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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
  • Patrick Chase

    Abolutely agreed. Then back up to infinity, do the same thing, and note that the entrance pupil now appears to occupy almost the entire front element regardless of where it’s located in the lens. My point was never about the depth/location of the EP, and I can’t understand why we’re arguing about that. It’s a canard.

    w.r.t. the FoV argument, these lenses do vignette wide-open, so again there’s no contradiction :-).

  • Brandon Dube

    I encourage you to hold a 70-200mm f/2.8 lens in your hand, look through the front of it, rotate the lens or do whatever you need to do to enhance your depth perception, and tell me where the aperture appears to be.

    It is very deep in the lens, far from the front element.

  • Turniphead

    I think the reason for the prevalence of zooms is that’s simply what the majority of users want in that focal length range. A high end full-frame 11mm lens would be a very niche lens – I probably wouldn’t buy it; but a high end 11-24 zoom covers pretty much the most interesting range of focal lengths for me – so if I had the money, I’d buy it!

    I think the main reason we don’t see many primes wider than 24mm equiv on MFT is that they’d be excessively large and bulky compared to the rest of the system. Most MFT buyers chose MFT because they wanted a smaller and lighter system, so I doubt they would sell as well as the somewhat less wide (but much smaller, cheaper and lighter) pancakes.

  • Patrick Chase

    Depends how much you like cropping. You can’t always zoom with your feet, particularly for things like landscapes.

    I worked that way for a long time (24-70 + 14 and 20 primes) and finally broke down and added a WA zoom.

    I still use T-S primes (17 and 24) for a lot of subjects though.

  • Patrick Chase

    OK, let’s use numbers. A typical 70-200/2.8 has a 77 mm filter thread and a 200/2.8 = 71.4 mm entrance pupil. You can hand-wave all you want, but the fact is that the entrance pupil does occupy almost the entire front element diameter on the long end at infinity focus. At closer focus the cone of light has nonzero angle and will be smaller at the front element than at the EP, so object location DOES matter in this context.

    You’re absolutely right that the FoV limits how deep the EP can be and/or how much of the front element it can occupy without vignetting, but that’s irrelevant to the point I was making.

  • Brandon Dube

    The entrance pupil does not know or care where the object is. It is the image of the aperture stop seen through the elements in front of the aperture stop. If you intend to test the lens more than just along the optical axis, you care a great deal where it is.

  • Patrick Chase

    The EP of a 70-100 most certainly ‘occupie’ the front element at infinity, where you test on the bench. It’s depth in the lens is irrelevant under those conditions.

  • Brandon Dube

    A larger pupil has the capacity for more resolution (there is ‘less’ diffraction). If you only increased its size without changing the aberrations, the resolution would go up.

    The EP of most 70-200s is buried deep inside the lens, I would not at all say it ‘occupies’ the front element.

  • David Bateman

    Thank you, it would be awesome to see the 43rds data. Since you haven’t tested it I think the fan boy claims have run crazy there.
    Also my self as a m43rds user would be good to know if the 12-100mm f4 is perfect claim, is true.
    I appreciate all you hard work

  • Omesh Singh

    For landscape you’d want good detail from corner to corner, but for portraits you’d have a more center-weighted subject. (i.e. more likely to have out-of-focus background in the corners of frame for portraits.)

  • We can test m4/3 and do sometimes. Part of it is volume. It’s easy to grab 10 copies of something we carry 300 of, but not so much when we only own 20 or so. There’s also the inevitable fanboy arguments about the different sensor sizes and what the tests show or don’t show. But I will try to do some more m4/3 comparisons soon.

  • Greg Timms

    I always imagined lenses were designed this way because of how they’re used. In the wide end you’re trying to fit in extra elements into the frame so better edge/corner performance is more desireable, and on the long end you’re pinning down a subject so edge/corner performance isn’t as important.

    There are probably going to be use cases that don’t adhere to that, but I imagine the most common use cases would follow the above statement

  • almeich

    I have the Sony 16-35 mm and an A7r2. I know the lens is less “sharp” at 35 mm than at 16mm, but that is no problem for me; I bought it for the wide end. And I have the 2.8/35mm.

    Some day I shall get around to do some test shooting with the 16-35. I’ll shoot (without moving the camera) at 35 and 28 and compare the 35-shot with a 28-shot cropped to show the same area as the 35. It shall be fun to see if more is lost by cropping than gained by avoiding the extreme long end of the zoom range. The 42 mp sensor has a lot of detailed information to play around with.

  • Thank you Roger. I’ve got very similar reflections after working with all my zoom lenses, But what you do is science and you got proof. Also thanks to yours tests I’m more aware at calibration. So two weeks ago I make a bunch of tests shots and send two my zooms to recalibration.
    What I see more that there is some extraordinary lenses there: Sony FE 2.8 Canon 16-35 III and Sigma 12-24 all of them are very blue!
    I saw test shots when Sigma 12-24 was sharper than Laowa 12 D just Wow.

  • David Bateman

    From your test we can clearly see that the olympus 12-100mm f4 is the best lens.
    Joking aside, how come I rarely see m43rds lens tested? I know you have them and post like this I was expecting to see a Panasonic or Olympus lens thrown in there. Are there testing issues with 43rds lenses on Olaf? Or are 43rds lenses not tested?
    Thank you for all the hard work! These posts really are interesting and you beyond the reseach paper minimum of 3 data points, or even 5. You must really like good p values.

