What does this mean?

[flores_g57]

I've been reading this book by Joe McNally where he made the statement " The shutter speed controls the ambient light & the light from the flashes gives you the f-stop ". Now I understand the shutter speed concept but I just don't get the flash and the f-stop . Could some one Please explain this to me.Thanks in advance!

[OsmosisStudios]

When youre using flash, changing the shutter speed alters how the ambient light affects the image, where as changing the aperture alters how the flashes affect the image. ISO is sort of 50/50.

[dlambert]

I'm not sure that makes sense without a little more context. Is he talking about balancing ambient & strobe lighting? This might just be a way of thinking about balancing these light sources, because the shutter & aperture are going to act together on all light sources equally, right?

[i speak in math]

The flash is very fast, upwards of 1/10,000 of a second. So, its much much faster than a shutterspeed of 1/200. Therefore, 1/60 or 1/200 or 1/5000*, they are all pretty much the same because the flash is still faster than that. (*You do have to stay under your sync speed, though, to have the whole sensor exposed at once) This shutter speed will have an effect on the regular daylight or ambient light where you are taking the picture. The shorter the shutter, the less ambient light gets in.

Aperture, however, is not time based. It either lets in more or less light. It has an effect on the flash. The more closed down and small the aperture, the less light from the flash gets in. The wider it is and bigger the aperture, the more light gets in.

However, the aperture also effect how much ambient light gets in. So, its a balancing act.

Lets say you like how the background looks at ISO 100, 1/100th, f/5.6 but your flash is overexposing the subject. You can always turn down the flash power, but lets say you have only a single setting flash that can't be adjusted. How can we reduce the flash power but not effect the background?

Easy! Close the aperture down and balance the shutter speed in the opposite direction. So now you will be at ISO 100, 1/50, f/8...TaDa...we have reduced the flash output by 1 stop without changing the background. Or if the flash was underexposed and we want to increase its power, ISO 100, 1/200, f/4.

[dlambert]

Originally Posted by i speak in math

(*You do have to stay under your sync speed, though, to have the whole sensor exposed at once)

That was one of the things that didn't make sense to me -- this concept is true only for a limited range of shutter settings.

It also seems like you're losing control of DOF this way, right? If you're looking for a shallow DOF, for instance, you don't really want to just crank down your aperture.

What about just changing the flash EV compensation? Even my manual flash has power settings -- wouldn't it make sense to start there?

[dakwegmo]

It's not quite as simple as Joe explains it, but it's a great rule of thumb to go by.

Essentially, ambient light is fairly constant so how much of that light gets recorded is controlled largely by the duration of time the shutter remains open.

Strobes, fire for a brief instant that is usually quite a bit faster than the amount of time the shutter is open, so the aperture is going to largely control how much of the light from the flash gets to your sensor.

[dakwegmo]

Originally Posted by dlambert

That was one of the things that didn't make sense to me -- this concept is true only for a limited range of shutter settings.

It also seems like you're losing control of DOF this way, right? If you're looking for a shallow DOF, for instance, you don't really want to just crank down your aperture.

What about just changing the flash EV compensation? Even my manual flash has power settings -- wouldn't it make sense to start there?

This is exactly right. If you set the aperture to get the desired depth of field, but the flash output is too bright, you would either have to decrease the power of the flash, or move the light further away from the subject to decrease its intensity.

[jdepould]

Originally Posted by dakwegmo

This is exactly right. If you set the aperture to get the desired depth of field, but the flash output is too bright, you would either have to decrease the power of the flash, or move the light further away from the subject to decrease its intensity.

OR use an ND filter, since moving the light can affect the quality of the light.

[Chip]

To muddy things a bit, manual and ttl flash work differently. Here's a link to a good article (by Neil van Niekerk) on this issue.

With ttl, you can use aperture, ISO and shutter speed to control the ambient light, and the flash will (pretty much) adjust automatically. That's the "good news/bad news" thing about TTL: it does a lot for you.

I'm pretty sure all the earlier posters were talking just about manual flash.

(I'm by no means an expert. If I have something wrong here, please correct me.)

PS - Should this thread be moved to the lighting section? That section could use some more traffic.