Yes I have read largeformatphotograhy.info site and some other resources, and still am confused in few things. And they are:
1. How to determine which focal lenght for what format? That is how to decide/calculate focal lenght of lens I need to have all movement of camera (including scheimpflug) and avoid vignetting?
2. How to determine/calculate ratio between bellows lenght/focal lenght of lens, that is to be able to focus lens from closest focus to infinity what bellows lenght I need?
3. How to determine wide lenses needed, and do I need long or telephoto lenses if I go to longer than normal lenses.
Basically, how to calculate what lens I need for particular camera format at same time keeping movement of camera, focusing through all focus range, and to have whole image and not cropped image or vignetting?
Next thing is reading lens charts. For example Schneider have lens info with telling "this lens is suitable for this format" and Rodenstock have lens charts which tells "this lens using with this format has tilt horizontal x vertical Y". How to read those charts, that is what shift in that situation means.
If answer will be too long, please, source of informations would be appreciated.
Thanks.
Last edited by haris; 05-07-2008 at 08:50 AM.
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It's simple really. The "normal" lens is defined as approximately the diagonal of the film format, and the needed minimum coverage is exactly the diagonal of the film format. Any more coverage than that will allow for movements.
With the camera focussed at infinity, the bellows length needed will in most cases be very, very close to the focal length. focussing closer will give more coverage, since the "cone of light" behind the lens gets wider as you move the lens away from the film to focus closer. The "extension formula" is 1/F = 1/u + 1/v, where F is focal length, u is distance from film to lens, and v is distance from lens to subject. There are a couple of caveats to this formula, since you really have to take into account things like internodal distance. But except for telephoto lenses, it's close enough.
Lens charts give how much rise/fall and lateral shift you can use with a given lens on a given film format without getting fuzzy corners from the corners of the film poking outside the edge of good coverage. That's fairly simple geometry.
Now e.g. a 13x18cm or 5x7" film has an image diagonal which is just about 210mm. So the "normal lens" for this format is a 210mm. But since not all 210mm lenses are equal, they can have very different coverage: A 210mm telephoto lens might cover only 24x36mm, a Tessar-type about 220mm, a Plasmat 300mm, and a wide-angle (like a Super-Angulon) 500mm. So a Tessar (like a Xenar f:4.5) will cover with very little movements, a Plasmat (Symmar) will cover with lots of movements, and a Super-Angulon is way overkill - very few cameras can take the treatment needed to get even close to the edge of the image circle.
Haris, I think the difficult part is realising which lenses fit into the different groups and categories. The lenses I mention give you something to compare others to.
It's worth trying to meet up with an LF user, and see just how the lenses behave and how much movement you really need to use.
I guess diagonal are calculated by Pythagora's theoreme, square of hypotenuse is equal as sum of squares of cathetes, right?
Right. Just remember that in LF, image format is a little bit smaller than film format, which again is a little bit smaller than the "name of the format". So that the image area of "13x18cm" film is about 12cm x 17cm...
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Originally Posted by Ian Grant
