This is one of four posts on the Nikon Z6 and Z7 fake ISO settings. The series starts here.
There are a few reasons why you might want to use the fake ISOs – or, if you prefer, the JPEG ISOs – on the Z6 and Z7.
Let’s say you are using the spot metering technique for ETTR. You find a significant highlight, meter it, and, to use Zone System terms, place it on Zone VIII. You note that, since the subject was fairly low contrast, that your intended exposure is a stop over what the gray card exposure would be for the subject at base ISO. The normal ETTR technique would be to set the exposure at base ISO the way your highlight on Zone VIII meter reading said to, expose at base ISO, and pull the image a stop in post.
However, there is another alternative. You could use the same exposure and set the camera to ISO Lo 1.0. the values in your raw file would be the same. However, when you opened the image in Lightroom, the 1 stop pull would have already been performed for you. Certainly not a compelling advantage, but it may be worth something to some people.
If you use the in-camera histogram, the benefits are less clear, since you trade in-the-field complexity for post-processing simplicity. You’d set the camera at base ISO, take a picture and examine the three-channel histogram (UniWB tweaked or not). When you found an exposure to your liking, and you noticed that the exposure was a stop over the gray card exposure, you could set the camera to ISO Lo 1.0 and make your real exposures.
Another issue is that the fake ISOs are useful to preserve what you see is what you get (WYSIWYG) in situations where you are intentionally overexposing relative to the gray card exposure. In this case, the EVF shows you a preview of the pulled raw file. An example is balancing natural light and flash.
I think this has merit in some situations as well, although I am not a big fan of using the camera’s EVF or review function to judge image quality in general. The one place where I do find it useful is in flash balancing. In the old days, I used to use Polaroid back for this. Now, I use the LCD display. Sure, the colors are wrong, and the image is too punchy to show you what you can do in the shadows in Lr, but you can get the flash ratios dialed in well.
Now let’s say that you’ve picked a wide aperture to get the background out of focus just the right amount, and you can’t crank up your shutter speed far enough to get a proper exposure at base ISO without getting above the camera’s maximum synch speed. You’ve also noticed that you can give the exposure a stop more without clipping the highlights. If you followed my earlier suggestions, you’d set the camera to base ISO, the shutter speed to max synch speed, overexpose by a stop, and pull the image a stop in post.
But you could also set the camera to a fake ISO setting a stop down from base ISO and use the same exposure. Then your preview image would not reflect the overexposure, and you could more easily judge the flash ratios. You’d also have the advantage that the one-stop pull in post would happen automatically.
I’m not going to use any of these techniques myself, but there may be some who could profitably employ them.