Canon’s HTP and ALO custom function settings
HTP disabled and ALO @ Low setting
Two of the Custom Function Settings available on the Canon EOS 5D Mk II, EOS-1D Mk IV are a little misunderstood in my opinion. The settings are HTP Highlight Tone Priority and ALO Auto Lighting Optimizer.
In the Instruction Manual for the Canon EOS-1D Mk IV, it states that Highlight Tone Priority improves the highlight detail. The dynamic range is expanded from the standard 18% gray to bright highlights. The gradation between the grays and highlights become smoother. However, the manual also states that noise may be more pronounced than usual if this setting is enabled.
Clearly, by optimizing for the highlights, something has to give. You don’t get something for nothing and higher noise is the result. However, this setting is not a cure-all for all situations. It works best when you are shooting landscapes but is not particularly useful when you are shooting a scene with highlights and shadows.
Consider Example 1 below. It was taken with a Canon EOS 5D Mk II at ISO 400. HTP was disabled and ALO at Low setting. The steps are totally overexposed. Had I reduced exposure in order to get the steps properly exposed, the shadow areas would have been totally dark including the interior.
HTP enabled, ALO @ Low setting
In Example 2, the settings are the same as the first image. It is quite obvious that in this scene, HTP is not going to save the day. The ratio between shadow and highlight is just too much for the sensor to handle. This is just an extreme example of some of the situations I am facing as I photography my project with as little strobe lighting as possible.
HTP enabled and ALO @ Low setting
In Example 3, I have disabled HTP but have kept ALO at Low setting as before. You can see that disabling HTP had practically no effect on the final image.
HTP disabled and ALO @ Low setting
So how does this affect the way I shoot? Firstly, I have to be very careful about my exposure and the time of day I shoot. For all of the examples above, I would have probably gotten a much more balanced exposure had the sun been lower in the sky. This would have allowed more sunlight to light up the interior of the shot. The sun would also not be bouncing directly off the steps which caused the highlights to wash out.
I did not shoot any exposures with ALO disabled but did so on another day but at a different time of the day. Following are two examples of the same scene shot with both HTP and ALO disabled.
HTP & ALO disabled
HTP & ALO disabled
You can see that the highlights in the examples with HTP and ALO switched off is still washed out. Canon’s manual states; If the images comes out dark or the contrast is low, the brightness and contrast can be corrected automatically. This is clearly a situation where HTP and ALO does not add any true value to the scene at hand.
Example 5 is under-exposed by 1/3 stop to illustrate how highlight on the steps is retained but in this case, the shadow areas are too dark.
As a general rule, I usually shoot with both HTP and ALO switched off. I prefer to have images that exhibit as little noise as possible. I also like to control how the sensor sees the image. I find that with ALO enabled, dark moody scenes which I want to preserve become too flat. Shadows are lightened up and highlights toned down.
There are some of you out there who might prefer to leave more of the thinking to the camera. Nothing wrong with that at all. What’s the point of a perfectly exposed image if you’ve missed the shot completely eh?
