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Just MTF Charts

Just the MTF Charts: 70-200mm f/4 Zoom Lenses

We did the 70-200mm f/2.8 lens MTF charts last week, so let’s do the f/4 versions of the same range now. Sports shooters and portrait photographers need that f/2.8 aperture, but many of us, most of the time, are willing to trade that off for smaller-lighter-less expensive f/4 models.

So, About f/4

Before we start, let me save someone from looking dumb on the internet. Not a week goes by that I don’t see someone use our MTF graphs to say something like “the f/4 is actually sharper than the f/2.8”, or “the f/2.8 is sharper than the f/1.4” yada, yada, yada. The complete sentence has to be “the f/4 is sharper at f/4 than the f/2.8 is at f/2.8”. Because stopped down means sharper, at least for the first stop or two.

So let’s take a quick look at that f/2.8 to f/4 difference. I’ll use the Canon 70-200mm f/2.8 IS II as an example. Here are the MTF results at 70mm taken at f/2.8 and f/4.

Lensrentals.com, 2019

You can see there is a pretty dramatic improvement throughout almost all of the field of view by stopping down just one stop – at 70mm.

Let’s make the same comparison at 200mm. This time things are a little different. At f4 things are clearly sharper in the center of the image, but in the outer half, there’s no definite improvement. The curves are different, but not better.

Lensrentals.com, 2019

 

So why must I smite your quest for the Holy Grail of “just give me one number to evaluate the whole lens at all focal lengths and apertures and shooting conditions so I can go online and say the Wunderbar 70-200mm scores 74.2 which is better than Ubertoy 70-200mm which scores 73.7”? Because of optical physics, that’s why. Well, and also because it’s a stupid quest.

The reason things don’t always improve the same when you stop down is pretty straightforward. Remember, lenses have aberrations, which among other things reduce MTF. Some aberrations are dramatically improved or at least significantly improved by stopping down. Other aberrations are markedly worse the further you go from the center and not influenced very much by closing the aperture.

So, closing down one stop will make a massive difference in the center of the image;  aperture-dependent aberrations (spherical aberration mostly, but also some types of coma) improve and the ‘distance-from-center’ dependent aberrations (astigmatism and some others) aren’t significant in the center. 

Out at the edges of the image, stopping down makes some difference for many aberrations, but very little difference for others. Unless you know what aberrations the lens has, you can’t predict how much improvement stopping down makes, especially in the outer parts of the image.

I always love when someone rants online about ‘the edges are soft even stopped down so my lens must be defective.’ Lots of lenses don’t get really sharp at the edges no matter how much you stop down, others do, but none ever get as sharp on the edges as in the center. 

So let’s do a more practical test and compare the 70-200 f/2.8 IS at f/4 to the Canon 70-200mm f/4 IS II at 70mm and 200mm (the full f/4 curves are coming up later).

Lensrentals.com, 2019

 

Lensrentals.com, 2019

 

The takeaway message, in this example, is that while the f/4 lens is arguably a tiny bit sharper than the f/2.8 at f/2.8, it is not sharper than the f/2.8 at f/4. It’s damn close, though.

The other takeaway message is you can’t be sure exactly how much and where (either image circle where or focal length where) a zoom lens will improve stopped down a stop. It will be better, but it’s difficult to predict precisely how much better without testing. And no, I don’t have the resources to do stop down testing on all the zooms. Even this little 3-copy example took a day’s testing and days are something I don’t have enough of.

A Quick How to on Reading MTF Charts

If you’re new here, you’ll see we have a scientific methodology to our approach, and use MTF charts to measure lens resolution and sharpness. All of our MTF charts test ten of the same lenses, and then we average out the results. MTF (or (or Modulation Transfer Function) Charts measure the optical potential of a lens by plotting the contrast and resolution of the lens from the center to the outer corners of the frame. An MTF chart has two axis, the y-axis (vertical) and the x-axis (horizontal).

The y-axis (vertical) measures how accurately the lens reproduces the object (sharpness), where 1.0 would be the theoretical “perfect lens.” The x-axis (horizontal) measures the distance from the center of a lens to the edges (measured in millimeters where 0mm represents the center, and 20mm represents the corner point). Generally, a lens has the greatest theoretical sharpness in the center, with the sharpness being reduced in the corners.

Tangential & Sagittal Lines

The graph then plots two sets of five different ranges. These sets are broken down into Tangential lines (solid lines on our graphs) and Sagittal (dotted lines on our graphs). Sagittal lines are a pattern where the lines are oriented parallel to a line through the center of the image. Tangential (or Meridonial)  lines are tested where the lines are aligned perpendicular to a line through the center of the image.

