Fuji GFX 100S exposure strategy, M and A modes

This is one in a series of posts on the Fujifilm GFX 100S. 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 “GFX 100S”. Since the strategy will work fine with the GFX 100, I’m also tagging it that way.

I have previously performed a quantitative analysis of the GFX 100 read noise performance versus ISO setting. That post also applies to the GFX 100S, which shares the same sensor. I also did a photon transfer curve analysis. 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 400, and even closer from 500 on up.
• There is a substantial improvement in input-referred read noise when you go from ISO 400 to ISO 500, 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 500 or as much as three stops over, set the ISO to 500 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 200 and 320, decide if you need the last bit of shadow signal-to-noise ratio (SNR). If you don’t, set the ISO to 200, take the shot, and get some extra highlight protection.
3. If that ISO is 400, decide if you could do with bit less highlight protection. If not, set the ISO to 400. If yes, set it to 500 and enjoy lower super-deep-shadow SNR.
4. If that ISO is between 500 and 3200, and you think you’ve got the highlights handled, set the ISO to 500, 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 3200, 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 320. 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 3200. 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 500.

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 GFX 100S 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.

What if you’re in a fluid situation and  want to use aperture priority exposure mode?

The cardinal rule remains the same: If you can, expose to the right (ETTR) at base ISO. With the GFX 100, you can use the live view histogram, or take shot and look at the histogram as you review the capture. Set your exposure compensation (EC) so that your meter will give you that exposure. You might want to bias the exposure slightly towards underexposure if the light is changing since it’s easier to deal with a bit more noise in the shadows than blown highlights.

But let’s ask what you should do if the ETTR exposure is too long for you or requires a wider lens opening than you want to use. If the lens opening is the most important thing to you:

• Set the ISO to base ISO
• Set the camera to aperture-priority exposure mode
• Set the aperture to whatever you wish
• If the ETTR exposure is acceptable, you’re done; take the shot
• If the shutter speed is too long, and you want some extra highlight protection, crank the EC down (make the numbers more negative) until you get an acceptable shutter speed.
• Stop when you get to two stops underexposed from the ETTR setting.
• If that gets you an acceptable shutter speed, you’re done; take the shot.
• If not, set the ISO to 500
• If the exposure is acceptable, you’re done; take the shot.
• If the shutter speed is too long, and you want some extra highlight protection, crank the EC down (make the numbers more negative) until you get an acceptable shutter speed.
• Stop when you get to three stops underexposed.
• If that’s still a short enough exposure, increase ISO until you get a shutter speed you can live with, keeping the EC three stops underexposed.

Sounds like a lot of fiddling, but after you’ve painstakingly followed the steps above a few times, you’ll get a feel for it and will be able to take many shortcuts. The idea is to make use of the highlight protection that comes with what would generally be called underexposure, and boost the gain in postproduction, which, thanks to the nature of the GFX 100S sensor, has practically no noise penalty.

If the shutter speed is what’s key for you, use the above procedure, but set the camera to S mode and swap aperture and shutter speed in the protocol.

You will be fiddling with the EC dial in the GFX 100S a lot if you use this procedure. It would be nice if the GFX 100S had a dedicated EC knob right on top of the camera, like the GFX 50R. So you’ll have to learn to push the EC button and twiddle the command dial to change EC.