Image Circle Coverage and Angle of View - how to calculate?

[ic-racer]

The diagram posted in #19 is a Vignetting Plot for Mechanical Vignetting, not Natural (cos) Vignetting. The curves look different.
Here is a clearer example:

[Tedman]

Hey thanks for your answers so far...
Am I right that in your example the effective pupil area is the diameter of the Circle of Coverage?
Would I really need to use an Optics Design software to find out about that?

My plan was to draw the lens with it's principal planes, pupil positions, image plane for infinity focus etc and draw some rays. The first part(drawing the lens) is no problem but I don't know what rays define the (maximum) Image Circle. I'll try to upload an example later today.

[Dan Fromm]

Tedman, given an unknown lens with unknown design all you can do to find the diameter of the circle of acceptable (to you) sharpness is to shoot film and (a) examine the negatives at the same magnification as the final print or (b) print to the size you're aiming for and examine the print. You can't calculate what you want with no data but you can always measure. There's no magic involved, just ask the lens what it can do. And ask it with film, don't look at ground glass.

[Christoph Schultz]

I'm late coming to this thread but I had to do some digging around to figure out what the hell you gentlemen were getting at. For those of us who are mathematically challenged, I made a simplified explanation that I could reference before purchasing new lenses. There will be more information like this coming to my new website in the future, just in case someone finds it useful and could use help in other areas of large format photography.


HOW TO DETERMINE THE ANGLE OF VIEW FOR VINTAGE LENSES

The Angle of View changes based on the size of the film. So the first step is find the diagonal of the film you're using.

Diagonal = √(width²+height²)

Next you might want to convert this to millimeters (perhaps because it is more accurate?). Then you use the Angle of View Formula below.

2arctan(diagonal of film/(2*focal length))

The last step is converting radians to degrees, then round to the nearest whole number.


RESOURCES:

1. Diagonal calculator: http://www.mathopenref.com/rectanglediagonals.html

[Dan Fromm]

The computational procedure you laid out gives the smallest angle covered that will cover a format given focal length. What does that have to do with anything?

Oh, and by the way, I don't know where you got your table of film sizes by format but it is wrong. You may have found a table of plate sizes. The usable area of a piece of sheet film is smaller than the usable area of a plate of the same nominal size.

[Christoph Schultz]

Dan, I found the chart on a blog located at http://jbhphoto.com/blog/2011/01/29/film-diagonal/.

As for the maths involved, I'm not completely confident, other than I used it to calculate known angles of view for lenses that I had specs on and the numbers matched. It has nothing to do with image circle or coverage, only the angle of view. I use this to determine how "wide", "normal" or "telephoto" a lens is compared to other lenses I own. Personally, I like a certain angle for portraiture and like to stick tightly to that range to maintain a certain look no matter what camera system I'm using.

If you have any additional insight regarding image coverage, or if my formula is incorrect, I am very interested to learn more. Also, if I need to edit my post above as to not send folks astray I would be obliged to do so.

[Dan Fromm]

Christoph, that something is posted on a site doesn't make it true.

As was pointed out earlier in this discussion, there are many definitions of coverage. Lens makers' coverage claims are often more narrower than users report.

Movements are very important to photographers who use view cameras. Your formulas, which start from the film's diagonal, give no information about a lens/format combination will allow movements.

[Christoph Schultz]

The above formula is an easier to read, more clearly written version of what Bruce Watson was saying on page one back in May of 2009. The formula is not meant to give any information regarding movements and image circle coverage. It provides a way to compare lenses and their focal length rendering across all film formats, which is why I found this thread to begin with.

Image Circle Coverage and Angle of View hardly relate at all. The coverage circle is determined by the design of the lens itself. Angle of view is a completely different thing; it's a function of focal length and format size.

[Taija71A]

Christoph, the following Chart...
Is always an excellent Reference (for 'Angle of View'):

Large Format Angle of View -- Format Comparison Chart.
http://lensn2shutter.com/angleofviewchart.html
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Thank-you!
Best regards, -Tim.