Traditional 35mm film cameras use 36mm x 24mm sized film ("full frame", Nikon FX or Canon EF). Most DSLRs use smaller (roughly 24mm x 16mm) "cropped" image sensors (Nikon DX or Canon EF-S).
Image Size
Cropped DSLRs can use full frame lenses. However, because the full frame lens needs to cover the larger full frame image size (wasted on a cropped DSLR), it will have one or more disadvantages when used on a cropped DSLR:
- more expensive
- larger and heavier
- less zoom range
- less resolving power per length (sharpness or lines per mm, total resolution on full frame DSLRs is similar due to larger image size)
Normal Lenses and Focal Length Sweet Spot
The easiest lenses to make (cheap, high performance), have a focal length equal to the diagonal length of the image sensor. These f1.8, f1.4 large aperture lenses are called "standard" or "normal" lenses.
- 45mm for full frame (most manufacturers use 50mm instead)
- 30 mm for cropped DSLR (some manufacturers use 35mm)
The more the focal length moves away from this "sweet spot", the more difficult it is to make a large aperture lens. This is why extreme wide angles and extreme telephoto lenses are expensive and have smaller apertures.
Full Frame Wide Angle Lenses on Cropped DSLR
A 18mm lens on a cropped DSLR will look like a 27mm lens on a full frame DSLR (Nikon 1.5x or Canon 1.6x crop factor).
Using a 18mm full frame lens on a cropped DSLR doesn't make sense.
- A 18mm full frame lens is 45 - 18 = 27mm away from the 45mm sweet spot.
- A 18mm cropped lens = 18 x 1.5 = 27mm full frame. This is 45 - 27 = 18mm away from the 45mm sweet spot, much easier to make.
So, image size and sweet spot issues mean that full frame wide angle lenses are not suitable for cropped DSLRs.
Nikon f2.8 Wide Zoom Lens Comparison
Wide angle professional level f2.8 lenses for Nikon are
- Nikon f2.8 24-70mm FX, not wide enough for DX cameras.
- Nikon f2.8 17-55mm DX, ideal for DX cameras.
- Nikon f2.8 17-35mm FX, DX cameras are better off with the more tele, lighter and cheaper DX lens above.
- Nikon f2.8 14-24mm FX, less than 2x zoom range, super wide angle on FX cameras. DX cameras are better off with the wider, lighter, cheaper and 2x zoom Nikon f4 12-24mm DX, only one aperture stop darker.
Canon f2.8 Wide Zoom Lens Comparison
Wide angle professional level f2.8 lenses for Canon are
- Canon f2.8 24-70mm EF, not wide enough for EF-S cameras.
- Canon f2.8 17-55mm EF-S, ideal for EF-S cameras.
- Canon f2.8 16-35mm EF, slightly wider than the 17-55mm, roughly 2x zoom range. EF-S cameras are better off with the 3x zoom 17-55mm f2.8.
Telephoto Zoom on Cropped DSLR
Cropped DSLRs are similarly better suited to cropped telephoto lenses. Professional level f2.8 telephoto zooms (like the Nikon and Canon 70-200mm f2.8) are not yet available in cropped format, but smaller aperture cropped telephoto zooms are.
Large Aperture Prime Lens on Cropped DSLR
Large aperture prime lenses (f1.2, f1.8, f1.4, f2) are also mainly available only in full frame.
Nikon does have a 35mm f1.8 DX, and Sigma makes a 30mm f1.4 DX for Nikon (and an equivalent EF-S for Canon).
Upgrading to Full Frame DSLR
Some cropped DSLR photographers buy full frame lenses whenever possible, hedging against the future when they might upgrade to a full frame DSLR.
Especially for wide angle lenses, they would be better served with the wider, lighter, cheaper, more zoom range cropped lenses; selling them when upgrading to a full frame DSLR.
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