Caltar II-N f4.5 75mm Lens - no light falloff?

BC wrote:
"Yes there are a few lenses like the famous Zeiss Biogon 75mm (a 3lbs
lens) which have little vignetting - not much more than the theoretical

light fall off which cannot be avoided."

Light falloff is not a theoretical inevitability. In fact, there are
wide angle lenses with zero rectilinear distortion which actually have
slightly *more* illumination in the corners than in the center of the
image. This happens fairly frequently with telecentric lenses having
no mechanical vignetting. The cos^4 "law" is often a meaningless rule
of thumb.

Brian
www.caldwellphotographic.com


I believe the Caltar is the same as my Rodenstock. I haven't checked
Rodenstock's website recently, but if I remember correctly, this
particular lens follows a cos^3 law rather than a cos^4 law, and that is
possible to achieve by lens design. But to say it has no fall-off is
nonsense.

BC wrote:
"Yes there are a few lenses like the famous Zeiss Biogon 75mm (a 3lbs
lens) which have little vignetting - not much more than the theoretical

light fall off which cannot be avoided."

Light falloff is not a theoretical inevitability. In fact, there are
wide angle lenses with zero rectilinear distortion which actually have
slightly *more* illumination in the corners than in the center of the
image. This happens fairly frequently with telecentric lenses having
no mechanical vignetting. The cos^4 "law" is often a meaningless rule
of thumb.

Brian
www.caldwellphotographic.com


I believe the Caltar is the same as my Rodenstock. I haven't checked
Rodenstock's website recently, but if I remember correctly, this
particular lens follows a cos^3 law rather than a cos^4 law, and that is
possible to achieve by lens design. But to say it has no fall-off is
nonsense.

"I believe the Caltar is the same as my Rodenstock. I haven't checked
Rodenstock's website recently, but if I remember correctly, this
particular lens follows a cos^3 law rather than a cos^4 law, and that
is
possible to achieve by lens design. But to say it has no fall-off is
nonsense."

You have misread my post. I never said that the Caltar lens in
question has no falloff. Inverse triplet type lenses of this sort do
in fact tend to have an approximately cos^3 falloff pattern as you
mention. My point was that falloff in general is not always
inevitable, and that it is possible to design and build a
distortion-free wide angle lens which has greater illumination in the
corners than in the center. I don't know what the ultimate limits are,
but I do know for certain that in practice an illumination "gain" on
the order of cos^(-0.1) is possible.

"I believe the Caltar is the same as my Rodenstock. I haven't checked
Rodenstock's website recently, but if I remember correctly, this
particular lens follows a cos^3 law rather than a cos^4 law, and that
is
possible to achieve by lens design. But to say it has no fall-off is
nonsense."

You have misread my post. I never said that the Caltar lens in
question has no falloff. Inverse triplet type lenses of this sort do
in fact tend to have an approximately cos^3 falloff pattern as you
mention. My point was that falloff in general is not always
inevitable, and that it is possible to design and build a
distortion-free wide angle lens which has greater illumination in the
corners than in the center. I don't know what the ultimate limits are,
but I do know for certain that in practice an illumination "gain" on
the order of cos^(-0.1) is possible.

It's just bull. The term "caveat emptor" was never so true as
on ebay. Many sellers are either uninformed or will lie outright
to get suckers to bid. Sometimes both.

Claim: "Does not require a center, neutral density filter... for light
fall-off"

It's just bull. The term "caveat emptor" was never so true as
on ebay. Many sellers are either uninformed or will lie outright
to get suckers to bid. Sometimes both.

Claim: "Does not require a center, neutral density filter... for light
fall-off"

Your lens undoubtedly has falloff as you describe. However, it is
possible to design a lens with identical field of view, relative
aperture, and distortion which has absolutely no falloff at all. There
is no "law" in optics stating that lenses must have illumination
falloff. Consider a perfectly telecentric lens which has no vignetting
and no pupil aberrations. Such a lens will have a perfectly flat
illumination curve.

Brian
www.caldwellphotographic.com

Your lens undoubtedly has falloff as you describe. However, it is
possible to design a lens with identical field of view, relative
aperture, and distortion which has absolutely no falloff at all. There
is no "law" in optics stating that lenses must have illumination
falloff. Consider a perfectly telecentric lens which has no vignetting
and no pupil aberrations. Such a lens will have a perfectly flat
illumination curve.

Brian
www.caldwellphotographic.com

I am not sure where you got this knowledge from.
If you take any lens, the projected profile of itself acting as apterure
changes with the lateral distance from the optical axis. Ad extremum, if
you consider a point in the infinity away from the optical axis, you do
see nothing more than a line with not opening, right?
Can you please email me at least one quote from an academic book on
optics, which states that the cos^4 law is a meaningless rule of thumb.
I would appreciate to receive it to understand what you mean.
If you feel that this conversation exceeds the framework of this forum,
you are most welcome to email me your response to my email address.
George
PS: The mechanical vignetting is a different pair of shoes....


BC wrote:
"Yes there are a few lenses like the famous Zeiss Biogon 75mm (a 3lbs
lens) which have little vignetting - not much more than the theoretical

light fall off which cannot be avoided."

Light falloff is not a theoretical inevitability. In fact, there are
wide angle lenses with zero rectilinear distortion which actually have
slightly *more* illumination in the corners than in the center of the
image. This happens fairly frequently with telecentric lenses having
no mechanical vignetting. The cos^4 "law" is often a meaningless rule
of thumb.

Brian
www.caldwellphotographic.com

I am not sure where you got this knowledge from.
If you take any lens, the projected profile of itself acting as apterure
changes with the lateral distance from the optical axis. Ad extremum, if
you consider a point in the infinity away from the optical axis, you do
see nothing more than a line with not opening, right?
Can you please email me at least one quote from an academic book on
optics, which states that the cos^4 law is a meaningless rule of thumb.
I would appreciate to receive it to understand what you mean.
If you feel that this conversation exceeds the framework of this forum,
you are most welcome to email me your response to my email address.
George
PS: The mechanical vignetting is a different pair of shoes....


BC wrote:
"Yes there are a few lenses like the famous Zeiss Biogon 75mm (a 3lbs
lens) which have little vignetting - not much more than the theoretical

light fall off which cannot be avoided."

Light falloff is not a theoretical inevitability. In fact, there are
wide angle lenses with zero rectilinear distortion which actually have
slightly *more* illumination in the corners than in the center of the
image. This happens fairly frequently with telecentric lenses having
no mechanical vignetting. The cos^4 "law" is often a meaningless rule
of thumb.

Brian
www.caldwellphotographic.com