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Painting Zoom Lenses with a Broad Brush – Roger’s Law of Wide Zoom Relativity

I’ve been writing peer-reviewed scientific papers for way longer than I’ve been blogging about optics. I value significant numerical information presented with methods that allow reproducibility as much as anybody. But way too many people who can’t define either spurious accuracy or spurious resolution believe (and unfortunately create) nonsense numbers on the internet and repeat them as though they mean something.

So I decided to write a post that presents some data from over a hundred lenses, but without any specific numbers, and nothing that says a given lens is better or worse than anything else. (And yes, I’m fully aware that tomorrow someone will link to this post to claiming I said one of these lenses is way better than another. As best I can determine I’ve only said about 30% of what I’m said to have said).

So why would you bother reading it? Because I bet by the end of it, I will show you something you probably didn’t know about zoom lenses. While it’s geeky, it might actually be useful to you. So this is a post for everyone. If you hate numbers, there aren’t any. If you want to learn a general law of lenses, I’ll show you one. If you like looking at beautiful landscape images and discussing photographic technique, well, OK, then it’s a post for almost everyone.

So What are We Going to Do Today?

Well, let’s take a pricey optical bench, add nine copies each of a bunch of zoom lenses. Let’s measure the MTF, not just across the middle but also from top-to-bottom and corner-to-corner. Rather than give you the several hundred MTF numbers that generates for each lens, I’ll just plot one frequency in a graph. (The frequency is 30 line pairs/mm, which is a good frequency because it’s relatively high resolution, suitable for today’s high-resolution sensors). And I’ll just map the sagittal numbers because it cuts the number of graphs in half and the conclusions are the same either way. So one lens, tested at one focal length would look like this.

Olaf Optical Testing, 2017

 

In the center, where things are blue, the MTF is pretty high, 0.8 or greater. At the edges, where things are red, the MTF is very low, 0.2 or less. This is actually a very good copy of this lens at 16mm, quite sharp in the center with the inevitable blurring in the corners that is the hallmark of it’s kind. But, as I’ve often said, zoom lenses vary. So let’s look at thumbnails of 9 more copies of the same lens, the Canon 16-35 f/2.8 Mk II at 16mm. Why thumbnails? Because we don’t need to look at details here; we’re just getting an overview.

Olaf Optical Testing, 2017

All of those lenses easily passed our screening tests and are good copies. On the highest resolution test charts on a 5Ds, even the top center one passes. But you can see each is a little bit different than the others.

Now let’s take those ten lenses above and average them together, so we get a picture of what a ‘typical’ Canon 16-35mm f/2.8 Mark II should look like. As you may have noticed, just to amuse myself, I chose the ‘most typical’ copy for the image above; it looks almost identical to the average one below.

Olaf Optical Testing, 2017

 

From here on out, when I show you a graph of a lens, it will be an average of 9 acceptable copies of that lens. The definition of ‘acceptable’ changes depending upon the lens and the focal length, of course, because not all lenses are equal, and a given lens isn’t equal at different focal lengths. But that’s what we’re here to talk about.

There is one thing I want to repeat. This is partial data; we’re looking at one MTF frequency and only the sagittal MTF at that. Don’t go fanboy and try to use this to do a lens comparison. It’s representative of the other frequencies, and tangential data follows a roughly similar path. But using these pictures to say this lens is better than that lens is, well, fanboy drivel.

So What Happens at Other Focal Lengths?

Well, we started with the Canon 16-35 f/2.8 Mk II at 16mm, so let’s look at it at 24mm, and 35mm, too. This should surprise none of you, we’ve known for a long time this lens was sharpest at 16mm and then softens up as you zoom in. It’s actually a tiny bit better at 35mm than it is at 24mm, but neither focal length is nearly as sharp as 16mm except at the very edges.

