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Lenses and Optics

A Quick Zoom Variation Demonstration

Published March 17, 2015

Sometimes I put up posts basically because I find myself answering the same email over-and-over. One of the emails I get a lot is: “I want the best copy of the xxxx zoom lens.”

People get irritated because my response is usually something like this: Which focal length do you want the best copy at? The long end, short end, or in the middle? By best copy do you want the highest center MTF, the best corners and edges, or is the middle of the image the most important to you? Or do you want the one with the flattest field? And again, at which focal length do you want whatever is the most important thing to you to be the best at?

Generally, people miss my sarcasm and just respond that they want the best overall lens. I could probably satisfy that request for a prime lens (it would be splitting hair, but it could be done). But there’s no way to do it with a zoom; there are just too many variables, and the variables interrelate and counteract each other in complex ways.

So today I thought I’d just put up some pictures and demonstrate how one single variable, field tilt, changes on a given lens at different zoom distances. We chose Canon 70-200 f/2.8 IS II lenses because they are excellent. So you fanboys that want to take this out of context and say Canon 70-200 f/2.8 IS II lenses have tilt variation, trust me, it’s less than the zoom you are a fan off. Every zoom lens made is like this.

Field Curvature Examples

All of these lenses had been tested just prior to running these field curvatures both with test-chart photographs and on our MTF optical bench. All of them were optically at least average, if not slightly better than average by my standards, which are quite a bit higher than what the factory standards are (we reject about 5% of lenses that factory service tells us are fine).

The first lens I’ll show you was the weakest of the 12 we tested (although I’ll mention again, it was still an “average” copy). It looked fine on all of our usual optical tests, but on the optical bench the MTF chart had some detectible astigmatism at 200mm. The field curvatures at 70mm, 120mm, and 200mm zoom lengths show why. There’s a bit of field tilt at the wider focal lengths (about normal for most zooms) but at 200mm the tilt becomes more dramatic and the tangential field is much more tilted than the sagittal field.

Β 

If you prefer numbers, our bench calculates out the actual tilt in degrees, and the table below shows the tilt numerically.

  Sagittal Tangential
70mm0.110.21
120mm0.130.38
200mm0.180.9

Could you have detected this at home? Probably if you were really into pixel peeping, did careful alignment, and were fairly experienced you might have detected it. Could you have seen it in a photograph? If you compared edges at 200mm with a subject that would reveal astigmatism, you might see it. Probably not, but maybe.

Here’s a second example; this lens optically looked fine also and passed our usual optical tests. On the MTF bench it had a bit of astigmatism at 200, but not nearly as much as the first example.

Β 

Β Looking at the tilt numbers in numerical form instead of pictures is probably clearer.

  Sagittal Tangential
70mm0.01-0.03
120mm0.130.02
200mm0.16-0.31

Could you have detected this one as a problem at home? No, I’m almost certain you could not unless you have an Imatest setup, perhaps.

A third example, just for anyone who is starting to think that this is something that just happens at the long end of the zoom.

I won’t bother making a numerical table this time; I think you’re probably getting the idea. This lens is perfectly flat in the center of the zoom range, with just a slight bit of tilt at either end.

What Does It Mean?

All I’ve done is show you one variable (field tilt) in a number of optically well-adjusted lenses.

We ran 13 copies of the lens through field curvature tests at these three focal lengths. Two of them had flat fields at all three test lengths, and two more were nearly so. But neither of those two had the best MTF, they didn’t resolve as well as most of the tilted lenses I just showed you, although, again, they were acceptable. If we had tested some other things – rotational variation, lateral chromatic aberration, MTF at various focal lengths, etc. every lens would have been slightly different from every other one in those measurements, too.

Of the 13 lenses, only the first example I’ve shown above could possibly have been detected outside of the lab, and even that would have taken some intensive home testing to find.

So, if I’ve done this properly, you should now be thinking, “If we can’t see it in a photograph, then it doesn’t matter, does it?” Which is the point. Asking for the sharpest copy, best copy, whatever, is pretty much a fool’s errand unless you have something specific you want. For example, I could give you the copy with the highest center MTF curves at 200mm. That would be easy (although probably useless). But I sure can’t give you the best zoom lens because they are all best at something and worst at something else — and in a photograph you can’t see the difference 99% of the time anyway.

So why don’t we make them perfect? Because we can’t. Optically adjusting a lens to improve center resolution at the long end may affect field curvature at the short end. Then fixing the field curvature at the short end might affect focus accuracy in the center. When you adjust these things for a living, you quickly learn to make compromises so that everything is acceptable. It can be complicated and it can take hours. And when you are finished, some things are still a bit better than others.

