...I'm thinking of building a 5x4 fixed focus camera with a spare 90mm...
Hello from France !
On our French MF/LF web site galerie-photo there is an article by Gilles Barbier about a 13x18 fixed-focus camera. He wanted to get circular images up to the very edge of the illuminated image circle of the good old angulon-not-super he had handy. But you can put whatever ground glass you wish in front of your 90 mm lens !
The text of the article is in French but you can have a look at the pictures.
http://www.galerie-photo.com/barbier-hybis-90.html
Gilles B. had asked me about the proper setting for the fixed-focus position. I indicated him what was, in theory, the proper offset to apply to the film plane with respect to the focus plane in order to be sharp on the hyperfocal distance. I agree that you'd better get the sharpest image you can get on your ground glass of a distant object i.e. no offser at all with respect to the actual focal point.
But it is good to know the magnitude of this offset, at least in order to know the precision required for this setting.
And since we are here on the UKLF group, I cannot but quote Sir Isaac Newton himself who provided us with the proper formula :
(offset with respect to the focal point) = (focal length)^2 / (object distance)
If you go for a 4x5" camera and demand a conventional circle of least confusion of about 90 microns, the hyperfocal distance for a 90mm lens @f/32 is 2.8 metre, about 9 feet.
Formula for the hyperfocal distance H :
H = f^2/(N . c)
where : f = focal length, N = f/number (e.g. : 22, 32..) c = diameter of the circle of least confusion.
The corresponding offset w/respect to the focal point, for seeing sharp at 2.8 metre with a 90 mm lens is (90x90)/(0.09) ~= 2.9 mm
In his camera, G. Barbier intended to use the full 13x18 (7x5") format hence he chose something less stringent for the circle of least confusion, namely a diameter of 150 microns.
@f/22, the hyperfocal in those conditions is 2.4 metre, the corresponding offset is 3.3mm.
So the conclusion is simple, if you intend to use your fix-focus camera at f/22 of f/32, with a circle of least confusion between 90 (for f/32) and 150 microns (for f/22) you have about 3 mm of tolerance to set the proper position of your your ground glass.
offset = 0 : everything is acceptably sharp between beyond infinity (this is actually very, very far
) and 2.5 metre, the hyperfocal distance
offset = about 3 mm : everything is acceptably sharp between infinity (this is already quite far, in indeed) and 2.5/2 = about 1.2 metre, 4 feet
There is a latin proverb :
in medio stat virtus so you can go for about one millimeter of offset and be happy !
And the conclusion is that there is no need of a swiss precision machine-tool to properly setup your ground glass provided that your camera allows some adjustable translation of the lens with repect to the ground glass !
Open the lens to its wider aperture, move the lens until a distant object makes a sharp image. This gives you the focal pont.
Then try with by spacing by one or two additional millimeters... by looking at a target located at 2 to 5 metres of distance. And see what happens on the ground glass.
With very old lenses thare is a possible "best focus shift" between the image obtained wide open and the actual image when the lens is stopped downn to f/22 or f/32. My feeling is this effect is very small with a super-angulon which is a modern lens.
Have fun !
