Using a 90mm on my 4X5 for a landscape shot and tilting the lens down for optimu m depth-of-field, at what distance should I focus on the ground glass? A midway point or at some other nearer or further distance? I need the flowers on the for eground at about 10 feet to be just as sharp right up to infinity. Would it be a similar application for a lens of longer focal length?
Paul: With the 90mm, you can get everything sharp by stopping down to f/22 and focusing at about 12 feet. At 10 feet for you nearest subject, I don't think you would need to use tilt at f/22. With the 90, only a small amount of tilt will be needed if you decide to use it. The wide angles need to be focused nearer than the longer lens to get everything within the depth of field range. On the longer lenses, focus about 1/3 the distance into the scene. With back tilts, I usually focus on infinity and use the back tilts to bring the near into focus. Just watch the ground glass carefully...you can see what it's doing. Incidentally, Schneider's web site has depth of field charts at different f-stops. The depth of field will be the same for any lens of the same focal length.
Or look at it another way: If you tilt your lens (or back), you can get everything on the ground in focus, all the way from 10 feet (or even 5 feet) to infinity. Then the question 'at what distance should I focus on the ground ground glass?' is pretty meaningless. The bottom part of the GG will be focused at infinity, the top at 10 feet, the middle somewhere in between.
This applies whatever the focal length of the lens.
When using tilt, you don't focus at a certain distance as you do with no tilt. Conventional DOF rules go out the window. The DOF is now shaped like a cone rather than a vertical slab/wall, and the plane of sharp focus can be tilted downwards or upwards (e.g., looking down a staircase or up...you use the same tilt angle). If the flowers are 1.5 ft tall and the lens is 5 ft off the ground and you want to set the plane of sharp focus at 3.5 ft below the lens, I'd guess about 5 deg of tilt to start, then adjust the bellows extension to set the plane of sharp focus and stop down the lens. The vertical rise you apply w/ the lens tilted (so the flowers 10 ft away are at the bottom of the frame) isn't going to affect the DOF...only the lens height above the plane of sharp focus, front/rear tilt angles, and aperture (given the focal length, of course).
For a longer focal length lens, the cone of DOF is much more narrow since its size a function of the hyperfocal distance, which depends on the focal length, aperture, and CoC. If say, you have the lens 1.5 ft off the ground w/ a 150mm lens, you'll need a lot of tilt...like 20 degrees! The pitfall is that after you apply front rise, you might hit the edge of the image circle! If you anticipate taking shots w/ lots of tilt, try to get a lens with the largest image circle you can.
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