Ideal iso, shutter speed and apperture for flash

[pilsburypie]

Just wondering if there was.

I understand you may want to use AV mode for correct backgroud exposure
I understand you may well have a particular shot where you want a fast shutter to capture action or a wide apperture for shallow DOF.

I just ask as when I'm inside and want to bounce flash and just forget ambient light, I seem to select Manual, iso 400, f8 and 1/125. Why, I'm not sure, except that I have heared snippets that I don't know are relevant to flash... eg. iso 400 as it is a balance between noise and sensitivity, f8 - because "it's great!" and 1/125 because it's fairly fast whilst not requiring as much flash power as 1/250.

See what I mean?! Very little science to my selection!

[[godfather]]

Get a Lightmeter :)

[Vad]

1. You should use the shutters speed that is equal to the synch speed of your camera. For your 50D it is 1/250 sec. The flash does not know which shutter speed you use and always emits for 1/1000 to 1/10,000 seconds depending on the model. There is absolutely no saving power is your camera is at 1/125.

2. The less ISO the better. The only reason to go higher is when it is not possible to use lower because for example the subject is too far, or when you want to save the battery power.

3. You were right that F number should be dictated by the required depth of field. Again you may want to deviate from the aperture that provides that DoF if the subject is too far or you want to save the battery power

[Wilt]

'ideal' is dependent upon how much ambient light there is, and what YOU want to achieve about capturing that ambient light! For example, you might want to capture the background details in a church looking toward the altar, but NOT want to capture the background details when the background is a messy table of dessert plates with half finished wedding cake visible (while the people are on the dance floor)!

In the former case, the higher ISO and slower shutter speed and wider aperture permit you to capture the background while using flash to expose the main subject in the foreground.

In the latter case, lower ISO and faster shutter speed and somewhat smaller aperture permit you to render the background darkly while the eye is drawn to the foreground subjects which are correctly illuminated.

[pilsburypie]

I understand the general principles - I read the flash guides on here and get the 2 seperate exposure idea. Probably haven't explained myself too well.

Inside my house, with natural light and the lights on, a typical non flash shot at say ISO 400 would be f4 1/30. This, when someone is moving can cause motion blur or even some hand shake if I've had too much strong coffee!

Using bounce flash, camera set in AV mode, will still have the same settings (or even slower shutter if I stop down the apperture) which will also have the same undesirable results in my example.

By setting to manual, f8 1/250, this will totally discount the ambient light and light the whole scene with flash. There will be no motion blur, camera shake and the scene will be pretty nicely lit. I could select a whole range of random shutter and appertures and the flash will kindly illuminate the scene appropriately.

My question really is: If f4 or f8 or f11 etc doesn't matter really to the shot, neither does 1/90, 1/125 or 1/250 second, neither does ISO 100, 200, 400, what is a good middle of the road sensible combination?

[pgruiz123]

Using bounce flash, camera set in AV mode, will still have the same settings (or even slower shutter if I stop down the apperture) which will also have the same undesirable results in my example.

By setting to manual, f8 1/250, this will totally discount the ambient light and light the whole scene with flash. There will be no motion blur, camera shake and the scene will be pretty nicely lit. I could select a whole range of random shutter and appertures and the flash will kindly illuminate the scene appropriately.


Seems to me that you have all the information you need, but are fighting it. The answer based on your shots is to set the camera in manual. By setting the camera in AV you get blurry shots. This does not happen in manual.

[bobbyz]

So your subject exposure in the room is f4, 1/30 at ISO400. Set your camera to same settings but put flash in eTTL mode. I would say use flash in bounced mode, bounce from corner or a wall to get directional light. You will need to adjust FEC for proper flash exposure on the subject.

Since 1/30 is slow (can cause blur due to subject movement), use f2.8 at 1/60. Or even bump ISO to say 800 so you at f2.8 and 1/125.

Now if you need to reduce ambient, increase ss to 1/200 or 1/250. If you need to increase ambient, reduce ss to say 1/100 or even 1/60.

