IS and IQ..

Why are some lenses with an image stabilizer sharper than the non-IS version, and others are just the opposite? I can't believe it's just the implementation of said stabilizer. Take the different versions of 70-200mm in both the f4 and f2.8. Same focal range, but opposite results. Is it a price issue? When making a product, you set a time/price limit. Maybe they reached the max value before finishing the fine tuning. Could it have been a bad copy or two that was used it the test for the site? He's pretty meticulous. If I was thinking ahead, I would have checked to make sure the IS system on the two versions of the 70-200 were the same, and also took the f2.8 model to f4 and seeing if it followed the same pattern. Just curious.

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Originally Posted by andnowimbroke

Why are some lenses with an image stabilizer sharper than the non-IS version, and others are just the opposite? I can't believe it's just the implementation of said stabilizer. Take the different versions of 70-200mm in both the f4 and f2.8. Same focal range, but opposite results. Is it a price issue? When making a product, you set a time/price limit. Maybe they reached the max value before finishing the fine tuning. Could it have been a bad copy or two that was used it the test for the site? He's pretty meticulous. If I was thinking ahead, I would have checked to make sure the IS system on the two versions of the 70-200 were the same, and also took the f2.8 model to f4 and seeing if it followed the same pattern. Just curious.

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I am sure if you wanted the answer to this question you would have to talk Canon. Most likely you would need to talk to the designers as I would bet you can’t find a sales rep that could answer it.

But…your question is confusing. Since there are five versions of this lens available. The 70-200mm 2.8 IS I & II, the 2.8 non IS, the F4 IS and non IS. Which are you comparing and which were the opposite results?

Could it be that the 2.8 non IS is the same as the version I and not the new version?

Sorry. I was comparing the 70-200 f4 versions and the old 70-200 2.8 versions. Both seem opposite in sharpness (affecting IQ) when adding the stabilization.

F4 with IS is better than the F4 non-IS
F2.8 non-IS is better than F2.8 IS

Lens design and often optical coatings differ between non-IS and IS lenses, partly as a result of when they were designed. Using the 70-200mm set as examples, the f/2.8 IS is probably the least sharp, while the MkII is the sharpest thanks to newer design and coatings.

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Originally Posted by andnowimbroke

Sorry. I was comparing the 70-200 f4 versions and the old 70-200 2.8 versions. Both seem opposite in sharpness (affecting IQ) when adding the stabilization.

F4 with IS is better than the F4 non-IS
F2.8 non-IS is better than F2.8 IS

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That is not what I am seeing ont he F2.8

http://www.the-digital-picture.com/R...mp=0&APIComp=0

I guess it depends on where you are looking...center or corners

Don't forget release-dates. If it's one of the first lenses released with IS, then it's likely to have not been as good an implementation as it is now.
The 70-200 f/2.8 IS (mk1) was 2001, after the f/4 (1999) and f/2.8 (1995), but that's when IS was just getting started, so maybe it was either rushed-to-market, or they just didn't know as much as they did now.
The f/4 IS was 2006, so it's a newer design, and by that time they'd probably ironed out all the creases in the IS.
The f/2.8 IS II is the most recent (2010), so it's had 10 more years of R&D compared to the f/2.8 IS mk1, and 15 years more than the f/2.8 original. The f/2.8 IS mk2 also has 1 Fluorite and 5x UD elements compared to the f/2.8 IS mk1 with only 4x UD and no Fluorite. It also looks like a complete redesign of the lens, the IS mk1 was probably just the non-IS with a few more elements jammed in for the IS, more glass is going to make worse IQ unless you change the rest of the elements to match...

I'm not sure if you can really quantify how it affects quality. Apples and oranges, really. The sharpness tests are taken on a tripod, which means that IS is off. From there you are looking at the quality of the glass it's self with IS taken out of the equation.

I say, IS greatly improves IQ. Because.. well.. it stabilizes what would otherwise be a blurred image. :P

I have no idea how the IS- thingy works (whether it ads more glass or just moves existing stuff around), but it had me befuddled as to why it didn't consistently affect sharpness throughout the 70-200 lineup. What the Dr. brought up made sense as far as the year it was brought up and the f4, while being an older lens, didn't get IS until after everyone else. Good call.