In our repair department, we call it Roger’s First Rule: Better is the Enemy of Good Enough. I can’t tell you the number of times we’ve spent four hours getting a lens optically acceptable, and then decided we’d make one more little tweak to make this thing just a bit better. After that tweak, everything else is totally awful and it takes another two hours to get back to where we were in the first place.

It’s different with prime lenses. There are far fewer elements, and especially far fewer moving elements, which makes for fewer variables. But finding the “best copy” of a zoom is basically a meaningless task; unless you know exactly what focal length and which features you want it to be best at.

(I know that 6,314 of you are about to suggest another lens for me to repeat this test on. Let me point out that this took nearly two full days, tying up a $200,000 machine for a non-profit blog post. It isn’t likely to happen again anytime soon.)

 

Roger Cicala and Aaron Closz

Lensrentals.com

March, 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 Lenses and Optics
  • Matthew Leeg

    … actually 3rd sentence of the 4th paragraph, but who’s counting, right? Oh yeah, we are.
    Take Care, M

  • Matthew Leeg

    Hey Roger,
    Just a quick grammar thing:
    It looks like the word “off” at the end of the 2nd sentence of the 4th paragraph is supposed to be “of”.

    Personally I like it when you tell fanboys where to get off, or what to get off of!

    Take care, and keep up the good work. We do appreciate it!
    Sincerely, Matt

  • JohnL

    My view on testing a new lens is:
    (1) Take a few photographs.
    (2) See if they look okay.
    (3) If so see it as “your” lens and don’t worry abouy any foibles it might have in the future.

    You might ask… how is this relevant here… My first 70-200 f2.8 IS II was really not good by this test… but the second one is fine… (Also the shop’s demo one was good, I so miss Jacob’s Pro dealership in Oxford St… gotta love any dealer that will just let you run out the door with a 2k lens to play with it.)

  • That’s why I’ve switched to primes after 25+ years of shooting with zooms.

    The only zoom I miss is Canon’s 16-35/2.8L. That thing was phenomenal, I still can’t quite substitute it with any given prime (or zoom for that matter).

  • Charlie

    Roger: We read all these tests when we buy a lens and camera, then found out from pros and this site that some of every model are better than others. I already chose “The Best” or whatever. I just want it to match what it is claimed to be. It’s really a matter of product quality and consistency; does it meet design spec? I don’t want one that doesn’t.

  • LeFred

    Hi Roger !
    What about a bit of lens teardown for a change ? Back in 2013 you said about Fuji lenses “I have less experience looking inside Fuji lenses, and to be honest find them a bit strange, but they are well constructed”.
    You made me curious. How can they be “strange” ? Isn’t there a Fuji lens lying around needing to be “fixed” ? πŸ˜‰
    Best Regards,
    Fred

  • Roger Cicala

    Richard, you’re confirming that I have my priorities all messed up. I’m buying these machines when I could be spending that money on Booth Babes for the next trade show. What is wrong with me????

  • I agree with Richard. Reading your blog posts over a period of time convinced me to buy a used lens from you guys rather than a new one. I live on a small island in the Caribbean, so returning or exchanging a new “lemon” lens is just not practical, no matter how good the warranty or exchange policy might be.

  • RICHARD

    These blog posts are not “non-profit.” They are the best kind of marketing because they help to reinforce LensRentals unique position as THE place to go when you need lenses that have been meticulously calibrated and optimized to perform their best.

    Yes, you could spend the equivalent amount of money on booze and booth babes at WPPI but it wouldn’t pay off the way these posts do in terms of building your brand. So if anyone gives you a hard time about this, you just tell them to go jump in the Wolf River.

  • Aaron

    @Roger
    Sure, you won’t be doing exactly the same thing (Well…maybe with super-zooms vs a couple of high quality primes? Maybe? Please? :P), but I’m sure it’ll illustrate something that you get bugged about, and be quite interesting for the rest of us.

  • obican

    Thanks for persuading me NOT to try to further adjust my Minolta 70-210/4 so that it “might” get better on the tele end.

    Long story:
    When I got the lens the first time, you could see it was a very bad copy by looking at the rear lcd without any magnification at all, it looked like a tilt shift lens. I could return the lens (the previous owner was nice enough to let me know about a thousand times that I could return it) but decided to try my chances at it.

    After going through a few disassembly guides for that lens, I took it apart. Luckily, this is one of the lenses which were designed to still function when partly assembled so I could adjust the groups by checking the live view from my A7. I could see the lens was badly decentered and decided to work on that, partly because it’s one of the things I could check the easiest.

    It’s a breeze to work on the lens as all the adjustments are can be made while the lens is mounted on a camera. To my luck, I had mastered the decentering test pattern about 20 minutes later. I did some test shots inside and outside the apartment at various distances and focal lengths and found the results acceptable. Put everything back on, went out for a drink.

