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strange blades

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strange blades

Old 13th Jan 2010, 07:47
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strange blades

A friend showed me same nice pics

I'm not shure if/how they're done. (photoshop i suspect)
Can somebody shine a light on this?

Regards



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Old 13th Jan 2010, 08:02
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It's most likely not photoshop, but has to do with the shutter speed (or frames per second if cut from a video cam) of the camera. It's quite a weird phenomenon to be honest, and I've never had it happen to me, but there's a technical explanation for it (I guess)

Here's a video where you have the same effect visible, although not taken with a DSLR:

YouTube - Landing in Samedan
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Old 13th Jan 2010, 08:49
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Same reason that the wagon wheels in old Westerns often looked to be going backwards, sure it's much more technical now, but a similar phenomenon.
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Old 13th Jan 2010, 09:28
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Strobing it's called,once saw it happen in when a Scotsman dropped a half crown.
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Old 13th Jan 2010, 09:38
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As Drapes says its to do with strobing, when theres a match between the shutter speed and the rotational speed of the prop, by chance I got a couple on a bog standard mobile phone.

First is a Kingair.



Dash 8-300



I might add copyrights mine, if anyone wants to use them please send a PM.
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Old 13th Jan 2010, 10:00
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i find its generally the cheaper camera's/mobile phones that do it. Better cameras or proper video cameras tend to show the prop disc as you see it
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Old 13th Jan 2010, 10:21
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Here is the effect, as seen from a relatively cheap spy cam attached to a model aircraft..

YouTube - RC Twin Gold Coast 31 July 08 0002

The bird way below..priceless

And

YouTube - RC Twin Gold Coast 1 Aug '08

Interesting the prop bend apparently forwards here


YouTube - Low Level Model Aircraft footage various compilation.wmv

Some effects here also
Fliegenmong is offline  
Old 13th Jan 2010, 11:39
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It's not strobing, strobing is caused by a pulsating light source and when the speed of the object and frequency of the light source is correct will allow an undistorted frozen or slow moving image object.

The effect seen here is caused by the way the image is scanned. It can be seen even in a regular camera with a moving blind type of shutter. With such a shutter the image is not captured all at once. There are actually two blinds which move across in front of the film, if the shutter speed is set to say 1/60 of a second the first one opens and then 1/60 of a second later the second one starts to close the window. This results in a moving slot across the film frame the width of which is determined by the shutter speed selected. When taking a picture of a moving object such as a propellor part of the blade will be in a certain position when the open slot gets to it, but since the propellor is moving as the slot moves across the film frame the it captures the blade in a different place resulting in the appearance of bent blades.

What we're seeing in these pictures is the same effect, the image is obviously captured bit by bit resulting in the same blade being seen in different locations as the image is built, piece by piece. I'm no expert on digital cameras but I imagine things which would affect these kinds of images would be how much of the image can be stored at once and the size of the image. Unless the entire image can be captured in one instant you will get this effect.

I used to have a camera book which had some good diagrams showing how this happens but I think I left it back in Australia, oh well.

Hope this helps.


To put it another way, in the kingair pic, imagine that picture cut into 10 horizontal slices with each being taken from the sensor and fed into memory one after the other from top to bottom. The first goes to memory then the second goes, but by then the image has moved, and so on. The dash 8 picture which I'm guessing was taken with the same camera shows fewer blades because of the different propellor speeds.

Note also that in slam dunk's pictures the blade images follow the orientation of the camera
Peter Fanelli is offline  

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