When I bought my first external flash gun, I thought I had bought the solution to taking all indoor and night pictures. At the time, about the only notion I had about flash was that it was a portable light to make it possible to take pictures when I otherwise couldn't. I thought that a good external flash was all I needed. Then I read about the strobist technique. I thought it was all about taking the flash off the camera. So I bought a low-cost radio transmitter for my flash to fire remotely (before I learned that my D90 has a very useful commander mode). I managed to take some good pictures, but still I was not able to take satisfactory "event" shots. Something was very wrong. Can one not get good flash pictures using all the default settings? Off course, the quick answer is "no" when you are using the built-in flash, but how about an external flash? I put all the lessons I have learned in the past three days to check this out.
(1) In the Auto or P modes in one shooting instance, the maximum shutter speed is automatically set at 1/60 and aperture at f4. In the S mode, the max shutter speed is 1/200s, which is the camera sync speed. At 1/200s shutter speed, my kit lens opens to the maximum aperture of f3.5. Default setting let me down. I was better off using the S mode, and dialing up the shutter speed to the maximum.
(2) Next, I mounted my f1.8 prime lens. In the Auto or P default modes, again, the setting was 1/60s and f4. In S mode, at 1/200s, the aperture opens to f1.8. Therefore in default setting (Auto or P), the best and most expensive lens would have done nothing for me.
(3) In daytime shooting with fill-in flash, there is no default setting on the external flash. It is recommended to use TTL-BL mode instead of TTL mode when the ambient light is brighter than the subject. Since there is no default option, the wrong choice can lead to a poor quality shot. Moreover, as fill-in flash, it is often necessary to stop down the flash to prevent getting the flash look. As the default is zero flash compensation, the user is let down once again.
(4) Untrained, one usually does not try to control how the image background appears. Using modern TTL flash technology, one can set shutter speed, aperture, and ISO to effect how the background looks, while maintaining correct flash exposure (within the limitation of the flash range). The default setting usually gives the flash look. Certainly very amateurish.
Conclusion: I believe it is very unlikely one can get good flash pictures in the default mode.