Warning: Wall of Text/Sample Pictures
I’m not extremely wealthy. I do, however, have a friend who owns a camera store and regular access to basically the full range of L lenses out there. A lot of them are very nice (some not so), but there’s something to be said for manual lenses. There’s (along with the obvious frustration of missing a shot due to your inadequacy) a bit more thought/planning/involvement required when you’re the one focusing and stopping down, and a lot of them have interesting optical characters. I love my 50/1.4 Takumar. However, I was having issues. First off, I think in a year there were two times I felt 50 was the exact focal length I needed. I don’t have this issue with my other primes, but for me 50 was awkward. That and my Takumar broke. Seeing as the manual 50s have gotten crazy expensive lately, I wasn’t feeling like dropping that kind of cash on a lens I felt awkward using, no matter how good the results. Enter FM, MFlenses, and PTON. There’s threads at all three about this supposedly magical Contax 35-70. Much deliberation, 550$, and a few days of anxious waiting later I have my own copy of this lens with mint glass (+ EOS adapter+AF chip+leather soft case). Canon may not be the sensor-tech champ right now, but the adaptability of alt-glass is super attractive.
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Strobist: Single YN-468, shoot-thru umbrella off of wireless. Tamron 70-300@161mm, 1/25 handheld, F5, ISO 1000. Could be sharper. Eh.
Disclaimer: This is not intended to be a series of factual statements. There’s a pretty good chance I’m in no way qualified to write anything about optics or cameras or the usage thereof. Most of these sample shots suffer badly from my incompetence at sharpening/resizing.
Impressions/handling
This is a small lens. It’s about 5 inches long when focused to infinity, and weighs about 500 grams. Compare to the Canon 24-70/2.8 at 950 grams and ~8+ inches. This is a beautiful walk around and travel lens. Handling is a bit unusual – there’s the well known one touch zoom/focus setup, and the zoom is reversed, that is, you extend to zoom out. The lens feels well constructed under tight tolerances. Aperture clicks smoothly and crisply between 3.4 and 8 at 50mm+, but in order to get down to F22 you have to zoom out below 50mm. Strange, but not an issue in realworld usage seeing as smaller apertures are most often used for landscapes which are also often wide. You can use F22@70mm if you zoom out, close the aperture, then zoom back to 70mm.
The one touch is well-executed – just tight enough to focus or zoom without changing the other, but very fluid when doing so. It’s not quite takumar-smooth but 90% there on a zoom. I hate the fact that it’s a dust pump, as it sucks a good 3 cubic inches of air into the mirror box zooming from 35 out to 70.
The lens is parfocal (or at least so close I can’t tell at 100% pixel peeping), extends while focusing (but remains about the same size when zooming), and rotates while focusing. There’s also a nifty macro mode that does ~1:2.8, which is also a nice touch for a walk around. When in macro mode (usable at 35 only), there’s a loss of light (inherent to all macro lenses) when focusing close, and as a 35 it gives you a bit of perspective distortion.
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Notice the textured look the close working distance gives. Also note absolutely no CA on any of the diamond facets in direct sunlight. Accidentally done at ISO 3200 wide open.
MFD is ~.7 meters, and at 70mm maximum magnification looks to be ~1:9, and at 35 (non-macro) it’s about 1:18.5.
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Example of maximum magnification at 35mm. Notice also strangely texturized bokeh wide open at close focusing distances.
I’ve not found an acceptable hood solution yet, so I’m using a protective filter right now. Most shots here are taken without the filter on. I’m not really able to tell a difference between the two using it in terms of sharpness/color, but the filter does flare. I had no mirror or infinity focus issues, and may or may not be using an old AF-confirm adapter. I’ve found it’s not any more accurate than I am, but it’s good when I don’t have contacts in/in the dark. As an F3.4 it is admittedly not the easiest to focus in very dimly lit rooms but is somewhat generous with the DoF. It hits infinity (hyperfocal) after ~20 feet, which is somewhat unusual given the labeling at 50 feet. I may have an adapter issue.
Optics
Wow. I read a lot about how great this lens is, but I’m blown away by it. I’m seeing no CA anywhere on a full-frame at any zoom and focus setting, and it’s sharp enough to create moiré on my 16.1 megapixel 1DsII sensor. Sharper than most L primes I’ve used, with very high global and local contrast. In fact, I find the global contrast a bit much. Colors are rich and Zeissish, with heavy blues and greens. I don’t think Canon has an equivalent lens right now. The 24-70 is a ton softer in the corners, worse overall, and has arguably worse colors/bokeh. I’m astounded by this. I’ll take this opportunity to remind you I’m not an optical engineer, simply a photographer who like to pixel-peep a tad much.
