Ok guys, I'm missing something...I read this article and I think it messed me up ....I'm going to copy/paste the excerpt from it and see if I can put into words what I'm confused about.
"You are outdoors in bright light and the camera’s exposure meter (or light meter) indicates an exposure of f/16@1/125 at ISO 100. You actually have many other choices for taking that picture. All of them will allow the same amount of light to reach the sensor and expose the image properly.
If your lens’ smallest aperture is f/16, you will only have one direction you can go. You can let in one more stop of light by choosing f/11 on the lens and cut out a stop of light by using a 1/250 shutter speed.
You now have an exposure with the exact same “Exposure Value” as the previous one, only your exposure settings have changed.
So, you may be asking yourself, why would I choose f/11@1/250 over f/16@1/125? Well, the f/stop controls something called Depth of Field, or how deep the focus is in the field of vision, and the shutter speed controls motion.
Using an f/stop one stop away may not change the look of the final image that much, but what if we use our knowledge of reciprocity again and again?
We can open the lens another stop to f/8 and change the shutter speed to f/500 and again to f/5.6 and the shutter at 1/1000 and, if your camera has the 1/2000 shutter speed, we can use f/4@1/2000 and get exactly the same exposure value that we had from the original f/16@1/125 setting.
Now the look of the finished image is greatly changed because f/4 has very little depth of field compared to f/16. The background of the picture taken at f/4@1/2000 will be soft, and the subject will be sharp and clear (if focused correctly).
You have isolated your subject from the background – a very popular trick used in portrait and floral photography. No longer is the background competing with the subject. You have successfully used the rule of reciprocity to change the look of your final image."
I understand how they're altering the aperture and shutter speed in this example, however, what I'm missing is, if you're in the field and you don't want to use the exposure/light meter, how do you know what the "base" setting should be? (Base setting in the example above was f/16@1/125 at ISO 100). And what if you wanted your ISO to be set at 3.5 from the get go, with an 85mm lens?
There's a piece of information I'm missing or overlooking here and it's driving me insane.