Flash and Shutter Speed

Tyler138

Tyler138

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Alex Tyler
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Hey I am looking into buying a flash in the near future and after playing with a fellow togs flash at an event i noticed that it was locked up to 1/200 shutter speed. (its was a speedlite 270ex)

I shoot mostly mountain biking and i found that some shots that this wasn't enough. Is it just the case of you get what you pay for and the more expensive ones give you the option of going higher or is it the case of freezing the action with the flash which i have heard being used before?

I am new to the whole flash thing so any info on buying one would be great.

Thanks

Tyler
 
It is locked at 1/200sec because that is the fastest shutter speed you can use and still cover all the sensor with the flash image.

The sequence of events when you trigger the flash goes something like this:

The mirror retracts, the lens iris closes down, the first shutter opens and then when it is wide open and the sensor fully exposed the flash triggers - the the second curtain comes across and blocks the sensor - the mirror falls down and the lens iris opens wide to view the next shot.

At higher shutter speeds the first and second curtain are never fully open - instead visualise a travelling slit passing in front of the sensor - the width of that slit determines the exposure the sensor receives - the smaller the slit the higher the shutter speed even though the slit may still take only 1/200 sec to go from one side of the sensor to the other.

If the flash were triggered at higher speeds than the set 1/200sec then only a part of the image wiould be exposed since the duration of the flash is much shorter than the time it takes the slit to go across.

It's rather like the effect you get when you photograph a TV at speeds over 1/25sec - all you get is a partial picture.

If you have a powerful flash you can freeze the action but only at night - during the day the daylight will usually be brighter than the flash and the flash will only act as a fill-in if the conditions are right.

.
 
Last edited:
petersmart said:
At higher shutter speeds the first and second curtain are never fully open - instead visualise a travelling slit passing in front of the sensor - the width of that slit determines the exposure the sensor receives - the smaller the slit the higher the shutter speed even though the slit may still take only 1/200 sec to go from one side of the sensor to the other.

If the flash were triggered at higher speeds than the set 1/200sec then only a part of the image wiould be exposed since the duration of the flash is much shorter than the time it takes the slit to go across.
Click to expand...
... and then there is High Speed Sync (HSS) whereby the flash will strobe on and off a number of times during the shutter opening to enable a picture to be taken at speeds faster than 1/200th (or whatever the cameras fastest sync speed is). I believe the 270EX has this feature - my 580EXII certainly does :)
 
Legacy said:
with reference to the flassh speeds of being locked at 1/200 th and HSS v's longevity battery power......

Have a look at this discussion and it has a great link in the first post which shows the flash duration v senso exposure.....

http://www.talkphotography.co.uk/forums/showthread.php?t=249110&highlight=hss
Click to expand...

Yes, that's a very good link in that link (not only for the stuff about flash :naughty: ).

The key to it is understanding how a focal plane shutter works (not difficult). Then it all falls into place :thumbs:
 
Cheers for the links will have a look.

And that Lightenupandshoot V-book looks really good will most likely buy that in the near future.

So HSS is when the flash strobes? So pictures that i have seen with a flash at 1/1250 shutter speed would be achieved this way?
 
Tyler138 said:
Cheers for the links will have a look.

And that Lightenupandshoot V-book looks really good will most likely buy that in the near future.

So HSS is when the flash strobes? So pictures that i have seen with a flash at 1/1250 shutter speed would be achieved this way?
Click to expand...

Yes.
 
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