Let's have a look at the Carl Zeiss Planar 50mm f/1.7 T*, the budget standard lens for the old Contax RTS system of the 1970s. It has always had a reputation as a snapper performer, sharper than the 50mm f/1.4 but of course not as fast. T* is Carl Zeiss' code for the special multi-coating they use, which is purple:
It's a manual focus, manual aperture lens. I stuck mine on a Canon 5D MkII with a generic Contax-EOS adapter, but it was an awkward fit, because the lens' aperture prong fouls the 5D's mirror. I bit the bullet and hacksawed it off. It will never father children. In common with the Contax 35-70mm it has an unmarked click stop slightly down from wide open, which I surmise is Zeiss' way of telling me that f/1.7 is for emergencies and that I should really think of it as an f/2.
Carl Zeiss Planar 1.7/50 T*
The basic arrangement of the lens elements dates back to 1896, although the 50mm f/1.7 feels surprisingly modern. I'm not sure if the body is plastic or thin metal, but it feels lightweight and robust (it reminds me a bit of the contemporary Nikon 50mm f/2).
Zeiss' history is long and complicated - by the 1970s the company had given up making cameras, but needed something to put its lenses on, and so Zeiss let Yashica of Japan use the old Contax name as a halo brand for its SLRs. The Yashica FR and FX SLRs shared a lens mount with the Contax RTS and 139, and there was nothing to stop you from putting a Contax lens on a Yashica body (in practice it seems that Contax fans used Yashica FXes as backups).
Yashica was eventually bought up by Kyocera in the late 1990s, and both Yashica and Contax were killed off shortly afterwards. The last batch of Contax cameras used a brand-new autofocus lens mount (the N Mount) which is incompatible with Contax/Yashica lenses.
Zeiss itself continues to make a range of lenses for most popular modern camera mounts; they are amongst the best lenses available for any system and are priced to match. The 50mm f/1.7 was, in its day, the entry-level kit lens that came free with a Contax RTS body. Mine is serial number 6503975. How old is it? I have no idea. I'm tempted to say 1975, but I have no idea. Some time between the mid 1970s and the mid 1990s.
Zeiss still has the MTF data sheet for the 50mm f/1.7 online, here. Looking at the MTF data, it seems that the lens is sharp in the very centre of the frame, not quite so sharp in a band around the centre, sharper again in a band towards the edge of the frame, and then not quite so sharp at the extreme edge. And indeed this is the case in real life, as we shall see.
For this post I took the lens to the local car park, and shot it alongside my Canon 50mm f/1.8. I used this lens because everybody has one and everybody knows what it's like. The Zeiss was mounted on a Canon 5D MkII with a generic eBay adapter. All the following were treated to the same unsharp mask settings of 150, 0.5, 0, which is very mild.
Here is the scene, and yes I realise that it's tilted. I should really have put the tops of the buildings across the middle of the frame, but in my defense I was very, very drunk:
The centre of the frame at f/1.8 - f/1.7, Canon on the left and Zeiss on the right:
The Canon lens looks a tiny bit sharper, and the Zeiss lenses has noticeable purpleness. Now the satellite dish half-way across the frame at f/1.8 - f/1.7, Canon at the top and Zeiss at the bottom:
As before the Zeiss lens has a purple glow, and the Canon lens looks a bit sharper. Canon's MTF chart for the 50mm f/1.8 suggests that the lens degrades gracefully from the centre outwards, speeding up towards the edges, rather than degrading and then getting better and then degrading again as per the Zeiss MTF chart.
Now the bottom-right corner at f/1.8 - f/1.7, Canon at the top and Zeiss at the bottom:
In contrast the Zeiss lens is noticeably better in the extreme corner. At this aperture neither lens shines, but the Canon lens shines less.
Let's move on to my second-favourite aperture, f/2.8. Here's the centre, again Canon on the left and Zeiss on the right:
As far as I'm concerned both lenses are as sharp as can be at f/2.8 in the centre, and if you just look at the brickwork you can't tell where one lens ends and the other begins. I won't include any more centre crops.
At f/2.8 the Canon lens still appears a little bit sharper in the mid-frame (Canon at the top - Ethel Merman at the bottom):
But again in the corner the Zeiss lens is better:
Let's look at f/5.6, Canon at the top and Zeiss at the bottom:
The Canon lens is again slightly sharper to my eyes. The corner:
Again, the Contax lens is sharper in the corner. At f/5.6 it is essentially sharp across the entire frame, whereas the Canon lens is soft in the last few hundred pixels.
And now f/8. The satellite dish doesn't get any sharper so I'll only show you the corner, which looks like this, Canon at the top etc:
At this point the Canon lens almost catches up although again it's not quite as sharp. Here's the top-right corner also at f/8, Canon at the top as before:
With this mass of straight lines the Contax lens stands out as noticeably sharper. Judging by the depth of field markings on the lens, at f/8 everything from about four metres to infinity is in focus. The Canon lens doesn't have depth of field markings.
What can I conclude? Neither lens has any obvious optical defects; CA is minimal and there's a little bit of barrel distortion but it's nothing of great significance. Both lenses are as sharp as can be in the middle from f/2.8 onwards; the Contax lens is generally sharper in the extreme corners, and at f/8 is sharp as can be across the entire frame. The Canon lens is for most practical purposes no worse, although pixel-peeping reveals that it's never as sharp in the corners. Oddly the Zeiss lens has a zone of relative softness around the middle of the frame, like a ring doughnut, but soft rather than sugary.
On a practical level the Canon lens wins, because it has autofocus and auto-aperture. During these tests I shot two sequences with the Canon lens; one with Live View autofocus and another with the manual focus ring. In practice the Live View autofocus was absolutely perfect, the manual focus wasn't quite as good, because the 50mm f/1.8's manual focus ring is terrible and jerky. If you're going to use it on a tripod and you need absolute sharpness at 50mm, or you're prepared to use magnified live view to focus, the Zeiss lens is the better of the two, and not much more expensive either.