flash and exposure

[sbakewell0508]

so I shoot in manual mode, but there are of course times when the only way to get a correct exposure would be to 1) have an obscenely long shutter speed or 2) use flash. my question is is when you are shooting and the in camera light meter says your still over exposed (my camera does not tell me how many stops over or under) and i use flash is there a set number of stops that even a flash wont compensate for? in that i mean if i am past the little triangle on my light meter for underexposure how do you know what power to put your flash at? short of a light meter. is it just trial and error? i normally use my flash on manual power and start at half power and go from there.. when i put it on ttl it always seems extra bright and blows everything out..

i have not had any problems with this as adjusting my flash is fairly easy, but how do you nail it the first time?

 

[KmH]

(my camera does not tell me how many stops over or under)

Sure it does. You need to read your camera users manual.

The default is each tick mark is 1/3 of a stop.

I recommend you learn how to use your camera before you think about moving on to more advanced techniques, like doing strobed light photography.

At any rate. Many pros use a hand-held light meter to measure the incident, and/or strobed light in a scene. The in-camera light meter can only measure reflected light.

However, by using the spot metering mode the reflected light ratio can be measured in many parts of a scene. From that and distance information, a strobed light exposure can be calculated. With practice, the calculation can be done in the photographers head in just a few seconds.

 

[Big Mike]

when i put it on ttl it always seems extra bright and blows everything out..

Ever try using FEC?

 

[sbakewell0508]

i know that my camera tells me about the exposure.. the little hash marks are a 1/3 stop and the bigger ones are a full stop.. but at some point i am either soo way under/overexposed it does not tell you cause its beyond the light meters length.

 

[Gaerek]

If you absolutely must know how many stops over/under you are, you could always count clicks of the wheel until the needle is back on the meter again, then set it back, and adjust flash as needed. 3 clicks per stop. Really though, the important information is whether you're over or under exposed in most situations.

 

[clanthar]

If your camera's internal meter indicates you are more than 2 stops underexposed then you'll be taking a flash-only photo in which case flash guide number divided by distance between flash and subject equals f/stop. Shutter speed then is set to sync speed and you're not concerned about ambient light.

Otherwise you could take the camera off manual and use the auto-flash-exposure feature of the camera; the above however would be more accurate.

Joe

 

[sbakewell0508]

clanthar... thanks!! that is the answer i was looking for ! sometimes there are situations of course when i need to be at 250 shutter speed so i will be more than 2 stops underexposed.

 

[sbakewell0508]

clanthar, so if i use this method (found the guide number tables online) and say my subjects were ten feet away from me and the guide number for me at the moment was 36, so i would need to use an aperature of 3.6 and i could shoot at 200 and i would get a correct exposure? (iso 100) is this at full power?? or half power etc?

if i chose to increase my aperature to say f8 or f11 for whole scene sharpness would i have to zoom my flash head into the corresponding gn in this case it would be 85 actually less..

i apologize to ppl who think this is a totally noob question, i am trying to research this stuff as well as get peoples opinions through the forum.

 

[clanthar]

Woah! OK then. What I was describing is how we used to do it. I used to shoot weddings back in the day (30 years ago). I used flash of course -- one of these: http://www.purplewaveauction.com/a/2007/20071023MLIO/5955.JPG We knew the strobe's GN and we were really good at judging distance. So 20 feet was f/8, 15 feet was f/11, 10 feet was f/16 and so forth. On the fancier units we could cut the power to half or quarter. So in close at 6 feet we could cut to quarter power and shoot at f/13. The strobe ran off a 510 volt battery and was always ready -- no recycle time. The camera was typically something like a Rolleiflex that synced at all shutter speeds so we'd either keep the shutter up to kill the ambient light or drop it down when we thought we could include stuff like the church windows. Seriously, we used to engrave, carve, paint, tape, etc. the f/stop values over the distance scales on our cameras.

No one today would be willing to use something like that. Maybe a small select few would use something like this: 200C Battery Portable Outfit - Norman : Portable Strobe/Flash -- in which case the same operation would apply.

Today everybody uses one or another variant of an ETTL, ATTL, TTL XXTTLL computer controlled hot-shoe flash that's less accurate and is otherwise a seriously ugly light by comparison. Hey that's progress for you. So I was really trying to suggest that you do the same and kick the sucker into auto, but hey, go for it. If you do, you'll need to make sure the flash is functioning under power control and not controlled by the camera. The math then is brutally simple: GN divided by flash to subject distance equals f/stop. If you zoom the flash the GN changes -- the math stays the same. The beauty of math is there's only one right answer. Correct exposure then is an absolute certainty which is more than you can say for XXTTLL mode.

Good luck,
Joe

 

[Big Mike]

Yes, you can use the GN to calculate your flash exposure...but it can be a tedious process.

In simple terms, you take the GN and divide by the distance...that will give you the F number to set your aperture to. Keep in mind that GN is given as a distance, and sometimes it's in meters, sometimes in feet.

GN will also be for a specific ISO (usually 100) and a specific zoom setting (if you have a zooming flash head). So if either of those things change, you have to recalculate. GN is always given for the flash at full power, so if you want to use a lower power setting, you have to recalculate.

There are some advantages to using your flash in manual. For example, it should be consistent from shot to shot...so if you change your aperture, you know that it will change your exposure.
But if you have a flash unit that is capable of TTL metering, I'd suggest trying it out. It does really make things easier...and since you're probably working with digital, if you don't like the exposure you are getting, you can just adjust the FEC and shoot again.