From there, the Sagittal and Tangential tests are done in 5 sets, started at 10 lines per millimeter (lp/mm), all the way up to 50 lines per millimeter (lp/mm). To put this in layman’s terms, the higher lp/mm measure how well the lens resolves fine detail. So, higher MTF is better than lower, and less separation of the sagittal and tangential lines are better than a lot of separation. Please keep in mind this is a simple introduction to MTF charts, for a more scientific explanation, feel free to read this article.

 

Canon 70-200mm f/4L IS

This is the original Canon IS version that we introduced above. While I don’t have test results for the Non-IS it was considered a bit sharper than this original IS, but not as sharp as the IS II version below.

70mm

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135mm

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200mm

Lensrentals.com, 2019

 

Canon 70-200mm f/4L IS II

This makes a good second lens to show you because the version II is noticeably sharper than the original version at 70mm and 135mm. You can tell the difference if you shoot with them, so this gives you a good ‘this much difference is significant’ comparison for the other MTF charts. It is better at 200mm, but more at the ‘you’d have to make a careful direct comparison to see a difference’ level.

70mm

Lensrentals.com, 2019

135mm

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200mm

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Nikon AF-S 70-200mm f/4 G ED VR

Nikon makes the best f/2.8 zoom in this focal length, but their f/4 version is better described as ‘fine.’ It’s about as good as the original Canon version, maybe a bit better, but not as good as the Canon version II. The more paranoid among you can now begin discussions about ‘did they dumb-down the f/4 version so it couldn’t compete with the f/2.8?’ I can’t imagine that is true; designing lenses isn’t like turning off a video codec.

70mm

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135mm

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200mm

Lensrentals.com, 2019

 

Sony FE 70-200mm f/4 G OSS

Sony tends to put out more lenses per year than anyone else, and they have improved their lens quality rapidly over the last several years. But some of the older Sony designs are not great, and this is one of those. It’s at it’s best in the middle range, not quite as good at the two extremes. Some of you more strident Sony supporters will now state how wonderful it is and that the tests are wrong.

70mm

Lensrentals.com, 2019

135mm

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200mm

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Summary:

The conclusions here are pretty simple. If you shoot (or adapt) Canon EF mount lenses the Version II Canon 70-200mm f/4 IS II is excellent. It’s so good that you should only buy the 70-200mm f/2.8 version if you need f/2.8. (Since lots of people want the narrower f/2.8 depth of field for portraits or need all the light they can get for stop-motion action photography, the f/2.8 still will have lots of takers.)

The Nikon 70-200mm f/4 is good at 70mm and 135mm. While it fades a bit at 200mm it’s a really nice walk around and travel lens. The Nikon f/2.8 version is so good, though, that most people who can afford it will be willing to deal with the heavier lens and higher price for the image quality.

Sony also has a 70-200mm f/4, and it’s OK. It’s not going to wring all the resolution you might like out of a high-megapixel camera, but it’s still a decent travel lens. From what I hear, though, a lot of Sony shooters prefer the Canon IS II f/4 on an adapter, and I can understand that option, too.

 

Roger Cicala and Aaron Closz

Lensrentals.com

July, 2019

 

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 Just MTF Charts
  • asad137

    given that curved sensors are used in astronomy

    Where? I’ve seen astronomical cameras that have curved focal planes but those are made by tiling individual flat sensors onto a curved mounting structure. I’ve never heard of one that uses acual curved sensors.

  • asad137

    silicon is stretchy and can just be pulled into the necessary shape

    Uh, not it isn’t. Silicon is a rigid crystalline material and is not stretchy at all. Are you confusing “silicon” with “silicone”?

  • Matti6950 .

    Nah i think it will be equal, maybe slight corner edge at f2.8 immediately gone at f4. Remember, the wider the angle, the more sharpness advantage Z mount has. But nikon has F-mount super sharp 28mm. Anything above is generally super sharp available. Sure nikon knocked it out of ballpark with FL ED. But it’s also tele (the range from where F-mount is not punishment that much). Much will depends also on how much they oversize the lens, and if they wanna dare the 3000 euro price tag again.

  • Thanks Brandon! I’ll see how far I make it through the paper!

  • Ertan Ozturk

    “Some of you more strident Sony supporters will now state how wonderful it is and that the tests are wrong”
    You are being humble. Some “strident” Sony supporters will burn your house if they know where you live 🙂 Fanaticism has gained a new meaning and level with “strident” Sony trolls.
    Thanks for your hard work by the way. I know it is quite time consuming and exhausting doing all these tests, taking the charts, writing sentences that make sense and that will not make strident fans unhappy 🙂

  • Andreas Werle

    Beautiful! 🙂

  • Brandon Dube

    Surfaces are not equal for different purposes. If you want to image on a flat surface, you absolutely must have both positive and negative powered elements for the Petzval sum to be anywhere near zero. If you curve the image, you don’t need negative lenses anymore because the image plane can curve as the positive lenses want it to. This means you can delete the negative lenses, which probably injected aberrations into your design anyway.

