Moving items and panning

[ericmark]

I was given the task to take two sets of images one high speed the other low speed of vehicles passing the college on the 30mph limited road.

Since it was an exercise rather than looking for good images I went to the limits with 1/15 second and ISO 100 and 1/2000 second and ISO 1600 letting the camera select the aperture.

The fast speed was as expected with very little blur and a load of noise the picture really showing the grain dropping to 1/1000 second and ISO 800 produced better results as expected.

However the low shutter speed of 1/15 second produced so surprising results with part of the vehicle pin sharp and progressively less sharp as one follows the length of the wagon. Not just one but a number had same effect. So thoughts as to why.

1) The image is also becoming larger and smaller and it could be the way it is zooming in! However since the sharp bit was not in the centre and there were no radial lines this seems unlikely.

2) Just plane out of focus. However with a 28mm lens well stopped down this also seems unlikely.

3) The relative speed me to wagon varying throughout the wagon length! Now this seems possible! The wagon went past around 50ft from where I was standing and is about 50ft long so the furthest part more like 70ft from me.

So started to calculate and at 44ft/sec (30mph) I need to turn 2.5 degrees in 1/15 second to keep far bit in focus and 3.5 degrees in 1/15 of a second to keep close bit in focus.

OK maths seems to show why. But is my theory sound? If it is then if a photographer is on outside of a bend then he should get less of the vehicle in focus than if he is positioned on inside of the bend.

Is this true? If so then what I have found is useful and if taking pictures of a race I should always try to get on inside of bend. Also likely safer position. However with cycles they bank and so on inside one would need to be low and on outside a higher elevation can be used and I don't like lying on the floor and anyway hard to pan lying on floor much easier when standing up.

So in some ways I hope I am wrong.

I know if I raise my speed to 1/60 second I can get most pin sharp and using more like 1/250 second I can get nearly all of the vehicle pin sharp but that is not the answer I am looking for. What I want to know is the relative speed theory I have related what is really responsible for having just part of the vehicle pin sharp.

I will try to add image once up-loaded.

[Kawarthabob]

My head hurts now

[asnow]

Mine does too. All I know is the rule of thumb I once heard. As a start point for panning ... the speed of the picture is the inverse of the speed of the object that is moving. Example, if the object you are panning on is moving at 60 (don't know if it is kph or mph) then the picture speed is 1/60 of a sec. If this is wrong then please correct.