Olaf Optical Testing, 2017

 

Now the question you should be asking, or at least the question I would be asking, is “Did they all get softer or did some get really soft and bring the average down”? I’ll just tell you that at 24mm they basically all got softer, but at 35mm there was a combination of softening and more variation. Here are the thumbnails of the lenses that went into making up that average.

Olaf Optical Testing, 2017

So there is more variation at the long end, but none of these 9 are as good at 35mm as they are at 16mm. The takeaway point is the Canon 16-35mm f/2.8 Mk II is best at 16mm and then gets weaker at longer focal lengths.

So let’s compare that to some of the other wide-angle zoom options. (Oh, and because someone will ask, I’m using an average of 9 because that’s plenty to show the tendency here. I’ve done it with lots more copies, and the averages don’t change much.)

What About Other Wide-Angle Zooms?

I know you all want your zooms to be even at all focal lengths, so let’s look at your shopping choices among the wide angle zooms and see if we can find that. Below I’ve placed the average graphs at 16mm, 24mm, and 35mm for the Canon 16-35mm f/2.8 III and the 16-35mm f/4 IS Canon lenses. Remember, the f/2.8 lens is tested at f/2.8; it would be somewhat sharper if tested at f/4.

Olaf Optical Testing, 2017

Both of these retain more sharpness at the longer focal lengths than the older design does. In fact, the 16-35mm f/2.8 Mk III is indistinguishable at 16mm and 24mm, losing a bit of sharpness at 35mm. The quick takeaway message is the new lenses are probably worth the upgrade from the Mark II; they don’t fall off as much as you zoom in. But I’ve got more of a point to make, so let’s continue

Let’s take a look at two third-party options in roughly this focal length; the Tokina 16-28mm f/2.8 AT-X Pro and the Tamron 15-30mm f/2.8. I’m doing these just at the two extremes of focal length here.

Olaf Optical Testing, 2017

 

You may be starting to see a pattern here; sharper at the wide end, weaker at the long end. How about some Nikon wide zooms. Nikon tends to design somewhat differently; maybe they don’t have this pattern.

Olaf Optical Testing, 2017

Well, the Nikon’s show the same typical pattern, sharpest at the wide end, softer at the longer end. Like the Canon 16-35mm f/2.8 Mk II, the Nikon 14-24 f/2.8 is better at the edges at 24mm, but the central half of the image is softer. The edge improvement may be an effect of less field curvature than anything else, but I won’t argue the point.

I have data on two more wide zooms I’ll throw up in the same graph. They have nothing in common, they’re just the two I haven’t shown yet, and two lenses fit reasonably well into one image. The Canon 17-40mm f/4 L is an old design; the Sigma 12-24 f/4 Art is a very new one. (BTW we now test 2X or fewer zooms only at the two ends, which is why there’s no middle data for the Sigma).

Olaf Optical Testing, 2017

 

At this point, I think, the pattern is pretty clear. For simplicity sake, I think it best we give this pattern a name, and I think the logical name would be “Roger’s Law of Wide Zoom Relativity” since wide zooms are relatively sharper at the wide end. Are there exceptions to this law? Yes, but they are few and far between. For a few of these sets of 9 copies, there’s one lens that’s better at the long end than at the wide end, but for most there are none. No set tested averaged better at the long end than at the wide end.

Is this useful to know? Yes, it is. If you’re going to test your brand new lens, either by taking pictures or using a test chart, test the long end. If the lens is weak, that’s where it will be weakest. With some of these lenses where the difference is great,  you might consider shooting at the wider end you can, either by foot-zooming a little closer or by changing to another lens when appropriate.

I know what you’re thinking now, though. You’re thinking, well, that’s just for wide zooms, right? Let’s take a look.

Standard Range Zooms

I’m not going to bore you with lots of text; you’ve got the drill now. I’m just going to show you graphs. Like the ones above, the wide end is on top, the longer end on the bottom.

