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Tangents

the answer to the brain-teaser: flash + ambient mix

November 26, 2012

brain-teaser: that balance between flash + ambient light

Last week I posted this brain-teaser, asking for comments on why the two images would look so different in terms of the balance between the flash and ambient light.

I closed the discussion there, since that thread had more than 400 replies! So in order to discuss then answer, a new post is necessary.

Just to bring everyone up to speed, here are the details that I gave about the images.

  • Both were taken with TTL bounce flash, and the same camera settings.
    (1/160 @ f/4.5 @ 1600 ISO).
  • Both are JPGs generated from the edited RAW files. Yet, they look very different in how the flash and ambient light appear to be balanced.
  • I didn’t adjust the Highlight / Shadow sliders in Lightroom away from the defaults.
  • I didn’t change the camera settings, nor the exposure mode of the camera.
  • My flash was bounced in exactly the same way for both images. I didn’t change the angle. I didn’t change my light modifier.

The brain-teaser asked for the explanation why would there be such a difference between these two images? Why would the flash and ambient light balance appear so different?

As mentioned in the original post, the answer isn’t all that complex …

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The camera settings are the same, hence the ambient exposure is the same.
- No off-camera flash or additional lights were used.
- Nobody else’s flash went off.
- The light didn’t change.

So if the ambient exposure was the same, then the only logical explanation for the difference in the backgrounds there, is that I adjusted the Exposure slider when I edited the RAW files.

That was the clue that was originally given – the images are JPGs from the edited RAW files.

If you think about it, for each image, the balance between flash and ambient is fixed during the moment of exposure. Nothing can be done afterwards. If the flash is over- or under-exposed, then the balance between flash and ambient will be different. If you then adjust the Exposure slider to get the flash exposure / foreground correct, then the rest of the image will be adjusted accordingly.

So to restate that, while looking at those two images:

The image at the top had the flash under-exposed. I was shooting a rapid sequence as the girls were catching the bouquet. The flash’s batteries (with battery pack), couldn’t keep up. So the flash was under-exposed. The ambient exposure would’ve still been the same though, since my settings didn’t change. To bring up the exposure for the image, the background became brighter.

The bottom image with the darker background, was somehow over-exposed. This one I can’t quite explain why though. My TTL flash exposures here were quite consistent, except for when I shot too fast during this sequence. However that may be, the image was over-exposed. Not the background!  That would’ve been the same for all the images because … my camera settings didn’t change, and my scene didn’t change. Now with the foreground / flash exposure being over-exposed, I pulled the Exposure slider down for the RAW file when I edited it. This will correct the flash exposure … but pull the ambient exposure / background down.

The difference in Exposure adjustments of the RAW files, amounted to nearly 2 stops difference. Hence the huge change in how the images appear.

And that is it. That’s the entire explanation. Nothing more, nothing less.

Discussion:

With the various replies, there were a few themes that came up a number of times. I’d like to go over those.
(Just to point out that this is not meant to pick on anyone’s answer or to deride the various replies. It really is meant so that we can all learn something here. So please take it in that spirit.)

 

  • first-curtain and rear-curtain sync.

The possible impact of first-curtain vs rear-curtain sync was offered as a possible explanation. The comments stated that this would explain the difference in ambient exposure, since the impression was that rear-curtain sync would allow more ambient light in.

This is not correct! If you think about it, the shutter speed would be exactly the same for first-curtain and rear-curtain sync. And since your aperture and ISO settings didn’t change, then your  ambient exposure would be exactly the same.

First curtain sync vs rear curtain sync is about the timing of the flash pulse … and has nothing to do with the flash exposure.

- first curtain sync vs rear curtain sync
- when NOT to use rear-curtain flash sync

 

  •  Inverse Square Law / light fall-off from the flash

I did write about how bounce flash photography is affected by the inverse square law, and how it actually works *for* us, and not against us when we bounce flash. But in these examples, my position didn’t change much. A foot or two. Certainly not enough to have caused this much of a difference in the background between the two images.

 

  • black foamie thing 

Using this (or not using this)  had no effect in the balance between flash and ambient light. It’s time to re-read what the black foamie thing actually does. Or doesn’t do.

 

  • evaluative / matrix metering vs spot-metering

Since I’m shooting in manual exposure mode, the metering pattern has no effect on the ambient exposure.  As for flash exposure, as far as I can discern, the choice of metering pattern doesn’t affect TTL flash.

- which exposure metering mode?
- exposure metering
- camera settings & metering

 

  • bounce off a wall close to me / further away

This wouldn’t explain why the background would be so hugely different between the two images. Remember, my camera settings remained the same.

My position relative to the wall behind me, might affect the reach of the bounce flash, a la the inverse square law, but not to this extent. Also, I didn’t change my position by that much.

Many did mention that there appeared to be a mirror behind me – and you can see it in the hard shadow of the Exit sign’s box in the top image (right-hand top corner). A mirror does throw off flash exposures. Really makes it more unpredictable. In this case though, going by other images in the longer sequence, this didn’t affect my exposures.

 

  • exposure compensation 

Nikon cameras: Exposure Compensation has no effect on ambient exposure. But it does affect TTL flash exposure. But … this still doesn’t explain why the ambient exposure would’ve been different.

Canon cameras: Exposure Compensation isn’t available as an option with manual exposure mode.

 

  • flash exposure compensation

Flash exposure compensation is used to bring TTL flash exposure up, or down. In this case, going be the way the images appeared … the flash exposure looked the same (at first glance)

 

 So who got it right? 

-  valeksensky - with comment #345

My guess: in first shot flash power was quite a bit lower (maybe there was a negative FEC – you said you didn’t change camera settings, and nothing about flash settings; or maybe it just randomly underexposed). Exposure is adjusted in postprocessing (again, you said highlight/shadow wasn’t adjusted, but nothing about the exposure). If my guess is right – first shot should have lower SNR/more NR, but it’s impossible to verify without fullsize image.

 

While many mentioned a change in flash exposure, very few took it a step further to explain why the flash exposure between the two images would then still (incongruously) be the same.

And that is where most nearly-correct explanations fell down – by not taking that extra logical step or two.

 

But there were a few who were definitely on the right track:

- Henry – comment #369, was onto it as well:

Insufficient flash power for the 2nd picture. The flash was fired before it had a chance to completely refresh its ready state.

- julia morgan

 Recycle time?

So the two winners of the Skype tutoring session:

- valeksensky  (who got it right)

- John (comment #42) .. who coincidently was on the right track as well.

The 2nd winner was chosen with the random number generator (on http://www.random.org):

Just to repeat the original rules:

To keep it interesting, I’m offering an incentive to participate in the challenge: An individual tutoring session (1 hr) via Skype each for two people who will be picked from the replies here.

The two entries that will be picked:
– One person from those that figure it out correctly;
– along with one other person who responds to this post, whether their reply is correct or not.

(The only restriction is those who win the tutoring session, need to be fluent in English.)

We’ll let this run until Monday, Nov 26th.

Finally … I hope everyone learned something from this. From a certain viewpoint, this might seem like a trivial comparison since I adjusted the Exposure in post-processing. And yet , it wasn’t a trivial exercise, because then the answer would’ve been very obvious to most.

There is one strong reason why this isn’t a trivial discussion – when someone tells you, “oh nevermind, you can fix it in post”, what they are neglecting to realize is that this gives an inconsistent look to the final photographs. It is always better to get it right in camera, where possible. If you want to balance flash and ambient light in a certain way, then doing so in camera is the best approach. Always.

So thank you to everyone who participated. And as always … the discussion continues. Let’s hear it.

 

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