCH with regards to coma. The question should be, is the coma tolerable for your wide-field astrophotography? Most of the time, this is a moot point for me, as the ambient sky glow from the nearby city limits my exposure.

  • Jeff Allen

    Sounds like Sigma has done a good job. The Canon EF 20mm f2.8 lens dates from the early nineties and it has bad field curvature and poor field illumination at f4 to f2.8. Its mot one of their L lenses and likely a poor seller.

  • preston

    Bob Pitt – see the review for this lens (and the Rokinon 14) at lenstip.com. They have exactly what you’re looking for.

  • @Lee Saxon:

    You should also take into account the fact that the Sigma is just sharper. Yes, that Nikon has somewhat better consistency in the center, but still: for both 50 lp/mm and 40 lp/mm (the ones I usually look at) the top of the Nikon interval is not any better than the bottom of the Sigma interval. That’s some hardcore beating…

  • The ‘how can they do it?’ question is maybe a question of marketing and pricing for Canon and Nikon. Before Sigma came along and disrupted the order of things with the Art series, you were locked into buying a Nikon or Canon lens at the premium end once you had the respective company’s bodies. Or go up-market to a Zeiss or whatever. So they could and did (and are still trying to) charge monopoly prices. The Sigma Art series blows the two lens monopolies sky high and exposes the decades-old rip-off. This is a wicked problem for both Canon and Nikon as those monopoly prices deliver fat margins and are a critical part of keeping their camera businesses profitable. Sigma has exposed the Canikon rort. I’ve interviewed Sigma CEO Kazuto Yamaki and he is one of the most impressive photo industry execs I’ve had the pleasure of meeting. More power to his and Sigma’s arm!

  • Brandon

    llya,

    I would be happy to discuss aberration theory in detail with you, but this is not an appropriate forum for that discussion. Please email me at brandondube@gmail.com if you would like to continue the discussion. I must warn you that I do not care much for the seidel polynomial and other representations of aberrations that are based on symmetry, as I was mentored by the late Dr. Thompson ( https://optics.synopsys.com/engineering-services/pdfs/resumes/2014Thompson.pdf ) and my research is primarily focused in freeform optics which break all symmetry.

    Regards,
    Brandon

  • Ilya Zakharevich

    @Brandon:

    First, your
      “aberration which is not symmetric with the optical axis”
    does not parse at all. I have no clue what you wanted to say here.

    Second, your description of coma is COMPLETELY “out of focus”. Sorry!

    **By itself**, coma is a very simple aberration. With coma, different “parts” of the lens have different “magnification” (here “part” means a certain distance from the optical axis). Say, for example, the internal circle of the lens produces a (focused!) image of a certain size, but the “external ring” of the lens produces an image (focused on the same plane!) which is 1% larger.

    As a result, with coma the image of a point source is an interval going in the radial direction. The length of the interval is (with the “linear?¹? coma”) proportional to the distance to the center.

    It is when coma **is combined** with OTHER aberrations, THEN it produces “cones” which you discuss in your remark.

      ?¹? With more advanced optical designs, a “cubical coma” may be visible too?—?if aberrations of smaller order are well-compensated. This is more applicable to mega-dollar designs (telescopes) than to photo!

  • Lee Saxon

    I don’t know, I’d argue the Nikon consistency chart is more attractive than the Sigma’s, even though the overall score is a little lower. The Nikon seems to have more center consistency, with much of its variation being in the outer frame where it’s less critical.

  • Randy

    Taking the long view, I think the real miracle is not just the lens performance, but that these lenses are coming from Sigma. How does a company go from making lenses that are fast, wide, cheap and “artistic”, to making lenses that outperform Canon & Nikon? I can’t think of a similar turnaround in the photo business.

  • Roger Cicala

    BegEater, if I knew that I’d go do it myself 🙂

  • BigEater

    You didn’t answer the question you yourself asked at the beginning of the post, namely: how is this company making such good lenses at such relatively low prices? There is no such thing as a free lunch, so besides missing the gold ring or the red stripe, what are we giving up by choosing Sigma over the competitors.

  • Bob,

    I’m sure many of us are interested in this lens for the same purpose if you can share images at some point it would certainly be appreciated.

  • Bryan Willman

    Having made much use of the Leica 21f1.4 on an M, and only really turning away from it as I stopped using the M, let me assure you that a 20f1.4 can be a very handy thing. Especially on fast cameras with high “cropping power”.