  • Patrick Chase

    This is a question for Brandon and any other OEs who might be lurking…

    Could the tendency towards lower resolution on the long end be related to the increased size of the entrance pupil? Even variable-aperture zooms tend to have significantly larger entrance pupils on the long end than on the short, and I can come up with totally hand-wavy theories as to why that might worsen some aberrations. I’m curious to know if that’s viewed as a significant complication?

    On a related note, what ‘turniphead’ says below about only using the entire front element on the wide end isn’t universally true. If you look at any 70-200/2.8 you’ll see that the entrance pupil occupies almost the entire front element diameter on the long end.

  • Many do. But a lot of people do carry the ‘two zoom set’.

  • wg

    Magnifying also makes for lower resolution, in principle, similar but opposite to adding a speedbooster to a FF lens for, e.g., MFT.
    The front group(s) of elements in principle create the image, the groups/lenses behind it magnify this image, and therewith not only magnify any imperfections as you state, but also decrease resolution simultaneously..
    In modern lens designs there may well be compensationary tricks for this, but never for the full 100%.
    I also think the fall-off is clearest with WA zooms, basically because the front elements are quite large in relation to the actual FL, and any imperfections from an optical view point are largest.

  • Sggs

    If we have to use the zoom at the widest and change 16/35 to 24/70, woudnt be wiser using primes?

  • My first career was in medical research.

  • “I’ve been writing peer-reviewed scientific papers for way longer than I’ve been blogging about optics.”
    I’ve been reading this blog for many years and didn’t know this. What was your previous career?

    I have to say thank you for your data-driven approach to lens reviews, which is something that other reviewers don’t do. I spend a lot of time making Python plots that look like yours–I’ve done some optical analysis and lens design, but most of my work is data analysis for detector characterization.

  • Deanaaargh

    You do have many great answers for questions some of us haven’t even thought to ask.
    Thanks for the blog.

  • Deanaaargh

    Thank you for your thoughts, I had not considered the inherent difficulties associated with back flange ratios, perhaps the newer mirrorless offerings will eventually have something to say about this. Though as you brought up astigmitization is likely to be an issue until something like curved sensors come about( if ever)

  • A good question and one I don’t have any answer for.

  • That seems logical and at least as good as any of my suggestions, maybe better.

  • Brandon Dube

    The zoom lens is, in a sense, the only new lens design form since the retrofocus design in the 1930s (or 1950, if you consider angenieux’s retrofocus the first “retrofocus” lens). An 11mm lens for EF mount has a telephoto ratio (ratio of its back focal length to effective focal length) of about 4:1. Historically, designs outside the range of about 0.66:1 to 2:1 ~ 2.5:1 have been considered extreme, both in specification and difficulty.

    The retrofocus design has a great deal of astigmatism and distortion that require great care and skill to reduce. It is also difficult to stretch the length of the lens (and its focal length) away from a starting solution. This largely has to do with the astigmatism and distortion I mentioned.

    There are an enormous number of zoom lens design types and strategies to develop them. But in general, a ‘good’ zoom lens will maintain a similar aberration balance as it is zoomed. A consequence of this is that, speaking generally, until you reach a mechanical limit you can often continue to zoom arbitrarily while maintaining reasonable image quality. Often, even if the IQ becomes bad, the optimizer in the lens design code can make it pretty good again, perhaps at the expensive of performance at a different focal length or the requirement to add an element or two.

    The zoom motions in ultra wide angle zoom lenses are mostly pretty small. E.g. the 70-300L extends about 2 inches from 70-300mm, and a 70-200mm lens is doing about that internally. On the other hand, the front group of 16-35mm lens may only move 1cm through its zoom range. ‘Stretching’ the motion another mm or two to reach 15mm or 14mm may be relatively easy.

    Distortion and astigmatism are still a challenge – they are in all wide angle designs – but as long as you have a good zoom that is well behaved, pushing it to shorter and shorter focal lengths is a lot easier than stretching your 21mm design to 14mm, or your 14mm design to 11mm.

  • Turniphead

    Actually it seems logical to me that they would be sharper at the wide
    end; as you’re using the whole of the front element at the widest
    setting (and probably the largest amount of the other elements too). As
    you zoom in you’re magnifying the centre portion of the front element,
    so any tilts/curvature variations/imperfections are made more visible by that
    magnification. I think all conventionally designed zoom lenses should be
    sharper at the wide end as a result.

  • Deanaaargh

    Given the obvious ( I assume) complexities of designing UWA lenses why are so many of the widest lenses zooms? of course there are exceptions Nikon Canon and Sigma all produce 14mm modern(ish) primes (the Nikkor is from 1999). But there are seemingly many new ever more extreme UWA zooms on the market most notably the 11-24 Canon. Even in the MFT world neither Olympus nor Panasonic produce a prime wider than 24mm equiv. Why aren’t these extreme wide angles being covered by prime lenses especially given the sacrifices in the corners the zooms make?

  • Panacea

    <- Edified.

  • Carleton Foxx

    You are a saint for doing this.

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