Digital photography

[Colloden]

I took this picture today with my new toy. As I did so the car below moved into the frame at about 30mph. Had it done so on an ‘ordinary’ film camera I would have expected it to appear normal or blurred depending on the shutter speed, film sensitivity etc. However the is clearly misshapen having oval wheels.

I can't find any web sites covering this. Anyone know how and why this happens ?

[Psi Baba]

Wow! Lorentz-Fitzgerald Contraction? I think you've underestimated the speed of the vehicle! :)

[Colloden]

There was a young fellow named Fisk,
whos fencing was exceedingly brisk.
So fast was his action,
the Fitzgerald contraction,
reduced his rapier to a disk.
;)

[richardm]

Ah yes. There is a famous series of photographs of high-speed cars that was taken in the 1920s, and they were deliberately shot to give exaggerated elongation to the wheels like that. Except the wheels are elongated in the other direction to enhance the impression of speed.

They are so famous that you don't need me to provide any sample images, or even the name of the photographer. Do you?
No.

Good.

Just as well, really ;)

[richardm]

I suppose it must be something to do with the way the object moves vertically across the image plane as the shutter moves horizontallyacross it - on modern cameras the shutter is usually a pair of curtains, with both moving at the same time, creating a slit through which light goes. Depending on the direction of the curtain, (up or down) the horizontally moving item will elongate left or right.

Things must have to be just-so to get the effect without blurring.

I suppose you've captured the same effect as the digital CCD scanned the light?

[Colloden]

Originally posted by richardm
I suppose you've captured the same effect as the digital CCD scanned the light?

Looking at the image I think you right. The car is moving to the right in the image as the camera is saving it to memory. If the copy from CCD to camera memory works from the top of the image down the bottom half of the car will be further to the right by the time it is copied.

.....++++++++
........++++++++
...........++++++++

As opposed to

.....++++++++
.....++++++++
.....++++++++

if you see what I mean.

Why do I bother with you lot?
I’m a genius.
GENIUS !
:D


I'm using capitals so it must be true

[jimlintott]

What richard accurately described is a focal plane shutter. They used to move slower than they do now resulting in a similar type of distortion. Yes, photos of early race cars often had oval wheels. A leaf type shutter doesn't have this problem.

I think that colloden has probably struck on the right answer which is a very similar effect to the shutter problem.

[ceptimus]

As it's a digital camera, it (probably) won't have a shutter at all. However, the image capture process does work it's way down the CCD sequentially, so there is a scanning effect very much like that which occurs with a focal plane shutter.

Although any one part of the picture is only 'exposed' for say 1/250 of a second which freezes the motion and eliminates blur, the scanning of the whole frame may take (say) 1/10 of a second, during which time the car has moved a significant distance.

It is not only the wheels that are distorted, the whole vehicle appears to lean backwards.

The same effect often makes helicopter blades appear bent.

[a_unique_person]

I bought a cheap digital camera, which I promptly returned, because the image aquisition processor in it was so slow that even a tall building was not straight. It was a Benq. Which is Acer of Taiwan trying to sound like they are really a European company. Don't buy anything they make, it is all cr@p.

I then went and bought a demo Kodak 2 megapixel with 3X optical zoom. It is cheap, but it works much, much better. For the price I paid, I am happy with it for the next few years. By then, the cameras will all be 10 Megapixel and $50. Hopefully I will be able to buy one that can take the lenses from my SLR, as I don't think good lenses will ever get any cheaper.

[rustypouch]

Yes other people beat me to it, but this effect is the result of a focal plane shutter.

And speaking of digital cameras, I have to brag. This week I have been using a Hasselblad with a Phase One back. It's worth more than I am!