dx vs. fx lens

randrx2

I always wondered and thought that if you took a pic with a DX camera at 200mm with a dx lens that it would be the same as taking one at 200mm with an fx lens.

The fx lens crops closer (300mm) than the dx lens.

I had always read that you still apply the crop factor to the dx lens. I guess not.

ziggy53

Lenses are labeled by their actual focal length, so a 200mm lens is a 200mm lens.

A DX body has a smaller imager which yields a smaller angle-of-view, compared to a FF imager. A 200mm lens will yield a field-of-view (FOV)/angle-of-view similar to a 300mm lens on a FF body.

A DX lens simply has a smaller image circle to match the smaller imager of a DX camera, but the focal length stated for the lens is always the correct focal length.

If you have a DX camera just remember this simple rule:

A 35mm focal length is about standard for a DX camera, and it will yield a FOV similar to a 50mm lens on a FF imager. Anything longer than standard is a telephoto lens on that camera and anything shorter than standard is a wide angle lens. The amount of telephoto or wide angle is roughly proportional to the variance percentage from standard.

randrx2

ziggy53 wrote:

Lenses are labeled by their actual focal length, so a 200mm lens is a 200mm lens.

A DX body has a smaller imager which yields a smaller angle-of-view, compared to a FF imager. A 200mm lens will yield a field-of-view (FOV)/angle-of-view similar to a 300mm lens on a FF body.

A DX lens simply has a smaller image circle to match the smaller imager of a DX camera, but the focal length stated for the lens is always the correct focal length.

If you have a DX camera just remember this simple rule:

A 35mm focal length is about standard for a DX camera, and it will yield a FOV similar to a 50mm lens on a FF imager. Anything longer than standard is a telephoto lens on that camera and anything shorter than standard is a wide angle lens. The amount of telephoto or wide angle is roughly proportional to the variance percentage from standard.

What I am saying is I took 2 pictures with a DX camera, one with a 200mm DX lens and one with a regular 200mm lens. The regular 200mm lens was cropped more (acting like a 300mm lens).

ziggy53

randrx2 wrote:

What I am saying is I took 2 pictures with a DX camera, one with a 200mm DX lens and one with a regular 200mm lens. The regular 200mm lens was cropped more (acting like a 300mm lens).

What are the 2 lenses that you tested? (Please be as specific in naming as possible.)

Do you have any image examples to demonstrate the phenomenon? (Preferably with full EXIF and metadata intact.)

If one, or both, of the lenses is a zoom, that could explain a little bit of a difference, but not the difference between a a 200mm and 300mm angle-of-view (equivalence).

randrx2

Camera is a Nikon D300.
DX lens, Nikon 18-200 3.5-5.6 G
Regular lens, Nikon 80-200 2.8 D

Here are the pics. I was about 12 feet away. Not an exciting subject, just something to experiment with.

DX lens at 200mm


Regular lens at 200mm

craig_d

I think what you're seeing here may indicate that the 18-200mm lens doesn't really go all the way to 200mm even if it says it does. I don't know that for certain, but as Ziggy explained, focal length is focal length. DX differs from FX only in the size of its image circle, which is to say, the DX image is a center-crop of the FX image, but objects within the two images should be exactly the same size if the two lenses are actually exactly the same focal length.

Another possible consideration is that the 80-200mm lens is about twice the physical length of the 18-200mm (7.4" vs. 3.8"). If the front elements of the two lenses weren't in the same place, that is, if the two lenses were collecting light at different points in space, then their fields of view may appear different.

ziggy53

Thanks for posting the examples. It's so much easier to understand what's going on.

I think there are 2 things at play here. The first is that lenses are measured at their infinity settings. If you test these lenses outdoors and with a scene at infinity, they will probably match up much better.

Manufacturers also often "round" the true focal lengths for marketing purposes. I'm betting that the Nikkor
80-200mm, f2.8D ED is closer to a "true" 200mm at it's maximum, while I would bet that the Nikkor AF-S 18-200mm f/3.5-5.6G IF-ED DX VR is more of a "Marketing" 200mm.

I tried to find a review which includes "measured" focal lengths of these lenses, but I couldn't find anything just now.

ziggy53

Popular Photography did measure the 18-200 VR to be 18.42mm-201.07mm, so it should be right on the money at infinity.

http://web5.popphoto.com/cameralenses/2763/lens-test-nikon-18-200mm-f35-56g-dx-vr-af-s-sqf-charts-page2.html

I did not find the 80-200mm, f2.8 but I do think it is accurate too.

randrx2

I did try to ensure the front of the lenses were at the same spot. I thought that might be an issue so I made sure of that.

I understand how DX and FX works as applied to the sensors on the camera. I was just seeing if there was a difference in the DX since it is optimized for DX (according to Nikon).

I am going to experiment at other focal lengths also.

Matthew Saville

Oh wow, this could have gotten ugly and confusing. Good thing we nipped it in the bud.

Basically, lenses are not always exactly what they're specified to be. If you're only focusing on a subject that is indoors and therefore pretty close, unfortunately that 18-200 is actually only getting to about 135mm. It's just a characteristic of the lens' engineering, and has nothing to do with the fact that it's a DX lens.

The physics behind it is, focal length can change with focus distance, depending on the lens' design. Super-zooms frequently suffer most from this phenomenon...

=Matt=

randrx2

Okay, so maybe I was too close. I took some new pics further away. These are through a screen so not the best quality.

So I was wrong. They both are the same 200 cropped to 300 on a DX sensor.

80-200 Nikon (non-DX)



18-200 Nikon DX

craig_d

Interesting. So it is just that the focal length isn't constant as focus distance changes, and that it varies differently for different lenses. Good to know.

ziggy53

randrx2 wrote:

Okay, so maybe I was too close. I took some new pics further away. These are through a screen so not the best quality.

So I was wrong. They both are the same 200 cropped to 300 on a DX sensor.

...

It's easy to see how you would draw that earlier conclusion based on your earlier results.

Thanks for posting the examples, both the closer focus results and the image results closer to infinity. That helps to solidify the concepts at work.