More basic Flash questions

Zarch

Zarch

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Mick
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Was messing around with my new SB-700 on my D3100 yesterday and have some questions.

I was in manual mode on the camera and the flash was set to TTL. I was indoors and I had to dial something like ISO 1600, f4.5 and 1/125 to get the correct exposure. I took the shot without flash and it came out ok. I then took the same shot with the same settings (flash on this time) and the flash brightened up the scene.

So messing about, I then took the ISO down to 100 and the camera lightmeter showed way under-exposed, as you'd expect. I took the shot anyway (no flash) and it was dark as expected. I then switched on the flash and took the shot. It was fine. Looked good at ISO 100 and was lit up by the flash.

So in manual mode, when you have a flash attached, do you basically ignore the lightmeter and let the flash fill in the gaps?

And in the above two examples, would the flash be working harder at ISO 100 than at ISO 1600? I'm assuming so? :thinking:

Thanks.
 
Yep, ignore the exposure meter in camera when using flash. The meter only takes into account ambient light levels, the flash in ttl will make up to the correct exposure level.
 
terryt said:
Yep, ignore the exposure meter in camera when using flash. The meter only takes into account ambient light levels, the flash in ttl will make up to the correct exposure level.
Click to expand...

Thanks Terry. :thumbs:

So will there ever be a scene/circumstance where the flash doesn't have enough power get the correct exposure? Is it just trial and error?

For example, if you shooting ISO 100, with a high shutter speed and say f16, surely something has to give?
 
I wouldn't ignore it. The meter will give you a good idea of ambient light (background mainly) and generally some light in the background is better than none. Yes there is a limit as with everything, it depends on distance aswell as your settings. If your using a canon speedlite in ettl mode there will be a green led that will light up at the bottom left of he flash if it has the power to make a correct exposure

Sorry just saw you said nikon, quite likely they have the same thing
 
Yeh, the test button/flash ready LED will flash if it is underpowered/underexpose the shot after taking the exposure, you should also see a lightning icon in the viewfinder.
 
You don't exactly ignore what the meter is telling you, it has very useful information about the level of ambient light, that you might choose to use and balance the ambient with the flash (usually a good idea) or ignore. But it doesn't affect the flash exposure that the camera has no idea about until a couple of milliseconds before the picture is taken, ie after the pre-flash is fired for flash exposure measurement.

If you have the flash firing direct, the camera can estimate the distance over which it is able to deliver good exposure, from near to far. That will change according to f/number and ISO, usually shown on a scale on the back of the flash. It can estimate that because it knows the power capability of the flash, and using the inverse square law it knows how far it can reach. The moment you turn the gun to bounce though, or use a diffuser accessory, it has no idea, so the scale will be disabled when you tilt the head.

There are many situations when the gun will run out of power. Bouncing is one, that uses a lot of power. The other common one is when using FP-sync, which is inherantly very wasteful of light.
 
If you wanna conserve power in your speedlight you might wanna consider shooting at a higher iso thus using less flash power it saves your flash if ever you were in a situation
 
Thanks for the replies. I've enjoyed reading Neil van Niekerk's posts and hope his "on-camera" book will help further when it arrives. :lol:

A couple of further questions if I may:

If you are in a very dark scene where no amount of changing shutter/aperture and ISO is going to get you a metered exposure, what do you do? Just dial in the settings you'd like based on subject/end result and let the flash do the rest?

I suppose the same goes for dimly lit scenes where dialling in the correct aperture/iso and shutter for ambient light could have you up at ISO3200, when the same scene could be lit by the flash at ISO100?


And when using bounce flash, how does it know how much light to throw out? As it has no idea of the height/direction you are bouncing the light? And if you've got you aperture/iso and shutter speed already dialled in, how does it exposure correctly? :thinking:


Thanks very much.

Mick
 
You're thinking about this too complicated.. As said above, there's 2 exposures, the ambient and the flash one.

You can think about it like the camera takes 2 shots, first one with the ambient light where aperture and shutter speed combined with ISO sensitivity define how it's going to look.

Second there's the flash exposure where the shutter speed really has no effect at all as the flash duration is under 1/1000s so it doesn't matter if you have shutter speed at 1/30s or 1/200s, the picture will be formed during the flash duration anyway.

These two combine to form the final picture.

Flash will rarely illuminate the whole scene except maybe indoors and even there the level of illumination will quickly dim with distance. The camera will instruct the flash to get a good exposure on the foreground "main" subject (or its guess as to what that is). The background will get whatever light falls there. That's why to get something other than a dark background, you usually want to set aperture and shutter speed + iso to something that will allow ambient light to make a difference in the image.

Zarch said:
And when using bounce flash, how does it know how much light to throw out? As it has no idea of the height/direction you are bouncing the light? And if you've got you aperture/iso and shutter speed already dialled in, how does it exposure correctly? :thinking:
Click to expand...

There's a measurement preflash from the flash which is measured by the camera Through The Lens (TTL for short) which is used to evaluate the effect of the flash on the scene and then immediately the shutter opens & the real flash goes off.

So it doesn't matter how you modify the light or bounce it, what matters is the light reflecting from your scene as that's what's measured.
 
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