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Sigma 120-300 f/2.8 Part II: Comparative Anatomy

Published June 20, 2013

We know the optical formula for the new version of this lens is the same as the old version. We know it has the ability to be reprogrammed and adjusted with the new Sigma dock and Optimization Pro Software. We know there have been some changes to the mechanics and structure of the lens, but we don’t know exactly what they are.

We have enquiring minds. We believe we have a right to know exactly what’s different when someone just says ‘it’s better, and worth an extra $1,000’. We also have Canon mount new and old versions of the old and new lens sitting on the shelf, at least for the moment. You know what happens next, right?

Plus, as I mentioned in Part I of this series, I really want to know if this lens, which has historically been less than reliable, has made changes that might improve reliability. Most repairs seem to involve the focus assembly, OS unit, or focusing motor. So I was eager to see if this area had been redesigned, changed, or beefed up somehow.

So let’s have a look inside. Yes, I know you don’t ever plan to disassemble your lens. You don’t plan to rebuild  your next car’s engine, either, but I bet you look under the hood before you buy it.

The outside is promising – there are clearly different external barrel assemblies and the new lens is a bit thicker at the back end.

When we take off the lens mount, differences become quickly apparent. The old mount has the usual 4 screws with shims underneath (probably to fine tune infinity focus).

Original mount


The new mount is quite different. First of all it mounts with 6 screws rather than four and it mounts to a polymer plate. The shims are located between the plate and the rear barrel. One thing to note, though, the 6 screws aren’t really holding the mount to the lens. Two of them simply hold the plate to the rear mount, only 4 pass all the way through to the lens body (as the shims clearly show). That’s not a negative, though. I don’t know of any lens that uses more than 4 screws to hold the mount on. I just want to point it out because I’m certain someone is going to be going on about ‘the sturdier 6-screw mount’.

New version mount


With the mounts off, we can see another difference with the new lens: the weather sealing is clearly more robust. The new lens has a foamed rubber seal (red pointer) between the mount and the barrel instead of a thin rubber ring.


Looking into the barrel we can compare the circuit boards. The new lens’ board is a complete circle, while the old lens has a gap (red pointer). This isn’t surprising since we know there must be more circuitry for the programmable switches and the dock. There are a couple of different / extra chips on the new PCB, but most of the circuit traces and all of the connections are identical.

The newer version has a complete circular PCB, while the older one is C shaped.


Looking close up (in the red square above), we can see an interesting difference in the flex cable coming from the switch panel. The connector is the same on both circuit boards. On the new lens all 10 connections from the switch are used. Not surprisingly (since it doesn’t have custom switches) the old lens only uses 6 wires. What is a bit surprising is it still used a 10 wire connector with 4 dead-end wires in the flex (red arrow). This makes me think that Sigma, a couple of years ago, knew they’d need those extra wires one day. Otherwise why not simply use a 6-wire flex and connector in the first place?

Other than that interesting tidbit there were no differences at this level,so we removed the rear barrel. First the good part: the new rear barrel has a second area of thick, water resistant rubber lip where it seals to the main barrel (red pointer). The original just has a thin rubber washer.

When we took the rear barrels off, the main mechanical / electrical package of the lens is exposed. This group contains the OS unit, the aperture unit, and the focusing unit. Looking at them side by side it doesn’t appear that there are significant differences.

The electronic unit on the old (right) and new lenses seem identical.

But changes could be subtle so we removed the units to examine them more closely. These units are interesting: the focusing/aperture/OS unit in this lens is quite self-contained and by itself is about as large as a Canon 24-105mm lens.

You might appreciate seeing this if you’ve ever sent one of these lenses in for a failure in either the AF motor or OS unit and the service center notified you they had replaced both. I would expect this unit is a single repair part (we don’t know because Sigma doesn’t release parts lists). It would probably be more cost-effective to replace the whole unit than to take it apart and replace just the one bad part, which would be very time consuming.

Those metal prongs you see sticking out just above the first element in this group are the manual focusing keys that rotate the large focusing element. (The front lens you see isn’t the one that focuses, it’s the group just behind it.) This is a big, heavy focusing group and moving it you can understand why AF is not quite as fast in the big Sigma zoom as in some other lenses; that AF motor is moving a big load.

