Why is DX lens longer on FX camera

OK...here we go....if it's been answered please point me to it...I probably won't understand it anyway. Having a tough

time understanding the whys of dx vs fx conversion of focal length. Yes I know you get more "reach" with a DX lens.

So I understand that I won't have as much with FX. (Never really had to think this much with film lol)

 

So...here's the thing. I've been reading my 12-24 DX will convert to 18-36. Doesn't that make it longer? A while back i

posted the question "What makes a lens digital" and from the answer I assumed it meant the diference is inherent in

the sensor of the camera based on the 35 m LR system...not the lens. The lens is what it is...so why would 12-24

convert longer and not to 9-18 mm on an FX? I understand my 70-200 will be "true" to it's focal length, as will the 28-

70....in fact al the non DX lens. I only have one other...a cheap walk around 18-135 ...what will that convert to? Can it

be used on the D700? It already vignettes on the dx!

 

Sorry for not understanding this. I AM sort of smart. I mean I shoot on manual and everything :) But this stuff is not

making sense to me.

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...

<br><br>

<i>So that means the 18-135 is really a 24-175?</i>

<br><br>

No, it means that those are the two zoom lenses that will produce the same useful <i>angle of view</i> 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.

<i>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.</i>

<br><br>

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:<div></div>

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:<br>

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.

<br>

<img src="http://www.robgulotta.com/12mm.jpg">

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

<img src="http://www.robgulotta.com/18mm.jpg">

and at 24mm:<br>

<img src="http://www.robgulotta.com/24mm.jpg">

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.