If you're a manual-only flash user, and you do any photography other than setup shots, I really would strongly recommend adding TTL flash to your repertoire. The great thing is, if you already know manual flash, you already have a leg up.
BUT TTL IS SO UNPREDICTABLE AND INCONSISTENT
Common criticisms by manual flash users about TTL flash include unpredictability and variability:
- Unpredictable in that "you never know what you're gonna get".
- Variable in that TTL uses the reflection from preflashes to guess the correct flash exposure and is thus subject to reflection-based metering weaknesses. For example, even though aperture, ISO, flash-to-subject distance, and the flash's guide number are held constant, flash exposure can vary depending on the reflectance of the image in the frame (which in turn can vary based on the reflectance of the subject or other elements).
OK, guilty as charged on both counts. In theory, you never know with 100% certainty what flash exposure you're going to get with TTL until you take at least that first test shot. In reality though, TTL gets reasonably close to an ok flash exposure. This is not that much different from using manual flash without a flashmeter or calculator (for GN calculations). The difference is that instead of you guessing the amount of power you need, the camera and flash offer a ballpark estimate. If you don't like the flash exposure you get, you simply adjust with flash exposure compensation - the same way you would adjust your power upward or downward if you didn't like the flash exposure you got with your test shot on manual flash mode.
It's also true that TTL can be variable even under conditions when flash exposure should be constant. Fortunately, if this theoretical variability is truly a dealbreaker for you, there are a few answers for that.
1. For Canon, Nikon and Olympus:
Spot meter a gray card when you take the shot. TTL will set flash exposure to render that gray card as middle gray (within the flash's power limits). This will work even in conditions that would normally fool TTL such as including a highly reflective object in the shot. Adjust flash exposure upward or downward if you wish using flash exposure compensation. Then use FV lock (aka FEL or FV Flash Lock) to lock the desired flash exposure. You will get the same flash exposure consistency that you will get from manual.
2. For Sony, Pentax, and other cameras that don't have FV Lock:
Include a gray card in your test shot, and adjust TTL flash exposure (with flash exposure compensation) to render the gray card as gray. When you do this, don't spot meter from the gray card because if you remove the gray card then the flash exposure will change. Just use the gray card in the frame as a guide for an "accurate" flash exposure.
In real life, TTL shooters don't use a gray card because we just use FEC to adjust whenever we don't like the flash exposure. Plus, most of the time (at least for me), the flash exposure doesn't really vary all that much anyway (except in unusual cases).
Ready to give TTL a try? Don't worry - it doesn't bite. :)
HOW TO USE TTL FLASH:
1. Set the ambient exposure as you would with manual flash.
2. Take a test shot with TTL flash.
3. Estimate the flash over- or underexposure and adjust the TTL flash accordingly, using flash exposure compensation.
4. Feel free to change shutter OR aperture OR ISO OR subject-to-flash distance OR any combination of the above. You can even change modifiers or filters or gels. TTL will keep the flash exposure the same!
It may help to think of TTL flash as a smart manual flash that can give you a reasonable guess of the right amount of power automatically upfront (which you can adjust), and also adjusts power automatically to keep your flash exposure the same from shot-to-shot, except that it isn't perfect so it needs tweaking upward or downward.
And if you're a TTL-only user reading this, check out these related posts:
TTL vs. Manual: a False Dilemma http://betterfamilyphotos.blogspot.com/2010/09/ttl-vs-manual-flash-false-dilemma.html
Therapy for TTL Addiction http://betterfamilyphotos.blogspot.com/2010/09/therapy-for-ttl-addiction-how-to-use.html