lenses specs at full aperturegreenspun.com : LUSENET : Large format photography : One Thread
Technical specs for Rodenstock and Schneider LF lenses indicate angle of view and image circle diameter at f/22. Consequently, the lens displacements are given in mm (shift, rise & fall) for the same aperture. What about specifications at full apertures ? Nikkor specs mentions both of them, at full aperture (i.e. f/5.5) and at f/22 medium range. It seems easier to appreciate if a specific lens will cover the 4x5" format and the remaining displacements, not only at f/22, but also at 5.6, 8, 11 ...
Who knows if a formula allows to calculate the angle of view and image circle diameter at full aperture for these lenses from Rodenstock and Schneider, as I've never found these indications anywhere ?
-- jean louis Llech (firstname.lastname@example.org), September 14, 1999
The image circle of a lens is not really smaller at high apertures, what you would get in the picture is more loss of light in the corners. This is why you have to shut down the aperture. Another reason is that the depth of field is generally too small for anything below f 16, unless you want the effect. So, don't worry about image circles at higher apertures - in my experience, this is a problem unlikely to arise in practice. Lukas
-- Lukas Werth (email@example.com), September 15, 1999.
The image produced by a large format lens wide-open will have lower quality, esp. in the corners. It might not stand enlargement. As the previous answer mentioned, there will be substantial light falloff in the corners. But it doesn't matter because you won't find yourself using wide apertures (unless you have a special application). The F-stop needed for a given depth-of-field scales with negative size, e.g., if f8 gives good depth-of-field in 35 mm, then you will need f32 in 4x5. In 4x5 one mostly uses stops like f16 to f32, and maybe f45 for closeups.
-- Michael Briggs (Michael.Briggs@prodigy.net), September 15, 1999.
Then again, those of us who photograph at night know which of our lenses can handle it or not. A 150mm at f5.6 focused on stars will show just how sharp your corners are. I have newer lenses that can make it, and older ones that can't. The older ones may be great at f32, but not wide open. Image circle is no where near as important as sharpness over the entire image. Besides, as mentioned, Nikkor tech sheets give both figures, and they are not the same. My 90mm barely covers wide open, but has more image than my 210mm when stopped down. The circle certainly does change with f-stop, if you consider the usuable part.
-- Chris Wray (firstname.lastname@example.org), September 15, 1999.
jean - to answer your question about a formula to calculate the angles: no, there is no standard formula, since this factor is dependent upon the lens design and number of elements. from my communication with the schneider folks, even the specs they list for image circle are somewhat subjective, depending upon percentage of noticeable distortion using test patterns at a given distance from lens center, and loss of illumination. if you have a question about a specific lens, it is best to call the manufacturer directly - they are usually quite helpful and certainly the most knowledgable.
-- jnorman (email@example.com), September 15, 1999.