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 Author Topic: How does Tilt Affect Hyperfocal Distance?  (Read 1027 times)
Alun
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 « on: November 16, 2012, 03:42:12 PM » Reply

As per the title, I have a 35mm lens with a Leaf Aptus II 7 back. The hyperfocal distance for that lens/sensor combination is 4 metres. How does tilting the lens down 1 degree (I use an Arca RM3di) affect the near focus (currently 2 metres) and does it leave the "infinity" focus alone or foreshorten it?

Alun
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Tim Gray
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 « Reply #1 on: November 16, 2012, 04:45:58 PM » Reply

It just shifts the plane of focus - see: http://en.wikipedia.org/wiki/Scheimpflug_principle
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duane_bolland
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 « Reply #2 on: November 16, 2012, 05:01:00 PM » Reply

It just shifts the plane of focus - see: http://en.wikipedia.org/wiki/Scheimpflug_principle

I was just about to post the same link!  Check out figure 2.  If the hyperfocal distance is 4 meters, I can't imagine a 1 degree of tilt makes much difference.
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Alun
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 « Reply #3 on: November 17, 2012, 02:41:31 AM » Reply

Thats great. Thanks for both of your replies

Alun
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buckshot
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 « Reply #4 on: November 17, 2012, 11:23:35 AM » Reply

@ Tim Gray: Yup, changing focus distance alters the focal plane angle about the hinge line.

@ duane_bolland: Nope, 1 degree of tilt can have a dramatic affect, especially with a wide angle lens.

In general, the HFD is a good place to start for some situations when introducing tilt and you'll find that, with the lens tilted (as mentioned above), changing focus changes the angle of the plane of focus about the hinge line.

So for example, assume the flange-focal length of your lens is 35mm at infinity. This means that if the centre of the lens is 200cm above the ground plane (which we'll assume is perfectly flat) and the camera sensor is 90 degrees to this (i.e. no other tilts in play), then with the lens focussed on infinity and 1 degree of tilt, the plane of focus will be along the ground plane. As you focus closer (lens moves further away) that plane will tilt upwards about the hinge line; as you focus further away, this will tilt downwards about the hinge line. There will be a depth of field associated with this plane of focus: it's 'wedge' shaped, beginning at the hinge line.

With wider angle, non-retrofocus, lenses (e.g. a 28mm) the flange-focus distance decreases (when focussed at infinity) and you start having to set angles of tilt like 0.6 degrees (depending on tripod height) to get the classic 'wedge shape' zone of focus from the hinge line to infinity. Getting close enough is usually good enough; the only system I'm aware of that accurately allows you to set tilts of 0.1, 0.2 degrees is Alpa's - but only with a very restricted set of lenses. Your RM3d has a nice tilt mechanism but it's really too coarse to set 0.1 degrees accurately and repeatably (not that you really need it this accurate for landscape photography - so what if the ground plane is 10cm above or below where it 'should' be, depth of field will usually save you).

When you start changing focus (with the lens tilted) to change the angle of the plane of focus - you do need to start looking at the effect on the ground glass - the tables that you commonly see are usually created only for the lens focussed at infinity. People say this is horrible with small a small ground glass, but it's not so bad.

Note, all of the above is for a non-tilted camera - if the camera is tilted, say, backwards, then the distance to the hinge line increases, and you have to focus beyond infinity to bring the plane of focus back to be coincident with the ground plane. Here a sytem like the Arca or Techno has an advantage over the Alpa, Cambo etc. since their fixed helicals generally stop at infinity (but can be adjusted to go beyond if needs be, but not on the fly).

Getting back on track - to answer your question, I'll make the following assumptions:

1. There are no other movements in action on your camera.
2. The flange-focal length of your lens at infinity is 35mm.
3. That the centre of the lens is 2m above the ground plane.
4. That you have focussed at exactly 4m, so that the flange focal distance is now 35.3mm.
5. That you have tilted the lens downwards by 1 degree.
6. That you are shooting at f/11.

What you will find is that the plane of focus begins on the ground plane, directly under your lens (I'll say 'just in front of your feet' from now on) and angles upwards in front of you at 26 degrees to the ground plane. The front edge of the depth of field associated with that plane will again start just in front of your feet and angle upwards at 58 degrees. Its back edge will again start just in front of your feet and angle downwards at 34 degrees.

Note: I have simplified this stuff to make it readable and applicable in a practical situation. See Merklinger if you really want to blow your socks off.

Jim
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