I've been shooting with the Nikon D4 now for about 9 months or so. Some of the cool features are:
- Auto-Bracketing with gaps between shots of up to 3 stops. You can shoot for an HDR, and in 3 exposures get either -2, 0, +2, or even as much as -3, 0, +3.
- Bonus feature when bracketing is if you use the timer setting enabled, when you click the shutter for the first time it will take all the shots for you without having to depress the shutter again.
- Being able to toggle the focus modes, and focus point count from outside the menus is nice. It took a bit to learn but now it is easy to recall how to adjust things.
- ISO adjustment is pretty straightforward with the D4, and the addition of being able to toggle Auto ISO on and off from the switches and dials works well. You can enable/disable Auto ISO by holding the rear-ISO button and then turning the Front sub command dial.
The way I have come to use it is in two modes: Aperture Priority and Manual Mode.
Auto ISO in Aperture Priority works as you would expect - you set the aperture and the shutter speed changes depending on the available light and value you've set for Auto ISO and minimum shutter speed. I have this set to 1/focal length. So at 200mm it tries to maintain 1/200th of a second. And at 500mm it tries to maintain 1/500th of a second. As with normal Aperture Priority mode, the EV adjustment will tweak the brightness of the exposure up or down as it is adjusted + or -EV.
The down side to Aperture Priority is all you get to control is the Aperture. And indirectly you can control the shutter speed if it gets low so that it will roughly match your focal length.
This will work with many cameras that support Auto ISO, but it works very well with the Nikon D4 since it allows greater control of how Auto ISO engages (focal length) and how clean the higher ISOs are. I have my max ISO set to 10,000. That's a pretty big number, and the image quality will suffer some, but in light that is that low, the ISO is upped and the camera does its job trying to get a sharp image based on focal length.
What I've described so far is great, you set the Aperture and the minimum shutter speed based on focal length (plus some EV adjustment) and the camera does the rest.
Here's how I really like to use Auto ISO though: I shoot in Manual mode! In manual mode I set the shutter, and the aperture. The Auto ISO will NOT enforce the focal length adjustment since the shutter speed is manually set. But it will cap the ISO at the max that has been set. The exposure will also be adjusted to + or - EV even though the camera is set to manual! The camera will operate like it does in aperture mode where EV is taken in to account, unlike in regular manual mode.
In Auto ISO and Manual exposure mode I control the shutter, the aperture and the exposure bias via the EV adjustment. The ISO will fluctuate up and down to meet the intended exposure. The ISO will be adjusted both up (as expected to boost sensitivity) and down (to not blow things out). This might be a bit unexpected, but it will allow you to go from say manual ISO at 400, to enabling Auto ISO and the resulting exposures might have ISO from 100 to 10,000!
What this then allows is for in normal to mixed light, you can get the camera to operate at the cleanest ISO possible. In manual mode with Auto ISO, the only thing the changes is the ISO, and it will go to the lowest value that attains the proper exposure. And if the conditions change greatly, so will the ISO (up the max already set).
Here's another example of full manual with Auto ISO enabled. This is shot at 1/60th of a second at f/13 and the ISO auto adjusted to ISO 140. For these shots what I really wanted to control is the shutter speed to get a good pan, and then the f/stop to get more DoF (so focus would be more forgiving at slow shutter speeds).
That's it, probably my favorite feature of the D4 - AUTO ISO!