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I'm trying to figure out the difference between the IS MKII lens and the IS STM lens.

Specifically, I am looking at the Canon EF-S 18-55mm f/3.5-5.6 lens. On that Amazon page, there are three options; of the EF-S, one is listed as IS MK II; the other is IS STM.

I'm still not sure which is best to purchase - given the comparable price - I have heard pros and cons of each.

I understand that STM is a newer lens technology w/ a stepper motor for focusing, if I understand correctly, on manual focus, rather than adjusting the focus directly, you adjust the focus ring, and the stepper motor brings the lens to that focus point.

I have read that STM can be slower to focus than USM - however, I don't think the MK II has USM. The STM is supposed to be quieter, and thus better for AF while filming video.

Since the STM lens is newer (I think), is there any reason to buy the MK II? (The MK II is actually more expensive on the page listed.) Are there benefits to it, or pitfalls to the STM that I should consider?

Side note; my camera is an older Rebel XS; I'm not sure if the STM will work fully with it.

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According to the Wikipedia article comparing all Canon EF-S 18-55mm version the STM version has (in addition to the new STM motor) a new optical design (13 elements in 11 groups vs. 11 elements in 9 groups), internal focusing and an extra diaphragm blade (7 vs. 6)

So, the EF-S 18-55mm IS STM is not just the EF-S 18-55mm IS II with the new motor but a completely new lens that seams to be slightly better than the old design in every way.

Even if you don't need the advantage of the STM motor for video I see no reason to buy the older version (unless it's cheaper), I also don't see a compelling reason to upgrade for someone that already has the IS II but the new STM version is better.

BTW, you can usually buy seconds hand kit lenses for next to nothing because a lot of people want to get rid of them when they upgrade, a quick look on eBay shows that the EF-S 18-55mm IS II has been sold for as little as $40

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    \$\begingroup\$ The other significant advantage of the STM version is that it had a non-rotating front element, which makes using polarising or graduated filters much easier. \$\endgroup\$
    – Philip Kendall ♦
    Commented Dec 24, 2014 at 14:53
  • \$\begingroup\$ thanks! good point about new/used though; i should look into used for a kit lens\ \$\endgroup\$
    – Nik
    Commented Dec 28, 2014 at 16:47
  • \$\begingroup\$ Another (possible) disadvantage of the STM lens is that EOS bodies introduced prior to about 2012 aren't fully STM compatible. \$\endgroup\$
    – Michael C
    Commented Jun 19, 2015 at 1:28
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The STM version is better because it has faster, smoother AF, and the front element does not rotate. The optics are also improved which gives you better image quality.

The STM focus ring is active during Auto Focus and you can small manual adjustments to focus. This is called Full Time Manual Focus.

The STM version also has a much nicer EW-63C lens hood.

enter image description here

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  • \$\begingroup\$ Honestly, with a lens that zooms 3x form 18-55mm, the shape of the hood is fairly irrelevant to all but the very widest focal length. \$\endgroup\$
    – Michael C
    Commented Oct 1, 2016 at 11:15
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In general STM lenses have quieter and smoother focus than their non-STM counterparts, especially compared to USM lenses. If you are shooting video with 2012 or newer EOS body (from the 650D onward) then this is a distinct advantage. If you are shooting stills then the typically faster focus speed of the non-STM lens is more of an advantage. In the case of the EF-S 18-55mm f/3.5-5.6 IS II and the EF-S 18-55mm f/3.5-5.6 STM though, the STM is also slightly faster to focus than the older IS II.

In terms of manual focus, the STM lenses are all "focus-by-wire". This means there is no direct link between the manual focus ring and the lens' focus mechanism. Rather, the focus ring transmits a set of electronic instructions to the camera body which in turn sends electronic instructions to the lens to move the focus elements using the same motor as when the camera is focusing the lens automatically. The smallest amount of focus movement possible is one "step" of the stepping motor in the lens. It is possible, however, to manually focus (even if in "steps" by wire) without turning off AF via the switch on the side of the lens barrel. This is known as "full time manual focus". To do this you must first half press the shutter button or press the AF-ON button to activate the camera's focusing system before you can manually focus the lens. Note: Many older Canon bodies are not compatible with the newer STM lenses in terms of being able to continuously focus while recording video. EOS bodies released prior to about 2012, including your Rebel XS, do not have this continuous focus video capability.

The older 18-55mm IS II lens can only manually focus when the AF/M switch on the side of the lens barral is set to "M" (for manual) but lacks any full time manual focusing ability when the switch is set to "AF". However, the smallest amount of focus movement is only limited by the users ability to move the the focus ring a minute distance and an infinite number of focus positions are possible. And older APS-C EOS bodies that aren't fully compatible with STM lenses can use all of the auto focus and manual focus capabilities of the EF-S 18-55mm f/3.5-5.6 IS II.

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