    I don’t like splitting hairs by shooting test charts but I could still tell the tele end was a bit weak compared to the rest of it. I did some shooting (brick walls, mostly) and compared the Beercan to Canon’s 70-200 2.8L IS, the Mark I version. Unbelievable for a lens which costs 80$ used, the Minolta can hold its own against the mighty white Canon L. On the wide end it is a bit sharper wide open than the Canon@f/4, with more CA and some kinds of fringing though. At around 135mm, it’s the same story. Only at 200mm, Canon pulls ahead, where it absolutely slays the Beercan in every criteria imaginable. Except for one thing, the Canon image feels more like a 160mm compared to Minolta at portrait distance or Minolta feels like a 250mm lens compared to Canon on the tele end.

    Anyway, I’ve been thinking about doing another session with the lens to see if I could improve it at 210mm, where I use it actually quite often. After reading your article, I’m much less willing to do so. My newest acquisition, Minolta 200/2.8 APO G HS is much sharper than both of those zoom lenses that I’ve mentioned ever hoped to be after all.

    Long story short again: Whatever you do with your zoom lens, there probably is a prime lens which is sharper at that particular focal length.

  • Ram

    Hey Roger,
    While I agree with your general premise that you wont usually detect minor lens tilt or other errors without sophisticated equipment, they are glaringly obvious in the work I do: astrophotography. Very few lenses in my experience have consistently been able to show stars as perfect pinpoints of light. A lens with even a slightly tilted element produces a terrible looking star image off axis. Photographing a star field is a severe test and weeds out 90% of all lenses I have tested.

  • Bryan Willman

    Bryan’s suggested CounterQuestion – Instead of saying “I want the best at …” where “…” is unrelated to any real project, instead ask “I do xyz which involves pushing the limits of pixels in way Q, I would like to rent/buy/build the lens most likely to succeed at this.”

    That probably won’t generate as much sarcasm, but it’ll be just much a pain to answer….

    BTW: Isn’t a large part of cine lens cost the TotalConsistencyThroughZoom range feature – which cine users actually use? That is, zoomed one end to the other, focus behaves, distortion is constant, field of of focus behaves, visible aberations are constant, and so forth? In a still camera you don’t care, in a cine camera it would drive you nuts, when these things change with zoom.

  • Roger Cicala

    Aaron, no question I will, and in way less than a year. But I bet I don’t tie it up to do the same thing over again πŸ™‚

  • Aaron

    “Let me point out that this took nearly two full days, tying up a $200,000 machine for a non-profit blog post. It isn’t likely to happen again anytime soon.”

    Really… Somehow I expect that within the next 12 months you will tie up the machine for at least 1 full working day just for something to post here on the blog. See if you won’t. Because you’re just that freaking awesome πŸ™‚

  • Roger Cicala

    KimH,

    That is indeed part of it – although there still will be a little bit of variation, even in one of those $30k Cine zooms. But no question there is less variation in them. The housing, the fact that they’re a low-volume item, and the extra design required to make them par focal through the zoom range all add to the cost too.

  • KimH

    Allright – Sir – I’ll be the one asking πŸ™‚

    When I look at some of the $30.000 cinema lenses i think “there’s a reason they cost that much!” Some have better zoom-range. Some are better F etc etc – But is it also things like this “less Tilt”, more consistency across a zooom range or is it somethign else?

    You’re probably the only one i would dare to ask and expect a propper answer. There you go – i’m that impressed.

  • Phil White

    I’ve been a fan of LensRentals for over two years. I spent hours reading your evaluations, posts and equipment summaries for at least a year before I trusted you enough to purchase a lens from you. You did not let me down. As a matter of fact you inadvertantly ‘guided’ me to a better purchase.

    The story: I wanted to buy a lens for my Canon 7D and after months of deliberation decided on the EF-S 17-55mm F/2.8 IS USM. I went to your site, hesitated . . . I decided not to purchase and left the site. Figured it was now or never so I returned. The lens was no longer there (or so I thought). Still needing a lens, the EF 24-70 F/2.8 L was there. More money, but it was now or never. I purchased it. You said it was an 8 and had miinor flaws (you fibbed a bit – I’ll get to that in a bit). After I finished the transaction I went back to the site and found the EF-S ON THE SECOND PAGE! YOU HAD GUIDED ME INTO A PURCHASE I HAVE NOT REGRETTED. Oh about those flaws – you must have been using an electron microscope ’cause I ain’t seen none!

    My meager point is: Your philosophy appears to be do it right ALL the time. Anyone who follows your site nows that.

    Yes, WE WANT THE BEST – and Roger you deliver. You and your team’s knowledge, wit, sarcasm and honesty surpasses any expectations.

    So for you guys who ask for,”. . .the best”, you don’t have to ask. It’s already there right in front of you.

  • I just love this scientific way of sayin’ “STFU, go out and take some good photos!”

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