Zoom Or Prime?

Upper left corner, 17-40 L Canon @ f16

Do you struggle to decide whether to go the convenience of zooms or the sharpness of single focal lengths. According to some articles, the war has been won by the zooms and resolution stopped down is now so close to good primes that the only reason for primes is if you shoot low light or sports.

For some time I have been unhappy with the corners of my images - reviews describe the corners as sharp, or slightly soft, I'd describe them as downright terrible. Oddly most disappointing has been my 70-200 Lf4 - though I have to confess I dropped it.

One defect of the current rage of backpack camera bags is that when you pick them up, the stuff automatically falls out. Unfortunately I happened to be standing on the concrete abuttment of a railway bridge over water at the time. Fortunately it rolled out of the bag as I picked it up, hitting the concrete but not actually smashing - in fact apparently no damage at all - and most importantly as it rolled to the edge of the concrete, I stopped it first. At that time I was shooting 10D and since then have graduated to full frame and notice that corners just aren't all that sharp - is this a feature of the lens (not shown by the frame cropping of the 10D, or did the lens not quite survive the fall. Oddly it's still plenty sharp at the centre and all four corners seem equally fuzzy so things still seem aligned.

Of concern is that what if the lens isn't damaged and this is just what it provides and everyone else is oohing and ahing about centre resolution or even edges but not corners - not sure I buy this arguement - some review specifically test the corners (eg SLRGear ) and that isn't what they are seeing.

Had a good look at the canon published mtf curves at Canon and the MTF curve for the 17-40 shows exactly what I was seeing - great centre resolution - terrible corner resolution - suggesting that the MTF curves could in fact be used for useful comparison.

In case you don't do a lot of MTF peeking, on Canon's site, the heavy solid blue line represents contrast at low frequency stopped down to f8 - and probably is your best guide to overall resolution. The heavy black line represents contrast wide open. The other lines refer to resolution (measured as MTF contrast in other directions of measurement - frankly I can't remember which is sagital and which radial and frankly it doesn't really matter. The ideal lens would be a perfect straight line across at 1 for all the lines. If you look at the MTF for the 135 you see just how close you can get to that. MTF curves don't measure at f16 so say absolutely nothing about problems of diffraction. My own experience is that resolution at f16 is a touch down from f8 and can be mostly hidden by a touch extra sharpening. The same cannot be said for smaller f stops on my full frame camera.

Testing the 17-40 has shown me that corners never get sharp and though I had indicated I could live with fuzzy corners because of cropping or stitching - some recent work where I needed the full 17 mm. wide meant I couldn't throw away the corners.

Anyway, I now have a 17-40 with fuzzy corners (and great central resolution), a probably defective 70-200.

I am in the process of figuring out what to do to replace the lenses. I tested a 135 f2 - very sharp but it's main characteristic is that it is sharp right from f2 - which as a landscape photographer I don't have any use for. I also tested the 24-70 - significantly fuzzy corners at 24 mm. but decent at 35 mm. and just plain excellent at 50 and 70 mm.

I'm wondering about the 70-200 f4L IS when it comes out in November - again I really don't need f.8 shooting landscapes- but a couple of times could have used the IS -= Vancouver Harbour and ballooning. I'm off on a major excursion on Friday though so have to come up with a solution this week - sigh.

I carry a beautifully sharp 300 f4L lens but seldom use it and only once have used it with a 1.4 extender - hardly worth the trouble of carrying it. The 70-300 is unaceptably fuzzy in the corners from examples I have seen so that isn't an option. Everyone raves about the 70-200 f2.8 L IS - but perhaps everyone isn't as fussy as me. Still, worth a test and I'm going to do so tomorrow.

Just to complicate matters it's been suggested that I test three different lenses to see which is sharpest - that's a bit scary, for a $2000 lens.

By the way - the f8 corner was even fuzzier - even though depth of field should not have been an issue at 17 mm.