Please give me a simple explanation in this.

Thanks for all the responses. It helps to read them.

However, I feel the need to provide samples. Here are they;

D700 + 35mm f/2 AI.

Auto ISO 200-6400. Min. Shutter Speed: 1/60

All shots are at f/2.8

Min. ISO: 200

Flash set to REAR

The first shot without flash. For lighting reference. The camera chose ISO 2000, so the parameters are; ISO 2000, f/2.8 and 1/60.



First shot with Flash ON-REAR. Min.ISO was 200 so the camera chose 200 and lowered the shutter speed to 1/6, which is not enough for me to take a proper handheld shot;

Second shot. I increased the Min.ISO to 400. So did the camera. It also increased the shutter speed to 1/13. So ISO 400 and 1/13. Still not enough for me to handhold;

Third shot. Here's where it gets weird. I increased the Min. ISO to 800. But the camera chose ISO 360 and 1/25. The image is quite underexposed.

Fourth shot. I increased the Min. ISO to 1600. The camera chose ISO 320 and 1/50 shutter speed. Severely underexposed.

Fifth shot. I increased the Min.ISO to 3200 in this one. The camera selection is ISO 400 and 1/100 shutter speed.

So, please help me from here on out

How do I guess the ideal settings for a proper rear curtain and shutter speed?