Shutter speed depends largely on what subject your shutting... size matters!
Let's start this little primer with a disclaimer; There are many ways to skin this cat. While there are technical theories and logic to be applied, opinions, styles and technique will vary. What is written here is my take on the situation based on my personal experience and philosophies. So, use at your own risk.
It seems any discussion of motorsports photography will ultimately make its way back to the pan shot. While it might be a "staple" shot in a shooter's repertoire, it can also find its way of becoming a crown jewel in your portfolio.
In reality, or at least to my mind, the pan shot is almost a genre unto itself. There are so many variations and "looks." In fact, the panning motion creeps in to many types of shots. It's simply a means of keeping energy and putting movement into your photos. It maybe a short quick pan tracking a tire changer or following a car coming off a corner almost directly toward you. It is for that reason, it's a required skill if you plan on calling yourself a "motorsports shooter."
Again, what I'm going to share is what works for me. Others may argue some of the points, but I think I've broken it down in a manner that will take some of the mystery out of the technique and give you a basis to build your skills.
Basic side pan at 1/80th produces good sharp results.
Let's start with the fact that there are no basic camera "settings." I see this question all the time on internet Forums... as if you can just dial in the settings and it will all work out. Like any shot, the settings can cover the spectrum depending on what you want from the shot. Even different vehicles will change the approach. I use much higher shutter speeds shooting bikes than I do with cars. Not because they're faster... but because they are smaller. Of course, your ability to adapt to each situation is what separates the good shooter from the average shooter.
And that brings up another myth... panning is NOT necessarily about the speed of the car... it is the motion of the camera. So, the trick is controlling that motion to create the effect and get the results you want.
The swing's the thing.
With a long lens, you can still get nice blur at a higher shutter speed. This is with a 500mm at 1/320 sec.
Panning is about YOUR swing. It is no different than a good golf swing, tennis swing... or any other activity where you "swing." It is all about developing a consistent swing. Sure, a golfer may swing a little harder or softer from one shot to the next... but for the most part he's choosing a club that fits his swing in order to accomplish the distance and shot he needs. The clubs are the tools that extend and generate the required results from his swing.
You need to do the same thing. Find a comfort zone. Swinging too fast will produce a jerky erratic movement. To slow will produce a wobbly shaky movement. You want a nice smooth movement. A rhythm... and follow through. And not necessarily across an entire 180 degree arc. But... it should be smooth, steady and COMFORTABLE.
A similar shot with a 200mm shot at 1/100 sec.
So, how do you keep up with the car's speed while controlling your swing? Once you've developed a constant smooth COMFORTABLE swing, you then shift your location and alter your lens (club?) choice to make the speed of the object your photographing fit YOUR swing. Get closer or move back... choose a longer lens... or shorter. But make the combination work so that you are in your COMFORT zone. In theory.... if you are in your zone... smooth swing... right lens, right distance, I should be able to reach over your shoulder and adjust the camera's shutter speed without affecting you... within reason, of course.
The further away the car is, the slower it appears to be traveling. Think about it... if a car came by just 12 inches from you and doing only 30 miles an hour, it's flying. It would scare the living daylights out of you. But, the same car could go by at 150mph 300 feet away and look like it's doing 55mph. You'd think nothing of it.
So, by positioning yourself properly and choosing the right lens, you can track the car while pretty much having the same swing rate most of the time. Of course you'll learn to adjust your swing speed up a bit or down as needed and as you improve. But ultimately you'll find that sweet spot where it just feels good... smooth and in control.
Hold the lens under the barrel... tuck your elbows in... breath...pick up the car... track it... then squeeze off a few frames while you are tracking the car... and follow through. It's all about rhythm. Once you find it, you'll get comfortable and start dragging that shutter slower and slower.
Don't look for fixed settings. It's different with every lens and every set of circumstances. Good panning is all about understanding the geometry. You can successfully pan with a 500mm lens at a higher shutter speed than you might with 100mm lens. Remember, it's the movement of the camera... so a little movement with a 500mm reflects a lot of movement when you extend the angle all the way out to your subject. A pan at 1/320 on a 500mm is probably going to give you similar background blur to a pan with a 200mm at 1/160.
So now, with the situation in our control, we can make creative choices that help tell the story we want to convey to our audience.
