Can anyone tell me if ISO playes any role in ambient light when using flash, or is the amount of ambient light restricted only to shutter speed?

I'm not sure if I totally understand the question, but here goes:

ISO always plays a role in setting your exposure, whether you're using ambient, flash, or a mix of both.

If you're using only flashes, the amount of light that your flashes contribute to an image is only related to the ISO and the aperture; a flash only pops for like 1/5000th of a second, so, with a flash, there's no difference between a shutter speed of 1/30th and a shutter speed of 1/60th - the light is still only present for 1/5000th of a second (you do have to pay attention to your shutter speed when you're using a flash, but that only has to do with making sure the flash fires at the right time. It still has nothing to do with the adjusting the exposure). So, when you're shooting flash, you control exposure with aperture and ISO.

If you're mixing ambient and flash, you're controlling exposure with a combination of ISO, aperture, AND shutter speed, because the shutter speed DOES matter with ambient light.

So that's a general overview - is there a specific lighting circumstance you're trying to figure out?

Yes....so what I am asking is... if I am shooting in a very dark area and need to use flash but want to bring in some ambient light, I would slow my shutter speed.. but would increasing my ISO also help?

I assume from this reply that it would. In otherwords...if I were shooting formals in a dark room, and I was using flash with a shutter speed of say 1/40th of a second, and say an f-stop of 5.6...would increasing the ISO from 400 to 800 make any difference in the amount of ambient light present in the final photo? Or would it be just as good to shoot at 100 ISO and avoid noise completely.

Hope that's clearer.

Well, if you're shooting a group formal, you don't want to open up to 2.8, so yes, increasing your ISO would be wise. It's all about balance and decisions... in your case, you don't want to go below f/5.6 (since it's a group and you are concerned with DOF), you don't want your shutter speed to be too low that you get blur from your subjects, so you'd be asking your flash to do more work to light your scene with a lower ISO... this will usually result in a very dark background and very flash lit subjects. If you want to include more ambient, in that situation, bumping up your ISO the way to go.

Something to think about:

The aperture and the shutter speed (in ambient light) both control the amount of light that gets into the camera body and falls on your sensor to create the image. The ISO controls how sensitive the sensor itself is: setting the ISO to 100 means that i'll take twice as much light to record the image as an ISO setting of 200.

So, let's say you shoot with a flash at 1/40th, f5.6, ISO 400. If you want more ambient light in the photo, you can can:

* slow your shutter speed - that'll let in more ambient light. But now you're at 1/20th, and you invite more blur;

* open your aperture - that'll let in more light, too, but it'll also let in more light from your flash, and you'll have to power your flash down to compensate. Also, opening your aperture will change your depth-of-field, and you might not want to do that.

* increase ISO from 400 to 800. Now you've told your sensor to be twice as sensitive, which is good for registering more ambient, and you didn't have to slow your shutter speed down. At a higher ISO, your image will be noisier/grainier. More importantly though, now the flash will contribute twice as much light as it did before, too, and now you're overexposed with the flash. So now you have to decrease the power of your flash by one stop.

ISO will be controlling the overall sensitivity of your sensor. Not ambient or light of the flash. On the other hand though, higher ISO will allow you to use less flash and save your batteries.

Thanks so much everyone.

So I shoot with ETTL and change my flash compensation as needed.

So...in the above scenario, I could up my ISO to say 800 (noise shouldn't be too bad if the exposure is right) and dial down the exposure compensation as needed and keep my shutter speed at a say 1/40th or 1/50 to avoid blur. This should add more ambient light than the same scenario with an ISO of 400 and not drop my shutter to unacceptable levels blur.

Correct?

I get blur at 1/40 and 1/50th, so i guess it just depends on the look your going for. 800 should be fine though. Your batts on your flash wont have to work as hard.

I assume you're shooting off-tripod? The kind of blur that's caused by camera shake will depend partly on how steady your hand is, but largely on the focal length of your lens. The general rule of thumb is that in order to get a sharp image from a handheld camera, the slowest you can shoot is 1/focal length. So if you're shooting with your lens at 70mm, the slowest you could go would be 1/70th of a second. If you want to be able to shoot at 1/50th, you wouldn't be able to zoom in much more than 50mm. If you have a tripod or a monopod, or you have a nice lens with image stabilization, you could shoot slower.

Yes..I'm assuming a group shot with a wide focal length...probably 35mm or so.

