DX lens focal length

This is probably a stupid question but I've never owed a lens designed for APS-C or DX cameras and I have to ask how the focal length ratings on them compare to full frame lenses.  That is, the Sigma 18-35 lens that seem to be so highly rated for use on APS-C or DX bodies, how does that focal length compare to a full frame camera.  Is 18-35 actually equal to 27-53 or is it actually equal to 18-35.  I full frame lens on a DX body will be effectively 1.5 times longer but I'm just not sure on the DX lenses.

I don't actually have any immediate plans to buy any DX lenses and expect to use my current lens kit on both my older D800E's as well as my new D7200.  The D7200 I expect to use primarily for action, sports wildlife and birds and for that the Sigma 150-600 Sport will probably be the main lens on it.  Of my other lenses that I plan to use with the D7200 I think the Nikon 70-200 f/2.8 VR (1st gen) will probably be the second most used lens.

So, just to repeat, I need to clarify what the focal length equivalence of a DX lens on a DX body is -- is it 1:1 or 1:1.5...


Brian

It's 1.5x (in your case D7200) what the DX lens specifies same as you would with a FX (FF) lens.

The focal length of this DX lens is 18-35mm, same as if it were an FX lens.  Focal length doesn't change because of format.  The image circle that a DX lens projects, however, is only large enough to cover a DX sensor.  Put it on an FX body, and it will vignette.  The 1.5X crop is due to the sensor on a DX body being smaller, so the field of view would be equivalent to a 27-53mm on an FX body (just like with FX lenses).  Many Point & Shoot cameras with very small sensors show the "equivalent" focal length, but this is not their true focal length.

That is what I thought but I just wanted to make sure in case I do decide to get a DX lens.

Thanks,

Brian

Keep in mind to use a DX lens on an FX body you will need to set your camera to DX mode to avoid bad vignetting of images. DX lens cast a smaller image circle onto the sensor since they are specifically made for smaller sensors. So even though you will be using a FX body you will not have the benefits of full frame images.

bradmangas wrote:Keep in mind to use a DX lens on an FX body you will need to set your camera to DX mode to avoid bad vignetting of images. DX lens cast a smaller image circle onto the sensor since they are specifically made for smaller sensors. So even though you will be using a FX body you will not have the benefits of full frame images.

Again, I have no immediate plans to get any DX lenses and even if I did I can't imagine any reason I'd want to put one on an FX body -- I have a complete lens kit for my FX bodies and wouldn't want to turn my 36MP camera into a 16MP camera when that wouldn't be needed.  Yes, I'm aware of the image circle being smaller etc.

I think the two most likely lenses for my D7200 will be the 70-200 and the 150-600.  In effect, those two lenses will cover from 105-900,  The earlier version 70-200 that I have, dating back to 2002, may not be as optically great as the newer version but it doesn't have the same focus breathing problems the newer one does and
 using it on a DX body crops off the worst part of the lens.

It's been about 10 years since I've done BIF shooting so I may start with the 70-200 to get back in the game and then switch to the 150-600.


Brian