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Gail's DSLR dilemma Blog
DSLR focal length multiplier
Understanding focal length multipliers seems downright confusing. It doesn't have to be. It's important to get the gist of it before buying interchangeable lenses for a digital single lens reflex camera.
The focal lengths of compact digicam lenses need to be explained in 35mm equivalent terms. My Nikon 5400 compact camera, when zoomed to 14.5mm. is the 35mm equivalent of a 50mm lens. But a standard 50mm lens on DSLR with a focal length multipier (FLM) effectively becomes mid-telephoto.
Why?
Sensors used the in the majority of DSLRs have a smaller area than the surface of 35mm film. As a result, each camera’s effective focal length will be different than the one listed on a lens.
Calculating effective focal length
To calculate the effective focal length of a lens, the focal length multiplier of the camera must be used. The three most common multipliers are 1.5, 1.6 and 2.0. Canon’s entry level camera's multiplier is usually 1.6. Nikon, Sony, and Pentax usually have a multiplier of 1.5. Olympus and Panasonic have a multiplier of 2.
A 50mm interchangeable lens on a DSLR with a 1.5 multiplier would have an effective focal length of 75mm. On a DSLR with a 1.6 multiplier the effective focal length is 80mm; it's 100mm on a camera with a multiplier of two.
Full frame DSLRs do not have multipliers or, more correctly, they have a multiplier of one. This is because their sensor format has the same area as 35mm film.
Implications of focal length multiplier
Wide angle shots
If you take a lot of wide angle shots, lenses with a 28mm focal length and the like will no longer suffice. When a standard wide angle lens is attached to a DSLR with a FLM, it becomes a mid-range focal length. You'll need a super-wide angle lens (eg. 12mm, 10mm).
Conversely, you may not need as long a telephoto lens as you think. Do the math before buying.
Shutter speed, camera shake and FLM
To help prevent blurred images when holding a camera, shutter speeds generally need to be faster than the focal length. For example, a lens on a digital camera set at a 35mm equivalent of 50mm requires a shutter speed of at least 1/60 second to avoid blurry images. However, this rule needs to be adjusted when using a DSLR with a multiplier. A 50mm lens on a camera with a 1.5 multiplier requires a shutter speed of 1/80.
Looking through the viewfinder
What you see through the viewfinder may be slightly smaller than the actual image saved onto the memory card. The actual percentage, usually more than 90 percent, is listed in the specifications found in camera manuals.
