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Tangents

off-camera TTL flash

May 20, 2011

off-camera TTL flash

This image of Amy, one of our models at the Treehaven workshops, came up for discussion with the group of attendees. As a straight-forward on location portrait using off-camera flash, it is ideal for an overview again of how easy the ambient & flash exposure metering is.

The basic approach with this portrait was to expose for the ambient light in the background, making sure our subject is somewhat under-exposed … and then to add off-camera flash with a softbox. The first question that came up was – how did I meter for the ambient light?

metering for the ambient light

With this scenario, one has to grasp that there are broadly two main areas in the background that we could expose for. There is the brighter sky area, and then the less bright trees in the background. We can not expose for both of those ‘correctly’. You have to decide what your background is, and then decide how you want to expose for it.

I chose to expose for the trees in the background. This would then let our model be under-exposed to an extent … and then we bring up the exposure on her with off-camera flash. In choosing the trees as my background, I allow the brighter sky area to blow out. But in the composition of the image in the camera’s viewfinder, I made sure the blown-out sky area is a relatively small portion of the background. My main background area is the patch of out-of-focus trees.

This is an approach that I have emphasized over a number of articles on this site – you have to be specific in how you choose the background in the composition of your photos.

Metering was simply done by looking through the viewfinder, eliminating the bright sky areas in the frame, and getting my settings in manual exposure mode. I purposely over-exposed a bit with this image, taking the exposure 2/3 of a stop up from the zero of my camera’s built-in meter.  Why 2/3rds of a stop? Because I chose it that way.

In a sense, there is a no ‘correct’ exposure for the background in this specific scenario. You can choose to make it bright and airy with pastel tones … or under-expose and make it moodier and darker. Your choice.

So that’s how I metered for the background areas. I fix these settings in my camera by using manual exposure mode. Then my composition won’t affect my camera settings.

The next question that came up was – how did I meter for the flash?

metering for TTL flash

The surprising answer was … I didn’t meter for the TTL flash. You can’t really. You have to rely on the technology. That’s the beauty of using TTL flash though – largely you can rely on the technology to get you there. The camera and flash will (via the pre-flash) figure out how much flash to add to the final exposure.

Then you can check on your camera’s preview whether you like the flash exposure, and then dial it down or up, via the flash exposure compensation (FEC) button.

That then is also the limitation of TTL flash exposure metering – you have to rely on the technology to give you results that you would like and need. And you can only really figure this out by checking the camera preview, and then adjusting the FEC to taste.

For a situation where you need to work fast, TTL flash is fantastically easy to use. But for a situation where you need numerous photographs consistently exposed, you will have a much slower post-production workflow than if you had shot with manual flash that you metered for.

manual flash vs TTL flash

With manual flash, you have to balance those 4 controls to give you correct flash exposure.
two of the controls are on the camera – ISO, aperture
two of the controls  are via your flash – power, distance

So you’d use a lightmeter to calculate the manual flash exposure. Or perhaps you could use the histogram to determine correct manual flash exposure. Once you have your flash exposure fixed for that specific distance, and camera settings, then every exposure will be nailed. Constant. But if you change one of those four parameters (ISO, aperture, power, distance), then you have to juggle something else to get correct exposure again.

But where TTL flash shines in comparison, is in the flexibility in changing your settings. TTL flash will follow your settings, and give you (what it deems to be) correct exposure.

Here are two examples where I changed my settings between shots, also showing the ambient-only exposure. This shows how the TTL flash will just follow my settings, and give me correct flash exposure. Simplicity itself.

1st image group: 1/320 @ f2.8 @ 200 ISO
2nd image group: 1/500 @ f2.8 @ 200 ISO

Not a large jump in ambient exposure – just 2/3rd of a stop, but it still shows the TTL flash exposure picking up the slack and giving me correct overall exposure for our model.


With this article I wanted to emphasize again how easy it is to use off-camera flash, especially when TTL flash is used. There’s a certain latitude we have in choosing our settings (while keeping our subject under-exposed to an extent), and then relying on the TTL flash technology to give us correct flash exposure.

So now we have to be less concerned about specific settings, and can concentrate on other essential elements of the final photograph:
- directing our model;
- composition;
- direction of light.

Equipment used with this photo session:

Nikon D3;  Nikon 70-200mm f2.8 AF-S II (B&H)
Nikon SB-900 (B&H);  Nikon SD-9 battery pack (B&H)
(2x) PocketWizard FlexTT5 transceiver (B&H)
Lastolite EZYBOX Softbox Kit (24″x24″) (B&H)

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Stay informed of new articles via the monthly newsletter.
Also join us on the Tangents forum for further discussions.

If you need more direct help or instruction on flash photography,
I do present workshops & seminars and also offer individual tutoring sessions.

If you find these articles interesting and of value, then you can help by
using these affiliate links to order equipment & other goodies. Thank you!
 

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