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Old Oct 20, 2007, 12:26 PM   #1
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At the 35mm-marked zoom position of a Panasonic FZ30 digicam, in AF MACRO focus mode the specified minimum Auto Focus distance is 5 cm (from the lens front surface?) For the usual (3:2) 35mm format equivalence, the angle of view should be 63.49 deg. Therefore, at 5 cm subject distance the field diameter (i.e., field diagonal) should be only 6.19 cm. With closer near focus, A 4.33 cm diagonal 35 mm slide could be duped, near full frame, simply by direct photography.

To confirm focus at the 5 cm distance, an image of a contrasty grid was taken at the closest focusable distance (about 5 cm). The captured image actually had a field diameter of about 9.6 cm. That corresponds to a field angle of 168 deg --- a really wide angle!

Auto Focus of this FZ30 at 5 cm was confirmed. However, the field was not that of a 35mm focal length as marked. Simplistic understanding of optical principles and camera terminology do not suggest that camera lens focal length markings apply only at "infinity" subject distance nor that at closer distances effective focal lengths are much shorter than marked. Fields are larger. The effect on effective focal length is greater at close range than at the tele end.
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Old Oct 20, 2007, 2:35 PM   #2
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Without going through the math, it seems as if you are using the front of the lens as the focal point when calculating angles. In order to get the correct angle of view, you need to use the sensor as the focal point, even if the focal distance is specified from the front of the lens.

Don't expect exact measurements, though. The 35mm equivalents are not meant to be exact, just a way to compare approximate fields of view.



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Old Oct 20, 2007, 9:04 PM   #3
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Your calculations are based on prime lenses and theFZ18 lens is not a prime lens. Prime macro lens move to macro by simply moving the optical elements further away from the sensor. Zoom lenses move into close focusing mode (which may or may not be "macro") by shifting the relationship of the optical elements which is how they change focal lengths (zoom) in any case. It's not surprising that any mark focal length has no relationship to actual focal length when in close-up mode.
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Old Oct 20, 2007, 10:58 PM   #4
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Daedalus3 wrote:
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Simplistic understanding of optical principles and camera terminology do not suggest that camera lens focal length markings apply only at "infinity" subject distance nor that at closer distances effective focal lengths are much shorter than marked.
Actually, it's standard practice to assume infinity focus distance when specifying focal length in lens specs.

Even when you're not talking about macro modes (with other nuances that impact them), a number of modern internal focus lens designs have much shorter effective focal lengths (they have a wider angle of view) at closer focus distances.

I see posts from time to time from people buying some of the new "ultra zoom" type lenses for their dSLR models, then complaining that their new zoom at it's 200mm setting is only 155mm when they compare the angle of view to a different lens on the same camera (and I'm talking subjects many feet away, not macros).

That's because their internal focus lens design changes effective focal length with focus distance. Some examples of lenses like that would include the newer Tamron 28-300mm and Sigma 18-200mm lenses (and there are more that behave the same way).
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Old Oct 22, 2007, 11:32 AM   #5
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Thanks, JimC.
That was precisely the point of my posting. The obscure subtleties of camera and lens terminology are not recognized by most buyers and users. Buyers are often disappointed that purchased equipment does not perform as expected from the specifications even though it is behaving normally. Therefore, the conventional concise, but simplistic, specifications are misleading to those unaware of the details.
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