Help me to understand a focus technique problem

[Roger Rouch]

Thank you all for the answers. I need to get under the darkcloth and try a few new things. It does at least look like I'm on the right track.

[Bill Glickman]

Roger, you wrote.... As you draw a horizontal line between closest and farthest objects, only the objects or parts of objects that touch the plane will be in focus. Everything above or below that plane will not be in perfect focus.

Keep in mind, this is true even when you do not tilt... there is only one plane of sharp focus regardless of tilt....how much of an area in front of or behind the plane of sharp focus is acceptably in focus is then determined by f stop. The only difference being that when not using tilt, all the planes are paralell, while using tilt the planes cuased by the f stop are a wedge, or cone shaped where the intersecting point is below the lens...

Now, to better address your question... you need to first decide if tilt is benefical to your scene... that means there must be a single plane in the foreground that extends outward awhile before it becomes 3 dimensional. For example, if you had a tall tree 15 ft away, titl is useless.... However, if you are shooting up a hill of flowers, tilt is ideal since there is one single plane near you for quite some time...

If you deem tilt is required, then your next task, as per Merklinger, is to determine how many ft under the lens the plane of sharp focus will be. This can be done by visualizing where you want the plane of sharp focus to be. It also can be done on a graph, but most people find this too confusing for the field... lets say it will intersect 10ft under the lens. Simple rule of thumb, fl/10ft x 5.5. So if you are using a 150 mm lens, you would need 3 deg tilt. That is an exclellent starting point and will should solve your near condition...then to align the far, just rack the back up and back until the plane of sharp focus is centered in the middle of the far subject. Then stop down as as required to acheive the remaining DOF by opening the wedge... to be safe, stop down to f32 if you have no polaroids or are unsure how to best determine this. As a general rule, if things look great in the gg, just stop down to the desired f stop, such as the sweet spot, say f16, if the far subject is very tall, and you can see the top and bottom blury, you have a lot of stopping down to do... setting the f stop does not have a simple rule of thumb like determining the tilt angle, unless you want to get out pencil and paper... Hope this helps...

[Jim Galli]

I'll stick my neck out here and oversimplify. Combine all available movements that will lessen the bed travel between your "nearest" item focused and your "farthest" item focused. When there's nothing else you can do with moves to lessen that travel, hold a fingernail on the rail at one "end" and travel the bed to the other "end" for a measurement. Best focus compromise is dead center of that travel and stop down. If you've still got more than 1/4" or so, you'll be up in the f45 world to equalize and you've got to do some soul searching about whether to take the trade-off in diffraction loss or go find a better layout. Sometimes the solution is to put the 50mm on the Mamiya where everything is sharp at f8!

[James Chow]

Bill has the most precise answer. It's basically a summary of Merklinger's articles. The DOF is a cone/wedge-shaped region about the plane of sharp focus. It opens up to infinity at infinity, as has a size of +/-J at the hyperfocal distance (measured along a plane parallel to the film plane), where J is the distance from the lens to vertex of the wedge of DOF (below the lens). Once you understand the theory of DOF with tilts, the application in the field becomes elementary. Two things I might add. One is to inspect the stuff in the foreground w/ a loupe, around 4x (say, maybe your subject is a group of rocks) plus anything tall like mountain/tree- tops in the foreground/background. Nothing is more annoying than having the top of a mountain or tree go soft. Second, sometimes, you need to set the vertex of the wedge of DOF underground so that it intersects the ground in front of the camera at the foreground subject. This becomes important if the foreground subject matter is very close to the camera and you still want to use tilt to ensure infinity is sharp.