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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 29 Aug 2011 (Monday) 00:19
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Shutter Controls Ambient Question

 
elguapo
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Aug 29, 2011 00:19 |  #1

I always hear, "shutter speed controls ambient exposure, aperture controls flash exposure." However, isn't it really "...aperture controls flash and ambient exposure"?

Without the flash, exposure is controlled by shutter speed, ISO, and aperture. Doesn't the aperture still effect ambient light even when using a flash?

Let's say I want to under expose the background (ambient light) by two stops. I do a meter reading and it says 1/60 f/4 at ISO 100. I can now underexpose the background by two stops while using flash in 1 of 2 ways:

1). Setup a flash so I get a meter reading of 1/60 f/8 at ISO 100. I then set my camera to these settings, and fire away. Haven't I accomplished my goal of under exposing ambient light by two stops even though I only adjusted the aperture?

2). Setup a flash so I get a meter reading of 1/60 f/4 ISO 100. I then adjust the shutter speed to 1/200 and fire away.

Of the two, I think #2 is better because the flash doesn't have to work as hard, but both are valid options, right? What would be the difference between the two images? Would#2 have less shadows?


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CronoDL
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Aug 29, 2011 01:53 |  #2

You're right in your thinking and examples. The one thing that's different between the two shots is the depth of field, due to the different apertures.

To have full control of depth of field (which many, if not most, would want), you need to set the aperture first. Then now you set your flash power to get the desired subject exposure, and shutter speed to "control" the ambient exposure.


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Aug 29, 2011 08:29 |  #3

This interesting because as I was learning flash I was trying to make sense of this as well. I figure everyone does during the learning curve.

Like CronoDL said you can control ambient with aperture but why would you want to? You have to separate the 2 elements in flash photography - subject and ambient. When working with the subject you want to control the DOF followed by flash output. That leaves ambient which you control with shutter speed.

When you said option #2 is better because the flash does not have to work as hard is true but this now takes away DOF creativity. I would not worry about flash power (unless you are maxed out) and concentrate on DOF.

So while technically the statement "shutter speed controls ambient exposure, aperture controls flash exposure." is not entirely 100% accurate it is the standard formula that provides you with complete control.


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BrandonSi
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Aug 29, 2011 08:31 |  #4

The shutter = ambient, aperture = flash is an oversimplification for teaching / rule-of-thumb purposes. There is only one exposure, but many people find that it's easier to think of as them as two.

The two are not equivalent as far as light goes, as 1/200 is not two full stops above 1/60, 1/250 is.

Ignoring the math error, both are valid, and similar, but like Danny mentioned, dof is now larger (more is in focus) in #1 due to the decreased size of the aperture. This may, or may not be what you want. You can mitigate this somewhat by adjusting ISO, but ISO impacts shutter speed and aperture size equally. My 5D has ISO 50, which can help in some situations.

Also notice in #2, you've now hit the max-sync speed for a lot of cameras (especially if you correctly use 1/250). I shoot at 1/160 as my triggers can't reliably handle 1/200 without catching the shutter.

There are other minor considerations as well.. increasing the flash power will drain batteries (if you're using batteries, or a battery pack on location) faster, and have a longer recycle time. If you're working with a model, they now have to look at and deal with a brighter flash every time.


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bobbyz
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Aug 29, 2011 08:55 |  #5

You are right.

But once things are setup and say ambient is changing, you can play with ss and adjust ambient really quickly. So that is why over-simplified rules like "shutter controls ambient" come into play.

ISO comes in play the same way as aperture.


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Aug 29, 2011 09:05 |  #6

bobbyz wrote in post #13018120 (external link)
ISO comes in play the same way as aperture

aperture+shutter.

I know that was a typo, just fixing it for you. :)

To be clear, ISO impacts ambient/flash equally, and to the exact same degree, just like a ND filter would.


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Hoppy1
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Aug 29, 2011 09:08 |  #7

It's not a very helpful rule, or even an accurate one.

