On Fri, 28 Aug 2015 19:01:08 -0800, (Floyd L.
Davidson) wrote:

Bill W wrote:
On Thu, 27 Aug 2015 23:02:25 -0400, nospam
wrote:

In article , Bill W
wrote:

I am going crazy with this. Two lenses, side by side. A Pentax DA L
70-300 lens, and Sigma 18-250 DC lens. The DA in Pentax means it's
designed for APS-C sensors, and the same with the DC on the Sigma
lens. Why do they have nothing close to the same field of view &
magnification at the same focal length? To get the same photo with
both lenses, I need to set the Pentax to 170mm, and the Sigma to
250mm. When I do that, and then look at the two identical photos in
LR, the exif data shows the Pentax set at 170, with a 255mm 35mm
equivalent, and the Sigma at 250, with a 375mm 35mm equivalent, which
is correct, I guess, but the Pentax has much greater magnification at
*equal* focal lengths.

that's called focus breathing, where the actual focal length changes as
you focus on closer subjects.

lots of zoom lenses do it, especially superzooms like the sigma.

try comparing with it focused at infinity.

This was indeed the issue. The field of view at 250mm is now nearly
identical when focused at, or near, infinity.

Just make sure you understand that focus breathing is a non-issue.
It just doesn't matter.

I do. The only issue is that it confused me.