Not the Focal Length I was expecting.

[SnapShot]

Hi all. I recently acquired my first lens designed for a full frame sensor that I will be using on my cropped sensor body. The Sigma 105mm f/2.8 EX DG OS HSM Macro on my Nikon D3200. Please forgive my ignorance in advance for the below . So in my research on this lens I read that it has the equivalent of 168mm on a cropped body. When shooting on a full sensor body wit a 1:1 focus setting the focal point should be ~1.02 feet. When I shoot on my cropped body being that it is acting like a 168mm I would presume I need to get back further away further from the subject to get the same object in focus at the 1:1 setting. This does not seem to be the case. Unless I am totally off on all of the above then my whole understand goes to pot. But if not my first question would be Is the 1.02ft. document distance from the sensor to the subject or from the lens to the subject? I assume its from sensor to subject. At any rate when I shoot a subject with a 1:1 macro focus setting and have said subject in focus my lens is right at 5 inches from the subject and the sensor is 11.5inches away. Ahh did I just answer my own question, LOL. someone please confirm - I'm still trying to grasp all of this. My sensor is at the roughly 1 foot from the subject but since it is artificially 168mm equiv but really on a cropped sensor I'm still at 105 its just cropped and I'm loosing part of the image not really having to backup to the 168, hence the hole cropped sensor definition? Meaning I cannot backup to get the same content extents in my frame with this lens on this body at a 1:1 and in focus, i would have to give up the 1:1 or blow the focus, correct? Also in that same thought, if correct is that If I put this lens on a full body reshot with all things being equal, the subject distance would be the same but I would get more content in my frame for the final picture?

[SnapShot]

Okay I think i just totally rambled and realized I probably should have posted the thread in a different section than the third party lenses and in a more appropriate section about understanding cropped frames. Sorry...

[Thomas]

The focal length does NOT change on a cropped sensor. It's as simple as that : a DX sensor simply crops 1.5x. It's as if you were shooting with a full-frame/FX-sensor and then crop away the outer 33%.
Thus the minimum focus distance is totally independent from the body you're using. The only effect when shooting from a given distance (which btw. is measured from the sensor, NOT the from of the lens) is that the FX-body shows 50% more of your subject than a DX-body. But that is completely OK as a magnification of 1:1 is not measured by how much the subject fills the screen but by the fact the 24mm of subject size equals 24mm of size on the sensor.
From that it should be clear that 24mm on a DX-sensor fills the width of that sensor, while it only fills 67% of the width of an FX-sensor.

hope that helps, SnapShot.

[RexGig]

Thomas is correct. When I shoot an image at work, with either of my macro lenses, 1:1 is at the same working distance, whether I am using a cropped-frame 7D or full-frame 5D. When actually viewing the image, of course, the 5D simply shows me more area.