Olaf Optical Testing, 2017

 

Olaf Optical Testing, 2016

 

Olaf Optical Testing, 2017

The graphs for Sony lenses can look a little different because at some focal lengths the built-in baffles cut off some of the edges, but the same thing happens – better performance at the wide end.

Olaf Optical Testing, 2017

Again, you can see the pattern; standard range zooms tend to resolve better at the wider end, not as well at the telephoto end. I didn’t show them, but 24-105mm and 24-120mm zooms have the same pattern. So the Law of Wide Zoom Relativity seems to hold true for zooms that go from wide to slightly telephoto. I can’t tell you if it’s true for superzooms, like 18-270s, because I will never, ever test them. Life is too short to test 10x zooms. I can tell you that it’s not true for 70-200 zooms, but that’s the subject of a future post.

So what does this mean for actual photography? For me, it means I shoot my wide zoom at the wide end as much as possible and reach into my bag for the 24-70 zoom when that’s an option. The choice is a little less clear when things approach 70mm, and the choice becomes a 70-200mm lens, but, as I said, we’ll consider that later.

Why Might This Be So?

This seems a bit counter-intuitive, doesn’t it?  Historically, it’s been harder to design wide-angle lenses, and with prime lenses, we tend to accept that wide-angles will be less sharp, at least in the corners, than longer lenses. So I would have expected the wide end to be less sharp in these zooms, or maybe that some would be better at one end than the other. But what we saw was 17 zooms tested, 17 sharper at the wide end. (You might argue about 2 of the 17 having better edges at the long end, but not better overall resolution.)

I can’t say for certain why this would be so, but you know me, I’m happy to speculate.

All zooms, whether they have an extending barrel or not, have at least two elements (and usually more) that move during zooming. The elements move along helical tracks, rotating as they go. Moving elements away from each other can magnify aberrations, which would reduce MTF, of course. The lens designer would attempt to correct for that, but lens design is always a compromise. The simple act of moving an element might tilt it or alter spacing from ideal.

However, there’s no reason I know of to think one position is better than the other. The movements of the zoom elements are usually complex. It’s not as simple as ‘when you zoom in, elements move further away from each other.’ If only the extending barrel zooms acted this way, then that might have something to do with it, but that’s not the case. A couple of these actually extend to get to the wide end and are ‘at rest’ in the center of the zoom range.

Lens design probably has more to do with it. Zoom lenses are designed, as best I understand, from a starting focal length. Then the design is modified to allow it to zoom, then corrections made for the aberrations that the zooming created and the cycle repeats until either the lens designer’s deadline hits or the marketing department is satisfied. It would make sense to start the design at the widest end which is probably the more difficult to design. That might, then, remain the better end when the design is complete.

The complexity of designing the wide end is probably less today with modern lens-design software, but in the optical industry, old habits die hard. If the practice was to begin a design at the wide end, that’s probably still the practice now. Not to mention the reality of lens design is that the designer usually begins with an existing lens, then modifies it. They rarely start the design from scratch.

Finally, and I know more about this than I do about lens design, there is the optical adjustment of the lens. For every zoom lens the adjustments are done at the wide end first, then a set of separate and more limited adjustments are done at the long end. But the rule of ‘get the wide end right, then tweak the long end’ is pretty universal for zooms.

So, to summarize: I don’t know why, exactly. The above was just me speculating on some logical reasons.

But I think it’s a useful and interesting thing to know, and something I’ve never heard talked about. With very, very few exceptions, every wide and standard range zoom is sharpest at it’s widest end.

 

Roger Cicala, Markus Rothacker, Aaron Closz, and Brandon Dube

Lensrentals.com

March, 2017

 

Addendum: Just because I know it’s coming, let me take a moment to comment on the inevitable people who will say, “I know you presented thousands of data points on hundreds of lenses, but you’re wrong because I have one that’s different.”

You actually might. Depending on the lens type tested between 0% and 15% are actually sharper at the long end. It does happen. It’s just not the general rule.

