a7RIII exposure strategy — manual settings

This is one of a series of posts on the Sony a7RIII.   You should be able to find all the posts about that camera in the Category List on the right sidebar, below the Articles widget. There’s a drop-down menu there that you can use to get to all the posts in this series; just look for “a7RIII”.

A few days ago, I performed a quantitative analysis of the a7RIII noise performance versus ISO setting. Then I made a series of posts showing visually what’s going on with the camera in that regard. The first of those is here.

Some conclusions:

• The camera is close to ISOless from 100 to 500, and even closer from 640 on up.
• There is a substantial improvement in input-referred read noise when you go from ISO 500 to ISO 640, but that improvement is material only in the deepest shadows.

So, assuming that you are setting your f-stops, shutter speeds, and ISO settings manually, what is the best strategy?

If you can, expose to the right (ETTR) at base ISO. This is a no-brainer and is true for just about any digital camera. What keeps it from working all the time?

• You can’t tell from the in-camera histogram whether the raw file is saturating. UniWB helps, but most people don’t want to go to all that trouble and look at green preview images, and even UniWB is not a panacea.
• You may need more light on the sensor. Depth of field (DOF) requirements and dealing with camera motion and subject motion often conspire to make the ideal exposure impractical.

So now what, assuming that you’re shooting RAW?

1. Decide on the f-stop you need based on DOF, not the light.
2. Decide on the shutter speed you need based on motion blur, not the light.
3. Then set the ISO; deciding that gets a bit more complicated.

The first thing to do is figure out the ISO that would place the significant highlights on the right side of the histogram; let’s call that the metered ISO. ) I want to reserve ETTR for base ISO exposures, and we’ve already decided that this scene won’t let us do ETTR at base ISO.)

1. If that ISO is 100 or as much as one stop over, set the ISO to 100 and make the exposure. If that ISO is 640 or as much as three stops over, set the ISO to 640 and make the exposure. You’ll get more highlight protection, and it won’t cost you anything significant in shadow noise.
2. If that ISO is between 250 and 400, decide if you need the last bit of shadow signal-to-noise ratio (SNR). If you don’t, set the ISO to 250, take the shot, and get some extra highlight protection.
3. If that ISO is 500, decide if you could do with bit less highlight protection. If not, set the ISO to 500. If yes, set it to 640 and enjoy lower super-deep-shadow SNR.
4. If that ISO is between 640 and 4000, and you think you’ve got the highlights handled, set the ISO to 640, and use that exposure. This will produce images that are, by conventional metering standards, more and more “underexposured” as the metered ISO rises.
5. If that ISO is over 4000, set the ISO setting to three stops under the metered ISO. Three stops of extra highlight protection should be enough for almost any scene, and pushing too much in postproduction can cause some color shifts.

That sounds complicated, but once you’ve gone through the steps a few times, you can figure out the camera settings you need instantly.

Let’s work through a few examples.

From the LV or sample histogram, you decide that that sunset-with-foreground you’re shooting needs f/11, and the wind means you need 1/250. But the histogram tells you that, in order to get a right-populated histogram you need ISO 400. The picture is all about the highlights. Shoot with the ISO dial set to 100, so you’ll get plenty of protection.

You’re shooting the Milky Way, and with the lens set wide open at f/2.8 and a 15-second exposure, which is all you can manage before the stars start to blur, the ISO setting for a right-loaded histogram would be 5000. But the stars are tiny, and the histogram might not be finding the brightest ones, or they might not make a big enough bump to see. Set the ISO to 640.

Now let’s move on to a slightly different situation, one in which you are not trying to get the settings for a single exposure or a set of exposures of the same subject in the same light, but a situation in which you want camera settings that will work over a modest range of lighting and camera angle changes. This is a situation in which your first instinct might be to use one of the a7RIII’s automatic exposure modes. However, you will see that there can be advantages to setting the camera’s exposure manually.

For your anticipated typical shot, use the techniques above to decide your settings. Then decide how much the scene and lighting is likely to change, and in what direction. If you think it will get brighter, bias the settings you came up with for highlight protection. If darker conditions are more likely, make the bias in favor of shadow SNR. The advantages of this approach over A, S, or P modes are:

1. You get the shutter speed you need to deal with the motion, and no faster.
2. You get the aperture you need to deal with DOF, and no narrower.
3. You don’t have to try to figure out what the heck the metering system is doing to your exposures.
4. You’ve got an ISO setting that does what you want it to do, even as conditions change.

This way of exposing is quite different than the way that most folks use, and is fluid and freeing in practice, removing worries about exposure and producing optimal files.