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Hi Guys... and thanks to those for their kind words.
Let me see if I can break this down and keep it clear.
Someone mentioned the film plane. That is key to this discussion. ONLY that part of your subject that is absolutely parallel to the film plane (or lens face) can stay in focus... assuming you're dead-on with your panning speed.
So... if the subject is going straight in front of you... from one side to the other... that car stands a decent chance of being sharp corner to corner.
Now... as you move into a situation where the subject is coming toward you at an angle... or going away, only a slice of the subject will be parallel to your lens or film plane. The sharper the attack of the angle, the smaller the slice.
Here is why. The part of the subject that is closest to you is traveling the fastest. For instance, if you shoot a 3/4 pan of a car 100 feet away it's going to be pretty simple for your camera to catch and your pan to be decent. Why? Because your not really panning... your camera movement is minimal in order to keep up with the car. A little tougher at 60 feet. A beast at 30ft.
Think about this... a car travels by you at 30mph on a road 50 ft. away. No big deal right. What if that car came by your at 30mph 12 inches from your nose? You'd be heading for the bushes.
So, if the car is coming at you at 45 degree angle, you could draw an imaginary line from the driver's headlight to the passenger taillight. Point your camera barrel so it's looking right down that line. Now draw another imaginary line from the driver's taillight to passenger headlight. Now you have an X through the car. This is a little bit off because a car is not a square... but I think you get the picture. That second line, is basically parallel to the face of your lens of the film plane. So, we want to try and pick a spot along that imaginary line and pan with it. If we pick up by the front of the car, our panning motion is going to need to faster than a spot near the back of the car. Because the close point to us is traveling the fastest.
So... when we shoot this shot, we're going to have to make compositional trade-offs to get what we want. The more we want sharp, the higher (safer) the shutter speed will need to be. The more radical and blur we want, the slower the shutter will be. With the slower shutter the job of getting something sharp becomes much more difficult. Even side-to-side pans are difficult at slower shutters. So it only makes sense, that a 3/4 will be tougher.
If you want the nose sharp, put your camera's focal sensor near the middle of the windsheld where it meets the hood. Almost on the passenger's windshiled wiper. Remember, though... your going to have to pan at that speed and stay with that spot.
Start out a faster shutter speed. These shots are fairly forgiving and you'll still get decent blur even at 1/200th:
Example here: (EXIF is intact) Here I was going for the driver's helemt... but as you can see, at 1/200 a lot of the car held focus... quicker shutter, less time for the car to travel off plane.
This one is at 1/125 and is softer in the nose.. but for me, in this shot, the energy is still going by the viewer and I like it... especially the background.
Lastly an example at 1/40th. Here you can see things getting extreme:
What's important, though... with each of these examples, if you draw the imaginary line from where I was standing, you can visualize an X at the point of focus. So, that's your challenge... putting the X where you want it and getting your panning motion to keep the film plane parallel to that spot.
Of course, all the basic panning rules then come back into play. Smooth motion, CONTROLLED motion.... not too fast or radical... not too slow and wobbly. Find the right postion and distance from the car that allows you to pan at a speed that fits your "swing." Choose a focal length that allows you to stay in control. It's all about your SWING. Just like in stick and ball sports... you have to have your swing and it has to be consistent. Choose the right club...err... lens for the distance and go for it.
Hope that helps.