# Size of Lens

Discussion in 'Large Format Cameras and Accessories' started by Gary892, Jun 16, 2004.

1. ### Gary892Member

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Greetings, I am trying to understand how to determine if a lens has enough coverage for a 12x20 camera. If I look at how to determine focal length, then the diagonal of the negative, which is approx. 23.25 would yield a 590.55 mm focal length lens. However, on this website I read someone's response that a 210mm f/9 would cover a 12x20 neative. What part of this formula am I missing and how can I determine by just looking at a lens if it will provide the coverage I need.

Thanks for any response.

Gary

2. ### JorgeInactive

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You are confusing image circle with focal lenght. I use a Nikkor 450 which has an image circle big enough to cover 12x20. There are lenses that have short focal lengths and big image circles as well as the opposite.

3. ### Donald MillerMember

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You are not considering the angle of coverage. This differs from lens design to lens design. While I don't know of a 210 mm that will cover 12X20, there may be one that I have missed.

The short lens that I have heard will cover is a 355 G Claron. I use the 450 mm Nikkor M on my 12X20. This focal length on 12X20 seems to give me almost the same angle of view as my 210 Dagor on 8X10. I do know that my 210 Dagor will definitely not cover 12X20.

4. ### jimgalliSubscriber

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Gary, to oversimplify, a cone of light comes out of any lens and travels to the film plane. If we know what angle that cone is it's relatively easy math to figure out what the lens will cover. So if you have a lens that gives 105 degrees you can draw on a piece of graph paper a 105 degree angle coming form a source (the lens) and see approximately how far away the lens would have to be from the film plane to cover a 600mm diagonal. A 60 degree lens like a tessar has to be quite a bit further away before that angle can cover 600mm diagonal. That's why it takes a 450mm Nikkor to get the same coverage as a 210mm Super Angulon. Symmar's are 70 to 72 degree lenses. So a 480mm Symmar would cover a 16X20 piece of film. G-Claron's are roughly an 82 degree lens at small f stops. So a 355 G-Claron will cover a 600mm diagonal at infinity. Hope that helps.

5. ### Gary892Member

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First off, thank you for the quick replys.
I don't mean to beat a dead horse but, how can one determine if the 450mm Nikkor M will cover a 12x20 by just looking at the lens? I am sure there is a tech sheet that would come with the lens but when trying to buy one and looking at them the tech sheet is not always available. I did some more research and found that to cover a 12x20 neg I would need a 180 degree coverage at minumum.
Any other thoughts.
Thanks
Gary

6. ### sankingMember

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Since you need a 590mm circle of illumination for 12X20 you determine the minimum focal length required to cover 12x20 using the following formula. To use this formula you must know the angle of coverage of the lens.
*
FL = (590 / 2) * (1 / tan(coverage_angle / 2))*
*
which is derived from the simple relationship:
*
tan(coverage_angle / 2) = (image_circle / 2) / FL

Lens specifications are mostly useless in that they are not made with contact printing in mind and often don't indicate as much coverage as the lens will really provide. A case in point would be the 450mm Nikkor M you mentioned. As I recall the angle of coverage given for this lens is about 65 degrees and if you do the calculations above based on 65 degrees the coverage indicated will not be nearly enough for 12X20, but it is a well known fact that the Nikkor 450mm covers 12X20 with many inches of movement.

The only modern lens of 210mm that will cover 12X20 is the 210 SSXL and its angle of coverage is something around 120 degrees.

Sandy King

7. ### Gary892Member

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Sandy, you actually elude to the source of the problem. You say as you recall the Nikkor 450 mm M has an angle of view of about 65 degrees, yet when I checked the tech sheet on addorama's website is states an angle of view of 52 degrees. This disparity is part of the problem I am having.

Thank you to all who have taken the time to responed.
Gary

8. ### Donald MillerMember

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The way that you can determine that a 450 Nikkor M will cover 12X20 is to take Jorge, Sandy King, myself, and any number of others word that it will cover. We have no reason to mislead anyone.

9. ### claySubscriber

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I'll corroborate as well. I shoot 12x20 and have the 450M. It covers with no problem. As does the 355 G-claron (little movement). The Cooke 222mm Series VIIa will also cover if perfectly centered and hyperfocally focused. The spec sheets from the manufacturers are generally conservative. Since we generally contact print a 12x20 negative, our 'circle of confusion' requirements are less stringent than somebody shooting an 8x10 who perhaps might entertain enlarging the negative.

10. ### Gary892Member

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Let me clear up any misunderstanding. I am not suggesting or implying or saying anyone is intentionally misleading anyone. I value each and every response. I am just trying to gain an complete understanding of how to determine the coverage of a lens.
That's all I was asking and several people have offered their knowledge, and I am greatful.

Thanks again for all the responses, and I will drop the subject.

Gary

11. ### JorgeInactive

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Gary, perhaps a read of Leslie Stroebel's book might help you get a better handle on this. Manufacturers many times include data that is far more conservative than the actual performance of the lens. They do this because they are talking about the size of the image circle that offers the best image. In the reference you site, they state 52 degrees as the circle that has best resolution with the lens wide open, all lenses as you get closer to the edges loose resolution and preformance, so instead of having people bitch at them they publish very conservative data.

You know how I find out about a lens coverage? I come to a forum like this and ask about the lens, most of the time people like Sandy, Clay, Thalmann, Don, etc know the answer. But if you really have to know how to make the math, Stroebel's book is the one to read.

12. ### markMember

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Don't worry about it. Some people get upset at the drop of a hat instead of thinking about the question. You are right that the spec sheet says less than what you will see others say here or on other forums. I see two reasons for this. (I did some research on this myself cuz it confused me too)
--According to calumet the manufacturer posts the image circle knowing that they should be able to get all of the movements necessary without image quality fall off.

--On a personal note I think publishing the smaller degree or circle size covers their butt.

Donald is right. Take the word of users over the Manufacturer The user will tell you what kind of movements you can expect on a given size. Ask about a particular lens here or on the LF forum. If there is a user they will tell you about it, and be honest in their descriptions.

This is another good source. http://www.graflex.org/lenses/lens-spec.html

13. ### Jim ChinnMember

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This page is on the Large Format Home Page. it is a good place to start for info on lenses by the major manufacturers. Does not specifically cover 12x20 but does have data on a couple of lenses mentioned here.

http://www.largeformatphotography.info/lenses/