Motion Blur

[Nigel G]

Hmm - I am happy that the wheel hub will be moving forward at the same pace as the train so in a panned shot it should be sharp (assuming I get the panning right).

But it was rotational blur in the wheels I was hoping to achieve.
Clive seems to indicte that the wheel rim is moving round at the same speed as the train goes forward with the spokes (which are the bits that will actually blur) traveling less distance as they get closer to the hub.
Mike - not sure I see how a wheel can be going at differrent speeds at the top and bottom ?

Somewher RPM has to come into this doesn't it? The 4 smaller wheels at the front of the engine must have a faster rpm than the 6 big ones?

[Birdsnapper]

It's a difficult one to get your head around. Imagine a vehicle with a caterpillar track: where the track is in contact with the ground it is stationary and the vehicle moves along it, but the top of the track is moving (at twice the speed of the vehicle - giving an average speed of the track equal to that of the vehicle). Now imagine the track getting shorter: the same principal applies even when the track is so short that it is circular, like a wheel. Still doesn't help with your problem though.

[petrochemist]

At least you shouldn't have the problem I encountered on many of my recent racecar shots at Knockhill, where one end of the car is sharp & the other shows considerable vertical movement.
I guess I must have picked a spot where the racetrack was rather bumpy. Train tracks are usually fairly smooth.

[Nigel G]

It takes 2 Steam Locos to haul a train up the hill out of Weymouth and for some reason Sir Lamiel wasn't available to help Oliver Cromwell so this weeks Weymouth Express was being diesel hauled Weymouth to Southampton.

So - in the 2 images below the leading diesel was the easier to pick up and get a consistant aiming mark on the front. Oliver Cromwell was at the back and travelling backwards so the target focus point was rather more hasty and was somewhere in the middle of the engine.

There's certainly lots of movement going on - probably too much - but I think that is because I am so close to a pretty big subject. Getting further away will cause fence problems again although at trackside a small pair of steps was great. At least the gloomy wet afternoon made getting a 1/50th easy.

Interested in your thoughts and observations.

[Gidders]

Interesting ... Looking at the diesel loco the front it's pretty sharp where you were tracking ... bit difficult to tell what's happening with the wheels.

For the Oli Cromwell, its nice & sharp in the middle of the engine particularly around the centre of the three large wheels .... where you were tracking and there's some great motion blur in the wheels.

So why isn't more of the image sharp? In this case the subject is tracking at an angle to where you are stood. Looking at the Oli Cromwell shot ... both the front (in advance of the tracking point) and the back (behind the tracking point) show blur. Because of the angle, parts of the subject in front of the tracking point are moving (relative to where you are stood) faster than the tracking point and those to the rear slower ... so both are blurred relative to the tracking point.

I would say your panning is pretty much spot on relative to the points you were tracking ... to get more of the image sharp - you need to press the button when the line of motion is at a 90 degree tangent to where you are standing - them the points in front & behind the tracking point are moving at the same (relative) angular speed.

The challenge is then that this is the fastest point of panning ... and you don't get any lead lines for your composition so you need to allow space for the train to move into. Practice, practice, practice - get on a street corner & photograph the fiestas & mondeos - lol