Wednesday 29 April 2009

Wedding Photography - exposure values

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Continuing on from yesterday's post.

Thanks for that detail David. So just to clarify - you did not use flash in that shot? I'm not too clear on what the thinking is with point 1 in your reply. I'm keen to know at what point you decide to turn off your flash outdoors. For example, in your full length portraits of the couple (over 100mm focal) would there be any point using flash on a bright day? I'm guessing the shutter speed would always be above the flash sync, so would even FEC +2 give enough power to make a difference?

Thank you, Neil

David, was there a reason you went with f/8 for this shot? With the crowd in a level line you could have gone wider right? Underexposing for ambient is difficult in daylight as the shutter speeds become so high, the Speedlite HAS to work in high-speed sync mode, reducing its range a lot - I think about 7ft is your limit. David what do you think?


Hi chaps,

Thanks for the comments. I'll address them one by one:

1. No flash was used with any of the group shots.
2. To address point 1 in my original reply let's look at some hard exposure figures (also check out my post on exposure values for some background info). To expose correctly for faces in the group shot (the subject) required 1/125, f/8.0, ISO 100 (we'll ignore the exposure compensation tweak) - this equates to an exposure value of 13. If I had wanted to expose correctly for the sky (the background) the exposure would have been in the region of 1/4000, f/8.0, ISO 100 - an EV of 18 and a 5-stop smaller exposure than that required for the group shot. If I'd wanted a bit of drama I would have needed to underexpose in addition. If I did so by a further 2 stops (ie 1/8000, f/11, ISO 100) the subject matter would now be 7-stops underexposed - and I'd have to make up this difference with my Speedlites! This is a huge amount of light which is beyond their capabilities. Their output is further limited under these conditions since the X-sync speed on my camera is 1/250 so we'd have to set the Speedlites to high-speed sync (FP) mode which reduces power output to about 1/3. Also the smaller the aperture of your lens the harder the flash unit has to work - and we're at f/11!
3. Using flash outdoors is a judgement call based upon the factors I mentioned in the original post - primarily the exposure balancing required and the subject area that needs to be lit (both of these determine the amount of power output required).
4. I probably could have used flash with the couple shots to just add a 'kiss of light' - I was shooting at f/2.8. Speedlites have a Fresnel lens for focusing light (up to a focal length of 105mm on my 580EX IIs). Having said this, I would probably have taken the flash off-camera though and had someone holding a Speedlite just out of frame and directing the light for me.
5. Flash units are quickly overwhelmed outdoors!

On to Peter's comments:

6. f/8.0 - f/11 is generally the sharpest aperture of a lens. If I can, I try and shoot groups at this aperture as well as the crowd-pleasing wider apertures!
7. High-speed (FP) sync mode reduces output to about 1/3. Range will depend upon the aperture you're shooting at but it probably won't be far from the figure you suggest.

Hope this helps! All the best, David

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