Why does a panned car stay sharp side-on but blur at the front and rear in a corner?
Asked 12/2/2021
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2 answers
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When I pan with a moving car that passes straight across my field of view, I can usually keep most of the whole car sharp. But when I pan through a tight corner, I often get only a small area sharp—usually around the driver or helmet—while the front and rear of the car are blurred.
For example, at about 7 m distance and around f/5, I expected enough depth of field for much more of the car to look sharp. The main difference seems to be that in the first case the car is moving perpendicular to me, while in the corner it is moving at an angle and changing distance as I track it.
Why does this happen in panning shots? Is it really depth of field, or is something else causing the front and rear of the car to blur?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
4
Panning only freezes motion that is parallel to the motion of the camera. Which means panning is most effective when the subject is travelling *in a straight line, and when the image is taken directly opposite from the camera position.
Additionally, relative speed changes along with changes in distance. So at shorter distances the relative speed increases (increased SS blur) and any additional change in distance is proportionally greater (DoF blur).
Using your situational drawings I added identical panning arcs. You can see that the straight line pan is more parallel to the camera path with less change in distance and less change in speed (required camera movement).
Whereas the converging corner pan shows much less parallelism; with a much greater change in relative distance and relative speed on the left side as opposed to the right side. Because of this you should see more blur on the left side than on the right side, with the greatest effectiveness (sharpness) near the center where the two paths/arcs most closely match.
Also note that, due to the greater convergence of the two paths, there is a greater change in parallax (angle of perspective). Which means details cannot overlay each other exactly. E.g. a wheel that appears more oval at a distance becomes rounder when closer and more perpendicular.
(*actually most effective when the two paths are exactly parallel; but that almost never happens)
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
4y ago
0
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It’s mostly not depth of field. In a corner, panning only matches the motion of one part of the car well—usually the part you’re tracking. The rest of the car is moving differently in the frame, so it shows motion blur.
Why:
- Panning works best when the subject moves in a straight line directly across your view.
- In a corner, the car is changing both angle and distance relative to you.
- That means the front and rear of the car don’t move exactly parallel to your camera motion.
- The car also appears to rotate as it turns, so areas away from the point you’re tracking blur more.
So the helmet/driver area can stay sharp because it’s near your tracking point, while the nose and tail blur from unmatched motion. Any change in distance can add a little focus error and relative-speed change, but the main effect is geometric motion blur from the car’s changing angle and path, not simply shallow DOF.
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