What is there to write about shutter speed ? If the speed is say 1/500sec it simply means shutter gets opened for 1/500sec and then closes - right ? Wrong !
I recently bought a SB-800 speedlight for my camera and was trying to go through sync speed related information. I wondered what is the limitation for supporting shorter sync speeds. This resulted in some reading. I read lots of articles about front and rear syncs - no where science part of it is clear till I came across this link which beautifully explained how the shutter works. With this explanation of how shutter works the limitation of flash sync speed made all the sense. This may be a useful information for you too !
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- Ganesh H Shankar
6 comments:
Superb article to share Ganesh. Thanks! Let me completely clarify with you as I have few grey areas. So, lets assume the shutter speed of SB50DX is 1/1800s as given. Dividing this by 3 gives as as below:
------|------|------|
0 600 1200 1800
Between 600 and 1800 is what the flash peak happens and to get the flash to fill the entire frame, it means that we should have exposed the film/sensor between 1/600s and 1/1200 ,Is that correct? Also, In few DSLR there is something called as first curtain sync and second curtain sync option given with 1/3 stop and 1/2 stop. I am wondering as to what can be a scenario where in you don't want the flash to fill the entire frame? And are about 2 options used for the same or used based on the flash that you use?
Thanks for the insight...though not completely understood it real fun to know these in and out...me being a technical person!
Pramod ! It is quite confusing I know. Nevertheless this is the best link I got so far about flash synchronization. I think you have some missing points related to the operation. This is what I understood.
1. Shutter speed is directly proportionl to width of the moving slit - this is a very important first point to note.
My guess on why shutters are designed that way is because of the mechanical limitation of time required to achive very high shutter speed. Opening and closing of the shutter window takes some time which can't be done at very high shutter speed. To explain, let us assume it takes atleast 1/300sec to move a curtain from one end to the other. If this is true then how can we support speeds lesser than this - for example 1/600sec ? Unfortunately in 1/600sec curtain can only traverse half the frame! Oh, why not then expose the entire frame two times covering 1/2 of the frame each time ? That led to a design with two curtains - front and rear - front moves first there is slit and rear comes next with an adjustable width which is proportional to the shutter speed we want.
With this design we have solved the first problem associated with the mechanical limitation of time taken for the curtain to move from one end to the other. But this created another problem for flash usage! Not a problem for ambient light.
2. Now for shutter speeds lesser than 1/300sec a slit moves to expose the frame. What happens if the effective flash duration is less than 1/300sec and we need shutter speeds less than 1/300msec ? The frame beneath gets un-evenly exposed. So in this case effective flash duration should greater than 1/300sec.
In reality camera's effective sync speeds vary between 1/250-1/500sec. Shutters speeds less than them will create special effects ! (whether we like it or not). Flash designs try to overcomes this with multiple firing during the exposure etc but they may not be very accurate.
Hope this helps - and you owe me a coffee for this !!
Ah! This is superb. How about you writing a full fledged article this? Its will be an awesome writeup, I am pretty sure!!
And coffee? I am ready to dine with you :-). Let me know the time when you are free and I am honored to treat you ! :))
If you are really interested in spending some time reading and understanding the concept of camera and off-camera flashs. This is regarded as the bible of it. A wonderful resource. Have fun reading.
http://strobist.blogspot.com/
Can i also be a part of the coffee gettogether ?
Let Shiv come back - we all will plan and go for a dinner - I will treat no problem !! I owe one to Shiv anyway..
Have not read the article in detail yet :( once i read will post my comments again.
On a side note have read that the mechanical limitation you mention limits the shutter speed to 1/250. This is true for canon. This is the reason guess nikon moved away from mechanical shutter to electronic shutter to achieve a flash sync speed of 1/500. But looks like the electronic shutter life is lesser than mechanical shutter. So nowadays a combination of both is used depending on if flash is used or not.
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