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10-30-2010, 04:00 PM   #16
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I am not saying this is useless. But what use is it?

Also, do you guys realize that what you see in Jpegs and RAWs is not the whole sensor??????
If you shoot DNG then use this too to expand the view to the whole captured data.

its called DNG recover Edge
http://www.luminous-landscape.com/legacy/recover_edges/
Convert to DNG and then use this if your file is not DNG already.


Last edited by Steelski; 10-30-2010 at 04:05 PM.
10-31-2010, 03:43 AM   #17
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QuoteOriginally posted by georgweb Quote
What is the real horizontal width of your imaging sensor ?
Georg,

most of the flaws of your approach have already been mentioned, like:
- why trust one vendor spec more than the other
- not applying the thin lens formula to correct for distance
- not even knowing deviations from the thin lens (distortion, float focus)
- ignoring the recoverable pixel area
- ignoring the masked pixel area (the outer pixels are black yielding a zero reference)

The 3-digit precise sensor sizes are given in the spec sheets and more often than not, they're not an exact 3:2. So, some verification may be interesting.

If you really want to know the exact (4-digit precise) figures, you have to do the following:

photograph the sensor with a mcro lens, mirror and shutter up in cleaning mode. I'm sorry to say, but you will have to drop an object of reference into the subject plane where the sensor sits. Either on the sensor itself, or after knowing the exact distance between lens and sensor, by photographing a reference chart at the exact same distance (4-digit precision requires to know a 50mm distance with 10Ám precision...).

Or you abandon this experiment and join the protests against Stuttgart 21
10-31-2010, 04:35 AM   #18
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What is the use of it? Satisfying ones curiosity is a worthy goal too, IMHO. Sometimes, it turns out that there are pratical applications, as with "pure" science (in general :-).
11-11-2010, 04:22 PM   #19
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Steelski, I don't have DNG with my camera .-) What would you think will be the difference in frame width from jpeg to raw to dng ? I would strongly suggest doing testing with jpegs but that's just me. It dawns on me that this is what Falk calls the recoverable pixel area. Had a look at my K100D and surprise, surprise, jpegs are 3008x2000px and PEF/Raw is 3040x2024. Hey that's 1% already.

Jolepp
, the use of it will be ultimately seen later. So far it seems useless because I have come up with a 2%, well no a 1% deviation which could well be within measuring tolerances of my 'setup'. I have fun with the curiosity part for sure.

Falk, maybe I should measure sensor widths right besides a Castor and see if there's any difference .-) Seriously though, I understand you are saying that with the sensor, what you see is what you get ? If that's so I could try to print out a strip of paper (or transparent foil) with a fine scale on it and put it on the sensor. I would print out a 20cm strip and crosscheck it with my existing measuring devices. If OK cut it to the sensor size and put it on the sensor. Then take a long macro and snap away. Good enough?

All, I still would very much welcome a growing list of real focal lenghts of lenses and I still have hope that this would be possible with just a few known lenses to calibrate one's camera and therefore be able to measure through all your lenses if you feel like it. Edit: What I am really looking for is the real horizontal FOV and that can be expressed (afaik + understood Falk) in a focal length of a thin lens set to infinity.


Last edited by georgweb; 11-11-2010 at 05:09 PM.
11-11-2010, 05:54 PM   #20
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QuoteOriginally posted by georgweb Quote
All, I still would very much welcome a growing list of real focal lenghts of lenses and I still have hope that this would be possible with just a few known lenses to calibrate one's camera and therefore be able to measure through all your lenses if you feel like it. Edit: What I am really looking for is the real horizontal FOV and that can be expressed (afaik + understood Falk) in a focal length of a thin lens set to infinity.
You don't really need to know the focal length to figure out the Angle of View of a lens.

Setup a large enough test target a known distance from a fixed camera position. Mount different lenses and use a bit of geometry/trigonometry to find the angle.
11-13-2010, 05:25 AM   #21
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QuoteOriginally posted by alohadave Quote
You don't really need to know the focal length to figure out the Angle of View of a lens.

Setup a large enough test target a known distance from a fixed camera position. Mount different lenses and use a bit of geometry/trigonometry to find the angle.
Exactly, Dave. I hope I have done that bit correctly in my excel sheet attached earlier. It is using the arcus tangens function to correctly give you horizontal viewing angles from a measured distance + width of an object.

Now, in order to make a qualified statement regarding either focal length (here: the theoretical FL of a thin lens at infinity) or the sensor width (here: the image-taking area in either JPEG or RAW or DNG mode) you need to know either of the two. Basically I only need the exact FL (in the a.-m. sense) of one lens and I'd be done. I'll try an M42 55/1.8 SMC Takumar next time and I am wondering whether a pinhole lens would be of any help. Of course everybody is welcome to do proper measurements and I propose the mentioned Takumar (and all its siblings) for this.

Last edited by georgweb; 11-13-2010 at 05:35 AM.
11-13-2010, 04:31 PM   #22
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QuoteOriginally posted by georgweb Quote
Basically I only need the exact FL (in the a.-m. sense) of one lens and I'd be done.
I don't know if the data is reliable (i.e., correct on an absolute scale). But I noticed that Adobe lens profiles contain focal length numbers. You can rename and open an Adobe lens profile with any text or XML editor. It contains focal length numbers for the different color channels (because CA makes the FL depend on the color).

I imagine that the relative ratio of Adobe lens profile focal lengths is precise. But their absolute values are not.
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