[bobbyz]

I
By setting to manual, f8 1/250, this will totally discount the ambient light and light the whole scene with flash. There will be no motion blur, camera shake and the scene will be pretty nicely lit. I could select a whole range of random shutter and appertures and the flash will kindly illuminate the scene appropriately.


Setting to f8 and 1/250 means ambient is reduced by 5 stops so yes, no more ambient now. Flash is the only light.

But I don't understand why would you now be able to pic any random ss and aperture setting? If you change say aperture to f4 and ss to 1/30, you back to the case of ambient exposure so flahs is just providing some fill.

[Wilt]

It is simply about striking about balance between the ambient exposure and the flash exposure, that permits you to get a sharp photo without sufficient ambient motion being captured! It depends upon the circumstances, there is not a single 'book answer'.

[pilsburypie]

Setting to f8 and 1/250 means ambient is reduced by 5 stops so yes, no more ambient now. Flash is the only light.

But I don't understand why would you now be able to pic any random ss and aperture setting? If you change say aperture to f4 and ss to 1/30, you back to the case of ambient exposure so flahs is just providing some fill.
What I meant was if I chose f8 at 1/250 and used my flash bounced, I could just as easy use f11 at 1/250 and get the same result. The flash would just blast a bit more power to corectly expose the shot. So what I meant by random apperture and shutter combinations is say anything from flash sync speed (or higher if I use high speed sync) to say 1/80 to freeze the action and any f stop I choose. I would get the same result except for DOF. Sure f4 and 1" aint gonna work;)

[jra]

What I meant was if I chose f8 at 1/250 and used my flash bounced, I could just as easy use f11 at 1/250 and get the same result.

Not true, your f11 shot will have a greater DOF then the f8 pic. There's really no right or wrong or "sweet spot". If you've got the flash power, and you don't care abour battery life or ambient exposure, use whatever ISO and aperture you desire. Just keep the shutter speed high enough to prevent undesired motion blur and overexposing the ambient. If you want to conserve battery life, use higher ISO's and larger apertures so that the flash won't have to work as hard.

[badgerW]

Battery life is one thing, also recycle time. I have a 430EX and it can take several seconds to recycle at full power or close to full power, depending on the state of the batteries. But at 1/32 or 1/64 power it can go repeatedly, pop! pop! pop! which is nice. I did this at my brother-in-law's wedding reception recently and I was using ISO800, f/2.0, varying shutter speeds depending on desired effect, with the flash set to 1/32 or 1/64 power and the flash fired every time, I never had a missed shot due to the flash recycling.

You can ask the same thing about exposures without a flash. What's to differentiate between f/8, 1/250, ISO 100 and f/8, 1/1000, ISO 400 in broad daylight on a nice modern DSLR where noise doesn't really become visible until ISO 800 or higher? Not much, really, unless you are photographing very fast movement. I personally could go either way, I might use the ISO 100 if I had a tripod or an IS lens while I would go with ISO 400 and 1/1000 if I was worried about camera shake, but that's only if I really stopped to think about it, and in non-critical situations I wouldn't really care.

[Curtis N]

Pilsbury,

Here's a thread that might help.
http://photography-on-the.net/forum/showthread.php?t=177622

[egordon99]

My question really is: If f4 or f8 or f11 etc doesn't matter really to the shot, neither does 1/90, 1/125 or 1/250 second, neither does ISO 100, 200, 400, what is a good middle of the road sensible combination?

The aperture selected WILL matter! First off, it determines your depth-of-field. Then it determines HOW much flash power is needed (assuming same ISO, room conditions). f/8 will require more flash power than f/4 (could be the difference between 1/1 and 1/4 flash power)

The ISO selected also WILL matter, as it also affects how much flash power is required, as well as how much noise is present. (if you increase the ISO from 100 to 200, your flash will only need 1/2 instead of 1/1 for example)

The shutter speed will have NO (absolutely NO!) effect on how much flash power is needed. It WILL affect the ambient exposure though, so once you start getting it slow enough to actually capture some ambient light, you need to be aware of it. Generally though, 1/250s at most ISOs/f-stops, the ambient exposure would be nil in indoor settings.

To summarize, FLASH exposure=flash power+ISO+f/stop
Ambient exposure=shutter speed+ISO+f/stop.