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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
  • Sean T

    This is fantastic, thank you Roger. How interesting. I appreciate the terrific variety of lenses examined too.

  • Patrick Chase

    So I’m a bit slow, and finally recognized something obvious about LR’s MTF testing methodology: The fact that the bench uses a broadband source means that lateral chromatic aberration degrades sagittal MTF.

    I’d been wondering why some lenses that I know to have very little astigmatism (particularly the Canon 200/2 and 300/2.8) appeared to have a fair bit in Roger’s tests, and I suspect that may be the cause.

    I’m not advocating that you do anything different. Simple narrowband MTF measurements don’t adequately penalize axial CA, so that wouldn’t be a reasonable solution. The only viable approach I can think to disentangle the impact of lateral color would be to do separate R, G, and B narrow-band measurements at the same focal point. If all 3 of those are high but the broadband is low in the saggital then you can be reasonably certain that lateral color is at fault. The obvious problem is that doing so would be bloody expensive and time-consuming.

  • Patrick Chase

    I don’t know of many (any?) cases where IS has been added to an existing resign, without significant rework.

    In order to function as such the IS group has to have a fair amount of optical power, so its very presence will ripple out through the optical formula. It is possible to *disable* IS in a design that was optimized with it by replacing the IS group with equivalent fixed elements, and Tamron seems to have done that in some versions of their 150-600 for example, but that’s very different from adding it to a “true” non-IS design.

  • I think that’s where you saw ‘mounted to 5D III and taking pictures of a proprietary test chart’.

  • El Aura

    No, your original post was correct. The Nikon 12-24 mm can be used on FF between 18 and 24 mm. Meaning, the image circle is larger at the long end.

    (I also had to edit my post a few minutes after posting because I got it wrong.)

  • Athanasius Kirchner

    You’re completely right! I should’ve checked instead of posting straight ahead. I’ll edit my reply.

  • El Aura

    Roger, you are probably right that this phenomenon has rarely been talked about. But I think every avid lens test reader pretty much knew this already.

  • DrJon

    But they do other tests for off-axis performance!?!

  • El Aura

    I have rather seen the opposite, at least for pure wide-angle zooms. An example is the Nikon DX 12-24 mm f/4 that can cover FF up to about 17/18 mm (something I have tested myself when I moved from Nikon DX to FX).

  • El Aura

    The Leica M has seen quite a number of extreme wide-angle lenses. The lack of a TTL viewfinder probably has something to do with that as well as the general trait of the M system of having pretty compact lenses (for what they offer).

    Some new manufacturers (Venus, Irix) have discovered extreme wide-angle primes (11 & 12 mm) on SLRs as a niche they could occupy.

  • Athanasius Kirchner

    Kinda OT: Does this in part explain the massive front elements used in retrofocal UWA zooms? In principle, if the effective FL is short, the need for a large physical aperture should be negated. I’m quite sure this doesn’t apply to retrofocal designs, I just don’t know why.

  • Athanasius Kirchner

    There’s an easy way to test that hypothesis, and it seems to be wrong. Image circles tend to increase in size towards the middle of the zoom range in retrofocal designs (common in SLRs). Many then decrease, but are still larger than at the wide end.
    How has this been tested? With APS-C lenses on 135 cameras. Nikon F, Sony E and Pentax K tests have shown this effect consistently. There are some Minolta tests floating around, too, but those were made with film. Now, with the GFX getting all sorts of nice adapters, I think we’ll have a definite answer for 135 format lenses as well.

  • El Aura

    There is the question whether one should plot the MTF numbers for twice the resolution per millimetre with m43 lenses (to achieve the same total resolution, m43 lenses need to have a ‘resolution’ that is twice as large as that of FF lenses, except that m43 sensor lag FF sensors in resolution, though maybe 1.5x the resolution would be more appropriate).

  • Athanasius Kirchner

    Great reply. One observation, though: I think you were meaning to say “longer and longer” instead of shorter focal lengths, in the last paragraph. Or did I miss something?

  • El Aura

    This hasn’t been pointed out so far, but all lenses whose test results you presented here are constant aperture lenses. I know that is not how variable aperture lenses work but one could argue that ‘stopping’ down a stop at the long end should improve IQ, everything equal, and thus at least partially compensate for the effect you presented here.