Second, we consider the fact that we see more detail when we zoom the lens to be the same as sharpness. If my subject fills up 10% of the frame and I zoom in, so it fills up 30% of the frame, I will see more detail. That’s not the same as sharpness. What the data I showed says is, within reason, if you shot an image at 70mm, then moved so that you had exactly the same framed image at 24mm, the 24mm image would have more detail.

And third, especially with wide zooms, if you shoot a test chart make sure you shoot different sizes of the same chart at the same distance. If you get close to a chart at the wide end, you may start approaching minimum focusing distance where lenses are less sharp.

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 Geek Articles
  • Kevin, correct on both counts.

  • There aren’t a whole lot, actually, except the obvious ones: Zooms have more variation than primes, etc.

  • Actually, I thought I suggested testing the long end since that’s where the weaknesses would be.

  • Thank you Brian! That makes much more sense than my speculations. I appreciate you putting the scientific theory to explain the observation.

  • Brian Caldwell

    I haven’t read all the comments here, so my apologies if I’m about to state the obvious. The reason that most wide zooms are better at their wide end has less to do with the desires of the lens designer or marketing people than the inherent properties of the basic wide zoom architecture. Almost all of these fall into NP, NPP, or NPNP types, where “N” refers to a negative-powered zoom group and “P” to a positive-powered zoom group. Unlike tele-zooms, these type of zooms generally have a moving rear group. This means that in order to maintain a constant f/# through zoom the marginal ray height gets bigger on the rear group. This is made worse by the fact that the magnification of the rear group is usually higher (e.g., closer to unity) at the long end than it is at the short end. Its worth noting that in some designs the iris diaphragm is actually cammed so the physical aperture gets larger at the long end just to maintain f/#. The net result is that its much more difficult to correct the long end than it is the short end, mostly due to spherical aberration.

  • Itai Basel

    If test is done correctly and not handheld with autozoom – I think apperant sharpness is less of a factor – but I am not as savvy as others on this thread

  • Itai Basel

    You suggest that testing the wide end will give a measure of how good a lens instance is – but is it so? were’nt there specimens that showed good wide angle resolution but lacked in the mid range or at the long end?

  • Don Farra

    Roger, thank you for the presentation. One question, what other general rules of thumb have you found regarding lenses?

  • Geoffrey Forrest

    Poor baby. Don’t you know here are no absolutes and therefore nothing is/can be perfect, except me, compared to the rest of the flat world.

  • Geoffrey Forrest

    Leica had a great name back in the 60s to a few years ago, because of the rangfinder system and the fact that they tests the lenses and rejected about 10%. Now the field has caught up with them, i.e., The Sony 7IIs, (To which you can adapt a, Leica Lens or any other.) and many of the other CSC camera. The new Fuji XGF takes care of all of the “problems” for $9,500 with a 25-52 zoom or wait for a wide non zoom, if you have
    the money. It will be much cheaper than the new H’blad and much-much cheaper than the, Leica S, at $21,000+, with a lens. The best camera/lens is always the next camera.
    The next few years will prove it. Wait until 2018 at Photokina in Cologne.

  • Geoffrey Forrest

    Test non-zooms like a100mm or 200mm and stick with that, depending on what you are looking for or should I say, depending on what you are looking through.

  • Geoffrey Forrest

    You are right. Also, what about the fact that WA lenses have an “Apparent Sharpness” and a deeper depth of field. One doesn’t need an auto-focus 18/21/24/28mm lens, when you can set it at 3Ft or 8Ft, more or less, depending on your needs, and at f5.6/6.1/8.
    and get faster shots, with no AF lag. Test Samyung lenses against Nikon/Canon/Sony,
    Remember the Sony Zeiss are not made by Zeiss and the Panasonic Leicas are not made by Leica and many lenses are made by Sigma, and most smaller Leicas are made by Panasonic in Thiland. The main difference, my Nikon 16- 85 is made in Thiland and says it on the lens. Most of the cameras are made, “Everywhere”. Leicas are not even, “Made in Germany”, They are made in Portugal and/or Austria.

