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

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    \$\begingroup\$ I'm not familiar with this camera, but looking online it seems to support EXIF v2.3. That data should have an entry for "Focal Length". Does your camera populate the EXIF data with an actual focal length? \$\endgroup\$
    – Peter M
    Jul 25, 2023 at 18:01
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    \$\begingroup\$ @Romeo Ninov So he' have to make some test shots, at different displayed magnifications, then check the EXIF for each. \$\endgroup\$
    – Laurence
    Jul 25, 2023 at 19:17
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    \$\begingroup\$ I guess the idea is that the test shots are done from the comfort of home, before setting out on any astronomical photographic journeys. \$\endgroup\$
    – osullic
    Jul 25, 2023 at 19:44
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    \$\begingroup\$ @RomeoNinov The OP isn't so far away that he can't post questions on here /s \$\endgroup\$
    – Peter M
    Jul 25, 2023 at 19:53
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    \$\begingroup\$ @Romeo Ninov Who said he's in the field? Anywhere, wherever he is, he's got the camera and some device capable of posting to this forum. Perhaps it can also cope with a picture file? :-) \$\endgroup\$
    – Laurence
    Jul 25, 2023 at 20:29

2 Answers 2


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.

LUMIX® Digital Camera DMC-FZ7 enter image description here

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

  • \$\begingroup\$ Wow! Thanks Osullic and Mike. When i found this site I knew I was in the right place. Great detail - very helpful. Crop factor was very useful info. Other camera is a MFT. Guessing the 300 rule would be better for MFT. I'm an old (old) photography enthusiast trying out a new hobby. It seems there will be lots of trial and error. The EXIF conversation was also helpful. I didnt think of it but by taking pics at each magnification and inspect the EXIF, I can map the magnification to the focal length, duh!!!! \$\endgroup\$
    – mbuk2
    Jul 26, 2023 at 20:01

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.


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