Back to my original point, though, as best we can tell the focusing/aperture/OS unit is identical between the two lenses. In fact, we could put the unit from the new lens in the old one, and vice versa, and they fit perfectly. (We didn’t hook them up because I don’t know if the electronic connections would be identical.)

Hoping we’d see some more subtle changes in electronics, we took things apart a bit more. Taking the OS units out, though, showed them to be identical, right down to the part numbers.

OS unit from the original 120-300mm OS
and from the new version


We didn’t completely disassemble the AF motor and aperture systems, but with the OS unit out of the way (it sits in the back of this unit) we could see inside quite clearly and could not find any difference between the new and old versions.

This doesn’t mean the new lens won’t be more reliable. There could be differences in quality control and assembly that we could not see. A slightly thicker piece here or there could be critically important but not something we would have noticed. Just because I couldn’t find any differences doesn’t mean there aren’t differences.

But to me, this means I’ll believe the new lens is more reliable when I see it is more reliable. Of course, I’ll be keeping statistics on failure rates of the new versus the old versions, but it will take 6 to 12 months before we have even a hint of any reliability improvements. At this point, there’s nothing that makes me confident things will be different.

While we’re here, you probably want to see the front of the lens; after all that’s the biggest part of the lens. It’s also the simplest part, containing just a few large elements and the zoom helicoid. I still find it impressive how large those elements are, even in the middle of the lens. There are no electronics in the front section other than a zoom position sensor, and nothing up there that has ever failed in our experience.

Front barrel showing zoom group and inner zoom barrel.



The 120-300 f/2.8 OS is a lens with unique capabilities, and as you can see some unique design features. The new version gives us some clear improvements that will make the lens more usable for a lot of people, and hopefully will end problems some users have had with back and front focusing.

The optics, and as best I can tell, electromechanical construction is unchanged between the original and the new version. Until we get some long-term reliability data, I can see no reason to think this lens won’t continue to be, shall we say, a bit delicate.

That’s not a deal breaker. To get a high-quality f/2.8 telezoom at this price (even the new version has to be considered reasonably priced compared to other supertelephoto lenses) there are going to be some compromises. Plus the improvements in Sigma Service and Repair make a breakdown less life-shattering than it once was.

Unless I knew I needed the programmability features, though, I don’t see a huge reason to pay for the new version. I’d probably take my chances with a copy of the old version.


Roger Cicala and Aaron Closz

June, 2013

All images copyright Roger Cicala and, 2013. You may reproduce as long as you credit the photo and link back to this article.

Author: Roger Cicala

I’m Roger and I am the founder of 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.

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  • Laser-x Alain

    any hints about how to disassemble the AF unit? I am stuck at getting the OS out and also the first element on the focus side does not want to move. why I ask?I got on that must have had a sever shock, the weakest point is the focusing unit and it got bend somewhere internally.

  • Nicolas Strauel

    Hi Roger,
    As this article is from 2013, do you now have any better reliability data about the Sport version you can communicate ?


    OK will do. Happy Eastern weekend

  • Mouette et Jambon

    Great ! contact : iffick at hotmail dot com


    Hi Mouette et Jambon,
    Right now the lens is still at the shop where I gave it in for repair ( in Brussels, Belgium ) . I am too busy at the moment to collect it before the end of April. I must warn you that there is a very small star-like damage on the front element since long ago. This however has no repercussion on the image quality. What I suggest is that after getting back the lens, I sent you a picture of the damage and one of the last photos I took before the lens went dead. So if you have some patience, I think we can work out something. If you are still interested, answer this email to exchange email adresses or mobile phone numbers

  • Mouette et Jambon

    Hi Jos, I’m currently looking for a front lens for a 120-300 OS HSM, that SIGMA doesn’t seem to sell as spare parts and is charging way too much for a repair. In case you would consider not keeping your dead 120-300, feel free to let men know !


    hello lens rentals,
    After excellent service for over 7 years on canon 7d and 7d II my 120-300 old version is dead: repair cost around 1500 euro. Any more info available on reliability of the new sports version? Any thoughts on using it with a mc 11 adaptor on a sony a9, a7rIII or a7III or the nikon version on a d500?