  • Athanasius Kirchner

    Are you sure that that’s the case? I get the feeling that the IS II was a complete redesign, and it’s possible that the design goals were simply much higher than before, and so even if impacted by the stabilization group it performs much better than older models. Of course, IS technology was improved in between, but the Nikon 24-70mm VR is using a state of the art stabilizer, and it clearly isn’t immune…

  • Athanasius Kirchner

    Which begs the question, why are telezooms generally better 50mm or 100mm before their maximum focal length? It could well be the same phenomenon that causes the “fade” on standard and wide zooms, and maybe it has little to do with design, and everything else with some other factor.

  • Brandon Dube

    Off-axis interferometry is enormously expensive, time consuming, and complex. They are certainly only doing it on-axis which only gives them information about the lens relevant to a very small region near the optical axis.

  • DrJon

    You mean current versions? Presumably they can do anything they want to in future versions?
    Also a source for how DLO currently works in DPP would be handy, as I’m unconvinced…

  • That’s already the case in some lenses (not Canon though, so far), and also the case for replacing IS units of AF motors, etc.

  • Brandon Dube

    They only know the aberrations on axis. DPP can only accurately work with information for like 10 pixels based on that data 🙂

  • DrJon

    Are you concerned for the implications of having aberration data in the lens for when DPP supports using this (as presumably it’s all invalidated every time you mess with a lens)?

  • Thom Hogan

    My experience with many of these is that they’re consistent and fine out to a point and then fade. The new Nikon 200-500mm is actually very, very good at 400mm. But at 500mm it’s weaker. Ditto the other telephoto Nikkor zooms, except perhaps for the new 70-200mm f/2.8E.

  • Thom Hogan

    And I think you might have part of your answer there, Roger. It’s something I’ve speculated on, too. Lenses that go from wide angle to less wide angle or normal or telephoto have an interesting property to them: they are often bigger than expected. A 70mm f/2.8 lens doesn’t need a 77mm front element, but for some reason 24-70mm f/2.8 lenses now need an 82mm front element. I think the designers are designing the wide end first and then finessing the long end for these lenses.

  • Yep! They’re using interferometers for on axis subassembly testing and perhaps whole lens testing. That’s top of the line. But then they apparently switch to target analysis on 5DIII for off-axis whole lens testing. But that was an impressive look at Canon’s manufacturing and assembly process. They lead optomechanics by a large margin.

  • DrJon

    BTW the odd picture of Canon’s in-house testing here:
    http://www.imaging-resource.com/news/2017/03/20/canon-factory-tour
    Especially:
    http://www.imaging-resource.com/manual-update/news/canontour/Z-IMG_0273_WB-625px.jpg
    Although possibly they didn’t let them photograph all the good stuff…

  • Lee

    Thanks for another interesting piece! Since we’re talking underlying design philosophy and how it directs lens offerings, I will jump in with a tangentially-related question I’ve always wondered about:

    Why don’t we see more moderate focal lengths with MTFs like, say, the 200/2 VR? The Zeiss 100MP takes on that nearly-flat shape (at a lower numerical level, of course), but it’s about the only I can think of.

    It’s hard to believe it would be just market taste. The 135 APO-Sonnar and 135/1.8 Art are like *half* the size of the 200/2’s. Does people’s willingness to deal with size/weight drop off *that* fast??

    So is that kind of MTF just not achievable below a certain magnification? That the Otus 85 needed to be so much bigger and exotic-glass-using than a typical 85/1.4 to approach that level just in the center (with a more typical rate of falloff) seems to suggest so. I mean, I realize the size difference is less than, say, between a 180/2.8 and the 200/2’s, but part of that is the aperture difference. (PS – I know a portrait-focused design for the Otus 85 is exactly what the market wanted, I’m just commenting on what it apparently took to get there)

  • Matt

    I’ve always noticed this. The 24mm of the 24-70mm is sharper than the 70mm end.

    And the 35mm of a wide zoom was always less sharp than 35mm of the 24-70mm.

    Sharpness alone doesn’t make the lenses better at the wide end though; Vignetting and distortion in particular are always strongest at the wide end of these zooms.

  • That’s my impression too, but I don’t have the numbers and graphs to show.

  • T N Args

    Thanks Roger. I was thinking more of the 100-400 and 150-600 types which (correct me if wrong) generally seem to fade at the long end, and manufacturers must know that is the end buyers are most keen about. Makes me wonder if your hypothesis about designing from a starting focal length is right, or if some other principle is in play.

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