What does Canon Highlight Tone Priority do, and how is it different from Auto Lighting Optimizer?
Asked 5/18/2011
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Many Canon cameras offer Highlight Tone Priority (HTP). How does it work, what effect does it have on RAW and JPEG files, and how is it different from Auto Lighting Optimizer (ALO)? Is it generally useful, and when should you use it?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
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Highlight tone priority is a camera mode that internally fiddles with exposure to preserve as much detail as possible in the "highlight range" of tones...the brightest tones in a photograph. It does this, however, at the cost of tones in the shadow range, as the ultimate effect is a shift of the histogram down towards the shadows. The cost of shadow tones is a bit less than the gain in highlight tones, however it is something to be aware of.
It should also be noted that HTP affects both RAW and JPEG images. This is in contrast to Automatic Lighting Optimizer, which applies an alternative tone curve when processing the sensor output for saving to JPEG. ALO only affects JPEG images, and has the tendency to compress dynamic range...thus costing you in your overall tonal range.
Originally by user124. Source · Licensed CC BY-SA 4.0
user124
15y ago
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Highlight Tone Priority helps protect bright highlights from clipping. In practice, it works like a mild underexposure (about 1 stop), then the camera lifts the midtones/shadows while leaving highlights less boosted. The result is more highlight detail, but with some trade-off in the shadows, such as more noise or less shadow detail. Because of this behavior, ISO 100 is typically unavailable when HTP is enabled.
HTP affects both RAW and JPEG output. Auto Lighting Optimizer is different: it mainly applies a tone-curve adjustment to JPEGs to brighten darker areas. ALO can reduce overall contrast and may increase visible shadow noise; it does not protect highlights in the same way.
HTP is most useful in high-contrast scenes where blown highlights are a risk. In evenly lit scenes, normal exposure usually gives better overall image quality. If you shoot RAW, you can achieve a similar effect manually by underexposing and then lifting shadows in post-processing.
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