focal lengths 18-70 and 70-300

[jonathan_chan4]

<p>Hi. I have a nikon d200, a nikon 18-70 lens and a sigma 70-300 apo lens. I undestand that my camera is cropped at 1.5 compared to a full frame slr, thus, if i were to use a lens at 50mm on a full frame, it would actually be 75mm on my slr. What i find quite interesting though is that my 18-70 (a dx lens) does not zoom in as far as my 70-300 (an fx lens) when both set to 70mm. Is this because fx lens focal length numbers are longer than dx ones? My nikon 50mm 1.8 also has a focal length close to the 18-70's 55 range.</p>

<p>thanks</p>

[andrewg_ny]

<p>I don't have either of these lenses. And the 'crop factor' should have no effect since its the same camera for all three of the lenses you mention. 1 mm of focal length is 1mm regardless of whether the lens was designed to cover DX or FX.</p>

<p>However I am not surprised that they don't frame exactly the same when set at the same focal length. A likely reason is that the Nikkor 18-70 is an internal focus design and these often have significantly reduced 'effective' focal length as they are focused closer. I believe this is also part of the design that allows modern lenses to offer reduced minimum focus distance & higher magnification. You might find that at infinity focus the fields of view are at least closer to the lenses you're comparing it to. Another possibility (again, don't know for sure on either lens) is that the nominal 70mm is slightly exaggerated on each of these lenses to make their ranges look slightly longer than they really are.</p>

[Michael R Freeman]

<blockquote>

<p><em>What i find quite interesting though is that my 18-70 (a dx lens) does not zoom in as far as my 70-300 (an fx lens) when both set to 70mm. <strong>Is this because fx lens focal length numbers are longer than dx ones?</strong></em></p>

</blockquote>

<p>No. Format does not change focal length. 70mm is 70mm. However what happens with many zoom lenses, particularly internal focus designs with a large focal length range, is that the focal length effectively shortens at close focus distances. On some designs (the new AF-S 70~200/2.8 VRII is one such lens) the focal length shortening is significant at close focus. The marked focal lengths are valid only at infinity.</p>

<p>Compare your two lenses zoomed to 70mm at a distant infinity subject, and you are apt to find that the field of view for each will be much closer than the fields of view for a close subject.</p>

[vick_vickery]

<p>To put focal lengths in perspective, remember that 34mm is about normal for your DSLR format, thus anything longer is tele, anything shorter is wide. 50mm, for instance, is always 50mm, but its a short tele on your camera.</p>

[jonathan_chan4]

<p>I honestly didn't realize (or didnt take any notice before) that focusing changes the focal length slightly. thanks everyone for your contributions.</p>

<p>Jon</p>