How do you use lens tilt to place focus along a receding street?
Asked 3/7/2021
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I want to understand the practical focusing method for a tilt/shift lens when the subject is a street receding into the distance. Specifically, how do tilt (or swing) and focus interact to place the plane of focus along the roadway using the Scheimpflug principle? Is the correct approach to set the tilt first and then adjust focus so the plane of focus rotates onto the street plane? Also, what if the hinge line/axis is outside the image area—does that prevent this from working?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
12
Firstly, I believe you have a misconception. You quote,
Tilt until G is in the image (green).
and you ask,
What if G is not in in the image or cannot be placed into the image by tilting?
Unless you are tilting with a super-wide fisheye (I can't imagine such a thing), the hinge/PoF axis (G) will never be in the image. In the example in your question, the PoF axis is located closely in front of the image plane, laterally to the left of the lens's optical axis, by a distance J = ƒ/sin() (per the Wikipedia article you linked to).
I believe Bender took his photo from the roof patio of the LondonHouse or River Hotels. Setting the PoF axis to be the front of the buildings along Wacker on the left, the perpendicular distance to Wacker Drive is about 75-90 meters or so, depending on how/where you measure.
Thus, you can pretty much fix your tilt angle by solving the previous equation for = sin-1(ƒ / J) ≈ 0.03° (for J = 75m) – 0.02° (for J = 90m). The point being, it's a ridiculously small amount of tilt.
In the image below, using PhotoTransit software, I've mapped a 35mm lens angle of view (, the green wedge) on roughly where I believe the shot was taken. The pink line is the image plane of the camera, and the lateral distance J to the PoF axis (G) is shown.
The PoF you're trying to get is roughly parallel to the optical axis of the lens, which just can't happen with any amount of focus.note You'd need to focus at or near infinity to get a PoF that is so heavily tilted. But as I showed above, there is virtually no tilt angle to the lens. Zero tilt at infinity focus is just... infinity focus with a regular 35mm lens.
Edit: Note: you can absolutely get the PoF to be parallel to the optical axis, or even tilt away from the optical axis, with enough beyond-infinity focus. Before this edit, my statement was assuming a perspective control lens, or tilt-shift adapter, on a SLR or mirrorless camera, where you have practically no beyond-infinity focus capability (if at all). But as @Bob’s states in point 12 of his answer, a view camera would be able to achieve this. Also, adapting a lens to a short flange distance camera, and using an adapter that is not long enough to properly adapt the flange distance, will get you beyond-infinity focus when the focus ring of the lens is set to infinity.
Originally by user11924. Source · Licensed CC BY-SA 4.0
user11924
5y ago
0
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Yes: in practice you set the tilt/swing direction to match the subject plane, then adjust focus to rotate the plane of focus onto that plane. The hinge line does not need to appear in the frame; it is usually outside the image, often close to the camera.
A useful mental model is that tilt creates a wedge-shaped depth of field. Tilt changes the wedge’s angle and hinge position; focus moves the plane of focus within that geometry. So the workflow is generally: choose tilt direction, add a small amount of tilt, then refocus and fine-tune tilt iteratively.
That said, for the example image, community answers noted tilt would not really recreate the desired result along that whole street from that viewpoint. The focus plane in the original appears to be set too close, and a regular lens with better focus placement and/or a smaller aperture would likely improve the street sharpness more simply. In other words: the hinge line being outside the frame is not the problem; the scene geometry and chosen focus/aperture are.
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