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! :)

"Remember high school math. a^2 + b^2 = c^2. (Pythagorean Theorem) That's how you determing the diagonal of the sensor/film size and the normal lens for your camera.."

oh jeez I'm in trouble now.

Nick writes: "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."

A 12-24 is marked 12 -24...with 35 mm as its reference.

Why does it convert to 18-36 on a FX camera (which has a 35mm sensor) or am I assuming. Or should I just go away and not think too hard about it.

Why does it convert to 18-36 on a FX camera (which has a 35mm sensor) or am I assuming. Or should I just go away and not think too hard about it.

It doesn't convert. On an FX camera, you would need to USE an 18-36 in order to see the same angle of view that a 12-24 gives you on a DX. That's all there is to it.

Melanie; the focal length on lens stays the same no matter what camera its used on. Just like your height doesnt change whether around 1st graders or a basketball team. To the 1st graders you are a giant; to the basketball players you are short. OK I am in hot water if you are a basketball player.:) The same goes for the lens; its considered long on a small camera; and shorter on a big camera of bigger sensor. What really matters is angular coverage. A 24mm lens covers a wider angle on a full frame FX camera than with a sub full frame DX camera. What folks are sometimes talking about is an equalvent focal length. Here with my Epson RD-1s its a sube full frame camera. Thus the 50mm lens for my film 35mm full frame camera when used on the Rd-1s is "like" a 75mm in ANGULAR COVERAGE. ie it covers less angle since the sensor is smaller; ie about 1/1.5. Since the sensor is 1.5 times smaller the coverage is like a 50 times 1.5 lens; ie a 75mm.

In point and shoot digitals some are actually marked/engraved with the equalvalent focal length of a 35mm film camera; with also or not the actual true focal length too.

Then the 12-24 on an Fx will be like a fisheye almost.

OK someone on this board it would be 18-36. Sorry.

Have a look at the diagrams here:

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

/Clay

Here is a chart I made a few weeks ago for the same question, which seems to be well received:

Melanie,

It's how people have come to describe what they see that is confusing you. As has been mentioned, the lens really doesn't care what camera it is on. It does the same thing. A 35mm is always a 35mm lens. A 200mm is always a 200mm. However, as Nikon went with a sensor that is SMALLER than the size of 35mm film, things changed in the camera. Due to the crop factor, which is very nicely illustrated in the above post, the image saved by the camera is smaller. Since nobody wants to print at a smaller size, they ENLARGE the shot to make it the same as 35mm film. Since the image was smaller to begin with, the field of view is still smaller. Almost like you were looking through a longer lens, but you're NOT. It would be better is people DIDN'T say you have more reach or the lens becomes longer or any of that, because it's really incorrect and misleading. The magnification of a 200mm is the same on a film or full frame camera as on a crop sensor, it's just being copped on the smaller sensor then enlarged to make up for it. Cropping always leads to enlarging to get back to the size we want.

The 12-24mm lens is still a 12-24mm lens. The difference is that a DX style lens puts a smaller image in the camera. On a crop sensor camera, it works fine. On a FX camera you see all the areas NOT covered with light by the lens. It's the field of view that changes, not the length of the lens.

"Then the 12-24 on an Fx will be like a fisheye almost"

Correct, except that the Nikon 12-24 isn't designed to produce fisheye distortion, it's what's called a 'rectilinear' lens designed to render straight lines without distortion. But if its image circle were large enough to fit the FX sensor, it would be a VERY extreme wide angle lens, whereas on the DX camera the field of view it provides makes it a less extreme wide angle. And, as others have said, if you wanted to recreate the same field of view on a FX camera as the 12-24mm provides on DX, you would need a lens of 18-36mm.

I believe Michael Kohan has an excellent picture. The DX has a smaller sensor. The image circle does not have to be as large compared to film or the new FX sensor. The focal length of the lens does not care about the sensor format. 12-24mm DX is the same for either. One of the important things about this is the image circle. You may put this lens on a full frame camera body but at 12mm you will get a circle on the image with in the file or print. This is your image circle and its not large enough to cover the full frame. Field of View is changed between the formats. You lose a bit on all the edges. The image size is the same all else being equal.

If you were to look at a 4x6 print of an image, consider that the size of the full 35mm/fx sensor/film frame. Let's say it's taken with a 50mm lens. The dx "crop" frame/sensor is smaller. Like you cut the edges off the print. If you were to simply enlarge it, you'd have a larger but trimmed print. However, if you took a 75mm lens and got the same original 4x6 "print" size, then trimmed the edges down to size, the print would look (almost but not exactly) like a smaller version of the original. That you could enlarge to the same original size and this 75mm on the dx print would look pretty much like a 50mm on the film/fx.

There are actually perspective differences when using different focal length lenses at the same distances so there would be some visual differences between the original 50mm 4x6 print and the cropped 75mm printed to 4x6. Depending on the subject, background, etc., they may or may not be apparent or even significant. You might see it if close with "wide" angles and the compression effects, even mild ones, of telephotos may show up some when shooting portraits. In many situations, the perspective differences don't have a lot of impact on the image is "seen."

