Film-Tech Forum ARCHIVE: f/number, focal length, throw, image size ?

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Posted by Iben Jimenez (Member # 3713) on 04-24-2006, 06:44 PM:

I just want a simple explanation of these terms: f/number, focal length, throw, image size. Why the short focal length lens can provide a bigger image?

Thanks

Posted by Larry Myers (Member # 738) on 04-24-2006, 07:12 PM:

1. f-stop= ratio of lens diameter to focal length
2. Focal Length= distance from the lens to the film when focused at infinity
3. Throw= distance from the film to the screen.
4. Image Size= magnification factor times the film size.

Posted by Dave Macaulay (Member # 813) on 04-25-2006, 12:19 AM:

The focal length is just the distance a pinhole "lens" would be from the film to give the same image size.
The image size is bigger with a shorter focal length because of simple geometry: as the pinhole gets closer to the film, the approach and exit angles get larger.
There isn't any way to find the focal length of a real projection (or camera) lens except by reading the markings or doing a projection test, the "pinhole" point is inside the lens's optical system somewhere and each lens design locates it differently.

F-number is the ratio of focal length to entrance pupil (f-number = focal length / pupil diameter). F-number describes the amount of light a lens lets through, basically. Exactly what the entrance pupil size is is tough to explain. It would exactly equal the diameter of a pinhole (since larger pinholes give poorer focus, a pinhole makes a bad real-world lens with either a really dark or really blurry image) but with glass lenses things get complicated. Generally the diameter of the rearmost element gives a good approximation of the entrance pupil.
Throw, focal length, and image size are proportional. You can calculate one given the other two. If you make a scale drawing you can even measure the focal length; in plan, draw a line from the right screen image edge to the left film image edge and another line connecting the opposite sides. Where these lines cross is where you'd place a pinhole lens, the distance from the film plane to the crossing is the focal length.

Posted by Iben Jimenez (Member # 3713) on 04-25-2006, 08:26 AM:

Thanks Dave.
But, the pinhole principle applies to projection lenses as well?

Posted by Larry Myers (Member # 738) on 04-25-2006, 09:35 AM:

The best way to tell exactly what focal length you have is to photograph stars. Star tables will tell you how far one star is from another. Measure the distance between two known stars on the film and do the calculation. My 50mm Nikon lens cames out to 50.6257mm.

Pinhole images are very interesting. The pinhole size will be the resolution of the image. Good pinholes are not cheap and could run 1000's of dollars for a low micron a high quality gold scientific pinhole. In an example, a $400 dollar 500 micron pinhole (1/2 millmeter or 1/2 millmeter resolution) will give a 2 line per millmeter image at 16 inches of focal length at f-812 with 660mm of depth of focus. It is best to use a 16 inch round drum as a camera with 16x20 inch reversal color paper. Exposure will be about 2 minutes in noon summer sun. The 16x20 image will look very good when viewed at about 3 feet away.

[ 04-25-2006, 01:31 PM: Message edited by: Larry Myers ]

Posted by Dave Macaulay (Member # 813) on 04-25-2006, 09:41 PM:

Yes, a lens is a lens, projection lenses do exactly the same job as camera lenses. They are obviously built differently because they do very different jobs; for example projection lenses don't have f-stop irises and camera lenses aren't made to take the heat of projection lamps.
A pinhole makes an interesting camera lens if you have a motionless subject - as Larry says exposure time gets very long with a pinhole size that gives a decent image focus. For projection, pinhole lenses are useless... but the principle applies in theory.

Posted by Michael Schaffer (Member # 1204) on 04-26-2006, 04:51 AM:

quote: Dave Macaulay
projection lenses don't have f-stop irises

Schneider make some.

quote: Dave Macaulay
Exactly what the entrance pupil size is is tough to explain. It would exactly equal the diameter of a pinhole (since larger pinholes give poorer focus, a pinhole makes a bad real-world lens with either a really dark or really blurry image) but with glass lenses things get complicated.

Could you elaborate on that some more?

Posted by Iben Jimenez (Member # 3713) on 04-26-2006, 08:24 AM:

Well, I understand that the focal length in a photographic camera is the distance between the intersection of the rays of light to the film plane or photographic film. For a single lens, the focal point would be the center of the lens itself and the geometry is like an "x" with the lens in the center or intersection and the two images at the sides. For a multiple lens system, the focal point may be in some part between the lenses. But my question in terms of the projection lenses is: What are the two points of reference of the focal length? The distance from the film to the focal point or the distance from the back of the lens to the focal point?
Thanks
IJ

Posted by Larry Myers (Member # 738) on 04-26-2006, 08:54 AM:

I believe the throw distance is from the film to the screen. If your back 50 ft and you measure from the lens to the screen, this is not a big factor as your just 2% or about 1 foot off at 50 feet. At 10 ft throw, this can be a big factor as you are now 10% off.