Simple Algorithm for Calculating Exposure Increase when Using *Front Rise*?

[Andre Noble]

What's your method for taking into consideration the extra exposure necessary when you use front rise - ie when using a wide angle lens on a tall achitectural subject - for example?

[vinny]

none.
a center filter would be helpful depending on the lens of course.

[Andre Noble]

Aren't you effectively extending the lens-to-film-plane distance?..

Using a Schneider 72XL, with 30mm front rise for example.

[Sevo]

Aren't you effectively extending the lens-to-film-plane distance?..


No, the distance is plane-to-plane and these are still parallel. Lens vignetting can be an issue - but this is design dependent and gradual, so that you need a matching gradual ND filter to compensate it, a plain compensation factor can won't do.

[Andre Noble]

You are right. The real light fall from extensive rise relates to lens optics and varies from center of lens axis (none) to edge of image circle. I'm sure Michael Briggs knows a little more. Thanks for the inputs.

[Chuck P.]

"Aren't you effectively extending the lens-to-film-plane distance?.."


No, the film and lens planes are still parallel........

Yes they are, but not sure I get this, the lens plane can be extended several inches forward of infinity and still be parallel to the film plane, requiring an exposure adjustment for bellows extension.

[Doremus Scudder]

"Aren't you effectively extending the lens-to-film-plane distance?.."

Yes they are, but not sure I get this, the lens plane can be extended several inches forward of infinity and still be parallel to the film plane, requiring an exposure adjustment for bellows extension.

Think of it this way: The lens projects a large image circle. When you use front rise, you are just raising the projected image circle along with the lens. Sure, the edges of the image circle are farther from the lens than the center, but this is only an issue with shorter focal length lenses. Manufacturers make center filters for these lenses to compensate for exactly this (and for other falloff inherent in the lens design). For most lenses, the distance and associated falloff are minimal.

Even with substantial front rise, the center of the lens is usually perpendicular to some point on the film. This is the point that gets the most exposure. You certainly don't need to use exposure compensation for this point. Other points get proportionally less exposure in relation to the increasing distance from the lens center. As mentioned, however, unless the lens is very short or has a lot of falloff, the effect is negligible.

When using a very short focal length lens and lots of rise, you will get darker top corners/top due to this effect, which you can correct with the appropriate center filter (which you would use for any shot with this particular lens where corner illumination was critical) or correct at the printing stage with dodging/burning.

When shooting with lots of bellows extension, you are moving the center of the lens so far away from the film that the decrease of light intensity from distance alone requires compensation. Note, however, that the relative difference in the distance from the lens center to the center and corners of the film decreases; illumination is actually more even with increasing extension.

FWIW, I shoot black-and-white only and correct for most falloff when printing. I've developed what I call a "center-filter burn" using a hole in a dodging board and moving the board from close the print upward to the lens, increasing in speed as I move for 20-25% of the basic exposure time (depending on the lens). I note the optical center of the image when shooting so I can center on this while printing. I do give a bit more exposure if exposure in the corners is critical.

Oh yes, while we're on the subject: you likely introduce much more illumination difference when using tilts/swings than when using rise or shift, since you are physically moving portions of the film farther from the lens (or the parallel plane), resulting in one part of the film receiving much less light. I don't know anyone who compensates for this, although I've had occasion to use burning/dodging to compensate.

Best,

Doremus

[ic-racer]

Depending on how much falloff you have with your particular lens (that you know through experience) you can add some exposure to your base exposure so that you have adequate shadow detail at the far edge of the image circle. Then burn in the center of the image circle when printing.

[evan clarke]

Don't overthink exposure.. There are two kinds, more and less..get the feel for your combo.

[al olson]

Think of it this way: The lens projects a large image circle. When you use front rise, you are just raising the projected image circle along with the lens. Sure, the edges of the image circle are farther from the lens than the center, but this is only an issue with shorter focal length lenses. Manufacturers make center filters for these lenses to compensate for exactly this (and for other falloff inherent in the lens design). For most lenses, the distance and associated falloff are minimal.

Even with substantial front rise, the center of the lens is usually perpendicular to some point on the film. This is the point that gets the most exposure. You certainly don't need to use exposure compensation for this point. Other points get proportionally less exposure in relation to the increasing distance from the lens center. As mentioned, however, unless the lens is very short or has a lot of falloff, the effect is negligible.

When using a very short focal length lens and lots of rise, you will get darker top corners/top due to this effect, which you can correct with the appropriate center filter (which you would use for any shot with this particular lens where corner illumination was critical) or correct at the printing stage with dodging/burning.

When shooting with lots of bellows extension, you are moving the center of the lens so far away from the film that the decrease of light intensity from distance alone requires compensation. Note, however, that the relative difference in the distance from the lens center to the center and corners of the film decreases; illumination is actually more even with increasing extension.

FWIW, I shoot black-and-white only and correct for most falloff when printing. I've developed what I call a "center-filter burn" using a hole in a dodging board and moving the board from close the print upward to the lens, increasing in speed as I move for 20-25% of the basic exposure time (depending on the lens). I note the optical center of the image when shooting so I can center on this while printing. I do give a bit more exposure if exposure in the corners is critical.

Oh yes, while we're on the subject: you likely introduce much more illumination difference when using tilts/swings than when using rise or shift, since you are physically moving portions of the film farther from the lens (or the parallel plane), resulting in one part of the film receiving much less light. I don't know anyone who compensates for this, although I've had occasion to use burning/dodging to compensate.

Best,

Doremus

As I am sitting here in Florida, separated from my library of optics books, I have to say that this is what is referred to as the Cosine to the Fourth Law. I.e. the light falloff is proportional to (cos(alpha))**4 where alpha is the angle between the axis through the center of the lens and a line from the center of the lens to a point on the film plane. Hence the light dims as it gets farther from the lens axis.

So if you can determine your alpha you would be able to compute a good estimate of the light falloff. I have tried to understand this formula by geometric construction and can justify cosine cubed, but I never understood the reason for the fourth cosine factor.

[cosmicexplosion]

Sounds like a small problem.
But if its a sky that is bright and potentially over exposed by a poofteenth then a. Your film will Handel it and b. just under expose a bit. C. Best to reel of a few sheets in a test situation.
My one cent.

[Sevo]

As I am sitting here in Florida, separated from my library of optics books, I have to say that this is what is referred to as the Cosine to the Fourth Law.

For a meniscus-and-aperture lens or simple symmetric double achromats, yes. YMMV with more complex lenses, even more so with wide angle ones - the Russar, Super Angulon and later successors entirely break with that rule.