lens chart? coverage???

[sammy_5100]

hi,

the lens chart here: http://largeformatphotography.info/lenses/LF4x5in.html

says the Super-Angulon XL 47 has 166mm @ f/22 and 11.25 deg of land tilt,
yet the Super-Angulon XL 58 also has 166mm of coverage and only 9.16 deg of tilt.

how can this be?

sammy

[Glenn Kroeger]

These numbers agree with the latest Schneider brochures... the 47 covers 98 degrees while the 58 covers only 96 degrees

[sammy_5100]

yeah, but how can they have the same coverage (166mm) yet the 47 has more tilt?

[Dave Moeller]

Simple geometry. The longer the focal length of the lens, the more extreme the effect of tilt on the movement of the circle of coverage. Here's another example:

The Grandagon-N f/6.8 200mm has a circle of 495mm, and a maximum front landscape tilt of 43.85.
The Fujinon CM-W f/8 450mm has a circle of 520mm, but only has a maximum tilt of 24.49.

If you visualize this in your mind, it's pretty easy to pick up the fact that the longer the focal length, the more the image circle moves with each degree of tilt. So the 58mm, being longer than the 47mm, will not be able to tilt as much even though they have the same coverage circle.

[sammy_5100]

thanx so much.

how can landscape tilt be different than portrait tilt?

[Todd Schoenbaum]

If you imagine projeting a coverage circle onto the negative plane and then superimposing a rectangle within it there would be extra room on the top and the bottom, and to the left and the right. In the landscape view (i.e. with the negative orriented horizontally) there would be more excess on the top and bottom than there would be if the negative were oriented vertically (portrait). Also, the amount of potential landscape tilt would be the same as the potential portrait swing.

[Mark Sawyer]

I've seen pinball machines with various themes (Space Captain, the Addams Family...) but I've never seen a Large Format pinball machine. They should make one, just so we can set off the "tilt."

[Julian Boulter]

Guys,

I have a few more questions on the chart:

1. Why is the diagonal of 4x5 measured at 153.7mm ? I measured the diagonal of a piece of 4x5 film and it came out as 161mm. Does the smaller figure take into account the available area after insertion into a film holder?

2. Why are there no figures for rear tilt? is this because rear tilt will always remain within the image circle? is that true for both rear base tilt and rear axis tilt?

3. Why are the Nikon SWs measured @ F16? does this imply the optimum aperture for these lenses? does the image circle get larger or smaller as the aperture closes?

4. How is it possible that 2 lenses of the same focal length and identical image circle can have different angles of coverage e.g. 135mm Rodenstock APO-Sironar-N and 135mm Nikkor W? is it to do with the position of the lens center?

5. how do we derive tilt degress from image circle?

That will do for now ;-)

Thanks

[Ernest Purdum]

Julian,



1. You'll see several figures for diagonals depending on to what extent the value is rounded off as well as how much allowance (if any) is made for borders.



2. Visualize the cone of light coming from the lens. When the front frame tilts, the cone moves. If, however, the back frame is tilted, it has no effect on the cone of light.



3. To provide a standard of comparison, the maker has to choose some aperture to use in stating the coverage. This does not imply the optimum aperture, which is probably larger than f16. The image circle gets larger as the size of the diaphragm is reduced.



4. There is indeed a contradiction here. It could have something to do with actual versus nominal focal length, I suppose. (I haven't looked at the data.)



5. For a non-mathematical type like myself, the easiest way to do it would be to draw it out on paper. Notice that the results will differ when the back is rotated from horizontal to vertical position.