Flash at 100 or 400 ISO

[tecboy]

When I use a speedlite in TTL with ISO 100, the image gets darker. With ISO 400, the exposure seems to be right, but with slight noise with post processing. Is it better to set to ISO 100 or 400?

 

[Overread]

Shift the flash into manual mode and up its output to full power and take a shot at ISO 100. If the shot is underexposed then it means that, even with flash, there is just not enough light with the scene and set settings to expose correctly - thus you will have to either:

a) Add more flash light or ambient light
b) use a slower shutter speed
c) use a wider aperture (smaller f number)
d) use a higher ISO (as you have already tried with).

TTL can give some variation in the powers it selects depending on how the scene is metered and it could be that its metering and giving a good output of flash for foreground elements; whilst background elements are being underexposed because the light is falling off fast (inverse square law of light falloff) and thus still can't render them properly exposed. Raising the ISO and lowering the flash output will thus be able to balance the exposure of the background (controlled more by ambient lighting) and the foreground (dominated by flash output). Of course this means that your flash output must lower (TTL is going that for you, if you use it manually you'd have to lower the flash output to compensate).

 

[cgipson1]

^This! TTL is not known for accuracy!

 

[cptkid]

TTL is silly, all manual baby!

 

[Robin_Usagani]

Really depends what you are trying to achieve. There is nothing wrong with using TTL on the on camera flash. Yes, it is true that TTL is not consistent and not accurate. But when you move around and bounce your flash, inconsistencies is what you will get even with manual mode. I shoot mine often with ISO800 and 1600. I really doubt it will be accurate with manual mode either.

 

[runnah]

I shoot mine often with ISO800 and 1600.

You're just a loose cannon!

 

[Big Mike]

I don't often shoot at ISO 100 when using flash. 400-1600 is more likely.

The main reason is that when you use a lower ISO, you require more power from the flash. Sometimes you need that extra power/distance from your flash...but it's also a consideration that you may not want your flash firing at high power every time.

For example, when shooting a wedding reception (dark room), I'm most likely using flash for most shots. If I use ISO 100, the flash will take longer to recycle and I'll have to change the batteries more often. But if I use ISO 400-800, then the flash doesn't have to fire with a much power, so the recycle time is shorter and the batteries last longer. There is the added benefit of increased ambient exposure as well.

Noise shouldn't be a problem with a T3i at ISO 400. If you are getting too much noise (especially if it comes out with processing) it very likely means that you are underexposing your photos and trying to make them brighter in post. Get your exposure correct in-camera (even if it means using higher ISO) and then don't brighten them in post....that will get you less noise.

 

[cynicaster]

I’ve got a T3i with a 580EX-II flash. For general “run n’ gun” type shooting indoors, I never have the ISO below 400, because I find that lower ISO speeds only give good exposures in the most “easy” situations (close subjects, low ceilings, close walls, etc.)

If you want to point the flash head right at your subject, you can get a lot more range, but honestly, who does that?

Bounce flash is a great technique, but it doesn’t come for free.

 

[johnson33]

Fuji X-TRA 400 ISO Film is suitable for daylight, twilight, and indoor shooting and for candid snapshots and sports action, available light or flash. have a good time