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Example of colors/bokeh. I relied on the AF-adapter which seems to be misfocusing a fair bit. Nose was very sharp though :P Only PP is skin smoothing (poorly done at models request) and web output. Bitingly sharp in raw, almost to the point of making it a bad choice for portraits.
This lens appears to outresolves my sensor from F4 corner to corner below 50mm, with slight improvements to contrast at F5.6. At 70mm you get great (but not sensor-beating) results at F4, switching to excellent at F5.6. I don’t see diffraction at F8, but it stagnates from an IQ perspective from there (admittedly it holds up well to smaller apertures, but I avoid F16/F22).
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Comparison at F8 and F3.4 at 50mm. I doubt you can tell on the websize, but there's almost no benefit in sharpness from stopping down. Poorly composted in paint. Please someone teach me photoshop.
The corners are especially impressive and free of curvature. I have no qualms about shooting wide open. There is some purple fringing on occasional specular highlights, but it’s at or below that of most of the Ls I’ve used. You can make it flare with awkward angles into the sun but it’s difficult, and I’ve never managed to do it in actual shooting.
The bokeh is overall quite good. It doesn’t have a lot of that millimeter-DoF/one eyelash in focus thing that everyone’s going for nowadays, having a conservative 3.4 aperture combined with a maximum 70mm focal length makes it a bit limited in that respect. It does, however, seem to have less DoF than I would expect from a 3.4 lens, closer to 2.8 in my opinion. I think this is due to the very fast drop off/texturization that happens wide open.
The bokeh it produces is good but dependent on focus distance. Nearer infinity (say, focused on a portrait with a background a few feet+ off) it’s very creamy and holds up well to tough scenes like shiny metal or foliage (especially stopped down a half stop to F4).
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Wide open bokeh at more distant distances in this torture test is just OK to distracting. Stopping down or getting closer makes it much smoother. This example is neither textury or smooth. How not to make a bokeh example pic. I also apologize for high contrast lighting, and missed AF-confirm focus, just testing the lens.
Up close, what I see in my viewfinder is different from what I see in the raw – the lens appears to very quickly make nearish background objects into textured circles (relatively steep bokeh falloff, but it keeps the general shape of the background). Definitely falls into the unique bokeh category wide open, smoothing out when stopped down just a bit to F4 and beyond or focused to further distances.
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Example of textured bokeh.
I’ve not done brick wall tests. There’s a small (but noticeable on a computer) amount of vignetting wide open. Distortion doesn’t seem noticeable, but I don’t do a ton of architecture. Performance seems fine in macro-mode, but I like to shoot at F/8 there a bit more for DoF reasons.
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I love the colors/contrast this lens puts out. For those of you watching at home I used the AF-adapter+my 1DsII's focus trap and it appears to be off a bit. This is one of those shots someone is going to have to teach me how to resize/sharpen, it looks terrible compared to the raw.
Conclusion:
It’s a very small, one-touch zoom with incredible IQ. Prices are unfortunately on the uptick, but I still think this is a value at 1K. You have to be able to deal with manual focus and aperture, but to me that’s no major complaint. It fits perfectly in the general purpose range, and is fast enough for most applications. The macro mode is great, as is the handling. If you’ve been thinking about it, it’s hard to go wrong. I can see a modern AF/IS version of this lens going for thousands (Get on that Canikon). Perfectly beautiful and if you decide it’s not for you prices are only going up. I’m still looking for a hood and better AF-confirm adapter if anyone reading this has suggestions. I can’t see myself parting with this lens anytime soon.
Sharpness: Outstanding
Handling: Good, some awkward points. Well-built.
Bokeh: Requires some work, but in general good. Very good when only compared to other mid-range zooms (ala 17-55IS/24-70L).
Color/contrast/Zeiss look: Very high, I can tell which shots were taken with this lens. Doesn't color match with my Canon/Tamron/other manual lenses, which can be an issue.
Price/value: Varies, but generally very good if you can deal with the manual aspects.
I don't know if this really is a 24-70 killer. They're different lenses. This is a better IQ/walkaround option, but lacks AF, a 2.8 aperture, and a red ring. I know which one I'd reach for.
Congrats on finding a gem of lens. 