    Or, in the more general case, it gives you a knob that is essentially a 1:1 control over field curvature. Where you may have had 5, maybe 10 elements all fighting each other to get near zero field curvature otherwise.

    This makes the curved image surface as valuable as several other elements, not just parameters, in the design.

  • Brandon Dube

    Any optical design program can accommodate a curved focal plane – just alter the curvature of the image surface. You can make it a more complex surface (a sphere, Freeform, […]), too.

  • Brandon Dube

    Making curved sensors is extremely difficult.

  • Brandon Dube

    “I’d have to refer you to the report on that”

    static1 dot squarespace dot com/static/578d10066a4963fd85e0aa32/t/5af25e61aa4a99ed9ecfa39c/1525833475522/bdd_ug_thesis_10.pdf

  • Misha Engel

    Would love to see the MTF chart from LensRentals of the FUJINON 19-90mm T2.9 Cabrio, against some competition (Zeiss, Canon, etc..)

  • bwoodahl

    Thanks. Crap, I think I need to check my f/2.8 FL ED against others. But I may just hold off for the Z-mount f/2.8 S. I’m guessing edge-to-edge it will (easily?) surpass the FL.

  • Dragon

    Hi Roger. It looks like in your Canon f/2.8 to f/4 (at f/4) comparison, the 70 mm f/4 lens curve is from the IS II version and the 200 mm curve is from the original IS version. So many charts so little time :-).

  • Søren Stærke

    The reason there is so many elements in modern lenses is among other things to correct for Petzval curvature or Spherical aberations. This means that using a curved sensor would reduce the amount of “correction” needed, BUT the ideal amount of curvature is entirely dependent on focal length of the lens. Wide angles gives best results with very curved focal planes (read: sensors) whilst telephoto lenses yield a rather flat focal plane.
    So to change to a curved sensor you not only need to determine which focal length do you want to optimize for, but you also have to ditch your entire current lens ecosystem, and you also need to change your entire sensor production facility because spherical silicon wafers are not something anyone works with.

    Basically there is a few advantages with curved sensors but so many disadvantages that it’s not reasonable to do.

    Btw, yes it is easy to simulate in optics software like Zemax. I’ve done it before.

  • Azmodeus

    Any chance of a test of the Tamron 70-210 f/4 Di VC for comparison Roger?

  • 16, it’s on the graph. If you have the new f2.8 ED I’d say definitely. If it’s the VR II, well, maybe you have a good f/4. I am assuming, of course, you’re not comparing your f/4 to the f/2.8 at f/2.8.

  • redwave18

    Now I feel inadequate for having the original f4 IS version. How can I rest easy knowing another lens in the same class resolves a few more l/mm and every photo that I shoot will have missing info because of my feeble eyed lens.

  • bwoodahl

    How many copies of the Nikon f/4 did you have open hand for this comparison? My f/4 is as sharp as my f/2.8, maybe I have a crappy 2.8?

  • I would white list you if I could. I think if you post your links it usually puts it in purgatory until someone releases it, I do have that power.

  • Curved sensors can work with interchangeable lens cameras. An adaptable curvature sensor would be great (it doesn’t need to be actuated, silicon is stretchy and can just be pulled into the necessary shape!), but this isn’t necessary. If a sensor with fixed curvature is used, all lenses could be designed for that standard curved shape. The necessary field corrections (and therefore associated aberrations) would still be reduced compared to those for a flat sensor.

  • There is active research occurring on curved sensors for a variety of applications. When I go to conferences and see these presentations, I try to talk to the people afterwards about when it might be seen in a commercial product and what form it would take.

    Curved focal planes are currently used in astronomy, though in most cases these are a bunch of flat sensors mosaiced together into a curved shape. This reduces/eliminates the need for a field flattener. I would post links to examples, but whenever I do that, my comment gets flagged as spam. Google “panstarrs focal plane” for one example. I think there are examples of monolithic curved sensors, but these are weird one-offs.

    I talked to a group that was taking full-frame CMOS sensors (Sony? I forget the manufacturer. There were your standard DSLR sensors though) and curving them. Dark current, read noise, etc are mostly unaffected. They then tested these with various DSLR lenses (modified to removed the field flattening elements) and had good results. I asked if zoom lenses can be made compatible with curved sensors, and the answer was yes.