Ok, normally to prevent the charcoal finish of a portrait at the indoor reception i drag the shutter about 1/20 to 1/30th of a sec with my f. stop at about 6.3 at iso 400 to pic up more ambient light and a little more DOFfor group shots. For close up portraits indoor i open up about 2.8f to f4 depending on lighting conditions and my shutter is on 1/50th of a sec at iso 200-400. my flash power always remain the same, I seldom make changes to it. I only work with f-stops and shutters most of the time and i always shoot on manual. and believe it of not I have never used more than eight(8) AA batteries on a wedding day. never.

Actually Jason, those 2 sentences are a contradiction. If ISO controls the sensitivity of the sensor, then that has a direct impact on the amount of ambient the sensor records, unless I missunderstand your statement.

In simple terms:
Aperture controls your flash exposure - larger aperture, less flash power you need; smaller aperture, more flash power you need
Shutter Speed controls your ambient light - faster shutter speed, less light gets recorded; slower shutter speed, more light gets recorded
ISO AFFECTS BOTH AMBIENT AND FLASH EXPOSURE - higher ISO records more light. Period. No matter if the light comes from your flash or the ambient.

As far as dropping the shutter speed below 1/50 or 1/40 not being good because of motion blurr, that depends on what the subject is. If the subject is a nice skyline of a city, then yes, get a tripod and shoot a several seconds exposure. But if the subject is something else, like a bride and groom, and the skyline behind is the background, I only care to have "the" subject sharp. So I am no afraid to drop my shutter speed to 1/15 or 1/10 when using flash, because under that scenario, which I believe is the same scenario Jennifer is describing, the flash will freeze the subjects. I was just at OSP Indy, in which we had a night shoot, and I shot a couple with the Indianapolis skyline as the background at 1/15 through the whole session. Then I even panned, rotated, and zoomed the camera during the exposure, that caused the lights from the buildings in the back to appear to be moving, but the subject were frozen in the exposure by the flash and were very sharp.

Another thing is that if Jen is going to be in a room, depending on the room size, she may be able to bounce the flash behind her at a high ISO like 800 or more, and create a nice light that fills the room without appearing like it's flash.

Jen, when using flash, don't be afraid to drop your shutter speed to 1/15 or even 1/10. Do try to remain very steady, but it's totally doable.

I think that this only holds true if there's not much ambient light lighting up "the" subject. If the ambient ONLY lights the background, then yes, a slow shutter is fine because you don't care if the background is a bit blurry; the flash will freeze the subject. BUT, if there's also enough ambient light lighting up your subject, and you shoot handheld at 1/15th, you can end up with an image that almost looks like a ghostly double-exposure because you will have recorded blurry ambient-light for 1/15th AND the flash for 1/zillionth. So at slow shutter speeds, the more you're using the flash as fill or supplemental light, and the more the ambient light is the "main" source of light, the more camera-shake blur you'll get.

Very good points. And I thank everyone for helping me with this.The last time I had posted about blurry subjects, this issue came up. Someone said that you must have at least a two stop difference between the amount of ambient light and the flash output for the flash to effectively freeze the subject.Is this true?Ramiro...this makes sense to me. I'm glad you clarified this for me. Thank you.

It would help so you don't need as long of a shutter speed to bring in the ambient.



This helps as long as the flash and/or camera is compensating the flash output. A proper flash exposure at ISO 400 @ 5.6 will be overexposed at ISO 800 @ 5.6 if the flash output isn't decreased by a stop. Basically, as you increase your ISO, you need less power from your flash.

Cheers,
Zack

Matt, you are correct, for my scenario to hold true, the ambient light cannot be contributing much to the exposure, if it does then the double exposure you mention will happen. Below are some examples, the first, the girls were in a room which was lit, but the ambient of the room was not contributing much to the exposure, so I shot it at 1/15 to bring in the ambient and balance the flash, which was bounced on the ceiling. All these were shot with my 120j on a light stand via remote triggers.

Here is something you can try
1) Find the area you want to use for the setting.
2) Turn off your flash and check your exposure of the background (i.e. iso 400, 1/30th @ f/4)
3) Quickly fire off a shot to check what your BG will look like.
4) turn on the flash and change your f/stop to f/5.6 or 8 to tone down the background and try using the second curtain fire for the flash. Many OC Flashes have this option, I know on the 580Ex it is 2 presses of the H button.
The slower shutter speed will allow you to slightly expose the background, the second curtain fire will expose the subject properly. You may have to play with Ex. Comp to get it dead on.
It is a neat effect if there is movement in the background, like a wide angle shot of the couple just off the dance floor, with the party going on in the background.

John