Need help to understand exposure times

[WyzMan]

Hello
I am sure that this is a basic question but how else can a boy learn?

Please can someone help me to understand how my camera works by commenting on the words below. All of this “desire to learn” came after purchasing a fancy flash gun and reading about high speed flash synchronisation – which I think I understand now. Thanks for indulging me, so here goes …

My camera has a focal plane shutter. This means that it has two ‘curtains’, which move in front of the sensor, one after the other in order to expose the sensor to light once the release button has been pressed.

Slow speeds ….
I press the shutter release button. Curtain 1 moves to uncover the sensor (horizontally or vertically – immaterial to me!). Some time later, at a time determined by what has been set by me or the electronics, curtain 2 moves over the sensor to cover everything up again.

OK so far – slow means anything longer than the maximum curtain speed which for my CANON 10D is 1/200 second.

High speeds ….
Curtain 1 moves over and before it gets to the other side, curtain 2 starts its journey resulting in a slit, (whose size depends on the exposure time), moving across the sensor, exposing light to the sensor surface.

So… high speeds, less light on the sensor over the period of 1/200 second - the maximum curtain speed - correct?

Is this right?
If so, it seems to me that it always takes 1/200 second to take a picture (at ‘high’ speeds). The difference between say a 1/500 second exposure and a 1/1000 second exposure is simply the size of the slit which traverses the sensor, NOT in the 'speed' of the shot. Each sensor pixel may only be exposed for 1/500 or 1/1000 second but the time it takes to assemble the image is the same - 1/200 second.

So … how come a fast exposure is used to freeze fast motion? If the subject moves significantly during 1/200 second, there is going to be some blur no matter what exposure time is dialled in.

Thank you - Alan

[scottbergerphoto]

It's kind of hard to follow your question. Just to make it simple(for me), the exposure time, is how long the sensor is exposed to light or how long the shutter is open. The shutter speed can go much higher then the max sync speed of 1/200 of a sec. The 10 D is good for 1/4000 sec? The max sync speed is the fastest shutter speed that will allow the flash to expose the entire frame. You can actually go higher then that with the High Speed Sync Mode on the 550EX. That give a series of lower intensity pulses of light.
Regards,
Scott

[WyzMan]

Scott,
With a Tv of 1/1000 second, is the entire sensor exposed at once for 1/1000 second?
OR (and this is what I understand to be the case)
does a 'slit' - between the two curtains, travel over the sensor (taking 1/200 second to do so), with the slit size set such that each pixel on the sensor is only exposed for 1/1000 second - the Tv setting?

If the former - sorted - I can appreciate how we can freeze fast action.
If the latter - stumped!

Thanks,
Alan

[scottbergerphoto]

Scott,
With a Tv of 1/1000 second, is the entire sensor exposed at once for 1/1000 second?
OR (and this is what I understand to be the case)
does a 'slit' - between the two curtains, travel over the sensor (taking 1/200 second to do so), with the slit size set such that each pixel on the sensor is only exposed for 1/1000 second - the Tv setting?

If the former - sorted - I can appreciate how we can freeze fast action.
If the latter - stumped!

Thanks,
Alan
The process was elegantly explained in this thread:
http://photography-on-the.net/forum/showthread.php?t=33872
The 1/200 is the fastest shutter speed on the 10D when both leaves of the shutter are wide open for the flash. At faster speeds, a slit moves across the sensor, exposing each part of the sensor for whatever the exposure is set for. If that wasn't the case, then you couldn't use the same settings on different cameras and come up with the same results. You couldn't use a separate light meter. There are leaf shutters with very high sync speeds.
I don't know how long that process takes. From what hmhm wrote, it appears that the process takes 1/200 sec, and the size of the slit is determined by the shutter speed.
Regards,
Scott

[cmM]

Someone please correct me if I'm wrong :!:
The exposure time is the time value you input (ex:1/1000), not 1/200 with a smaller 'slit' than 1/500 with a bigger 'slit'
So, the sensor is only exposed for that 1/000 of a second. Think about sports photography, drag racing for example… A car moves at 200 mph => 200/3600 => 0.055 miles per second * 1660 = 92.22 meters per second / 200 = 0.461 That is the distance a car moves in a 1/200 of a second if it travels at 200mph. If the actual exposure time is 1/200 of a second, then how do you explain the photographs where the action is frozen completely ?

[robertwgross]

For purposes of metering exposure, you can forget about how long the entire shutter process takes. The only thing relevant is how long each sensor pixel is exposed to light while the shutter slit passes over it.

For non-flash purposes of controlling subject motion blur, there isn't much you can do. You crank up the shutter as fast as you can and then shoot.

For flash purposes of controlling subject motion blur, you can use high-speed flash sync. As has been discussed, the flash unit puts out a quick sequence of flash bursts that are staggered out over the entire shutter process. So, it is possible that one sensor pixel is illuminated by flash 1, and another pixel by flash 2, and another one by 3, etc. If you do this enough, you can see a little bit of "curve" in the subject in the finished image.

Also for flash purposes, there is first-curtain sync versus second-curtain sync. For a fast moving subject, second curtain seems to provide better results in the finished image.

---Bob Gross---