There are many factors that come into play: date, starting design, design priorities, cost target, available materials, and chief of all: the lens designer. Imagine Canon wants to come out with a 24-70 f/2.8 IS, and they give the exact same task to two different lens designers. One may be more talented and experienced, and his turns out sharper than the original 24-70. The other is an apprentice, and his turns out with more aberrations than the original 24-70.

Let's try varying design priorities for a different example. Same lens designer, but they tell him to optimize bokeh. He turns out a 24-70 IS with creamy, smooth bokeh, but it's softer than the original 24-70. Then they come back and tell him to optimize sharpness instead. So then he turns out a 24-70 IS with excellent sharpness, but now the background bokeh fell out of the ugly tree and hit every branch on the way down.

There are a lot of ways that design priorities can affect each other. For example, faster lenses tend to have thicker elements with the same spacing, which means less room in between elements. Adding I.S. generally means adding a new group of elements to the design. Adding it to an f/4 lens that already has a lot of room to play with might be a lot easier than adding it to an f/2.8 lens where there is no room. If the only way to make room is by removing one of the aberration-correcting lenses, then you might end up with I.S. but at the cost of more chromatic, spherical, or field-focus aberration (for example).

Hope that helps.

I work in a product design role and can say that at my company we obviously have budgets, but we never release marginal products for production just because the budget has been exhausted. I wouldn't think that Canon would either. From what I've read, all of the 70-200 versions are tremendous optically.

Quote:

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Originally Posted by andnowimbroke

Why are some lenses with an image stabilizer sharper than the non-IS version, and others are just the opposite? I can't believe it's just the implementation of said stabilizer. Take the different versions of 70-200mm in both the f4 and f2.8. Same focal range, but opposite results. Is it a price issue? When making a product, you set a time/price limit. Maybe they reached the max value before finishing the fine tuning. Could it have been a bad copy or two that was used it the test for the site? He's pretty meticulous. If I was thinking ahead, I would have checked to make sure the IS system on the two versions of the 70-200 were the same, and also took the f2.8 model to f4 and seeing if it followed the same pattern. Just curious.

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Yours is the good company. Others choose the "beg forgiveness rather than ask permission" approach. I would hope Canon is more like yours. Sometimes I question though...

@daniel
Good points about room available.

I'm sure canon wouldn't mind me stealing images from their Camera Museum for this discussion, so here's some block diagrams:

http://www.canon.com/camera-museum/c...28l_usm_bd.gif
70-200 f/2.8L non-IS

http://www.canon.com/camera-museum/c...lis_usm_bd.gif
70-200 f/2.8L IS mk1

http://www.canon.com/camera-museum/c...lis_ii_usm.gif
70-200 f/2.8L IS II

http://www.canon.com/camera-museum/c..._4l_usm_bd.gif
70-200 f/4L non-IS

http://www.canon.com/camera-museum/c..._is_usm_bd.gif
70-200 f/4L IS


Now, I'm not a lens designer, but still, here's my thoughts:
- Going from 2.8 to 2.8 IS, they've just added in the 'triplet' that moves during stabilising, and changed an element or two on either side to direct the light rays through it properly. Also note that they've "bent" the rays more towards the centre of the lens, because all the light rays have to go through the small IS-group, then they have to bend them back out (that's how a telephoto works).

- In the non-IS version, where there's a big gap where there's no IS group, the light rays can go through unimpeded, and with less bending.

- Going from the IS mk1 to IS mk2, they've put the Fluorite element right before the IS group. The light rays still get bent a lot to go through the small IS-group elements (red square), but they're bent through the fluorite. Fluorite elements bend light a lot more, and a lot more cleanly than normal glass (but it costs a packet). They've also brought in 2 more UD elements (between Fluorite and normal glass in bending quality) at the front, where the light gets bent the most (the 'telephoto' design). That's probably accounting for a lot of the extra quality in the IS mk2 from the IS mk1.

- In the f/4 lenses, they're pretty much the same design, and they're also not that dissimilar in IQ, they've moved a UD lens around to bend the light better to go through the IS unit, but besides that they're practically the same lens.

- In general though, more elements in a lens makes for a 'darker' lens (and can introduce some problems), but the more elements the more it is corrected for aberrations, distortion, CA, field curvature, whatever (look at what you get out of a lensbaby, that's what a single or double-element lens looks like with no aberration correction...). So the IS unit in some lenses is helping out the IQ, in some cases it's actually making it worse. In general though, the newer and more expensive lenses (especially the case with IS) are going to outperform the older ones...