With all the obsessive paranoia (sic) over VCM Typos, Steve's sign-off here is a most-excellently-delivered intentional Zinger!Originally Posted by steve simmons
If it wasn't premeditated, all the Zingy-er!!
With all the obsessive paranoia (sic) over VCM Typos, Steve's sign-off here is a most-excellently-delivered intentional Zinger!
If it wasn't premeditated, all the Zingy-er!!
I think others have pretty much covered it, but let me review the procedure adding one wrinkle which may not have been mentioned.
First, suppose you know what you want the plane of exact focus to be. That would be the case if there isn't any signficant vertical variation about that plane which you want in focus. Choose a near point and a far point. Choose a moderate tilt of about 10 degrees to start. Usually that is the right order of magnitude. Then follow the following rule for refining the tilt. Focus on the far point, and notice the positions on the rail where the standards are. Then refocus on the near point and do the same. If you had to increase the distance between the standards to get the near point in focus, your tilt undercorrected and you need to increase the tilt. If you had to decrease the distance between the standards to get the near point in focus, that means you overcorrected, and you need to decrease the tilt. Using this method, you will always be correcting the tilt in the right direction, and you are unlikely to find yourself making things worse. The worse that could happen is that you find yourself oscillating between overcorrecting and undercorrecting, and in that case, you would just split the distance in the tilt angle.
Most situations, however, do involve some vertical extent above and below the plane of exact focus. To understand how to deal with that, you need to understand the nature of the DOF region when tilting. It is a wedge shaped region bounded above and below by planes extending from the so-called hinge line which lies (usually) below the lens. You can estimate the position of the hinge line if you look to see where you think the plane of exact focus should go. In choosing your near and far points, you should place them so the prospective plane of exact focus will lie somewhere in what you envision the DOF region to be. Usually this will be such that the section of a vertical plane intersected by the wedge will be bisected by the plane of exact focus. Usually the near point will be close enough to the lens that there won't be too much possible vertical variation in its position, so it is best concentrating on the far point in this regard.
Once you have fixed the tilt, the whole wedge swinges about the hinge line as you move the standards. If you got the plane of exact focus exactly right, centered vertically as above, nothing more should be required, but often you find that further adjustments in focus can improve the situation. To help with this, locate high and low points above and below the plane of exact focus that should be in focus. (They need not be at the same distance from the lens.) Focus so that each is in focus, note the position on the rail of each, and set the standard half way in between. You can then use the methods decribed in the LF Photography web page for determining f-stop in terms of this focus spread. (The original spread between near and far points is not relevant for this.) In some cases, you may want to favor the top or the bottom so you may decide to place standard close to one of the two vertical extension points.
Leonard,
If I move the focus from the point where the original near/far points are both in focus, and do this in order to get the verticals in focus, don't I then change the degree to which those earlier points will be in focus? How do I take into account both near/far and vertical focus points in order to a) decide what f-stop to use and b) where to place the focus?
for beginners... focus far - tilt near. Then add depth of field to increase sharpness everywhere else seems to be a simple way to figure it out.
Robert Oliver
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