Confusion on Crop Factor

[pjgoswami]

Presently I am using a AF-S DX NIKKOR 18-55mm f/3.5-5.6G VR (3.0x) Lens & Viviter 200mm telelens (Manual) with my new Nikon D5000.

As crop factor from 35mm film camera to APS-C(DX) is about 1.5 so I am getting 300mm(200mmX1.5) for the Vivitar 200mm lens. Now my confusion is that if I will buy a Nikon 70-300mm Telelens whether I will get 105-450mm (when used with Nikon DX format digital SLRs is equivalent to a 105-450mm on a 35mm format SLR: Nikon site) or the actual 70-300mm.

My confusion arises after reading a line in the Book "Nikon D60 Digital field guide" Chapter 5: Selecting & using lenses. Page no. 92. "Camera and lens manufacturers went to work creating specific lenses for DSLRs with digital sensors. These lenses are known as DX format."

As per the books which are the DX lenses.

If I will get 450mm at extreme end for 70-300mm lens then I can start shooting birds by that lens after purchase for the time being, but suggested lens starts from 400mm & which I can't effort now.

[sybren]

There are two things at stake here:

- Crop factor: read up on Crop factor - Wikipedia, the free encyclopedia and linked articles, they explain it better than I do

- Lenses specifically made for digital cameras. A digital filter reflects more light back into the camera body than a negative (you know, the old-fashioned chemical photography) does. This means that digital cameras are more sensitive to flares, which is compensated in "lenses for digital cameras". There is also another difference, which is linked to the crop factor. I'm sure that once you read the Wikipedia article you'll be able to understand it or ask more specific questions ;-)

Cheers,
Sybren

[OsmosisStudios]

Here's the thing.

The sensor in your D5000 is half the size of a frame of film. What that means is that it only "sees" the middle part of a frame of film (cut out 1/4 on each side and 1/4 on top and bottom of a frame of film). What this means is that it is like CROPPING out a section of the image.

The side effect of this is that the sensor "sees" a scene similar to that of a longer lens. Your 200mm lens, on a full-frame camera, sees a 200mm field-of-view. On a DX camera, it sees the same as if you had mounted a 300mm lens onto a full-frame camera.

Even with DX lenses, though, you have to multiply the focal length by the crop factor because all lenses are still measured the same way. So your 18-55 is roughly equivalent to 27-85mm.

In your example, the 70-300 on a full-frame camera sees 70-300, but on a DX camera the sensor only sees the centre of the frame and thus sees an equivalent of 105-450. So, if you use the 70-300 on a DX camera (like your D5000), you'll get 105-450. Now, this may sound fantastic (hey, a 400mm lens = 600mm DX! That's HUGE!), but it's only really good for telephoto: on the wideangle you get the same effect.

As such, my 10-24 is equivalent to a 15-36mm lens. 10mm, were it on a full-frame camera, would almost see behind the front of the lens: but it's almost impossible. That being said, Im bound and determined to try shooting my 10-24 on a film camera, just to see what effects I can get.

DX lenses are different from FX lenses in that they re DESIGNED for digital. What this means is that, while they are measured in normal terms, they really only project an image circle the size of the DX sensor, instead of the full FX. You get the same field of view, but in a much smaller, lighter lens.

Practically, all you need to know is whether your camera is FX or DX. Your D5000 is DX, so any focal length (ANY!) you see has to be multiplied by x1.5 to give you the equivalent field of view. This is really only of value if youre comparing shots from a DX camera and an FX camera using different lens

[pjgoswami]

Thanks to sybren & OsmosisStudios for the step by step guidance. Now I could understand that in wide angle DSLR with APS-C is not giving good coverage.

Thanks again for your kind information.

[sybren]

Hold your horses. You can get a proper wide angle lens for a APS-C camera as well, but you just need less millimeters. My 10-22mm lens really is a wide angle lens, equivalent to a 16-35mm lens on a 35mm film camera. Sigma even make a 4.5mm fisheye lens, especially for APS-C cameras. That should be wide enough for pretty much everybody.

[OsmosisStudios]

Originally Posted by sybren

Hold your horses. You can get a proper wide angle lens for a APS-C camera as well, but you just need less millimeters. My 10-22mm lens really is a wide angle lens, equivalent to a 16-35mm lens on a 35mm film camera. Sigma even make a 4.5mm fisheye lens, especially for APS-C cameras. That should be wide enough for pretty much everybody.

Very true: I mentionned the 10-24 earlier.

The Sigma 4.5, though, is a circular fisheye: not a diagonal.

[Pen]

Whats the difference between a diagonal and circular fisheye?

[OsmosisStudios]

Circular fisheyes create a circular image in the middle of the frame (the corners of a photo are black). A diagonal fisheye fills the scene with the fisheye image.

[sybren]

Generally a fisheye can see 180 degrees. A diagonal fisheye has those 180 degrees from the top left to the bottom right corner - on the diagonal.

[Pen]

OH, ok, that helps, thanks!