Why do I get inconsistent results with Canon ETTL flash?

Question

When using any flash I have ever bought on whatever camera I ever had I get inconsistent results. So I am assuming it is my lack of knowledge. I am now using the canon 6d and a 600ex-rt flash on camera. ETTL auto on everything or sometimes I use manual.

My results vary. If I use my 24-70 or my 70-200. It doesn't matter. I will shoot a photo of people dancing in a dark room using ambient light and a flash. They will either be super blown out or super underexposed. When shooting up close the images is super under exposed. I say It is user error since it happens with whatever camera and flash I am using. Here are some samples.Also my color balance is ALL OVER THE PLACE.

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Answer 1

The example photos you posted look fine to me, both in terms of exposure (which is easy to judge) and color balance (which is more difficult to judge).

Color balance is always hard to get right when you shoot with mixed light sources, such as for example ambient and flash, especially if they provide a similar level of illumination in the final composition. For equipment and situations available to happy amateurs, this is just something we have to live with, and try to compensate for (usually, but as pointed out by inkista, not always) in post-processing. Shooting in RAW (if supported by the camera) can help here by not forcing you to pick one specific white balance setting in the camera, allowing white balance adjustment in post. In extreme cases you can then even bracket white balance settings if needed for different parts of the image (for example, an outside window illuminated by sunlight and an indoor scene illuminated by flourescent lights).

There are no obviously blown-out parts of your example images (even the girl's white dress is largely correctly exposed, and has enough texture that even a quick application of Levels on the JPEG posted brings out slightly more detail), and even the histograms look pretty good (the left histogram is for the adult couple photo, and the right is for that of the children; both are linear histogram views, which should roughly match what your camera would show):

(Remember that the histogram plots each pixel value on the horizontal axis, against the number of pixels with that value on the vertical axis.)

Normally you want to aim for the histogram to taper off toward both the highlights and shadows end, whereas these only taper off on one end each. However, in this case, I would argue that that's not a problem, at least not by itself. (Keep in mind that rules of thumb are great, but you are allowed to break them; even more so than rules of non-thumb.)

The number of pixels at either extreme value (0 and 255, since this is 8 bits per channel) is trivial, which is another indication that outright over- or underexposure, even of portions of the images, is not a problem in your case. In case of over- or underexposure, the histogram would show a large spike at either extreme value, either in a single color channel or in the combined "value" channel.

There are two possible explanations that I can think of:

• Your monitor isn't calibrated well, or at all. If you don't want to buy a calibrator (which is recommended for higher-end work), this guide can get you quite acceptably close to good using just the adjustments on your monitor, or you can use this quick and dirty cheat sheet if you want to just have something reasonable and don't care about fine adjustments. For comparison, on the latter, with normal ambient lighting I have no trouble discerning even the lowest-contrast fields. Neither of these guides are actually about calibrating your monitor, but they are about getting as much color and greyscale detail as possible out of it.
• You are forgetting that light falls off with the square of the distance to the light source. This applies doubly when photographing using a flash, because the light source at the camera illuminates the subject and the light is reflected back to the camera to produce the image. By doubling the distance from the flash to any part of the image, you reduce the illumination of that part of the image by half; this half, then, is halved once more on its way back to the camera sensor before the image is recorded, for a total illumination of one quarter when the distance is doubled. This allows you to use the flash to isolate a subject from the background, especially if the flash provides a significant fraction of the overall lighting in the scene, but also makes it very difficult to produce consistent illumination of a deep scene when using a flash. This effect is particularly visible in your photo of the adult couple, where the couple is correctly exposed but the background is very dark. This is normal when using a flash as a significant source of illumination in a photo.

Answer 2

Any time you rely on an automated metering-based system to handle exposure for you, you're going to get inconsistent results shot-to-shot, because the scene change and the exposure changes. In addition, we don't know how your camera is set, and flash behaves differently depending on the camera mode as well.

In M mode, the assumption is you know what you're doing and the camera won't interfere with how the flash is set.

In Av and Tv modes, however, the assumption is that you want flash for fill, and that you're balancing the ambient (non-flash) light against the flash so that the majority of the light will come from the ambient. You can nix this behavior in Av mode by messing with the custom settings and nailing down a faster shutter speed in Av mode. But in general, exposure settings will be very close to those you'd use without any flash at all, and only a little bit of flash illumination. This could be why your first image seems underexposed.

In the P mode, the flash will behave more like you probably expect it to from experience with built-in flashes on P&S cameras. In good light, the assumption is that you want the flash for fill (i.e., to "fill in the shadows"), but that in low light, you want to use the flash as your main source of illumination. In this case, you tend to get an image much more like your second one--where the majority of the light in the scene is coming from the flash and not so much from the ambient.

Every flash image is basically the sum of two exposures: the ambient light (all the light that isn't from the flash), which is controlled by iso, aperture, and shutter speed; and then all the light from the flash, which is controlled by iso, aperture, flash power output, and flash-to-subject distance. How you choose to balance the flash against the ambient will determine the "look" of your image.

As for your white balance being all over the place--yes, this happens. Flash tends to be a different color from ambient lighting. Typically cooler/bluer. With mixed color light sources, it's difficult to white balance easily and simply. The method to get around this is typically to "gel" the flash--that is to put a colored filter over the head of the flash, so that its light more closely matches the color of the ambient lighting.

See also: How can I get started with a first flash gun? And the neilvn.com (Neil van Niekerk's website) "Tangents" blog, most notably the "Flash Photography Techniques" section.