Why is DX lens longer on FX camera

The problem is everyone says the lens is longer which is great for marketing. What happens is the smaller sensor CROPS you image and creates an image that would be the same as if you used that lens on a full frame but since you lose the edges of the picture it makes an image that appears to be magnified.. The magic word is cropping, the rest of the image spills over the sides. The image size on the sensor is the SAME for either sensor. This is why these new small sensor lenses dont do well on full frame since the lens design does not have to support the increased area of the large sensor.

It's really not as complicated as you might think. Take two camera's. Lets say a d700 (full frame) and a d300 (1.5 crop). Stick a 100mm lens on both and take a picture. The photo with the crop sensor will look like your shooting with a longer lens. But if you just crop the photo from the d700 (full frame) it will be exactly like the picture from the d300. On a DX body your just using the center of the lens. The advantage of the DX body is if you use telephoto lens you get more "magnification." Not that you couldn't do it with a full frame camera but you would have to crop the picture with the full frame to look like the DX. So if you have a D700 and I shoot a D300 and you crop your photo, we will have the EXACT same photo. But mine (d300) will have more mega-pixels because I started with 12 and you started with 12 and cropped down to 6. As far as the lens goes. It's easier/cheaper for them to make a lens for a DX camera because your "only using the center." So by making a lens that only covers the center a full frame sensor, but all of the DX, you can get a better lens for less cost/weight. But if you put that lens on a full frame body you'll see the black circle (kind of like a fisheye) because the picture isn't covering the whole sensor. The whole convert your lens or 35mm equivalent is to try and create a level playing field. For example: If i tell you to go to the grand canyon and stand in spot X , a 15mm lens (full frame format) will cover from point A to B. Well if your shooting a DX body, you'll need wider lens because your only using the center of the image. In this case a 10mm lens (10 x 1.5). Does that help?

Well it helps if this statement is correct. the 12-24 is realy an 18 to 36 but on a DX body you are just seeing the 12-24 center portion?

So tht means the 18-135 is really a 24-175? (aprox)

Melanie: Nope. A 12-24 is a 12-24, regardless.

The job of the lens is to project an image into the camera body. A 12mm lens that's been built to work on both formats of camera is STILL a 12mm lens on either camera... but it's been engineered to project the image over a larger area inside the camera - which is why it will work on an FX body.

A 12mm lens that's been made specifically for DX bodies is still the same focal length, but projects a smaller image into the camera body. It's not that a smaller image is good or bad, just that a larger image is wasted when the sensor that it's hitting is smaller. For people who have no need for the larger projection out of the lens, a DX-only lens can be made physically smaller, lighter, and less expensively. But 50mm is 50mm no matter what. The difference between 50mm on a DX vs FX is just that the size of the sensor allows a different percentage of the projected image to be recorded. A DX sensor is just taking a smaller crop out of the middle of that projected image.

And since we know who MUCH of a smaller crop we're talking about, and what angle of view it will end up producing in your final frame, it can be helpful (for people who come from shooting 35mm film cameras or FX bodies) to talk in terms of what focal length lens on an FX will produce the same ANGLE OF VIEW as a given lens on a DX body. So, someone who is used to shooting with a 50mm lens on an FX body - and likes the "normal" angle of view that such a lens provides - may find it useful to know that a 30mm lens on a DX body will produce essentially the same angle of view. The perspective is not quite the same, but that's a separate issue.

So, if you have a 50mm lens that will work on both bodies, it will SEEM longer when you use it on your DX body because your DX body is only looking at the center part of what the lens captures... if you wanted that same center part on an FX body, you'd need a longer lens (say, closer to 80mm).

The main problem with using DX-specific lenses on an FX body isn't the focal length, it's that they've been built (in the interests of size, weight, and price) to be well suited to DX, and don't producre a large enough image to cover an FX sensor. You can still use it, of course. And the center part of that image, on an FX sensor, will look EXACTLY like the DX image would. But the outside margins of the image on the FX sensor would be vignetted, or simply black (since the DX lens doesn't cover it well). Either way, 12mm is 12mm.

And to be clearer...

So that means the 18-135 is really a 24-175?

No, it means that those are the two zoom lenses that will produce the same useful angle of view on the two different bodies.

ALL lenses are marked with reference to 35mm, full frame, i.e., "FX" dimensions. As everyone has said the focal length doesn't change when a lens (FX or DX) is used with a DX body--just the field of view.

>"ALL lenses are marked with reference to 35mm, full frame, i.e., "FX" dimensions. "

No. All lenses (for SLR cameras) are marked with their *real focal length* - 35mm, FX, DX, etc is irrelevant.

A 50mm lens is a 50mm lens on a DX, FX, 35mm, 645, 6x6 or 6x7 camera body. The angle of view will change on each body, and the lens may not have an image circle large enough to cover the format, but it will still be the same 50mm focal length.

Lenses are not marked by format.

A 50 mm lens for a RB 67 is a wide lens for that format. A 50mm lens for 35 mm (film) is considered a normal lens, for DX a 50 mm is a slight telephoto. Focal length is focal length. It has nothing to do with what format it is shot on.

Its like asking why one is taller when around 1st graders than around the LA Lakers. In each case you DO NOT grow or shrink in height! :)