Focus Question

[Justin F. Knotzke]

Here's the dilemma I had. I was up on a walking path shooting down on some rapids. Picture the following:

WATER WATER WATER WATER WATER WATER 2

ROCK ROCK BIRD ROCK ROCK BIRD ROCK ROCK 1

ROCK ROCK ROCK ROCK ROCK ROCK ROCK ROCK 3

ROCK ROCK ROCK ROCK ROCK ROCK ROCK ROCK 4

ROCK ROCK ROCK ROCK ROCK ROCK ROCK ROCK 5

The numbers on the right indicate how close they are to the camera. The higher the number, the closer to the camera.

Now as you can see, level 1 is the bottom of the rocks before the water. But because rapids are rapids they were actually higher in the air and therefore closer to the camera.

So what would happen is I would tilt the front lens to focus on the rock at #5, then focus to get the water (since it was at the bottom of the glass) and repeat until the top of the glass and the bottom of the glass were in focus. The problem was the rocks (and two birds) were out of focus. I had no idea how to fix this so I just stopped down.

Any ideas?

J

[Donald Hutton]

Movements alone cannot solve every situation. Frequently, stopping down is the only solution for getting everything into focus.

[Ralph Barker]

I'm having trouble envisioning the topography, but if we assume the water had a high mineral content, it sounds like you may have been between a rock and a hard water place. ;-)

While I agree that stopping down is sometimes the best, perhaps only solution for a specific situation, sometimes a less obvious placement of the plane of sharp focus can help. That is to say, placing the plane in a compromise location that is near or between the primary elements in the scene produces a better range for DOF to work within.

[Justin F. Knotzke]

I'm sorry, I did a horrible job of explaining what I was shooting. I'll give you made up example that explains the scene hopefully better.

The camera is on top of a hill that is 20 feet up from the ground below. The camera is pointing down the hill. The hill is fairly gradual in that the camera is able to record the entire slope.

At the bottom of the hill, about 10 feet from where the hill flattens out, is a rock, say 5 feet high.

The camera is positioned so that the entire slope of the hill is in view and the last element in the frame is the rock.

The element which is farthest away from the lens however is the bottom of the slope, not the top of the rock. Yet the rock is at the top of the frame.

My normal way of focusing is to tilt the lens until the top part of the glass is in focus, then move the lens board so that the bottom part of the glass is focused. I repeat these steps until all is focused.

Except, the bottom of the hill is not in focus.

That make any sense?

J

[Alan Davenport]

First, let me say that your original description of the image really ROCKS. (Sorry, I had to say that.)

Don is right in his response. The point being, there is only a single plane which is in focus, and anything not on that plane will be defocused. Unless your entire subject lies in a single plane, you can't get it all in perfect focus. All you can do with movements is adjust where the plane of focus lies in the subject space. Once you've gotten that done as best you can, you need to stop down until the depth of field is sufficient for the rest. See Merklinger's work (http://www.trenholm.org/hmmerk/index.html) for a discussion of what happens to DOF when the lens is tilted.

[David Vickery]

Hello, If I understand what you are saying then it sounds right that the bottom of the hill would not be in focus with the lens wide open, because you have tilted the plain of focus. If you have decided that you want your plain of focus to be inclinded from the foreground to the background then you do have to decide which point in the background and which point in the foreground to focus on , just as when you use no movements you have to decide which single point to put the plain of focus on.

You are reducing a three dimensional subject to two dimensions.

" Yet the rock is at the top of the frame." Do you mean that the rock is at the bottom of the ground glass??

[Bruce Watson]

So, you focus on the far, tilt for the near, and leave the middle ground out of focus. Yep. This isn't terribly rare.

There are, I think, two major ways to go with a scene like this. First, you can re-zero your camera and rely only on stopping down. Second, after you define (using tilt) the plane of sharp focus, you can move the plane to a compromise position (make the middle ground better at the expense of both near and far) and use stopping down to bring the whole image into focus.

And sometimes, you just have to pack up and walk away...

[Justin F. Knotzke]

I've got my answer.

I just wanted to make sure I wasn't missing something. Coming from 35mm, the whole focusing thing with LF has been a learning experience.

Thanks to all who replied.

J

[Leonard Evens]

I'm going to respond to your hill example, although I'm still not sure I completely understand what the problem is.

First, let me say that if you have some significant vertical extent fairly close to the camera, then you aren't going to be able to make much progress by using a lens tilt.

But from your description, I'm not sure that is your situation. It would depend on how close the rock is to the camera, and it sounds as if it is not supposed to be specially close.

The first thing to note is that the top and bottom of the frame have nothing to do with the matter. You have to identify a near point and a far point which are appropriate for your situation. They could both be in the middle of the frame or anywhere else for that matter. If the whole subject is close to being in one plane, you just adjust the tilt so that the near and far points are both in focus, and then everything else will also be in focus because the subject is close to being planar. Without the rock, it sounds as though the hill surface might qualify for that. But the rock probably extends somewhat out of that hilside plane, which creates a problem. So here is what you do. First adjust the tilt so that the hilside is in focus. Next refocus so that the top of the rock is in focus and note the distance you have to move the standard. Finally focus halfway in between and stop down far enough so the hillside and the top of the rock are still in focus. Alternately, you could tilt so your near point is the hillside close to the camera, your far point is the top of the rock and both are in focus. Then see how far you have to move the standard to bring the hillside in focus and focus halfway in between. Or try some other variation of this.

The point to keep in mind about all of this is that the region in adequate focus is wedge shaped. It is centered on a line, called the hinge line, somewhat below the lens, determined by the amount of tilt. The plane of exact focus splits this wedge in the middle. As you move the standard back and forth, with the tilt fixed, the whole wedge rotates about the hinge line. The wedge opening is determined by the relative aperture. It has to be large enough to cover both the top and bottom of the rock. When you are done, the exact plane of focus should pass through your near point and somewhere between the top and bottom of the rock, most likely its middle.

[Justin F. Knotzke]

Ok.. A picture is 1000 words.. so: http://www.shampoo.ca/tmp/thescene.jpg

That's the scene.

Now please keep in mind which forum this is. I am rather new to this.

I was working under the impression (and I think now having read Mr Even's post that my impression was wrong ) that one should focus on the top part of the glass and the bottom part of the glass and that will have everything else inbetween in focus.

This oversimplified approach, I am starting to understand, assumes that which is at either end of the glass is in fact closest and furthest from the lens.

Using my above example, this clearly isn't the case. What is furthest from the lens is in fact the bottom of the big rock.

So my question then is what should I be focusing on? My guess at this stage is the top of the sloping rocks and the bottom of the big rock.

Thanks again

J

[Alan Davenport]

Here are my 1,000 words:

http://home.comcast.net/~w7apd/public/forum_reply.jpg

[Ralph Barker]

Finding the way to Oz in this case is a matter of following the red rocky line, instead of the yellow brick road. ;-)

Most commonly, Justin, the lens is tilted forward so that the camera back, lens board, and plane of sharp focus all converge on a point below the camera. But, it works the other way, too, with the convergence point placed above the camera by tilting the lens board back. By using that method, the plane of sharp focus can be placed below the horizon, angled down the hill.