On 8/28/2015 10:58 PM, Floyd L. Davidson wrote:
PeterN wrote:
On 8/28/2015 10:45 AM, PeterN wrote:
On 8/27/2015 10:59 PM, Floyd L. Davidson wrote:
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.

Can someone explain this to me? I am thinking of buying the Sigma
18-300 - also a DC lens, but it now appears that the Pentax would
still have greater magnification (375mm equivalent). How can one
compare the actual magnification of different lenses on paper? It's
obvious in this case that one needs to multiply the Pentax focal
length by 1.5, which is expected, but not the Sigma. How can I know
that going forward, and looking at other lenses?

Which cameras are you using?

Note that at the same focusing distance and the same actual
focal length, the "magnification" will be identical. Just be
aware that magnification means the comparison of the projected
image to the actualy object's size. It has nothing to do with
the size of the image you get!

You might post a couple images, one from each camera/lens, to a
webpage or to alt.binaries.photos.original. If we could
download the image and look at Exif data it would help.


Your theory is spot on. However, some lenses have what I would call
optimistically labeled focal lengths, especially at closer distances. My
Nikon old 18-200 is an example. At close distances, I would estimate
the longer end as being closer to 160 than 200.


I shold clarify, that issue exists with zoom lenses. I
have not noticed it with prime lenses.

Zoom lenses have no lock on focus breathing. But don't
miss the point that the focal length at the minimum
focus distances make no difference at all! Who cares
what it is!

What you want is at least some minimum magnification.
If the lens provides that...

The fact is that focus breathing is what allows the lens
to focus that close with fewer aberrations. If the
design target required the focal length to remain
longer, the lens simply would not be able to get the
same quality at the same magnification!


Ah!


--
PeterN