Here's the easiest way to see this:
12-24 at 12mm on D3: Its a 12mm lens, but the useful area is roughly equivalent to what you would get out of an 18mm lens because of the DX format of the lens. A real 12mm lens would look the same, but the vignetted areas would actually be image, not black.

Here's the same lens, same camera at 18mm, the first point that its not vignetted:

and at 24mm:

so, when someone said that the 12-24 would be about an 18mm lens on FX, its true, but not because of magnification, because of the DX image circle... the 36mm part is utter nonsense, though, as its pretty much a regular 24mm lens at 24mm. So its more like a useful 18-24 on an FX sensor.

The image circle grows larger when you zoom out apparently, but that has something to do with the design of a zoom lens, not because it was ever intended to be an FX lens too, I assume.

This whole discussion is not new to digital. From 1972 I've shot with one or another Olympus Pen F. The Pen F has a 18X24 mm frame size (AKA half frame or cine frame) on 35mm film and is an SLR with interchangeable lenses. With adapters I can use various lenses designed for full frame 35mm SLRs. I multiply the focal length by 1.4 to arrive at the equivalent angle of view compared to a FF 35mm SLR. Thus my 135mm Takumar attached to my Pen F has roughly the same angle of view as a 190mm attached to a FF SLR. Oh! And does anyone remember the Konica Autoreflex, a 35mm SLR that could shoot both full frame and half frame...on the same roll of film? Must have driven photofinishers mad. One difference between lens designed for crop sensors compared to FF sensors is that they should have higher resolving power to compensate for the greater degree of enlargement to arrive at the same final print size.

I think the answser to your initial question "So...here's the thing. I've been reading my 12-24 DX will convert to 18-36. Doesn't that make it longer? " is as follows... An FX camera will recognise the lens as being a DX lens automatically (unless you force it to discard this info) and will thus take the picture in DX format. ie. it will only use the middle part of the FX sensor. In effect this will result in the same picture you would get on a D300. The only difference will be the number of pixels used. 6Mpixels used by the cropped D3/D700 image contra 12Mpixels by the D300. That is why it converts to 18-36.

Craig wrote: "There are actually perspective differences when using different focal length lenses at the same distances so there would be some visual differences between the original 50mm 4x6 print and the cropped 75mm printed to 4x6. Depending on the subject, background, etc., they may or may not be apparent or even significant. You might see it if close with "wide" angles and the compression effects, even mild ones, of telephotos may show up some when shooting portraits. In many situations, the perspective differences don't have a lot of impact on the image is 'seen.'"

Perspective is a function of subject distance and angle of view, not lens focal length. If two images are taken of the same subject, from the same location, with the same subject distance, with two different focal length lenses, it should be possible to crop the image taken with the shorter lens to match the perspective of the longer lens.

"Compression effects" with a long lens do not appear because a longer lens is used, but because of the long distance to the subject and the narrow angle of view. A similarly cropped image taken with a wide-angle lens would have the same perspective, including the "compression effects". (Caveats: the resolution would be lower, because a smaller portion of the frame was used; some slight apparent dissimilarities might exist owing to different corrections used in the lens construction; also this assumes a non-macro subject, such that the difference between the two lenses in distance from lens nodal point to subject is trivial.)

Never understood why this matter confuses people.

Shoot a picture at any focal length on 35mm film (or FX digital) and print it 6"x9". Then crop away 1" each from top and bottom, 1.5 inches each from both the sides. What's left is your DX 4"x6" print for the focal lengh that focal length.

It's as simple as that.

Thanks everyone for your input. i really appreciate the effort.

yes hashim i could just go ahead and shoot and not question but I am a curious person as to why things work the way they do.

Now if both the D300 and D700 are 12 mp wouldn't the full frame spread out the pixels more? It looks like FX has more detail so how is that possible?

All issues of angle of view aside, the full image from a D300 and from a D700 are going to have essentially the same number of pixels, it's true. But because the D700's light-sensing pixels ("photosites" - the little spots on the sensor that actually collect the light) are larger, they're going to be able to record the image with lower noise. You can research the sensor-size-to-noise-ratio issue separately if you'd like - there are many threads to be found that explain the laws of physics involved.

So, if you took a 50mm lens and used it on a 12MP FX-sized body, and shot a picture... and then put a 30mm lens on a 12MP DX-sized body and took (pretty much) the same picture, you'd get pretty much the same results. But when you do that in very low light, and need to really crank up the ISO to make the camera more sensitive, the FX body will result in a better quality (less noisy) image.

Further, you don't need to go so such engineering extremes to get a wider-angle lens to work well on an FX, which means that for certain kinds of shots, a lens at a given price point is more likely to have better-resolving, or less- distorting GLASS for the same angle of view, and thus you get a better image per lens-buying dollar in some situations. In some ways, the exact opposite is true when working with longer lenses (say, for wildlife, in OK light)... the DX's higher pixel density, coupled with a quality long lens will get you "out there" better.

only if it takes a folded kipper half the time it takes a peeled prawn to swim through half a mile of custard whilst reading a newspaper less the volumn of the density times a single egg.. hope that helps.