  • Andrew

    Hmm….Is Sigma learning from VW? Could it be they have secretly installed a defeat device in OLAF? Sounds painful!

    Perhaps not – just amazing again!
    Andrew

  • And you have to go to the end to reply.

    Also, re. Loxia 21mm: comparison only makes sense if they are both measured at the same aperture. Being two stops slower gives the Loxia a huge advantage when both are shot wide open.

  • Frank Nachtman

    Having your comments sorted in most recent first order is counterintuitive. If a commenter refers to a previous message, you have to read the conversation backwards. Most UI experts will agree that flowing the comments down the page in the order that they’re published (i.e., most recent at the bottom) is easier to read. Worth considering.

  • Yair

    Thank you for a fantastic article.

    The biggest question is how it compares to the Loxia 21mm?

    I guess you are the only one that can tell us.

    Thanks
    Yair

  • Samuel H

    Ok then this is me begging:

    Please please please pleeeaaaaaase

    Thank you for tuning in

  • Brandon

    Samuel,

    As tilt plays the biggest role in determining stopped-down “losses,” the OLAF procedure is to pick the most symmetrical of the 10-25 copies measured to record stopped down data.

    I do not believe any lenses have been measured stop down since I left, so you’ll have to beg Roger and Aaron on that front 🙂

    -Brandon

  • Samuel H

    For this particular lens, I’d also like to see f/5.6 results…

    Maybe pick an “average sample of each model?

  • I wonder about following technique to get perfect star picture with lens with astigmatism: take two pictures, one at optimal sagittal focus and other at optimal meridional focus (potentially a lot of pictures in between because optimal focus may be different for different distances from image center and do some alignment). Combine light values from both using MIN function to avoid cross appearance of stars.

  • Brandon

    Some general comments on so-called “coma,” which online seems to have been confused for *all* aberrations that affect the performance outside the center of the frame.

    Most all of today’s photographic lenses are superbly corrected for coma.

    Coma originates as a spherical aberration which is not symmetric with the optical axis, producing a cone as different annular sections of the lens’ pupil are further and further from the optical axis, each producing a “blur circle” the sum of which is the cone seen in images with coma.

    The aberrations typically labeled as “Coma” are oblique sagittal spherical aberration and especially astigmatism. A particular review website that uses a diode test frequently shows a diode which gets imaged in a way that looks like wings.

    If the wings are a perfect line at 45 degrees in the image, this is almost pure astigmatism. Such a condition decreases slowly as you stop the lens down, as astigmatism grows linearly with aperture and to the second power with image height.

    If the wings are rather “fat,” and flared this is typically oblique sagittal spherical aberration and the fatness will disappear very quickly upon stopping down, leaving an underlying line or X shape.

    If astigmatism drives the performance in the edge of the frame, as is rather typical these days, you may muck with it by adjusting the focus. At the tangential focus you will get a streak that points towards the center of the frame. At the sagittal focus you get one that is normal to the frame (so at 45 degrees to the image in the corners). In between you will find an “X” which is probably smaller and less disturbing compared to either of the other focus positions. This may compromise the center resolution; it depends how much field curvature there is.

    ———————–

    In terms of deducing the level of what is called “coma” from the MTF; the low spatial frequencies are far more telling than people give them credit for. The performance in the edges at 10 and 20lp/mm is quite similar, on average, to both the 24L II and the 24G. This would indicate that the “coma” performance is fairly similar. The 20A is “significantly” less consistent than the 24L, which is rather good, so an excellent sample is probably noticeably better in that regard.

    Happy Holidays everyone,
    Brandon

  • Roger Cicala

    Bob, we can’t exactly tell that from MTF charts, although Brandon sometimes finds strong hints of it. But he knows a lot more theory than I do.

  • Peter Simonov

    As always, thank you so much for this helpful information.

    But I’m very interested in this lens capabilities for astrophotography.
    Can you comment about the levels of coma aberration here?
    Is it too bad for shooting night sky at f/1.4?

  • Bob Pitt

    Thanks Roger for your excellent reviews and discussions. Is there any way to determine coma from the MTF charts or from any of your other test setups? I am a widefield night sky photographer and just received my Sigma 20/1.4 a few days ago. Under preliminary tests under cruddy skies, there appears to be more coma than with my Rokinon 14 for example (and the Sigma 35/1.4 also), but will be further testing under better conditions as soon as possible.
    Regards,
    Bob

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