In article ,
RichA 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.

A horrible SCAM perpetrated by them, Nikon and others. The Nikon 70-200mm at
200mm is actually close to 115mm when used close-up. What a con.

it's not a scam and it doesn't actually matter.

most lenses, including whatever you use, have focus breathing.