  • Geoffrey Forrest

    A basic law of design: If an product/object has to perform/provide one function it is easier to produce than a product that has to perform/provide two functions and much better than one that has to perform/provide three. Meaning, If you are looking for a rtelationship with a very good looking person, then there is no real problem. If you, also expect them to be smart, more of a problem. If you expect them to be a good cook or good at something specific, less of a chance, and if you expect them to love you, you have
    a real problem. Get it?

  • Geoffrey Forrest

    The earth is closer to egg shaped and you are flat.

  • Geoffrey Forrest

    What about this: To test the difference between a photo shot with a non-zoom,
    16/18/21/24mm, and cropped to 20/24/28mm, Meaning, I can save a lot of time/money
    buying a very fast non-zoom WA of 21/24/28. Or, for example, what about a test of the
    Sigma dp0 w/21mm, fixed lens?

  • Kevin Sparks

    Fantastic results and observations I haven’t seen elsewhere. Thank you, Roger!

    Curious if the Sony-Zeiss 16-35/4 also fits the pattern, and if it likewise tests as favorably overall as the Sony standard zooms included here?

  • Dragon

    Roger, the EF-M 11-22 might be an exception. It is amazingly sharp at the 22mm end. Actually sharper than the 22mm pancake.

  • GF

    Roger, I know you said we shouldn’t be doing comparisons, but it’s hard to ignore that 16-35 III from Canon.

  • Ralph Wallace

    Roger – Please test the Canon EF-M 11-22mm f/4-5.6 IS STM

  • Rob. S.

    While I do like the 17/1.8 and 25/1.8 Olympus lenses myself, too, I actually have gotten the impression that the 12-40/2.8 zoom is sharper and more consistent across the frame at both focal lengths…

  • pmpm12345

    Long article. I’ll summarize in case people don’t have time to read it:

    Roger compared many wide angle lenses. He found that they were rated as (from best to worst):

    * Canon 16-35mm f/2.8 III
    * Canon 16-35mm f/4 III
    * Nikon 14-24mm f/2.8
    * Sigma 12-24mm Art
    * Nikon 16-35mm f//4
    * Canon 17-40mm f/4
    * Tamron 15-30mm f/2.8
    * Tokina 16-28mm f/2.8

    For normal zoom lenses, he found Sony was best, followed by Canon/Nikon, and Tamron was the worst.

    He found that the lenses behaved better wide-angle than zoomed in. He hypothesized that this was because a zoom lens acts kind of like a built-in adjustable teleconverter, and fundamentally crops the image, leading to worse performance. He repeated this result for standard lenses, but then said he couldn’t replicate it for telephotos, which he’d write another article about.

    I think the point of the article was that Sony has now caught up and passed lens giants Canon and Nikon, while, despite occasional good copies, Tamron is still a low-cost low-quality alternative. Of course, Roger didn’t say that outright since he does business with all these companies, and didn’t want to appear smear one. So there were a lot of disclaimers about how he wasn’t saying that, and a lot of verbiage about correction of aberrations and so on, designed to make the article look like it was about something else. But we could all read past that.

    Roger: Please correct me if I’m wrong about anything.

    Thanks for the great lens review article! This was wonderful.

  • Magnar W. Fjørtoft

    Great work! And also educational and fun to read your comments! Also, I am impressed what modern lens makers can do, and the quality of affordable mass produced lenses! 😉

  • Perry Winkle

    Oh no, Lensrentals has a cuck infestation.

  • Apples and oranges. You’re comparing manufacturer’s computer generated ideal MTF with multi-copy actual tests. The MTFs on the computer are always excellent. Actual lenses not so always.

