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Author Topic: Standard lens size equivalents on superzooms  (Read 748 times)
Alvysinger
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« on: September 04, 2013, 06:36:35 PM »
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Wondering if anyone has any advice. I just bought the Lumix FZ70 (not my primary... I have the Sony RX1 for 'proper' photos). For my work it's necessary for me to have a long zoom range (the camera seems amazing, by the way). BUT it would be incredibly helpful to know what the 35mm equivalent is at every stage of the enormous zoom. Right now it just has a zoom bar with 'x3...x4... Etc. But I need to know if I'm looking at a 75mm or a 200mm, as precisely as possible. My Sony RX100 has this information. Is there any trick to this? Or an easy way to decode what the exact length of the zoom is?

Thanks - hope this is clear to someone smarter than I.
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BartvanderWolf
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« Reply #1 on: September 05, 2013, 05:39:21 AM »
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Is there any trick to this? Or an easy way to decode what the exact length of the zoom is?

Hi,

The exact focal length is what it is. The relative focal length, 35mm equivalent, is scaled by the difference in sensor size. The sensor of the FZ70 measures 6.17 x 4.55 mm, the 35mm size is 36 x 24 mm, so approx. 5.83x in horizontal direction, and 5.27x in the vertical direction, or 7.67 mm diagonal versus 43.27mm IOW 5.64x in the diagonal dimension.

The lens is marked as having a real focal length of 3.58 - 215mm (and the FL used is also recorded in the file's EXIF data), so multiplied by an average of 5.64x that would be 20 - 1213 mm equivalent to 35mm focal lengths.

Cheers,
Bart
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Telecaster
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« Reply #2 on: September 05, 2013, 02:45:33 PM »
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Just out of curiousity, what benefit does knowing the "35mm equivalent" focal length give you? Are you doing work that requires using specific fields-of-view? If so you should be able to determine FOV in degrees, and then set your camera's zoom accordingly, without needing to refer to an image format you're not using.

-Dave-
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Alvysinger
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« Reply #3 on: September 11, 2013, 12:45:35 AM »
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Thanks... Yes -- I'm a director and I bought the camera specifically for location scouts, where its useful to approximate the field of view of as many different lens sizes as possible.

The trick is to take pictures at different focal lengths (at which the camera is a revelation) and then label the pictures at the correct 35mm focal lengths so the director of photography can prep that correct lens sizes in his lens package.

The lumix is an amazing tool for this. Previously I've been using my 5d2 with a whole suite of lenses, but image quality is less important than lens size information for me, so if I can figure out a way to calculate tha information from the lumix then it will be quite literally life changing.

How would I do this with degrees?

Thanks for your help - I really appreciate it.

Michael
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BartvanderWolf
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« Reply #4 on: September 11, 2013, 09:35:06 AM »
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How would I do this with degrees?

Hi Michael,

Depends on how exact you want to calculate it.

At infinity focus, it's simply:
2 * arctan( X / (2 * f) )
with 'X' being the horizontal, vertical, or diagonal, sensor size, and 'f' being the true focal length (as recorded in the EXIF metadata). The sensor of the FZ70 is supposed to be 6.17 x 4.55 mm.

For closer distances you also need to calculate the Magnification factor, and the formula becomes:
2 * arctan( X / ( 2 * f * (M + 1) ) )

The magnification factor is, when approximately measured from the front of the lens (exact position depends on lens design):
M = f / (So - f)
where 'So' is the object distance, and 'f' the true focal length.

You can also plug the various dimensions in my DOF output quality planner as introduced here.

Do note that the cameras that you mentioned, have slightly different aspect ratios, so their field of view angles for a given relative focal length differ somewhat, depending on which dimension you calculate.

Cheers,
Bart
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