The Side Pan
This is the standard pan shot and probably the best place to start honing your panning skills. It's also the perfect shot to demonstrate what changes with different shutter speeds. Remember, your aperture setting is fairly inconsequential when panning. Other than stopping way down to handle the amount of light a really, really slow shutter allows, the aperture is not going to change much in the outcome of a pan shot.
Find a nice gentle corner where you can shoot from the inside of the turn. This is nearly the perfect scenario since your movement (which is an arc) has a better chance of syncing up with the cars movement (also an arc). If you're on the outside of the turn, your panning arc will be directly opposing the cars arc, so choose the inside. All we want to do at this point is get some success under your belt and work on developing your swing.
If you're in a corner where the cars pass through at a good constant rate of speed, start with your shutter at 1/250th. Use a single sensor in your cameras focal point settings. Put that point right on the car's side panel... follow... shoot... follow. Burst two or three frames. Take a look at the preview... magnify it and see if any of the frames look sharp. Look at the seams on the body work or decal edges. Shoot some more and start dropping down the shutter. Don't go crazy... but as the frames improve in sharpness see if you can get down to 1/100th or 1/80th.
Now download those images and study them. Depending on how tight in the frame you have the car, the "corner-to-corner" sharpness should be pretty good at 1/250th. And, it will soften if you got down to 1/80th. Any lag in your swing and tracking of the car will be reflected in the softness of the image. It will be more exaggerated at 1/80 than at 1/250th. Put aside your best shot at 1/250th... assuming it's sharp. Now practice until your arms are sore and see if you can make a 1/125th (or even 1/80th) shot as sharp as that 1/250th shot you put aside. That will show you your progress in improving your swing and your marksmanship. Believe me, it takes practice.
The 3/4 Pan
This 3/4 pan was shot a 1/160 sec.The 3/4 pan is a shot that's somewhat more "stylized" ... it's a pan where the car is coming at you.. as opposed to coming past you. Keep in mind, in order for everything to be in focus, the subject would need to be traveling on a plane perfectly parallel to the lens face. In a pan where the car is coming at you at an angle, only part of the car will be parallel to the face of the lens at some point of your swing. Try to picture a point where the car is intersecting with an imaginary line that is parallel to the face of your lens. The intersection is where things will be most in focus. Creatively, that's where you'll want to make your shot. Most shooters go for the nose, or in open cockpit cars, the helmet.
It's also important to note, as the lens sees it, the front of the car is traveling across the lens face faster than the rear... because (as mentioned above) the rear of the car is further away. So, we know only a specific portion of the car will be in focus. By really dragging the shutter.. 1/30 sec. in this case, we'll really get a lot of blur but we'll also give up a lot of "in focus" area. Ideally, if we manage to get the nose the sharpest point, we give our viewer the sense of a what a bullet this race car really is. The viewer has a visual sense of the speed and energy.
Same location at 1/30 sec. You'll see I was able to hang onto the nose, but the rest of the car shows much more blur than the image above shot at 1/160 sec.
These are just two types of pan shots... I'll write more on other types and other styles at a later date. What's important at this point is getting YOUR SWING consistent. Your swing should "feel" nearly the same with both of these shots... maybe a little more travel or a little less, but the motion and rhythm should feel the same. Like my original analogy... it's similar to golf... you develop your swing... then based on where your standing and the speed of the car, you choose a lens (club) that will put you in your comfort zone and get the job done. You only adjust your swing... you don't drastically change it.
Once you're feeling kind of lucky, put your focus on manual, drop the shutter to 1/15 sec. and point a 200mm through the trees. See what happens?
Forget that. If you're looking for great shots, there is no such thing as keeper rate. Sure, you're not going to be swinging for the fences when you first start out. Understandably, you'll start at a higher shutter rate. But, seriously, who cares about HOW MANY you get? The only thing that matters is the shot that makes you go
WOW. Get the shot. I'd rather have one mind blowing shot at 1/15th or 1/30th of a second than 100 nice pans at 1/125th. Unless it's an historic moment in time, no one will ever remember the shots at 1/125th.
Obviously, if you need things for a client or something specific... sure, get the safe stuff done. But then, go swing for the fences... go nuts. If you're getting too many keepers, you're not trying hard enough. That's why you have a delete key.