What's happening here?

Just a brain teazer.
16-45, 6th of a sec, hand held, manual focus, SR on, any idea why the wheels look like that? I could understand if it was lit by street lights cycling at 50hz but sunlight?


Ignore the bad pan looks like I was panning too fast amongst other things...

I think its owing to the way the focal plane shutter exposes the top part of your picture first and comes down to the bottom to finish, just like a narrow slit moving down, SO the bottom of the wheels have moved across the screen as the shutter has moved down.Regards.Spill.

Except that at 1/6s, there's no narrow slit - the whole of the frame is exposed.

It's only for exposures shorter than the flash sync speed that the trailing curtain follows the leading curtain more closely than that.

What I find puzzling is the strobed appearance of the wheels. If they were rotating smoothly during the exposure, I'd expect the holes in the hub caps to get smoothed out.

The only thing I can think of that might cause a strobing effect is the SR, but it seems a bit far-fetched to me.

I dunno what's causing it - but it's making my tummy feel wobbly the more I look at the photo

[quote=
Except that at 1/6s, there's no narrow slit - the whole of the frame is exposed.



[/quote]Right ,But the shutter is still exposeing the top of the picture first is it not.Regards.Spill.

I've had similar results sometimes with shake reduction on (never when it is off). I think it might be caused by shake that exceeds the shake reduction capabilities. I get that result sometime when riding in a car (as a passenger) and trying to take pictures of car passing us.

Just thinking about this, the wheels are shiny and shaped, there is a chance that they are reflecting light only from certain positions as they rotate. Because the SR is compensating for movement this has frozen the motion and acted like a strobe flash type effect.

It's the best explanation I can think of!

Nevertheless, for an exposure as long as 1/6s, the whole frame is exposed for a long time (approx 30x as long) in comparison with the time it takes for the curtains to go past.

So the amount of exposure difference between the top and the bottom is very small. I don't think it's enough to account for this.

And in any case, effects due to the curtain moving slowly cause circles to become ovals. Not at all what we're seeing here, I think. But I don't know what was causing it.

Antilock braking is another far-fetched idea that occurs to me that would cause a strobing effect

Probably the same effect as the old cowboy films when the Chuckwagon wheels went backwards!..lol..................

keep it up guys the answer is out there somewhere!
I like this they were certainly braking when they saw me, but maybe not quite that hard ChrisA
I'll nip out today if I get a chance and turn SR off, I'm fairly certain that that is the cause.
I quite like this mage actually, if that dog in the back had been pin sharp I think I would have flooked a pretty cool image.

Maybe Photoshop a dog in

I also kindof like the image.

Almost, but not quite. A motion picture camera takes pictures at 1/50 second at 25fps in countries where the mains electricity is at 50 Hz or 24fps where it is at 60 Hz (USA for example) If a spoke is slightly behind the one photographed in the previous frame then the wheels will appear to be turning backwards.

Oval wheels are indeed the result of the focal plane shutter's 'slit' moving across the image. I know a photographer who was commissioned to photograph a very technical golf book (swing techniques and stuff) He had to reshoot using a lens with a leaf shutter.

I think beakynet was closest but SR probably has little to do with it.
I would have to say it is very simply the overlapping of highlights as the wheels turn, move and bounce. If the wheel hub was solid you'd probably get a smooth blur, but the wheel is spoked (in a way) and you will get cyclical highlights that overlap or give a strobe effect and expose greater than where they intersect only with empty space.
You'll notice that at the edges where the wheel is smooth, the image is more consistently blurred, other than a few wavy patterns from the wheel bouncing on the road.

If you try to take pictures of car passing yours at dusk with said car lights on, the same effect happen were the lights are concerned, but with SR off, it won't do it. For that effect to happen, your car needs to be rolling while the other car passes you. I use that trick on my K10D as "special effect". Sttrangely, I haven't been able to get the effect out of my K20D. It doesn' mean it can't happen with the K20D, it might happen with the right speed combination.

If this is true, you wouldn't need SR to produce the effect - effectively the wheel is causing its own strobe effect (ie a flash only in certain orientations).

If this is the same as what amoringello meant, then I agree, but I didn't really understand what he was driving at.

I do find this a lot more plausible than SR - the freqency at which it shook the sensor back and forth would have to be the same as the number of spokes going past per second, as well as leaving it stationary for long enough to get each successive 'freeze' and this is too far-fetched for me.