    Don’t get me wrong, those are great lenses. And I have not tested the 24-105. However the 24-35 behaves exactly the same as the other lenses I presented here – slightly sharper at the wide end. As to ‘confirmation in independent review’ since I don’t know of any review site with access to an MTF bench, please link us up. I could be wrong, happens all the time, but I can’t believe I’m missing someone publishing MTF bench results.

  • ?ukasz Moszczy?ski

    “(…) every wide and standard range zoom is sharpest at it’s widest end.”. Really?
    Roger, what about that data:
    http://www.sigma-global.com/en/lenses/cas/product/art/a_24_105_4/data/
    http://www.sigma-global.com/en/lenses/cas/product/art/a_24_35_2/data/
    http://www.sigma-global.com/en/lenses/cas/product/art/a_18_35_18/data/

    These manufacturer charts also have confirmation in independent reviews.

  • Marcello, for the minimum focusing distance loss of sharpness, it is not universal. Macro lenses, of course, work best at minimum focusing distance and lenses with a compensating element also maintain sharpness very well. It is something to consider on other lenses, though.

    I think the common ‘lens swap’ is going to be for the person who shoots a wide zoom, like a 16-35. There’s good reason to consider that a 16-20 in practice and reach for your 24-70 for anything in that range. Or, as many people are doing now with the numerous excellent wide-angle primes on the market, carrying a 14mm or 15mm prime and a 24-70 zoom, etc. The similar question comes up with 24-70 zooms at 70mm if you have a 70-200mm zoom.

    The reality is, though, the convenience of a zoom is the reason for a zoom. The long end may not be quite as sharp, but a shot done at 35mm on a 16-35 is still going to have much more detail than one taken at 16mm and then cropped.

  • Second question (had a problem with post lengh…): should i stay away to the MFD? should i choose a lens that can focus as close as X if i need to be 2 or 3*X close? Can i compensate stopping down, like in the Nikkor 60mm micro?

  • Ok Roger, so you put in our minds two precious statements:
    wide and standard zoom performs better at the wide end
    lens do perform less good near their MFD

    Lets come to put in pratical this 2 statement. How this lead me to the choice of my lens and the way to use it?
    First question: I will pick a wide lens to use on the wide end, and that is ok. When the situation allow me to do so, i will swap for my standard zoom, but… Should my 2 lenses have an overlap or it is a better strategy to not overlap. And when it comes to the moderate tele, should i put aside my zoom for a prima to keep the quality high as possible?

  • tresemes

    I’m talking about Nikon’s 24-70s, which are the ones you mentioned. You do realize that blue is better and red is worse, right? There’s no red color on the VR’s graph, it does seem to have a wavy field curvature but resolution gets better towards the edges whereas the original only gets worse. Here’s a more thorough comparison by the man himself, where they arrive to that conclusion, that center resolution has been sacrificed for better corners.

    https://www.lensrentals.com/blog/2015/10/nikon-24-70mm-f2-8-ed-af-s-vr-sharpness-optical-bench-testing/

    I wasn’t going to talk about other lenses but since you did…the stabilized Tamron 24-70mm matches or betters the 28-70mm, the Canon 16-35mm f4 IS is much better than the 17-40mm f4, and even CaNikon’s first stabilized 70-200s were better than the non-stabilized lenses they replaced.

    Anyway, my point is that even if it were true that VR messes up the original MTF I’ve yet to see a brand that just takes an old lens, throws in a stabilizer and calls it a day; they make whole new designs to overcome that or even improve on the original.

    The stabilizer, from my experience, DOES add a variable in real life that can result in uneven sharpness sometimes; but when it comes to “controlled” tests like this one I fail to see an example where “resolution has been sacrificed for optical stabilization”, the only thing that’s sacrificed for stabilization is our wallets.

    https://uploads.disquscdn.com/images/7c6e8bec169baf0616e804820745c2584a9ee36a3b48a9b672295a721fa6de88.jpg

  • Athanasius Kirchner

    That’s really interesting, thanks for sharing. I wouldn’t have guessed.

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