Here's my 17-40 f4L compared with my new Olympus 21mm f3.5 that I intend to use for landscapes. Thought you would like to see it back to back.

Rather than bog you down with a huge write up and lots of work for me cropping and verbally comparing at all manner of fstops, have a read of these notes and let the images do the talking by downloading my screendumps. They are 100% crops and if you scroll the images to the right you can see the section of the image in the Photoshop navigation box that the crops relate to. I dont claim to be some expert and I dont know all the jargon, but I hope my notes are agreeable and the test may make you reconsider the quality of the glass you use in comparison to lenses of the past.

Olympus 21mm
Its old and second hand and well looked after fortunately. An Ebay purchase without a mark on it, cost £150 which is cheap in the world of modern wide angle primes. I bought a Fotodiox adapter ring for £20 and it fits really well. There is no protruding metal work to hit the mirror or any horrors like that. Its tiny, looks really bizarre on camera and is so light. Its totally manual of course, no AF, AE but you can get an 'AF confirm' adapter ring that lets the lights flash inside your camera to confirm your manual focus is precise, very handy.

Canon 17-40 f4L
I have owned my copy in particular for about two years and I have been really happy with the versatility and the quality of the shots it is capable of producing, especially at f1-f16 where the shots seem to really sing. Centre sharpenss is always excellent, edges are not so good but at f11 to f16 the reuslts are very acceptable, just what I require from a landscape lens. Many people hate this lens for its soft edges and terrible control of distortion, but other photographers who have looked at it seem to agree I have an exceptional copy I should never get rid of it, which I wont of course. Its a great travel lens too and is on my camera for 8-% of my images.

Why then bother with a 21mm prime? Distagon envy? Well perhaps. I am always keen to get the maximum out of my camera's sensor, and after reading posts on the Fred Miranda Alternative Lense Forum, Manual Focus Forum and other sites, lenses such as Zeiss and Olympus bring together a community of digital users that look for alternatives to Canon L and other very expensive AF equipment. Many results are very surprising, with old designs humiliating the modern equivilants. But each comes with its compromise, each its trade off. But it your like me, taking tripod mounted landscapes primarily, a set of light, very sharp and low cost alternative primes or zooms are an attractive idea, especially if they embarass the hell out of what you already own.

Test conditions

Camera - 5D
Tripod mounted on concrete, mirror lockup, 2sec timer, RAW mode.

Post processing
Abslutely nothing other than a unsharp mask of 100, 0.4, 3 that 16-9 seem to do on all their tests (well one I looked at!) as an initial capture sharpen.

Observations and Results

My garden, you can tell I dont like gardening, note the 48 year old all terrain photo vehicle on the drive.

Click on each catagory for a downloadable comparison, some notes I have written to campare the crops will help.

Sharpness – f5.6
Centre Canon by far. For some reason the centre of the Oly is worse in the centre than the edges.
Left Mid Olympus is a clear winner not contest, but only the extreme Oly edges are failing
Left Top neither look good in the top left corner but the Oly has terrible smearing. But look in the bottom right, Oly is way sharper.
Right Mid I would go with the Oly
Right Bottom Olympus the winner again, but the extreme edges suffer on both

Sharpness – f8
Centre Canon has it by a nats whisker
Left Mid Olympus is a clear winner, 17-40 is almost embarrassing!
Left Top Olympus wins again but bad smearing in the upper edge
Right Mid Olympus wins with better definition in the fence detail, but way less detail to resolve than the left mid comparison
Right Bottom Olympus the winner again, but not by much

Sharpness – f11
Centre I cant tell the difference
Left Mid Canon has sharpened up in comparison to f8, but Oly is easily the winner
Left Top Again Oly is just excellent except the far edges which smear (but this represents such a small area of the image)
Right Mid I can barely see the difference but if I had to give it to anything it’s the Canon
Right Bottom Olympus the winner again, but not by much

Sharpness – f16
Centre Olympus is sharper (look at the gate in the hedge)
Left Mid Oly again wins, but the Canon is very good at f16
Left Top Again the Oly, but the 17-40 is resolving detail very favourably on the Canon. The smearing is still there in the Olys extreme edges.
Right Mid Again the Olympus, but only just
Right Bottom Olympus is barely any better

My conclusions from this my very first lens comparison.

I am going to enjoy using the Olympus 21mm as a landscape / coastal lens as I am sure the test shows that at f11 and f16 where I spend most of my time hanging out, the sharpness is far better. The very extreme corners of the lens fail worse than the Canon with smeaing and CA, but this seems to represent such a minimal area that I can't see myself trading back for the 17-40 when shooting at 21mm, simply because the sharpeness is so much better overall. Its a much higher percentile. My copy of the 17-40 is very nice indeed and at f16 and f11 its at its peak, but it has highlighted the following conclusions at the following fstops -

f5.6 - Olympus is not sharp in the centre. Canon is sharp. You would expect both to be sharp so I wont use the Oly at this aperture.
f8 - Olympus centre detail is now sharp. Olympus edge detail at f8 is so much better than 17-40 which is rather bad.
f11 - Centre detail is almost indistinguishable. Edge detail is still far better on the Olympus.
f16 - Canon starts to lose centre sharpness but corners are at their sharpest. Olympus is still sharper overall.

Its a prime the Canon is a zoom, no doubt winner could be the prime, but if like me you find your landscapes hovering around 21mm most of the time you may want to consider this as an alternative as its -

Sharper overall
More neutral
So incredibly light, great for hiking.
1/4 the price of a 17-40 (well thats not very comparible as its 21mm only)
10th the price of a Zeiss 21mm Distagon
Looks weird on a 5D and especially on a 1DsIII

The lens is fully manual, the Canon has AF and AE. This poses no problem to me whatsoever but it may to you. My 17-40 is focused hyperfocally 100% of the time for landscapes. If you hate stopping down, have a try. Its only arranging the shot at f.3.5 and then turning the dial to f11, meter and then take the shot.

Another thing to consider is filters, they are 52mm so its going to be cheap on polarisers etc. If you have a step ring you can use all your larger filters without an issue.

Colour overall –
Oly is more neutral looking
17-40 is warmer

Contrast overall –
The 17-40 seems like it has more contrast. This is of couse totally unimportant after images go through RAW processing as far as I am concerned.

Distortion -
Its well controlled on my copy of the 17-40 (I have seen some terrible shots with this lens) but the 21mm is much better with only the very edges suffering as expected. (no test results for this, just viewfinder observations so excuse my basic approach)

CArberration -
Better on the 17-40 overall, bad colour smearing in the extreme corners of the Oly which I guess is down to coatings or something? Those proficient about lens technology will be far more up on this than me. This is not present on the 17-40 at all, but as this is the absolute corner I am not too worried about this (yet!)

Thanks