Poor Exposure ≠ SAB

Started Feb 8, 2012 | Discussions thread
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Najinsky Veteran Member • Posts: 5,739
Poor Exposure ≠ SAB

Having a camera in the early stages of exhibiting SAB has led me to do a lot of analysis of my images, and also perform a lot of experimentation with how the camera operates.

The take away is that there are curve balls all over the place that can impact exposure (and perceived exposure) and it would be wrong to assume that exposure issues are always caused by SAB. To conclude that you have SAB, you really need a sequence of exposures, consistent light, and a fully explained EXIF before concluding SAB.

Among the curve balls that can mislead your exposure are:

  • Optical View Finder

  • Shutter Speed Limit

  • Auto ISO

  • Auto DR

  • Adobe Lightroom

Optical View Finder

If you try to use Spot Metering via the Optical Finder, parallax errors can cause you to meter the wrong spot. Fujifilm advice against spot metering via the OVF.

Shutter Speed Limit

At wide Apertures, the shutter speed is limited. For example to 1/1000s for F/2.0. So in bright light at big Apertures, the metering may calculate 1/2000s or 1/4000s, but the exposure is taken at 1/1000s leading to overexposed images.

This is fairly widely known, but it can unintentionally get triggered by the settings described below, and that's the curve ball.

Auto ISO

The cameras base ISO is 200. When shooting JPEG, an artificial LOW ISO of 100 is available but this is a firmware trick and not available when shooting raw or when using other firmware tricks, so for most purposes lowest ISO is 200.

Auto ISO allows the camera to keep a defined minimum shutter speed by raising the ISO up to a defined maximum. By itself, it is a useful feature (although not without its own quirks) however, it can unwittingly combine with the feature below to bad effect on the exposure.

Auto DR

This is the real villain in the story.

DR100% is standard dynamic range, using a normal contrast curve. DR200% and DR400% are useful for high contrast scenes where you want to retain both shadow and highlight detail. It does this by underexposing by 1 stop (DR200%) or 2 stops (DR400%). This helps prevent blowing the highlights, and the shadows are recovered using a tone curve designed for the job.

The problem comes from how it underexposes for DR200% and DR400%, by reducing the ISO used for the exposure, but only for the capture, not for the metering.

So if an exposure is metered as F/4 + 1/500s + ISO 800, the capture for DR400% is taken as F/4 + 1/500s + ISO 200. The EXIF will show F/4 + 1/500s + ISO 800 and will also have a MakerNote field set to identify DR400.

So it can only use DR400% if ISO is at least 800 (or DR200% if ISO is at least 400).

So here's what can happen when you shoot in bright contrasty light with Auto ISO + Auto DR (or DR > DR100%).

1) The X100 detects a contrasty scene and AutoDR chooses DR400%.
2) DR400% requires at least ISO 800 so Auto ISO supplies that.

3) The scene is then metered with ISO 800 as a minimum. Depending on the Light and / or Aperture setting, this can push the shutter speed dangerously close, or over its limit.
4) If the shutter was limited, the image is overexposed.

However, that isn't the end of the story. See below.

Adobe Lightroom

Remember, for an image shot in DR400%, while the metering was done for a capture at ISO 800, it was actually captured at ISO 200. The embedded JPEG will have been processed with the DR400% tone curve and will look 'correctly' exposed, but the raw data will be from an ISO 200 capture and will be 2 stops underexposed.

So in LR, DR400% images will appear 2 stops underexposed (1 stop for DR200% images).

So now what happens in LR with a DR400% image which also happened to get overexposed due to the high ISO and limited shutter speed?

The underexposing can cancel the overexposing and save your bacon. Had you shot JPEG, the overexposure would have been baked into the JPEG, but the raw is 2 stops under, and usually has an extra stop of headroom recovery.

However, if viewing with raw software that respects the MakerNote (DR400 marker), the image is processed with the higher exposure (Aperture does this, as I believe does SilkyPix).

So JPEGs can look different than their raw.
Overexposed images can look overexposed or correctly exposed.
Correctly exposed images can look correctly exposed or underexposed.

It's no wonder people think they have exposure problems or SAB issues!


Fujifilm FinePix X100
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