我用50D的相机，在使用430EX的时候，如果相机使用自动ISO，那相机会自动选择ISO400，无论环境光线多亮。
查了好久没有看到相关问题的讨论，只在外文论坛看到有讨论。可是我的英文水平实在没看懂，想请教各位摄友，能帮解答一下吗？
下面我把链接放上来，要不帮我翻译一下？
http://forums.dpreview.com/forums/read.asp?forum=1019&message=32178752

About using the center-AF and reframing: if you use all AF points and your subject is the nearest thing covered by one of them then the camera will focus on it. No need to center and reframe.

About avoiding ISO-400 in sunny days: ISO-400 brings no relevant degradation in IQ compared to ISO-100 if there is plenty of light.Your 430EX features high speed sync (no problem with shutter times below 1/250s). Unless you get close to 1/8000s I see no reason to be concerned.

Now, coming to your actual problem...

In Av mode (same for Tv and M) the camera will expose for the ambient light and use the flash to lift the subject. If you have few ambient light then you will get long shutter times. Period.

At this point you shall ask yourself: is the background important for you?

(1) if yes, apply high ISO (1600, 3200...);

(2) if no, apply a negative exposure compensation. This will make the background darker but not affect your subject, which will be iluminated by the flash.

You might also ask yourself: is the selection of a particular apperture important for this picture?

(3) if yes, go back to (1) and (2);

(4) if no, consider shooting in P-mode. It sets the shutter time to avoid shaking and adjusts apperture and flash power to get the subject well exposed. Using flash in P-mode disables program shift (you can't set a particular apperture).

In my humble oppinion: if your background is rendered black then it is not important to ensure that it also gets blurred... and if you are getting 10s @ ISO 3200 in Av-mode then your background is quite dark anyway.

Yes, you can shoot in M and set Av,Tv and ISO at your will. In this case the camera will adjust the flash power to get your subject well exposed. But again: if you set a low ISO and/or a fast shutter speed then not much ambient light will make to the picture and your background will get dark. There is no free meal out there...

Now, there is still one thing that could help: to bounce the flash.

If you shoot in P-mode and fire the flash straight to the subject, the camera will apply low flash power / small apperture to avoid an overexposure of the subject. This separates the subject from the background, which gets few flash light and is rendered dark. But if you bounce the flash e.g. to the ceiling than the whole room gets flash light (your subject being just part of it). There is less separation between subject and background and you might get a better balance. Similar arguments apply to M-mode and to Av-mode + negative exposure compensation.

I hope that this helps...

Best Regards

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嗯，说明书上确实有些的，固定为ISO400。

看一遍说明书就行自动Iso就这样