One of my favourite lenses was a Sigma 70-200 f2.8 lens I purchased secondhand, but was like new from Camera Jungle.
Recently the autofocus started to play up. I noticed it was almost dragging, like it was trying to focus but struggling.
As the lens is not that cheap, even secondhand, I decided to take it to B&W to see if it was worth repairing. Needless to say after about a week they reported back that the lens had been dropped... The issue was the front 'barrel' had gone slightly out of shape, hence the issue with the autofocus. I have to say I was a bit puzzled as I can't recall dropping the lens and it certainly was unmarked. To add to the issue the quote wasn't a worse case scenario, as if the motor has been damaged as a result of the attempting to focus, there would be additional cost. I was looking at at least £200 providing they found no further issues.
The first dilemma is, do you spend that money on repairing a secondhand lens, hoping nothing else goes wrong, or cut your losses...
I did a little research to see what was around that wouldn't cost me an arm and a leg. A replacement for the Sigma was going to cost me more than I wanted to spend.
I read a review on the Canon 70-200 lenses, of which they do two in my budget, an f2.8 and an f4.
In one review something was pointed out to me that I had never really thought about. I was a little hung up about having a 'fast' lens, but in the review the photographer took a photograph of his dog, at a distance, both at f2.8 and at f4. This was to make the comparison between the two lenses.
The photograph taken at f2.8 although OK, the dogs head was not totally in focus from the nose to the top of the head, whereas the f4 was. In fact the photograph taken at f4 was much better.
With regards to the Canon lenses the f2.8 version is much bulkier, heavier and a lot more expensive. Second dilemma solved, go for the f4 version...
I sold my faulty lens on eBay for £200 (let someone else worry about the repair...I did advertise as faulty, spares or repair) and purchased an 'as new' Canon 70-200 f4 off good eBay for £300... Result!!