Fx lenses on a DX camera?

[Jessieh04]

I know a bit about what will happen when a DX lens is put on a FX camera, but what about the other way around? Say, the 14-24mm Nikon lens on the D300? Dare I suggest the outcome might not be bad? The center of the image is usually the best in terms of sharpness and quality right? All that will be preserved in the photo while the edges are removed. Am I correct in the assumption?

[OldClicker]

You are correct in that the center of the image is all the sensor records and that the 'non-APS' lenses work just fine on a camera with an APS sensor. - TF

[cm_mtb]

The center of the image is usually the best in terms of sharpness and quality right?

Yup. This is a definite advantage of using a lens designed around a full frame on a crop sensor camera (not to say that there aren't reasons to use DX lens, of course).

[Jessieh04]

And what of the megapixels? Any major differences?

[Sushigaijin]

megapixels are a function of the camera, not the lens. There will be no difference.

There WILL be an apparent "crop" factor - APS lenses usually convert around 1.5x - so a 20mm lens will behave more like a 30mm lens, because the center is effectively cropped from the image circle by the smaller sensor. This could be good (long telephotos will seem even longer) or bad (wide lenses will seem less wide). There is also an apparent change in depth of field - a smaller sensor will appear to have more depth of field than a larger sensor, all other factors being equal.

It isn't that the lens has changed - a 20mm lens is still a 20mm lens - but the amount of the image has changed. A 12 mp image is also always a 12mp image, be the sensor a cell phone camera, compact, APS, or full frame - if that wasn't also true, we'd just be looking at smaller images coming out of APS cameras because they'd just be crops of the full size photo.

[Franglais]

....There is also an apparent change in depth of field - a smaller sensor will appear to have more depth of field than a larger sensor, all other factors being equal.
...

I agree with the rest your post, but a smaller sensor gives smaller apparent depth of field.

Put a 50mm on an FX camera and make a full-frame 8x10 print. Put the same lens on a DX camera and make a full-frame 8x10 print. The image won't be the same because the FX camera gives a "normal 50mm" view and the DX camera gives a "moderate telephoto 75mm" view. But also - on the print the DX image will appear to have slightly less depth of field.

The image produced on the sensor by the lens is identical. However as the image from a smaller sensor is enlarged more than the one from a bigger sensor, detail becoming out-of-focus is more noticable. The Circle of Confusion is smaller

Check it out here: http://www.dofmaster.com/dofjs.html

[OldClicker]

To further confuse things on the DOF - from my POV, the sensor size ONLY changes the field of view. DOF (and others) is a function of the lens, not the sensor. If you are the same distance, with the same focal length and print the same image at the same size, the DOF will be the same. It is only when you react to the different field of view that things change.

Erik chose to react by moving further or decreasing the focal length with the smaller sensor to give get the same image in the field of view and the DOF increased.

Charles chose to blow a smaller (different) image from the smaller sensor to the same size print and got a smaller DOF.

I chose to ignore the cropping that I have to do to get to the same image. I justify this by saying that it is just cropped in a different place. :-)

TF

[Sushigaijin]

To further confuse things on the DOF - from my POV, the sensor size ONLY changes the field of view. DOF (and others) is a function of the lens, not the sensor. If you are the same distance, with the same focal length and print the same image at the same size, the DOF will be the same. It is only when you react to the different field of view that things change.

Erik chose to react by moving further or decreasing the focal length with the smaller sensor to give get the same image in the field of view and the DOF increased.

Charles chose to blow a smaller (different) image from the smaller sensor to the same size print and got a smaller DOF.

I chose to ignore the cropping that I have to do to get to the same image. I justify this by saying that it is just cropped in a different place. :-)

TF

I agree here totally. The only real important numbers are magnification and f/stop - everything else is a function of crop. This is why "apparent" is an operative word; a 50mm lens at f/1.8 will ALWAYS give you the same DoF and FoV (within some tolerance for design) but the size of the crop usually determines how we frame and shoot. I think the "apparent" FoV is probably more important than the actual numbers on the lens - I know that I'll have to step back to get the same framing with a smaller sensor, and that's going to effect which lens I choose for the job.

Thinking of lenses in focal lengths without a reference sensor size is like considering buying a boat without regard to what body of water you'll be piloting it in. Obviously an ocean liner and a dinghy have different purposes. Knowing when to use one, and when the other is easier when you think of FoV than focal length.