Fake ISOs, ETTR, & WYSIWYG

I posted my test results and conclusions on the a7RII’s fake ISO settings on the DPR alpha 7 forum. To put it mildly, a lively discussion ensued.

The SNR was not particularly high, but even so, as is usual in Internet discussions, I learned a couple of things. In my original post, I took the same stance I took on this blog: don’t even think about using the fake ISOs. But, based on the discussion, I am reconsidering, at least for sophisticated photographers and two corner cases. One is a possible symbiotic relationship between ETTR and  fake ISOs.

The particular fake ISOs I’m talking about are ISO 50 on the a7RII, and presumably on the other alpha 7 cameras, and L 0.3, L.07, and L 1.0 on the Nikon D810. It doesn’t apply to ISO 80 and ISO 64 on the a7RII, because those ISOs actually reduce the dynamic range from that achievable at base ISO.

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 50 in the case of the a7RII, or ISO L 1.0 in the case of the D810. 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 (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 50 or L 1.0 and make your real exposures. If you’re using he a7RII, and adjusting the exposure with the exposure compensation wheel, then noticing that you’re giving the picture a stop more than the meter reading is easy, but you have to remember to dial the exposure compensation back a stop if you switch to base ISO.

My own personal opinion is that these games aren’t worth the candle, but you may have a different opinion.

Another issue that was brought up on DPR 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. 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 ou 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 either of these techniques myself, but in the interest of explaining the limitations of my previous recommendations, I felt it desirable to write this post.