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Tangents

08 – flash exposure comp


flash photography techniques
metering techniques ~ flash exposure compensation ~ more examples

Flash exposure compensation

There are two different kinds of exposure compensation :

- overall exposure compensation.
This is set on the camera body – and affects BOTH ambient and flash exposure for Nikon; but only the ambient exposure with Canon cameras.

- flash exposure compensation.
Setting flash exposure compensation affects the flash output only. Ambient exposure is unaffected. This can always be set on the flashgun itself, but some cameras have a button on the camera body itself where the flash compensation can conveniently be set without taking your eye from the viewfinder.

Exposure compensation is used with the automatic metering modes, however …
with most Nikon cameras, dialing exposure comp in manual exposure mode will bias the meter.
With Canon, you can’t dial exposure compensation in manual exposure mode.

Flash exposure compensation is used to compensate for the flash output when the flash is used in Auto or TTL mode. It obviously can’t be set when the strobe is used in manual output.

Firstly, exposure compensation in general ..

What many new photographers have trouble coming to grips with, is the concept of :
1. adding exposure compensation when the scene / subject is light in tone,
2. and decreasing exposure compensation when the scene in front of the lens is darker in tone.

The reason for doing so, is that your camera’s meter tries to expose for everything as a middle grey tone.

Hence, if you are using one of the auto modes (or Auto / TTL flash), the camera will expose any light toned scene as if it should be of an average tonality. In other words, the light toned subject / scene will be exposed as middle grey. Eg, someone in a white dress against a white wall, will appear under-exposed. So you need to bump the exposure compensation up for lighter toned scenes.

The same reasoning goes for darker toned scenes. A man in a dark suit against a dark brick wall, will have skin tones which over-expose if you left the camera to its own decision. The dark tones would fool the camera’s meter.

To make it even more clear, let’s think about this scenario:

We have a setting where the light is consistent and even. So there will be an exact combination of aperture / shutter speed / ISO settings which will give correct exposure for skin tones.
Now, if our subject dresses in all black or all white clothing, our meter reading will change … yet, the light didn’t change. In other words, we would still need the same exposure, regardless of the variation in our camera’s light meter reading.

So if you insisted on using automatic exposure, then you would have to use exposure compensation. And you would have to vary your exposure compensation depending on your composition – because the size of the light / dark patches of clothing and background will affect your meter reading.

The same reasoning goes with using Auto or TTL flash. You have to continually adjust your flash exposure compensation, dependent on the tonality of the scene in front of your lens.

Also, please read the pages on exposure metering using your camera’s meter, as well as the explanation of why using exposure compensation in an auto exposure mode, is much harder work than using manual exposure mode.

This is the reason why I use manual exposure mode nearly exclusively.
But then you may very well ask why I use TTL flash (or Auto flash) instead of manual flash …
… and the reason why I use TTL flash is that TTL flash is easier to control when I am constantly changing position in relation to my subject.

And as I explained on this previous page, it is easier for me in these situations, to use the camera in manual exposure mode, and the flash in TTL / Auto mode. But this means that I have to constantly change my flash exposure compensation.

Which finally brings us to the rest of the discussion on this page …

Flash exposure compensation when using fill-flash ..

With fill-flash (using TTL or Auto flash), you will most often dial down your flash exposure compensation to give only a tiny bit of fill light. So in this case, your flash exposure compensation will be around -1 to -3 EV. But it depends on the tonality of your subject as well.

But when your flash is your main source of light, you will usually hover your flash exposure compensation around 0EV to +0.7 EV depending on the camera and camera system … and of course, the tonality of your subject and scene. So your flash exposure compensation could still range anywhere from around -2 EV to +3EV.

Once again I want to stress a particular point – there are no specific or fixed settings.
There are just too many variables for anyone to give specific ‘do-all’ settings.

There are a number of factors which would affect how your camera and flash meters TTL flash, and would therefore affect how much flash exposure compensation needs to be dialed in:
- reflectivity of your subject,
- how much of your frame is filled by the subject, and
- how far the subject is from the background, and
- whether the subject is off-center or centered in the frame,
- the individual camera’s exposure algorithms that the camera designers came up with,
- available light – (this ties in with how the camera’s metering algorithms work),
- back-lighting – (strong back-lighting always require a lot more flash exposure compensation).

Therefore you have to juggle all this when figuring out how much flash exposure compensation to dial in. A seemingly tough task that gets easier with experience.

But here’s a hint – when your flash acts only as fill light, then the actual flash exposure compensation can vary a lot without affecting the quality of the final image much.

An example:
Flash exposure compensation of say -2 EV will look slightly different than -3 EV, but in the end the actual photo won’t be incorrectly exposed with either setting. If your flash was the main source of light, then a full stop incorrect exposure would be a lot and might very well mean the image is a flop in terms of exposure. But when the flash is just fill-flash, it is less crucial – although careful and subtle use of flash should always be the aim of course.

With Nikon’s flash system however, you have the choice between TTL and TTL BL, (ie standard TTL and matrix TTL). With TTL BL, the camera and flash take into account the available light and will reduce the flash output accordingly. In my experience, more subtle fill-flash is possible with Nikon’s flash system than with the current Canon flash system.

The conclusion here is that ultimately it is best to know how your specific camera and flash reacts in various scenarios and various lighting conditions. There is only so much that can be learnt outside of actual experience and continual practice. You have to know your own camera.

Cumulative exposure compensation with Nikon cameras ..

The Nikon bodies (that I have experience of), allow you to set overall exposure compensation even when you have your camera set to manual exposure mode. This allows you to bias the metering.

With Nikon, the overall exposure compensation and flash exposure compensation is cumulative .. to an extent. For example, if you were to dial in +1.0 exp comp and -1.0 flash comp, it would cancel each other – but only for this scenario where the ambient light is low, and your flash is your main source of light.

Where the ambient light levels dominate, and flash is used as fill-flash only, then different algorithms come into play, and you have other factors such as max sync speed and available apertures affecting the scenario as well .. and hence the flash and exposure compensation might affect ambient light exposure differently then.

But with Canon, flash exposure compensation and general exposure compensation aren’t linked, as they are with Nikon. So with Canon, in manual exposure mode, you can only set flash exposure compensation and not overall exposure compensation.

(It is no use asking me how it handles this in any of the auto exposure modes, since I use my cameras nearly exclusively in manual exposure mode. You’re on your own there.)

Okay, this has become far too wordy. Time for some more images …

 

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