focal lengths

<p>I was playing around with a friends d300, trying some of his dx lenses vs some of my full frame lenses when I noticed odd angle of views between the 18-200 VR and my Sigma 180mm macro:<br>

<strong>Nikon 18-200 @ 200:</strong><br>

<img src="http://www3.telus.net/picbin/18_200@200.jpg" alt="" width="480" height="319" /><br>

<strong>Sigma 180mm:</strong><br>

<img src="http://www3.telus.net/picbin/180mm_prime.jpg" alt="" width="480" height="319" /><br>

This is definitely not what I expected. I've read on reviews that the 18-200 is a little short at the telephoto end, but this is a huge difference. Does anyone know what focal length the Nikon lens actually is, or if the Sigma is actually greater than 180mm?</p>

<p>The 18-200's actual focal length changes depending on how close you are to the subject. With very close focus (as in your example), the effective FL at the long end is much less than 200mm. In the case of your Sigma example, it looks like you're actually too close to get the near part of the ballhead in focus, so you'd need to back off just a bit - which would also change the apparent magnifaction.</p>

What Matt said. Focal length is measured at infinity focus.IF and zoom lens tend to have shorter FL at less than infinity.

<p>Subjectively, the bottom image is much more convincing as a 200mm shot than the top, so my money too is on the macro lens being accurate and the zoom being much wider when you focus close. You could play around with the focus while looking at the amount of background included in the viewfinder for each of the two lenses. If Matt and Snr. Novisto are right then you should see much more of a zoom effect with the zoom lens, even if the zoom is racked to full. </p>

<p>You might find the front nodal point of both lens moves around by different amounts too which would mimic moving the camera backwards or forwards. </p>

<blockquote>

<p><em>"Does anyone know what focal length the Nikon lens actually is ..."</em></p>

</blockquote>

<p>The Nikon lens is 200mm (within tolerances) ... at infinity focus. At shorter focus distances, the focal length is shorter than 200mm. The closer to minimum focus you get, the more pronounced the focal length shortening. That is because the 18~200mm is an internal focus (IF) lens. Internal focus works by shifting one group of elements internally within the lens for focus while the other element groups remain in a fixed position. When you do this, the focal length changes (think about it ... thats how zoom lenses change their focal length). And when the zoom range of the lens is extreme (11x in this case) the effect is usually even more pronounced.</p>

<p>If you were to set the 18~200mm to 180mm and focus on a far distance subject, and focus the 180mm Sigma on the same subject, you would likely find the the field of view is nearly identical for both lenses.</p>

<p>Thanks everyone. <br>

Michael, I did that. Shot them both at a range of about 20 feet instead of 5 and the fields of view were nearly identical. Interesting.<br>

Cheers,<br>

Aaron </p>