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#1
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Large format 10x8
Coverage for LF lens is always given at f22. Is there some reason for this. Would the coverage change much at other stops. I want to find the optimum f stop on my Fujinon 300/5.6. In 35mm terms it is usually 2 stops down from wide open. Would this be the same for LF 10x8.
Pete |
#2
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Generally speaking, the coverage of a lens improves the further it is stopped down. For example, a 90mm Angulon won't quite cover 5x4 wide open. But when stopped down to f/16, there is enough coverage to allow a little bit of movements.
As to why f/22 is used as the point where image circle is stated, I don't know. That said, the Fujinon 300 CMW/WS has an image circle of 412/420mm - More than enough for movements on 10x8 even wide open. As a side note, I have used a Fujinon 300C on 10x12, and it gave plenty of coverage. |
#3
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Image Circles get larger and more clearly defined (a harder edge to the vignetting) as lenses are stepped down.
Normally 35mm and MF lenses tend to give their optimum performance 2+ stops down from wide open Most LF lenses are quoted at f22 just as a sort of unofficial standard for comparison. Wide open, LF lenses are sharp in the centre and soften considerably as you get towards the edge of the image circle. So LF lenses tend to be optimised around f16~f22~f32 to get sharpness across the image circle. It is worth visiting Schneider and Rodenstock web sites and looking at the typical MTF curves for lenses they show to understand this more clearly. In addition, with LF, depth of field is an issue for most shots, so we tend to need small apertures to be sharp front to rear. Its one of the great technical joys (!) of LF, all those clearly defines "rules" that work OK for MF or 35mm don't apply any more and you have to understand at a working level the physical performance of your lenses - its what makes the LF world so demanding to use and so rewarding to master. Hope this helps Martin ps - if you let me know exactly what lens you have, I'll see if I have any info on it - your shooting at 10x8 I take it. Last edited by Martin Aislabie; 15th January 2016 at 05:40 PM. |
#4
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I use a Fujinon-W 300mm f5.6 lens as standard on my Tachihara 8x10 camera. Since I don't enlarge from 8x10 negatives but rather contact them out as same size positives every aperture on the Fujinon is effectively "optimum". By this I mean that every aperture from f5.6 to f90 delivers fineness of detail better than the eye can resolve. So I select the aperture in order to deliver the depth of field required; sharpness is a given.
As for coverage at extreme movements I tend to get vignetting from the camera bellows before the Fujinon runs out of image.
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The word Photography first uttered and defined by its author Sir John Herschel at Somerset House, London on the evening of March 14, 1839: quote "Photography or the application of the Chemical rays of light to the purpose of pictorial representation" unquote. |
#5
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It was Carl Zeiss Jena who first optimised their large format Tessar lenses for f22 before WWII. I use a few different Tessar and Tessar type lenses and experience has shown that's true, there's a slight difference (softening) in edge/corner sharpness at f16.
With modern LF plasmats there's less of a difference between f16 and f22. Lenses for 35mm cameras are optimised differently because in general they are used hand held at wider apertures, and they don't need coverage significantly wider than the format. Ian |
#6
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Thanks everyone, most informative. Time to get the beast out again.
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#7
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one thing not mentioned is that the manufacturer figures @ f22 are also focussed at infinity. Normally you aren't focussed at infinity and the lens will be extended and that means the useable image circle becomes bigger than the infinity spec.
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An old dog learning new tricks |
#8
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With some lenses coverage is given for different f stops. So the 120mm f6.8 Dagor I've just bought covers Quarter plate wide open, 5x4 at f16 and Half plate at f45. In this case it's what the manufacturer CP Goerz deems to be acceptable sharpness rather than the illuminated circle.
Ian |
#9
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#10
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Quote:
I'm used to working with some lenses with very tight image circles, my 65mm Super Angulon and 90mm Angulon mostly, also a 150mm Tessar (not quite as tight), and you quickly find it's simple to adjust to get the best when using movements. I'd add that's with my light weight 5x4 kit and I'm working hand held probably more than 50% of the rime, I still use movements. Even with lenses with slightly larger than average coverage it doesn't take much to run out of the image circle on the corners. I use a coated 12" (300mm) Dagor with my 10x8 camera, that covers 15x12 at f45 and I've lost the corners using rise. I was shooting a building higher up a hill. Instead of using rise I should have let the verticals converge and corrected during printing, my De Vere allows tilts. Ian |
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