Focal Length Variations

So I have 2 lenses that are 17mm-XXmm zooms. Two different manufacturers. I'm doing a lens comparison as controlled as I possibly can.

One of the first things I noticed, was that the frame was filled differently between the two lenses at 17mm. One definitely gives a larger FoV. When you look at the 2 images, the one that gives a great FoV has the same amount of additional area captured on both the left and right sides of the frame, and top and bottom. Top and bottom is not as great of a difference as the left and right.

Is it possible to determine which lens is actually giving a 17mm focal length/Fov/AoV? All shots were taken on a tripod, indoors, and nothing was moved (as evidenced by the equal amount of additional subject in the frame all around).

If it's a detailed mathematical equation, I'll skip it, but is there a chart or anything that could be used from X distance to the chart?

Very common... It is rare any lens is exact to the specification.

For example, the 70-200 II at 70mm and the 24-70 II at 70mm are drastically different... The 24-70 appears less than 70mm and the 70-200 more.

If you look at any lens patent, they round... So a 25.2-65.4mm may be billed as a 24-70... For whatever reason... A 98-372mm on a parent would be a 100-400 and so forth.

Ok, so I should ignore it? I was trying to make a layered .gif for the comparison, and they don't line up like I hoped (same exact crop locations). I guess it'll have to be side-by-side comps then.

Yeah, ignore it. There is no issue, especially between two different companies and such. No lens is exact to what is stamped on it.

Agree that it s a common problem/issue, and has no substantive real world effect in most situations, but I would like to know if there is an easy way to work out the actual focal length of a particular lens (as opposed to "this 35mm has a wider FOV than that 35mm", e.g.). Anyone?

Would this apply to zooms only or primes as well? Just out of curiosity. I would think primes would be easier to make exact, or the same meaning 3 copies of a 50mm from same company are all 51mm etc.

On top of what's been said above, most if not all lenses' focal length is calculated or measured at infinity focus. Focus any closer and all bets are off comparing two different model lenses at the same marked focal length.

Manufacturers also tend to round off focal length to commonly marketed values. For example, a lens marked "50mm" could easily be anywhere from 46 to 54mm or so in reality (again, measured at infinity focus).

In addition, there are both design and manufacturing tolerances that figure in to the errors that are sometimes notices.

In regards to the FL being calculated at infinity, how does one actually focus to infinity? Always wondered about that...

... you spin the focus ring to the infinity marker

I seem to recall seeing 70-200's compared. Both set at 200mm, brand 'X' was about 190, brand 'Y' was closer to 205. But it's been a while.

Canon 24-70 MkII @70 mm tripod mounted

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