ETTR is only a useful exposure strategy at base ISO for ISOless cameras like the a7. What’s the maximum ISO you should use on the a7? There are three ways that I know to find out.
The first is to minimize read noise as referred to the input of the pre-ADC amplifier. If you’re trying to do that, turning up the ISO in the a7 past 1600 or so is fruitless.
The second is to maximize the signal-to-noise ratio in the lower parts of the midrange or the brighter parts of the shadows. If that’s your goal, you may be just as happy gluing your ISO knob to 100.
The third is to look at the Unity Gain ISO, and not use ISOs more than a stop or two over. Since the 13-bit ISOug of the a7 is about 1000, 2000 or 4000 should be your top ISO.
Here’s what I do: I give myself a stop or two extra headroom if I don’t have enough light to do ETTR at base ISO. After that, I increase the ISO as necessary to maintain that headroom until I get to 1600. That way my finder, which I usually leave in normal (non-automatic gain control) mode, doesn’t get too dim to use.
I don’t exactly understand your phrase “I give myself a stop or two extra headroom.” Can you explain what you mean in operational details?
Jerry, by that I mean I let the histogram slide to the left for a stop or two before I touch the ISO control. “Underexposure” gives more room for bright objects that affect the right side of the histogram — thus, more “headroom”.
Jim
Thanks! Let’s see if I can restate (in slow-motion reasoning) your standard way decently well: You’re in manual mode with manual ISO. You want ISO 100 whenever possible, so you try it first. You set aperture and shutter speed hoping to be consistent with ETTR. The result of a perfect ETTR, when conditions allow it, is generally considered overexposure by the camera.
If you cannot achieve ETTR with your desired aperture and shutter speed, then you are willing to expose within a stop or two below the ETTR and keep ISO at 100 (or whatever the base is). If there is no better term, let’s call this “middle exposure.”
If you would get an exposure below middle exposure at your desired aperture and shutter speed, then (and I’m guessing now) you keep shooting a middle exposure, but now at a higher ISO.
However, if that ISO would be above 1600, then you shoot at 1600 anyway and deal with the underexposure in post processing.
Do I understand?
“Do I understand?”
Perfectly. You said it better than I.
Jim
Thanks, though I feel I was rather long winded.
I just today discovered your blog, I am hoping to read as much as I can this weekend about the a7r. Thanks for such a terrific resource!