How do I convert the Panasonic FZ70's zoom factor to focal length for the 500 rule?
Asked 7/25/2023
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My Panasonic FZ70 is labeled 20–1200mm equivalent (60x zoom), but while zooming it only shows a zoom multiplier such as 7x rather than a focal length. For astrophotography, I want to use the 500 rule, which needs focal length. Can I estimate focal length by multiplying the zoom factor by the wide-end focal length, or should I use the camera's actual focal length and crop factor instead?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
2y ago
2 Answers
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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.
Originally by user38159. Source · Licensed CC BY-SA 4.0
user38159
2y ago
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Generated from our catalog & community — verify before relying on it.
Yes. On the FZ70, the displayed zoom factor can be converted by multiplying from the wide end.
- 1x = 20mm equivalent
- 7x ≈ 140mm equivalent
- 60x = 1200mm equivalent
So the equivalent focal length is approximately: zoom factor × 20mm
That said, the 500 rule is based on full-frame/35mm format. The FZ70’s actual lens is about 3.58–215mm, which Panasonic lists as 20–1200mm equivalent. For the 500 rule, you can either:
- use the actual focal length with crop factor, or
- use the 35mm-equivalent focal length directly
These give the same result.
Example at the wide end:
- actual focal length: 3.58mm
- crop factor: about 5.58
- 500 / (5.58 × 3.58) ≈ 25 seconds
This matches using the equivalent focal length:
- 500 / 20 ≈ 25 seconds
So for practical use on this camera, just use the 35mm-equivalent focal length derived from the zoom factor. Keep in mind the displayed zoom number is rounded, so intermediate values are only approximate.
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