Tamron 17-50 and Canon 17-40L

Sorry about the misleading title, but this is not a pros and cons debate about them..
So please do not flame each other for that sake.

I was told by some photographer that..
Canon 17-40L's 17mm field of view angle is larger than Tamron 17-50's 17mm field of view angle..
Does anyone know about this and confirm it either way?
According to him.. there is a chunk of difference in them..
104 degree vs. 78 degree..

That can't be right..
I'm thinking.. he must have read the specification of 17-40L with FF as a reference.. and Tamron 17-50 with APS-C as a reference..

just because it says 17mm on the starting point doesn't mean they are all exactly the same - i believe some lenses show significant variation from others at the 'same' focal length.

So it might well be partially true, though the figures you give seem way out. I only have the 17-40 so cannot comment, but the Canon lens was designed just for Eos, the Tammy comes in different mounts for different makes, and remember that Canon is the only one with a 1.6 crop factor - so if anything the tammy should 'look' wider on a Canon body ( 1.5 x 17 as opposed to 1.6 x 17)

Your suspicion is correct, your friend was probably reading FOV data from a chart. Since the 17-40L is designed to be an UWA lens on FF cameras the data set is for FF. For sure it is not going to give you 104 degree diagonal FOV on APS-C.

That said, it is also true that many lenses do not give exactly the focal length they state. Some 50mm lenses are actually 49mm or 52mm. This kind of error is generally really small and of no interest to most photographers. It certainly is not the kind of error that would give 104 degree FOV on one 17mm lens and only 78 degree FOV on another 17mm lens.

I also heard something like this with the Tamron 70-300mm and the Canon 70-300mm. I've only used the Tamron 17-50mm so I can't say that it is.

I too think your friend is thinking about a field of view of the 17-40mm L on a full frame body. The Tamron is made for a cropped sensor and it's field of view is less.

Better way to consider it is that at 17mm on a cropped body, the field of view would be equivalent to a 17mm x1.6 crop factor= 27.2mm field of view for a 35mm film format. I would bet that if you zoom back the 17-40mm L on a full frame to 27mm and also compare it to a tamron at 17mm on a cropped body, the field of view would be about the same.