What is the magnification value of a camera lens?
When you buy binoculars, you make your purchase decision partly based on their magnification - such as 7x or 10x. How does this equate with camera lenses?
When you buy a consumer digicam or a video camera, you usually see the included zoom lens advertised as 5x, 10x ... But wait - this is not the same value. This ?x value is the zoom range factor.
For example, the Canon EF-S 17-85mm IS USM Lens has a 5x zoom range. Divide 85mm by 17mm to get the zoom range of 5x. The Canon EF 28-300mm IS Lens has a zoom range of 10.7x.
To get the binocular-like viewfinder magnification value of a lens, divide the focal length by 50.
A formula that is easier (for me at least) to calculate in my head is to divide the focal length by 100 and multiply by 2. Just move the decimal point two places to the left and multiply by 2.
Here is an example: For a 300mm lens, divide 300 by 50 to get 6x magnification.
Or divide 300 by 100 to get 3, then multiply 3 x 2 to get 6x.
Another example: For a 100mm lens, divide 100mm by 100 to get 1. Multiply 1 x 2 to get 2x magnification.
Keep in mind that viewfinder magnification varies across Canon's EOS line (for example: .72x in the Canon EOS 1D Mark II Digital SLR, .7x in the Canon EOS 1Ds Mark II Digital SLR, .9x in the Canon EOS 20D Digital SLR and .8x in the Canon EOS 350D Digital Rebel XT SLR/ Canon EOS 300D Digital Rebel SLR) and must be taken into account as the subject size in the viewfinder is affected. The viewfinder magnification does not affect the final image (the FOVCF of course does affect the final image framing).
Please note: This is just a simple, approximate subject magnification amount in the viewfinder.
I am not referring to the size of the subject relative to the size of the subject on the sensor. This is generally reported as the lens magnification spec and is often reported as .14x, .29x ... It is also reported as 1:4 (final image size on the sensor is 1/4 as large as the subject), 1:1 (subject is life-size on the sensor) ...
I am also not taking about the pixel density of the sensor. A higher pixel density sensor will (all other qualities being equal) result in more detail available for enlarging (or viewing on a monitor) a specific subject in part of the photo.
I am also not referring to output (display, printer, ...) size or density. Any picture can be printed or displayed at any size. For example, a 1:1 life-size macro picture will appear far larger than life-size when displayed on a 19" LCD monitor at 100% pixel size due to the relatively large size of LCD pixels. Of course, the amount and quality of pixels available for printing/viewing make a big difference in the results.