90mm lenses and their need for a center filter.

[Navy Moose]

Hi Ron,

Thank you for clarifying the image circle for me. The math I've been seeing in my study of large format is pretty daunting, especially to math challenged people like me.

I'm familiar with the inverse square law. Are you talking about vignetting with the cosine ^ 3? I was digging through a reference and saw mentioned there is a calculation for that.

[Ron Marshall]

Hi Ron,

Thank you for clarifying the image circle for me. The math I've been seeing in my study of large format is pretty daunting, especially to math challenged people like me.

I'm familiar with the inverse square law. Are you talking about vignetting with the cosine ^ 3? I was digging through a reference and saw mentioned there is a calculation for that.

That's correct; cosine^3 or cosine^4.

The cosine is taken of the angle between a line on the axis of the lens (straight through) and the point on the image circle where you want to calculate the light intensity.

[Ben Chase]

I also have the Rodenstock Grandagon-N 90mm f/4.5 and shoot landscapes as well as interiors with both color transparency film and black and white.

I don't have the center filter for this lens and really don't see the need to get it, even with about 20 degrees of rise in some of the shots.

[Leonard Evens]

It was my understanding there would be no math in this debate. Can someone please explain what these calculations mean? I think I understand what the image circle is, this determines the size of the circle of light hitting the film plane. The cosine stuff just about made me cross eyed.

Thank you.

Sorry to confuse you. It might have been better just to give the results of the calculations: that even with the best design, if you use maximum rise, you may have as much as 4 stops drop off in light intensity from the center to the edges. But if your image is centered and you don't care that much what happens at the corners, you may not notice much difference.

You should take the trouble to understand what the image circle is. Ron gave a good explanation. The reason it is important is because different lenses of the same focal length can have different size image circles, and this can be important when deciding which lens to buy.

For example, the f/ 5.6 72 mm Super Angulon XL has an image circle of about 226 mm, where my 75 mm f/4.5 Grandagon N has one of only about 196 mm. That allows for considerable more movement, and in principle the Super Angulon would be a better choice where a larger amount of rise is necessary. I've encountered the limitations of my 75 mm Grandagon in this respect when trying to photograph some nearby Chicago skyscrapers from Millenium Park. Unfortunately the Super Angulon XL is much more expensive and much heavier, and also it wouldn't work with my camera.

It is important to distinguish the angle of coverage from the anglue of view. The former refers to the total image circle where the resolution is good enough for a decent picture. The latter refers just to that part subtended by the film frame.

[Nick_3536]

Image circle is the diameter of the circle of light that a lens will project.

That's circle of illumination.

The image circle is inner part of that circle that provides good image definition. Some lenses casts a large light circle with edges that aren't worth much. At least not if you're enlarging. Might be okay with a contact print.

[Navy Moose]

Sorry to confuse you. It might have been better just to give the results of the calculations: that even with the best design, if you use maximum rise, you may have as much as 4 stops drop off in light intensity from the center to the edges. But if your image is centered and you don't care that much what happens at the corners, you may not notice much difference.

You should take the trouble to understand what the image circle is. Ron gave a good explanation. The reason it is important is because different lenses of the same focal length can have different size image circles, and this can be important when deciding which lens to buy.

For example, the f/ 5.6 72 mm Super Angulon XL has an image circle of about 226 mm, where my 75 mm f/4.5 Grandagon N has one of only about 196 mm. That allows for considerable more movement, and in principle the Super Angulon would be a better choice where a larger amount of rise is necessary. I've encountered the limitations of my 75 mm Grandagon in this respect when trying to photograph some nearby Chicago skyscrapers from Millenium Park. Unfortunately the Super Angulon XL is much more expensive and much heavier, and also it wouldn't work with my camera.

It is important to distinguish the angle of coverage from the anglue of view. The former refers to the total image circle where the resolution is good enough for a decent picture. The latter refers just to that part subtended by the film frame.

Hi Leonard,

Thank you again. I am doing reading on the image circle. I bought the Caltar II-N 90mm because it had a decent image circle and was faster than the f/8 lens I was looking at.

I have Steve Simmons' book and Jack Dykinga's book as well that I'm reading.

Navy Moose