Focussing Question;What am I missing?

[gary_meader]

Hi everybody. I've been using LF for nearly 20 yrs. and there's a

situation in the field that I just don't do vey well. Maybe you can

help. Picture this: Low camera angle ( 4 ft off the ground ),

looking across a picturesque mountain stream. Nice boulders

close by, rushing water, get the picture? OK. With an axis tilt

8x10, I can use front and rear tilts to get the near boulders in

focus all the way across the stream. Here's the rub: at the other

side of the stream is a vertical bank with very interesting trees

and rocks that must be inthe photo. They are on the bottom of

the GG. When I tilt to get the near in focus, those trees go out

because they're not on the same plane as the stream. Is this

simply a case for DOF? I've always had a problem with this type

of scene and seem to have a mental block or something

keeping me from shooting a scene like this simply and quickly.

Can anybody shed some light here? Many Thanks

[chris_jordan3]

Gary, i think you already have the answer-- it's because you are laying the focal plane down flat, and so vertical things in the distance that point up out of that plane, will no longer be in focus (and if you are using the rear rather than front camera movement, then distant tree may also show a perspective distortion).

 

i think the standard technique in such situations is to lay the focal plan down about halfway inbetween the water and the trees (for example, instead of focussing right on the water's surface, maybe focus on the tops of the boulders, or the opposite stream bank), and then stop the lens down so both the water and distant trees become sharp.

 

the focal plane itself (with the lens wide open) is a narrow plane, but when you stop the lens down it becomes a big wide fat plane. when all your camera movements are upright at 90-degrees, that means things near and far come into focus (i.e., the big fat focal plane is vertical). but if you think of the big wide fat plane lying down at a 45-degree angle, it turns into a wedge-shape, like shining a big wide flashlight beam-- it gets fatter the further away it shines. so, if you angle it halfway inbetween the two things you want to cover, it will cover them both. does that make sense?

 

~cj (Seattle)

 

 

p.s.: or you could follow cj's focussing principle:

 

Fine-tune the tilts, fine-tune them some more,

Awfuckit, just stop down to f/64!

[ken_lee11]

You are right: after your tilts, the trees lie far outside the new plane of focus. You might consider adopting a less extreme set of tilts, in order to get it all in focus, if you can stop down the lens enough. Try setting up a similar shot when you are not under the pressure of the moment - you'll get the "swing" of it <groan>

[dave_karp]

Gary,

 

It will be interesting to hear what others do.

 

Here is what I do. I use front or back tilt to bring the foreground and the stream into the plane of focus. Then I stop down to get the tops of the trees in focus.

 

I use a depth of field chart that I laminated and carry with me in the field to help select the f/stop. Remember to consider the expected enlargement size when selecting the f/stop. Most of the depth of field charts assume a fairly small enlargement. (See http://www.englander-workshops.com/documents/depth.pdf for a good explanation of this.)

 

Sometimes when you can't stop down enough to get the treetops acceptably sharp, you might need to take the foreground a drop out of focus by reducing tilt. Then use your depth of field to bring the foreground and the treetops into focus.

 

I hope this helps.

[qtluong]

By experience, I'd say the optimal plane of focus would go through about 1/2 to 2/3 of the trees, and a boulder approximatively between 1/2 and 1/3 down your ground glass. It's tempting to focus on a

boulder further up the ground glass (ie closer to you), but that

won't be optimal, since you'll waste the top part of the DOF cone.

If you want to experiment more,

you can try the method II of the article "how to focus" on lfphoto.info. Then read "what f-stop" there to find how how much you

need to stop down.

[robert_a._zeichner1]

The rule of thumb for depth of field is that you will have twice as much behind as you will in front of the plane of focus, hence the "one third forward, two thirds back" idea. When you tilt the rear standard back, you are tilting the plane of focus forward. The trick is to establish the plane of focus such that it intersects the foreground rocks and the background trees at about 1/3 down from the tops of those objects. The "front" of the plane of focus is now above (toward the tops of the trees and rocks) and the "back" of that plane faces the ground. No matter how much you close down the iris, you will only ever be in perfect focus at the plane. You will however be able to take advantage of the added depth of field as you close down to the point at which everything appears to be in acceptable focus. There are certain situations where you simply will not have adequate DOF even with careful placement of the plane of focus. In those situations, you may elect to use a shorter lens.

[gary_meader]

Thanks, you guys for all these responses. All my images haven't

been out of focus or anything,( or I'd have quit long ago ) but I

always try to get it ALL in focus before stopping down. I'm

realizing now that that's like looking for the Holy Grail. Anyway,

thanks again, I'm sure all this will get me there.

Gary

[pete_andrews]

Gary: You're describing a flat, or near horizontal, plane of focus, which always goes to infinity. And then you're expecting objects above the 'infinity line' to be in focus. It ain't gonna happen. You MUST compromise with the plane of focus, to get the tops of the trees, or whatever, into focus, as well as the near point of interest. THEN stop down the lens to a sufficient DoF to get the midpoints in acceptable focus too.