50mm "Normal" Lens... Ok, but "Normal" on FF or APS-C Sensor?

Started Nov 10, 2012 | Discussions thread
Barrie Davis
Barrie Davis Forum Pro • Posts: 21,460
Re: Not the 50mm myth again. That one is tough to kill!

tkbslc wrote:


How can viewing size and position change the relative size of objects in a two-dimensional photo?

It was I that already went into that very point in my first posting.

Changing viewing distance (relative to nominal 'normal') doesn't change any image elements printed into the image, of course. But different viewing distances to the nominal normal for the taking lens f-length DOES change whether the shot appears of normal perspective ´╗┐or not at the distance viewed.

So, the perceived  perspective in ANY print will appear to be normal if the viewing distance is adjusted to equal the focal length of the lens shot with, multiplied by the degree of magnification in the image in question.

Example: Using a 2" lens (50mm) on 1" inch high film (35mm) and make a print 8" high (8x mag)

2 x 8 = 16. Therefore for normal perspective, view from 16."

Obviously, to get normal perspective from a 100mm lens, (would presumably have been shot from somewhat further away) the calculation is..

4 x 8 = 32. View from 32 inches..

.... and perspective will appear normal. But view from the original 16 inches, and perspective will show the typical compression we associate with longer lenses.

Conclusion: It is the DISTANCE when viewing the final image that creates the sense of perspective...

..... just as it is when shooting in the first place.

I'll let you work out the neutral perspective viewing distance for a 1" taking lens (25mm). Hint: nearer than 16".

As I wrote in my first post to this thread...

  • Viewing the picture from CLOSER (than nominal "normal") creates the compressed perspective associated with having shot with a telephoto lens.
  • Viewing the picture from FURTHER (than nominal "normal") creates the expanded perspective associated with having shot with a wide-angle lens.

Make sense now?

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Regards,
Baz
"Ahh... But the thing is, these guys were no ORDINARY time travellers!"

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