35mm Film sets standard 35mm film measure 35mm x 24mm When the first Digital SLRs came out, they had the form factor of the film cameras, but the sensor size was half the size of a film negative…resulting in a change in the field of view.
35mm was NOT perfect 35mm film measure 35mm x 24mm which is approximately a 2:3 aspect ratio This mean that what you see is NOT what you will get EXCEPT for 4x6s. –2:3 = 4x6 20x30 (or.66 factor) –2.5:3.5 5x7 = (.71 factor) –3:4 = 6x8 (or.75 factor) –4:5 = 8x10, 16x20 (.8 factor)
Lens Focal Lengths A stated lens focal length was based on the size of the image area. So, a 50mm lens (the yellow circle) on a 35mm negative (blue box) was designed to cover the entire negative.
Lens Focal Lengths, cont. But the same 50mm lens was MUCH larger than the DX sized sensors. So, what the user saw (I.e. field of view) was similar to that of a longer lens, specifically, a 75mm lens.
Lens Focal Lengths, cont. Then lens manufactures “got smart” and designed a lens with an opening just big enough to cover the size of the sensor…thus the DX lens was born.
Lens Focal Lengths, cont. The only problem is if you try to use a DX lens on a full sized sensor. You literally get a circle from the heavy vignetting of the lens on to your image.
FOV Crop – A benefit? This resulting 1.5x crop factor is perceived by most as a benefit…if you are wanting to shoot LONG…and a disadvantage if you are wanting to shoot wide. 70-200mm lens has an effective FOV as a 105-300mm. 300mm is good for sports and wildlife. 18-55mm is actually a 27mm- 82mm…so not really a wide angle lens. 10-20mm lens is actually a 15-30.
Camera Companies Release FX In the last couple of years, both Nikon and Canon have released professional level full sized sensors. (ok…Canon was first). But the consumer cameras have YET to see a full size camera resulting in even more confusion to the focal length Field of Fiew Crop. Some lens are designed only for DX sized sensors…other lenses will work on both. The smaller sized lens is good for portability…but bad if you ever upgrade to a different camera.
Not all cameras have the same FOV Crop Factor Crop factor simply means you take the stated focal length and multiply it times the factor number. I.e. FL x 1.5 = effective FOV. Nikon has been consistent with all of it’s APC sized bodies with a crop factor of 1.5 Other camera manufactures have produced: 1.7, 1.6, 1.3, etc.
Popular Nikon DX lensesvs. FX lenses 18-200= 27-300 18-55= 27-82 12-24= 18-36 28-105 24-120 24-70 14-24 300 The above lenses will not work on a FX body. If the above lenses are used on a DX body, then the FL has to be multiplied by 1.5 * Note: There are other lens manufactures with even more focal length…and some of those are designed to work with both FX and DX…or just DX. Generally, DX lenses are smaller and less expensive.