My Panasonic FZ70 zooms to 60x. The only focal length indicators on the lens are the minimum (20mm) and the maximum (1200mm). There is no indication of focal length as I zoom in or out; the viewfinder only displays a number for the magnification as the magnification is changed(e.g. 7X). For astrophotography I would like to apply the rule of 500 which requires a focal length in the calc. Is it a good assumption that the focal length would simply be the % of the numerical distance from the minimum (20mm) as indicated my the zoom multiplier (e.g. 1 to 60). Thanks, M
The Panasonic FZ70 has a focal length of 3.58-215mm (20 - 1200mm in 35mm equiv.)
This is right from the Panasonic website for your camera.
The actual “500 Rule” formula was written for 35mm “Full Frame” cameras. If you don’t have a 35mm camera, you would need to include the crop factor in the formula.
500 / (Crop-Factor x Focal Length) = Ideal Shutter Speed
The FZ70 has a crop factor of about 5.58 so the formula should be:
500 / (5.58 x 3.58) = Ideal Shutter Speed
500 / 19.97 = 25 seconds
If you were to use the 35mm equivalent focal length of 20mm you would still get 25 seconds.
500 / (1 x 20) = 25 seconds
20mm is the wide view. A 2x zoom lens would zoom to 40mm. A 3x zoom lens would zoom to 60mm. A 10x zoom lens would zoom to 200mm. And a 60x zoom lens would zoom to 1200mm. That's basic multiplication of course, and that's all there is to it. Just multiply the current zoom factor by the starting point (20mm) to get the current (equivalent) focal length.
This isn't going to be exact of course, because when the lens is zoomed to any focal length from, for example, 590mm to 609mm, then by virtue of rounding, these will all show 30x in the display.