Struggling with Flash and Flash Exposure!! :-/

You need to give a lot more info...

What type of flash did you use?
Some studio flash heads have a pretty long flash duration. In effect, the flash fires and then tails off gradually. As it nears the end of the tail the colour of the flash becomes progressively more red and there is little power left, but different makes and models do vary quite significantly, the older ones and the cheaper ones tending to be far worse.

With this type of flash, setting a higher shutter speed (but obviously not more than the maximum that will work) will cut off the tail, affecting the exposure.

You can test it yourself with a flash meter, setting the shutter speed at different settings will affect the exposure readings if the higher speed is clipping the tail of the flash.
 
sorry i meant with a flashgun

Its a 420ex

Basically im interested in putting together a strobist set up so thought id get a little more accustomed to my flashgun first, TBH i havent used it or any other flashguns for a while as eveything i do is with daylight, so ive been reading up and the constant i get is that shutter speed does not affect exposure, but it seems to be affecting it in this case? would my gun have a max synce speed? im not getting a curtain of black just a lower exposure as the shutter speed increases

thanks
 
If you're shooting in daylight, then your total exposure will be part flash and part ambient. In that situation, the shutter speed will obviously affect the ambient component, and therefore the total exposure.

I'm guessing that maybe the flash component was actually quite low, and therefore the effect of the shutter was quite noticeable. Do the same test in low light, eg indoors room light, and the shutter speed will make no visible difference.
 
ok so ive just tried it in my pitch black front room and theres a significant difference in exposure on me between a 1/500th and 1/1000th, the settings are iso 400, f3.5, and im marking the same spot on the floor so im no further away.... could it be that 420ex just has a max sync speed and 1/1000 is too much for it? sorry im just trying to understand it so i can confidently use on and off cam flashguns
 
Richard is right, if there is quite a bit of ambient then the shutter speed is going to have an effect. What you've probably been reading is the usual studio setup where apertures are quite narrow (f8 - 16) in a lot of case, this will mean that your normal indoor ambient is going to have basically no effect. However, if you are in a pretty well lit area, or high ISO/wide aperture then ambient comes into play. If the flash still more than the ambient then you will notice your subject is going to remain exposed correctly no matter what you do with the shutter speed (well until it is so slow that there is more ambient than flash), as the subject is illuminated by the flash, but the background is going to change as you allow more or less ambient light in with longer or shorter shutter speeds.

I've just read this again and while it makes sense it's not worded too well but I'm dead tired and can't think straight enough to tidy it up..... hope it makes sense.

I suggest having a play. Firstly make sure you are using the flash in manual mode and the same with the camera. If you set up where the flash power is high and the ambient light is low then you can play around with the shutter speed and you won't see a change. Now go for something, possibly with some natural light (evening is good) and again light with mostly flash. Now slowing that shutter down will allow more and more ambient in bringing the background up. Slow the speed the shutter up and you go back to a darker background. This is the way you would balance a subject shot against a sunset for example, you want to control the ambient as the sun goes down more and more but by only changing the shutter you can keep shooting with the same flash power making the shoot much quicker.
 
mattmatt001 said:
ok so ive just tried it in my pitch black front room and theres a significant difference in exposure on me between a 1/500th and 1/1000th, the settings are iso 400, f3.5, and im marking the same spot on the floor so im no further away.... could it be that 420ex just has a max sync speed and 1/1000 is too much for it? sorry im just trying to understand it so i can confidently use on and off cam flashguns
Click to expand...

Max sync is going to be 1/200 or 1/250s depending on the body with most Canons.

If you are using high speed sync to get over the lower shutter speeds then the faster you go the more you reduce the power of the flash.

If you use the same settings without flash are you getting any visible exposure?

Is ISO on Auto or are you in full manual?
 
full manual so iso at 400 and at 1/500th its a pitch black exposure without flash but a nicely exposed shot with flash, the 420ex doesnt have full manual capabilities its e-ttl, i imagine its just a case of the flash losing power as i demand more shutter speed from the ettl system??
 
mark1616 said:
Max sync is going to be 1/200 or 1/250s depending on the body with most Canons.

If you are using high speed sync to get over the lower shutter speeds then the faster you go the more you reduce the power of the flash.

If you use the same settings without flash are you getting any visible exposure?

Is ISO on Auto or are you in full manual?
Click to expand...

Yes. If you are shooting at 1/1000sec, or anything above your max x-sync which is 1/200sec, then if you are getting any flash exposure at all, you must be in high speed sync mode on the flash.

That is effectively not 'flash' as such, but actually ambient light in exposure terms, albeit for a very brief period so responds to shutter speed in the same way as normal ambient - it's basically like switching the room light on and off very quickly, just long enough to cover the full shutter cycle.

You need to bone up on how a focal plane shutter works, and high speed sync. There's a good video on the Pocket Wizard site. Edit: it's here :) http://www.pocketwizard.com/inspirations/tutorials/pocketwizard_controltl_optimiz/
 
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awesome stuff i thought it must have something to do with that but it was driving me mad not knowing!! the last time i used flash on a job they were full manual sb-24's and once on full power you knew what you were getting but this e-ttl and high speed stuff has thrown me out!! well thanks a lot for helping out guys :)
 
yeah bit pants but all i could afford at the time, bought a couple of sb24's now and found an old vivitar at a fair (all came to less then the 420 did at the time >_< ) so gonna get fully clued up on manual flash now!! :)

thanks again for your time
 
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