Panning Oddity

[Jay Beckman]

Ok all you physics majors, help me figure this out.

How can the nose and "STANG" be sharp but as you move aft, things progressively become a blurry mess?


What physical properties are at work?  Is it because the fuselage isn't completely tangent to the sensor plane of the camera?
An enquiring mind (mine!) wants to know.

[Dave S.]

Jay,

My $0.02:  I believe there are two things at play causing the referenced blur.

Your assessment of the focal plane is part of the equation.  I believe that motion, relative to the focal point, and the DOF play big roles as well.  If 'STANG' was the focal point, anything in front or behind that point (now part of the focal plane you referred to) will not be tack sharp.  Obviously, the aperture setting impacts the DOF and resultant sharpness as you move in front of or behind that focal plane.

The other part of the equation that has caused your 'blurry mess' is the relative motion of all the parts on the airplane as you panned for the shot.  Since the airplane was not traveling parallel to you at the time of the shot, just about all of the other parts of the airplane (wing leading edges, wing tips, tail) are not tangent to the focal plane.  During the time frame the shutter was open, while the focal point mostly moved left to right, all of the other parts (wing leading edges, wing tips, tail) also moved left to right, but they also had some additional relative motion towards and away from that all important focal plane.  I think this additional motion that moved through the DOF while the shutter was open played a significant role in causing said 'blurry mess'.

I think the motion of the wheels on the main gear is deceiving, in that they are still spinning (from the take-off roll), but they are close to the focal point, both in relative distance from the focal point and relative motion from the pan).

As I sit hear trying to coherently explain this, I am able to use my two hands to walk right through the explanation.  It all boils down to: DOF, angle of incident, relative velocity, focal point ('Stang') and distance to the focal point.  I'll see if I can find some time to do an analysis that combines all these variables to come up with some numbers for the relative motion for the different parts.  Shouldn't be very hard, they are all just numbers, .

Anyone have other/different ideas?

Dave

[Joe Copalman]

Nothing to add to what Dave said, but this is a problem with a lot of my shots at Coolidge.  On several of my front-quarter shots of departing aircraft at Coolidge, the front of the aircraft will be rivet-counting sharp, but the rear will be can't-read-the-N-number blurry. 

This happened on a handful of shots at El Centro as well, and I think Bill Osmun mentioned that higher/narrower apertures tend to give more even focus than lower/wider ones do, which got me thinking about my long-running problem with Coolidge shots.

[Paul Dumm]

No,  just ask anyone who lives out near Florence and Coolidge,  there is a BIG Blackhole out there someplace that does weard thing to normal people.

[Jay Beckman]

Based on some info I received on another forum from the resident motorsports gurus, Dave is on the right track with the idea of relative motion.

If you were to draw lines from the spinner, nose art, cockpit and tail code to the nodal point of the lens, each is moving at a different relative speed.  The cure is a higher shutter speed but of course this is anethma to what we plane shooters are trying to do.  The closer you are to your subject, the more pronounced the effect.  I guess racing shooters see this a lot when panning from the inside of a turn at a race track.  Drivers door is tack sharp but as the car is turning (and the extreme ends of the car are moving to/from in a relative way) they get blurry sponsor graphics at the nose and spoiler ends.

So I guess the cure is back up and shoot longer, and wait 'til the plane is more tangent.  Catch the prop arc at 90 degrees and no one can tell how fast you shot it anyway, right?   

[jslugman]

Shouldn't be very hard, they are all just numbers, Wink.

Lol....

[Joe Copalman]

So I guess the cure is back up and shoot longer, and wait 'til the plane is more tangent.  Catch the prop arc at 90 degrees and no one can tell how fast you shot it anyway, right?   

I've actually been practicing something like this at Willie this week - when the aircraft is at a front-oblique angle and the prop disc is still very visible, I'll shoot at 1/160th to 1/200th, then as I'm panning and the disc is less and less pronounced, I'll ride the wheel on the XTi a bit to increase the shutter speed to 1/250th or 1/320th.  It's much easier with taxiing aircraft (where I'll go as low as 1/60th for nose-on shots), since they're moving slower.