  • Jura Štok

    Is there any new data regarding reliability of 120-300S?

  • Hi Roger-
    I am an owner of a now out-of-warranty 120-300 OS HSM (non S version) for nikon that is going through its second drawn-out and super annoying repair (first was AF module, second is OS module). Your tear-down revealed much of the same guts in the S version as the non S version. After providing the new S version of the lens to customers for a few years now, I’m curious about the reliability of the S model. Any different than the non S? I really like my lens but am nervous about future repairs now that its warranty is done. Is it worth upgrading to the S version? Or maybe its time to find an alternative lens…

  • Liz

    Looking at this lens in 2015, can you comment on reliability? What is the average cost and turnaround time for repair?

  • Chuck Lantz

    I used a borrowed older version of the 120-300 for a few weeks, coupled to a Sigma 1.4 TC. I was very impressed with the sharpness of the lens, even with the TC, where sharpness often falls off a bit.

    I found this review while trying to find an easy way to identify the version I used when compared to the newest Sports version of the 120-300 (the one with docking capability.) Besides the gold band on the older version, and the slightly thicker “small end”, is there any way to tell the difference when checking for sale ads?

  • Roger Cicala

    It seems to be better, although we still so AF Motor and Stabilizer failures with more frequency than other Supertelephoto lenses.

  • Roger, now that we’re half way through July and the Sport lens has been around for a while, is there any evidence that the Sport version is any more reliable than the prior version.

  • Kerry Pierce

    Okay, thanks, Roger, I appreciate your help. I’d like to have OS and be able to use the dock, but I don’t see how that is worth the money it would cost me to upgrade. So, I’ll just muddle along with the old guy. 🙂


  • Roger Cicala

    Kerry, optically I don’t think there’s any difference whatsoever. Although some people seem to disagree with me on that.


  • Kerry Pierce

    Great stuff, Roger. I have an older, non-OS version of the 120-300 that I’ve used for about 8 years. It still works okay, but I’m wondering if the Sport is sharper and, of course, there are times when the OS would be advantageous. Do you have an opinion on the comparison of my version to the new Sport?

    thanks, Kerry

  • Roger Cicala

    Jeremy, they still have trouble with the AF and IS units, but overall it is better than the first version.

  • Jeremy

    Now that this lens has been out for a while what is the verdict on reliability? Is it improved from the first version?

  • bossa

    Thanks for answering my question Roger. The consensus appears to be that the new version is the same optically as the older unit but on doing a bit of research it turns out that the new lens has an extra FDL element (same number of elements it’s just that one of the front ones is now made of FLD glass). According to The Digital In Picture review this lens is much better than the older one and I’m looking forward to trying it ASAP.

  • Roger Cicala


    The lens is thicker in diameter, but the metal used for the barrel is not thicker. The weather sealing is much more robust, though.


  • Did you noticed if the barrels are thicker than the previous OS version ?
    Does it fell more robust ?

    Thank you.

  • Roger Cicala

    Keely, there’s been no change in the back glass, so i don’t believe the Nikon converters will fit.

  • Keeley

    Roger, I rented one of the older Sigma 120-300mm lenses and a Nikon teleconverter. The lens was not compatible with that teleconverter. Is the new lens compatible with the Nikon brand teleconverters, or is the Sigma brand one the only choice for Nikon users?

  • Roger Cicala

    Bossa, we had a lot of electronic issues — OS and AF motor failures almost entirely. Mechanically it held up quite well.


  • bossa

    Hi Roger and thanks for all your great work.

    I am very interested to know what kinds of failures the ‘old’ version of this lens suffered. If the guts of them are identical but for a few electronic, mount and seal features it might be possible to predict if the new version would fail in a similar manner.

  • Bad news, then. Thanks for checking it.

    BTW: Leica-R lenses (even the small ones) have a 6-screw mount.

  • Fake Name

    I would guess that the extra pins on that connector are for another lens.

    That way, they can share the same controller PCB across multiple lenses, with the differences only being in the firmware.

    It would be interesting to compare a cross-section of sigma lenses, looking to see what design aspects or electronics they share.


    I would think the new complete circular PCB would be more rigid; perhaps making the assembly more reliable. Also provides another path around for those electrons.

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