Why is DX lens longer on FX camera

[john_robison3]

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.

[hamish_gray]

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.

[david_martin2]

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.)

[hash]

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.

[mksnowhite]

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?

[Matt Laur]

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.

<br><br>

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.

<br><Br>

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 <i>per lens-buying dollar</i> 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.

[andrew_cole2]

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.