Forgive me if this is repeating earlier threads, but thought I should report my specific circumstances on this one.  Based on a couple of items I have already browsed I am wondering whether I have spotted the likely culprit, but want input from wiser, more experienced heads than mine. 

I have owned my EF-S 17-85mm IS zoom lens for approx two years now (having purchased it second-hand).  It has been a really worthwhile purchase for me, covering the majority of what I wish to achieve when out and about with my camera.  However, just recently I ran into problems.

A couple of weekends ago, whilst on a short break, I ran into the much discussed Error 99 when using the lens with my EOS 50D.  In this instance the message advised me to switch the camera off and then back on and also check the batteries/clean contacts and try again.  I followed the error message guidance with some degree of success.

This seemed to be an intermittent fault chiefly brought about by low light conditions.  As I also had the EF-S 17-55mm with me, I swapped the 17-85mm out and decided to investigate later at home.  No problems ensued with the 17-55mm other than restricting the ability to zoom in further.

Trying the two lenses back at home, both on the 50D and my 400D, the error seemed to have gone away (I had recharged batteries, cleaned contacts, etc, in the meantime).  However, last week, whilst on holiday, the problem returned with a vengeance.

This time it was the 400D that experienced the problem.  Again, it seemed as though the problem was linked with lighting conditions, but I suspected it might be more specifically associated with extremes of light or dark and contrast.  Strangely, at first this was restricted to the range of 17mm to 24mm and, occasionally, at the other end of the scale: 85mm.  As long as I stayed away from those zoom areas, all was fine. 

Unfortunately, this did not last.  Eventually, the lens ceased to function at all.  Previously, it would auto-focus and lock on, providing metering information right the way up the moment the shutter was fully depressed (at which time the error message would appear and no image captured).  Finally, the viewfinder showed a darkened view through the lens, almost as if the depth of field button was being pressed, and there was no response at all for light metering or focussing.

Turning to the ‘bog standard’ kit lens as supplied with the 350D carried by one of my travelling companions and everything was fine – the lens definitely appears to be the rogue element.

So, I ask: has anyone else experienced this?  Any diagnoses or advice?  Should I see if Canon or someone else can repair it?  Or, more interestingly, given that I have long lenses to take me from 70mm upwards, should I look at a different lens?  All suggestions welcome.  Thanks for any help.

Thanks for the response, KeithB.  Spooky how these things coincide!

I will drop a line to Canon shortly in the absence of any other suggestions.  It is certainly worth getting the lens serviced.  Mind you, although I find the 17-85mm a great workhorse, finding that extra 30mm a definite advantage over the 17-55mm, I am now tempted by the newer 15-85mm lens!

Well, I suppose the 17-85mm has been around a while and things have moved on a tad.  I must say I am wondering whether to get the 17-85mm serviced and then sell it to offset the price of a new 17-55mm sometime before the summer.  The problem is that I am sure the resale value on eBay, etc, of the 17-85mm will be falling due to the arrival of the new lens. Doh!

Thanks for the guidance.  I will get in touch with Canon UK and see what they can do...and start saving. 

Indeed, I think I may consider selling both the 17-85mm and the 17-55mm lenses, replacing them with the 15-85mm.  That seems like a plan.