[ptx] panosifter problems & questions

[Terje Mathisen]

Pablo d'Angelo wrote:

> On Thu, 12 Feb 2004, Terje Mathisen wrote:
>>Instead of depending upon having both real and effective focal length in 
>>EXIF (or real + crop factor), otherwise require an override parameter on 
>>the command line, a useful fallback would have been to combine the focal 
>>length field with a command line supplied crop factor:
>>
>> -crop 4.8
>>
>>would turn my Olympus D40Z at 7.5 mm into a 36 mm equivalent lens, with 
>>HFOV (in portrait mode) of just under 40 degrees.
> 
> Actually, I'm not completely sure what the crop factor really is.
> the ratio of the diagonals? Or the ratio of width, or height? Have seem
> different interpretations on the web.

You're right, I've even suspected PTAsm of getting this wrong for 
cameras which don't have a 3:2 aspect ratio, i.e. almost all digicams 
which have a 4:3 TV style chip.

IMHO, the proper way would be to calculate all crop factors relative to 
a film/chip plane diagonal, since this diagonal determines the size of 
the optics, right?

So a 24x36 lens has a 43.27 mm diagonal, while a "standard" small 
digicam uses a 1/1.8" chip which should have an 8.88 mm diagonal:

43.27/8.88 = 4.873

Using Google, I find various sizes given for chips that should all be 
1/1.8", but 7.1x5.3 mm seems typical, this corresponds well to the 
7.104x5.328 which I get when starting with an 8.88 mm diagonal.

If I take the ratios of the long sides (36/7.1) I get a crop factor of 
5.07, while the short sides ratio is 4.53, so it seems like PTAsm does 
use the diagonal to calculate this factor.

On the gripping hand, you still have to take the aspect ratio into 
consideration when calculating HFOV and/or VFOV!

Terje
-- 
- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"