A curiosity, just for experts…

Gentlemen,

I shot through the airplane´s propeller many times, and observed this:




Comparing whit images from a Sony DSC-W1, I noted that in the K10D shots, the propeller looks curved toward the rotation sense, when in the Sony shots look without this curious effect. I have hundreds of this kind of images all with the same curiosity and appearance.

As you can see in the image, the exposure time don´t affect this extraneous behavior, just change the visible blade angle (at faster shooting speed narrower visible angle). The propeller RPM are more or less the same in the four situations.

My vague idea about this effect is the different obturation systems; in the K10D the obturation perhaps is progressive (as the curtain of the film camera) when in the Sony perhaps is instantaneous, I don´t know…

Can you help elucidate these phenomena?

Thanks for your participation.


Best regards.
________________
Dirk TROTTEYN.
K10D+ Sigma 18-50 1:2.8 DC Lens.

horizontal vs vertical shutter?

I knew somebody would know
Now can you tell me what obturation means hahaha

I'm not certian on either...the pentax curved look is definately the shutter travelling vertically/
we know the shutter speed, but I don't know what the heck the other word means. other variables would be prop speed, angle of light, intensity of light, direction of rotation......is the sony a point and shoot? that would explain it too.....

The pentax has a focal plane shutter, in which two curtains allow a slit of light to progressively travel across the CCD. The faster the shutter speed, the narrower the slit. Thus, the propellor is in a different place as the slit moves, causing the effect we see. There is a famous old photo of a racing car where the wheel becomes oval for the same reason. Focal plane shutter distortion.

The Sony I presume is a compact or bridge camera with an electronic shutter that combines a shutter that is opening to show all the frame at the same time, some sort of leaf shutter, combined with a "freeze frame" where the CCD itself is part of the shutter process. The result is no distortion.

I knew someone who was commissioned to photograph many illustrations for a book on golf with a famous golfer. The pictures were of a 'how-to' nature. Using his latest SLR with a very high shutter speed off he went with the embarrasing discovery after the shoot that the golf club shaft was bent in every case. A costly re-shoot had to done using a leaf-shutter Hasselblad.

You can often get rid of such distortion by simply rotating the camera through 90 degrees. It depends how the moving parts of the subject and the moving parts of the shutter relate to each other.

Exactly right... the Cony is a fixed lens 'p&s'.

It can be a little irritating that you get this sort of thing. at 1/180 does it still happen? That is the maximum shutter speed for full frame exposure on the K10D.

Gentlemen,

First at all, many thanks to the participants for his rewarding concepts and his valuable time.

I forgot to mention than last night I did some basic maths about the blade tip travel at a given shot time for consistency with what we see in the images:

- Propeller circumference= 1.8 mts (dia) x 3.14 (phi)= 5.65 mts/turn
- Propeller rotation speed= 2500 RPM
- Propeller tip linear travel in 1 minute= 5.65 mts. X 2500 RPM= 14,130 mts.
- Propeller tip linear travel in 1 sec. = 14,130 mts./min / 60= 235.5 mts.
- Propeller tip linear travel @ 1/4000 s (obturation time) = 235.5 mts./sec/4000=0.0588 mts.
- Propeller tip linear travel @ 1/250 s (obturation time) = 235.5 mts./sec/250=0.94 mts.
- Propeller tip linear speed @ 1/1600 s (obturation time) = 235.5 mts./sec/1600=0.147 mts.

The final conclusion is, that the values obtained here, are coherent whit the images. So, the only meaning for the shooter speeds is the visible blade angle (not related with the curved prop pattern).

I will do some shots (will try) whit the propeller at the horizontal travel and the camera in normal position (or prop vertical and camera in portrait).

This phenomenon is not an issue for my and don’t bother in absolute, just a curiosity I wanted share with the forum members…


Best regards.
________________
Dirk TROTTEYN.
K10D+ Sigma 18-50 1:2.8 DC Lens.

Well thank you for that.
needed to exercise my noodle.
It was nice to get my brain fixated on something other than plotting asassaniation attempts on the "leave Brittany Alone" youtube freak.

Conclusions:

Two images I took this morning (31-OCT-2007).

In the PP I exaggerated a bit the black level and gamma for better sees the propeller.






So, no “Focal Plane Shutter Aberration” when the object moves parallel to the shutter path.

Just as our experts explained above in this tread!

Thanks for share your knowledge.


Best regards.
________________
Dirk TROTTEYN.
K10D+ Sigma 18-50 1:2.8 DC Lens.

A golf club's swing is moving through almost 180 degs. so no matter what the orientation of the camera it will be 'wrong'

My word... you must have lightning reactions, to be able to time the shot to catch the prop horizontal

Hi Chris,

I would like to have a sight and the reactions you mention, but I have just the same kind all human have.

For achieve my challenge, yesterday I did 49 shots in flight (of course not all for this purpose) but 9 with the intention to catch the prop in the desired location. When shot I don´t see where demon the prop is, so it is a hazard question…

Here is one of the failed shots, bat look what nice clouds I get in return…




At 1/500s you can see in the image more or less a 1/8 circle occupied by the prop pattern, so with a little patience, the challenge really is not so difficult.


Best regards.
________________
Dirk TROTTEYN.
K10D+ Sigma 18-50 1:2.8 DC Lens.

49?

Two or three, and it's an extremely good and careful look round first, in the south of England.

Ah, for such light traffic as the Argentine skies.