Angle of View Question: Lens Made for FF vs Lens for APS-C

Started Jun 13, 2012 | Discussions thread
Great Bustard
Forum ProPosts: 20,681
Re: Just go by the focal length.
In reply to Joseph T Lewis III, Jun 13, 2012

Joseph T Lewis III wrote:
Thanks to those who responded; I appreciate it.

I understand focal length being the same; no issue there. However, some of the replies seem a bit contradictory (it may be because it's very late and I'm tired). I'm not trying to argue; I just want to understand things correctly. The reason I want to make sure I have this right is because if the angles of view are the same on my APS-C camera I'm probably going to sell the Sigma lens, because the Tamron is lighter, smaller, sharper and faster.

Here's why I'm still perplexed.

Great Bustard wrote:

A 17mm lens, whether made for FF or crop, will have the same AOV when mounted on crop.

I would interpret that to mean the Sigma would therefore maintain the advertised 104 degree angle of view if mounted on an APS-C camera. However, GB then went on to say:

So, your Sigma 17-35 which was made for FF, will have the same AOV as the Tamron 17-50, which was made for crop (up to 35mm, anyway).

This is saying the Sigma will have an AoV of 78 degrees, the same as the Tamron. I'm obviously missing something here, because instead of 104 degrees we're now down to 78. Where did the extra 26 degrees go?

Mr. Bustard goes on to say the following:

However, a lens at 17mm mounted on FF will have a 1.6x wider framing than a 17mm lens mounted on 1.6x.

Is that what caused the angle of view to go from 104 degrees to 78 degrees? Is the difference in AOV that I get due to the difference between a FF and APS-C camera?

Thanks, and apologies for being so obtuse.

OK, for infinity focus, 17mm on FF has an AOV of 104°, and 17mm on 1.6x has an AOV of 77°.

So, where did the extra 27° go on 1.6x? Off the sides of the 1.6x sensor.

But, a 17mm lens, whether manufactured for FF or crop, will have an AOV of 77° for infinity focus when mounted on 1.6x.

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