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Image circles explained... a little bit... |
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Written by Patrick Jan Van Hove
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Wednesday, 23 November 2005 |
One of the challenges of ULF is finding lenses with proper coverage. The fact that many of the lenses used in ULF are not originally designed for photography means that their specifications as given by the original lens maker do not necessairly apply to the photographic world, such as Process lenses, for which the "wide-angle" designation doesn't mean the same as for lens marketed for photography.
Let's start with the image circle needed by the various formats. These are minimum values, calculated simply by calculating the theoretical diagonal of each format. Of course, due to film holders, the actual image area is slighly smaller than the values given here, by a slight margin.
| Format | I.C. in mm | I.C. in inches |
| 8x10" | 325.3 | 12.8 |
| 5x12" | 330.2 | 13.0 |
| 7x11" | 331.2 | 13.0 |
| 10x12" | 396.8 | 15.6 |
| 11x14" | 452.2 | 17.8 |
| 7x17" | 467.0 | 18.4 |
| 12x16" (30x40cm) | 508.0 | 20.0 |
| 14x17" | 559.4 | 22.0 |
| 12x20" | 592.4 | 23.3 |
| 16x20" | 650.6 | 25.6 |
| 18x22" | 722.0 | 28.4 |
| 20x24" | 793.5 | 31.2 |
So now it's just a matter of matching the lens' image circle with the one needed to cover the format and voilà! ... Right?
Well, I wish it was that simple. You see, It depends on your use of the camera a great deal. If you do for example architechtural photography, you are likely to need a lot of movement to allow for parallax corrections, which requires a significantly larger image circle than he minimum needed for your format. And beware: the larger the format, the more movement you are going to need to get to the same result, since movements are proportional to the size of the image.
On the other hand, ULF has one forgiving feature : with such a large image area, you easily get into the close focus range, where subject area is five times the size of the image. It is the range where bellows extension factor comes into effect, and for a good reason: the lens is further away from the image plane. This has an interesting side effect, the actual image circle is larger. That's why the image circle if often given at infinity: it is the point where the image projected by the lens is the smallest. If you plan on doing portraits, of close-up still lives, you can actually settle for a lens with less coverage that what is needed for your format. If you want to work at 1:1, you can actually use a lens that has only half the infinity coverage needed for your format !
Now, comes the easy part: how to figure out the coverage of a lens. In short you need one of two informations: the actual image circle, or the angle of coverage. Just from the focal length and lens designation, it's impossible to tell: different lenses with the same name have coverage which depends on the focal length.
 From the angle of coverage, if you want to calculate the coverage from an angle here is the needed formula:
coverage= 2x(Focal length x tan (1/2 angle of coverage)
This should give you the diameter of the image circle projected by the lens. Now a quick example:
The Schneider Super-Symmar XL has an angle of coverage of 105 degrees. At a 210mm focal length, this gives a :
2x(210mm x tan (105degrees / 2) = 547mm image circle...
I think this should cover the basics... Any questions?
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Last Updated ( Sunday, 27 November 2005 )
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