    Canon and Nikon have tons of patents for curved sensors. They are clearly researching these. I won’t post a link, or else this comment will never see the light of day (Roger, can I maybe be put on a whitelist or something?), but a search of CanonRumors will turn up some of these.

    Before the latest lines of mirrorless cameras came out, I really was hoping that Canon or Nikon would incorporate curved sensors into them. A curved sensor would greatly simplify (and therefore lighten and cheapen) lenses, and it would improve corner sharpness and vignetting. However, since Canon and Nikon have recently released their mirrorless systems and committed to flat sensors for the next couple decades, I suspect that we won’t see curved sensors in interchangeable cameras in the near future. They may turn up in some compact cameras or cell phones, though.

    Edit: I don’t know this for a fact, but given that curved sensors are used in astronomy, I believe that your standard optical design software like Code V or ZeMax can accommodate curved focal planes.

  • Andreas Werle

    Of course, a curved sensor is no solution for interchangeable lens cameras. But a “adaptive sensor” would be. Imagine a sensor which does not consist of one piece of flat silicon, but of thousands tiny pieces, each one sitting on a small actuator which can move it backward and forward and also tilt the surface. It would result in a sensor surface which can be adapted to every form needed and so compensate some lens errors.

  • Ilya Zakharevich

    @JLW:

    I would believe this statement only if the author would come with a convincing quantification. There are two serious issues; I doubt the author took them into account:

    • First, look, for example, at Sigma 35mm F1.2 DG DN Art. It has 17 elements in 11 groups. This is 17+11=28 positions-and-shapes of optical surfaces to tune up (for the purpose of increasing image quality). The more tune up parameters, the better is the best image quality one can obtain.

    Now add a curved sensor to this mix. This is an extra surface which you may involve in the optimization process. So instead of 28, you get 29. (In fact, you cannot change the position of the sensor, so it is more similar to adding ½ of extra parameter!) This does allow more improvement?—?but is 28½ vs 28 a big enough change to deserve the trouble?! (Increasing instead the number of surfaces in the lens by 2% would increase price by what?—?$30?)

    • Second, every lens would require a particular shape of the sensor. Forget about it for interchangeable lens cameras!

  • With zooms it’s testing at 3 focal lengths. With some lenses setting the aperture is easy, just put it on the camera, hit the button to close to set aperture, take it off. With some, like the older Nikons, they don’t hold a camera setting so each lens has to be manually adjusted for proper aperture by ‘pinning’ the aperture lever. 3 or 4 zooms can take an entire day.

    In the bigger picture, the machine is used for required in-house testing. So to do stop downs I’d need to buy a new machine ($200k) and hire a tech to run it just for that purpose.

  • Max Manzan

    While I see that the Canon 4,0/70-200 II has excellent image quality on the whole, I really dislike its strong drop in tangential performace at the edges/corners of 70mm.

  • Yeah, I was just trying to give you a hard time! I think we all appreciate whatever information you can share, but one can always hope … The process sounds pretty time intensive: what’s the thing that takes the most time now?

  • The theory is true, although not just a simple curved flat sensor, like those curved TVs and computer displays; it would have to be curved in both dimensions. That wouldn’t make lenses aberration free, although it would simplify some things. But all lenses (and the software to design them, I think) would have to be totally redone. I doubt any interchangeable lens camera manufacturer sees that as a great idea. It might be we see it on some fixed lens or (more likely I think) cell phone cameras.

    But this is rather out of my area of expertise. Brandon might be able to give better comments but I think he’s pretty swamped right now.

  • Andre, you are exactly right. Stopping down would give us a host of information. The trouble is, this whole setup exists to detect bad lenses and help us optically adjust them, all of which is done wide open. I can share those tests we have to do anyway, which are the wide open results. I can do a little free-time exploration, like the stop-downs above.

    But the blog (other than some positive PR) doesn’t generate enough revenue to begin to cover stop-down testing; best estimates are between 1.5 and 3 million dollars just to do the majority of lenses one and two stops down. I haven’t got that kind of funding, and no blog or testing site is likely to generate it.

  • Roger, thanks for the f/4 vs. f/2.8 comparisons: you read my mind!

    But don’t your results also argue for doing stopped down measurements in order to understand which aberrations are limiting a lens’s performance? That is, it’s too simple to say that a lens stopped down will be better. Are there other ways to divine a lens’s objective performance from just the wide-open MTF results?

  • J.L. Williams

    Re center vs. edge sharpness, I remember reading this years ago: “Lenses could be sharper, smaller, simpler and cheaper if they were designed to form an image on a curved surface, and making a curved digital sensor wouldn’t be difficult. The only reason it hasn’t happened is because nobody wants to go first.” Still true, you think?

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