Shutter speed question

[Florin]

Hello! I am playing with a shutter speed tester and while looking at the graphs I noticed that there is a tiny difference in time from the moment the shutter starts to open to the moment the shutter is wide open (obviously). I was wondering: does the speed that I select (1/125 for example) represent the time my shutter is wide open or the speed from the moment the shutter starts to open until it is fully closed?

Thank you!

 

[Dwig]

What type of shutter are you testing and where are you placing the tester's sensor?

 

[Florin]

What type of shutter are you testing and where are you placing the tester's sensor?

Right now I'm just messing with it so I can see how it works and how I can measure the shutter speed as accurate as possible. First tests are with a focal plane shutter but I will try with a diaphragm shutter and maybe a leaf shutter. I tried to place the tester sensor in the center but I did not do a very accurate placement. I will probably make a jig for more placement accuracy.

 

[Big Mike]

I was wondering: does the speed that I select (1/125 for example) represent the time my shutter is wide open or the speed from the moment the shutter starts to open until it is fully closed?

It's the time that each part of the sensor/film is exposed to light.

Remember that a typical focal plane shutter is two curtains. The first one opens, followed by the 2nd one which closes the shutter. At slow or moderate shutter speeds, the first one opens until it's fully open, then the 2nd one closes....but at faster speeds (above the camera's max sync speed) the 2nd curtain may start closing before the 1st one is full open. At 1/8000, the 2nd curtain follows closely behind the 1st, only exposing the sensor/film with a strip of light across the frame.

 

[bazooka]

I don't intend to threadjack, but this is a directly related question... does this mean that the curtains each change direction with each shot? Because I'm thinking that on the first attenuation, let's say the curtains both move downward, with the front curtain "opening" and the 2nd curtain "closing", but on the second attenuation, the roles reverse (curtains both move up, but 2nd curtain opens firing first, then the 1st curtain closes firing last? Or do they reset somehow after the exposure?

 

[Big Mike]

I don't intend to threadjack, but this is a directly related question... does this mean that the curtains each change direction with each shot? Because I'm thinking that on the first attenuation, let's say the curtains both move downward, with the front curtain "opening" and the 2nd curtain "closing", but on the second attenuation, the roles reverse (curtains both move up, but 2nd curtain opens firing first, then the 1st curtain closes firing last? Or do they reset somehow after the exposure?

It's my understanding that the shutter resets after each exposure, thus the curtains would always travel in the same direction during the exposure. Although I can't confirm that, so I might be wrong. It would mean a very fast moving mechanism on a camera that can shoot 8 or 10 frames per second.

 

[Florin]

I was wondering: does the speed that I select (1/125 for example) represent the time my shutter is wide open or the speed from the moment the shutter starts to open until it is fully closed?

It's the time that each part of the sensor/film is exposed to light.

Remember that a typical focal plane shutter is two curtains. The first one opens, followed by the 2nd one which closes the shutter. At slow or moderate shutter speeds, the first one opens until it's fully open, then the 2nd one closes....but at faster speeds (above the camera's max sync speed) the 2nd curtain may start closing before the 1st one is full open. At 1/8000, the 2nd curtain follows closely behind the 1st, only exposing the sensor/film with a strip of light across the frame.

What about the diaphragm and leaf shutter?

I found a slow motion video of the Canon EOS 5D shutter in action.
SLR shutter tests

@ bazooka If you watch the part under the mirror after the mirror has dropped you can see some movement there (seconds 13-14) so I guess the shutter goes back to the initial position.

Here is a Canon Elan 7. You can see here exactly how everything happens.
SLR shutter tests

 

[Florin]

Played with it a bit more. Managed to get very accurate readings up to 1/2000th of a second for the focal plane shutter. I think I can get more out of it but I don't have a camera with shutter speeds above 1/2000th of a second. Can someone explain the role of the battery in the setups described on the internet? As far as I've seen, everything works as an electret microphone.

 

[Hamtastic]

let's say the curtains both move downward, with the front curtain "opening" and the 2nd curtain "closing", but on the second attenuation, the roles reverse (curtains both move up, but 2nd curtain opens firing first, then the 1st curtain closes firing last? Or do they reset somehow after the exposure?

Almost all focal plane shutters reset and move the same direction each time. Most modern 35mm FSLRs and DSLRs (35mm and APS) sort of move in a diagonal. At higher shutter speeds, when a slit is used to create very fast shutter speeds, if the subject were moving in the same direction as the slit then it may distort in the photo. The idea is that subjects are more likely to be moving side to side or up and down than diagonal.

The classic example is a fairly famous, very old photo of a race car speeding by. The slit in the shutter moved from top to bottom. Since the image is upside down in the camera we can think of it moving from bottom to top of the photo. The car was moving fast enough that the bottom of it's wheels were in a slightly different place when the shutter went across than where the top of the wheels were when the shutter reached that point. The result was round wheels distorted into forward leaning ovals. Cartoonists saw this and have been using it to signify a speedy moving vehicle ever since.

A modern example with video is rolling shutter.

 

[KmH]

Most modern 35mm FSLRs and DSLRs (35mm and APS) sort of move in a diagonal.

Say what?

All the modern focal plane shutters I've seen move straight down and when they reset they move straight up.

Am I missing something?

[ame]http://www.youtube.com/watch?v=7CHgBuKmfog[/ame]