Without TTL, how to use a flash to light up an entire room with correct exposure?
My friend has asked me to take a few large B&W group photos at the Christening of her child this weekend (tomorrow).
She says she expects about 50 people to be in the shot which is going to be in a very small Church.
I am going to try and use my Hasselblad 501CM (no TTL metering possible) with 80mm lens (the widest I have) with my Metz 45CL-4 flash. To get the maximum DOF I need to set my lens to the smallest aperature possible but obviously if it's in a Church and the light is low, that is going to give a really slow shutter speed.
Now, usually, for portraiture, I use my Sekonic Flashmate L-308S light meter to meter the scene, then set the flash to one or two stops below that to provide soft fill in flash. e.g if the meter say f8 1\60th sec I set the flash to f4 or f5.6. But in this scenario I expect I am going to rely on my flash to light up the entire room. But I'm worried that in the Church the light will be low anyway and using a small DOF might result in a metered reading of, for example, 1 second or something and so setting the flash as I usually do won't help the matter because it won't be making the exposure any faster. What I don't know how to do is use my flash to override that reading?
So my question is - how do I meter the scene to get a good overall light effect but use my flash with more power, given that I don't have TTL metering?