The only complete truth is, with normal x-sync, shutter speed does not affect flash exposure. But everything else does, equally, as per normal.

In practise, the best way to do it is usually to set the ambient exposure how you want it, then and adjust the flash power output. Either on auto with +/- compensation, or on manual.


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Curtis ­ N
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Aug 29, 2011 09:32 |  #8

Like many other "rules" in photography, the rule you referred to is oversimplified, overused, and tends to confuse more than it helps.

If I'm shooting in late afternoon I start at x-sync speed so I can slow down the shutter as the light fades, without the need to adjust flash power. If it's early morning I start at a slow shutter speed to do the opposite.


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Aug 29, 2011 09:54 |  #9

Hoppy1 wrote in post #13018177 (external link)
It's not a very helpful rule, or even an accurate one.

The only complete truth is, with normal x-sync, shutter speed does not affect flash exposure. But everything else does, equally, as per normal.

In practise, the best way to do it is usually to set the ambient exposure how you want it, then and adjust the flash power output. Either on auto with +/- compensation, or on manual.

So if I was shooting on manual and had a shutter if 1/15 and aperture of f4 and then I increased my shutter to 1/200 but made no other changes including flash output the subject would have the same exact same exposure in both images?


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Aug 29, 2011 10:05 |  #10

I'm not sure. There was once a time I was just down right petrified to use my flash and the camera would go on Program mode. The part I had trouble with was that I would get a decent exposure (no DOF control) and was not sure why. I hated not knowing what was going on.

Breaking it down to two exposures - shutter = ambient and aperture = flash really helped me get over a big hurtle. I know there is a lot more to it but it was a good starting point for me.


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Aug 29, 2011 10:21 |  #11

Curtis N wrote in post #13018315 (external link)
Like many other "rules" in photography, the rule you referred to is oversimplified, overused, and tends to confuse more than it helps.

If I'm shooting in late afternoon I start at x-sync speed so I can slow down the shutter as the light fades, without the need to adjust flash power. If it's early morning I start at a slow shutter speed to do the opposite.

Between reading Niel van Niekerks stuff and all the great help around here that has made it much easier. When to use x-sync speed or when to slow it down. I sure treat low light level events a lot differently than I did a year ago.


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Aug 29, 2011 10:35 |  #12

digital paradise wrote in post #13018436 (external link)
So if I was shooting on manual and had a shutter if 1/15 and aperture of f4 and then I increased my shutter to 1/200 but made no other changes including flash output the subject would have the same exact same exposure in both images?

If the subject were lit only by the flash, that is absolutely correct. Slower shutter speeds make the ambient light have more effect on the image, though, even though the flash exposure is not affected by the shutter speed.

This assumes, of course, that the shutter speed is never set to a value that is faster (shorter time) than the camera's "maximum sync speed".


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Hoppy1
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Aug 29, 2011 10:40 |  #13

digital paradise wrote in post #13018436 (external link)
So if I was shooting on manual and had a shutter if 1/15 and aperture of f4 and then I increased my shutter to 1/200 but made no other changes including flash output the subject would have the same exact same exposure in both images?

The flash exposure would not change. And if you were shooting in total darkness, flash only, the total exposure would not change (becuase there would be no ambient component).

However, in reality there is usually some ambient light, often a lot of it with fill-in and slow-sync techniques when the whole point is to balance the flash with the ambient.

So in that case, going from 1/15sec to 1/200sec you would be taking out more than three stops of ambient exposure. The background would go much darker, and also, because almost inevitably some of that ambient would also be spilling on to the camera side of the main subject, you would darken that too by a certain amount too, depending.

Edit: crossed post with Skip


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Aug 29, 2011 10:59 |  #14

That makes perfect sense. I should have been more clear because thinking about ambient light present when I asked the question.


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elguapo
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Aug 29, 2011 20:22 |  #15

I understand that, in most circumstances, it is prefrable to use shutter to control ambient. I was just making sure I wasn't missing something because people keep saying "aperture controls flash exposure" like it doesn't impact ambient exposure.


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