Histogram / RAW conversion comparison

[neuroanatomist]

You mentioned you did not shoot with HTP on because of increased shadow noise. Is it really that visible? I have not shot with the 7D but it is hard to see any difference in files from the 50D.

It's rather situationally dependent. I have seen it in some shots, not in others. I suspect it's a combination of factors. Even Bob Atkins stated (from the article you linked), "Looking closely at images shot with Highlight Tone Priority turned on and off, there is an increase in the shadow noise if you look closely enough." As I stated, I don't usually use it. But, I do use it occasionally (in fact, it's one of the settings I put in My Menu) in situations where looking at the scene I expect issues from blown highlights at the exposure I plan to use.


Daniel summarized it nicely in a previous discussion on 'tweener' ISO settings:



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It's a pretty minor effect by itself. Just like HTP is a pretty small effect, and ALO is a pretty small effect. But when you start combining all these small factors, it results in a big factor. For example, ISO 250 by itself may not increase shadow noise enough to notice. And ISO 200+HTP by itself may not either. But combine 250+HTP and the result will be much more noticeable: the shadow noise is as bad as ISO 640. Add ALO into the mix and you can get people wondering why their ISO 250 shot looks like ISO 1600.

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[Daniel Browning]

You mentioned you did not shoot with HTP on because of increased shadow noise. Is it really that visible?

It depends on the ISO. HTP works in the context of a constant display brightness and a constant exposure. In that context, low ISO has more noise than high ISO. In other words, when you keep your exposure the same, and the raw conversion is made to come out with the same brightness, higher ISO has less noise (sometimes a lot less) than low ISO. This is the opposite of how we normally think about ISO because we don't normally keep exposure fixed. In essence, it separates ISO-as-a-camera-setting from ISO-as-an-exposure-index.


So ISO 3200+HTP (which is actually just ISO 1600) has the exact same noise as ISO 3200 without HTP, but ISO 200 has less noise than ISO 200+HTP (which is actually just ISO 100).


The way HTP works is pretty simple. Say you shot a scene at ISO 200 f/4 and developed it with default raw converter settings, and it came out with too much blown highlights. Then you shot it at ISO 100 f/4. Now the highlights are better, but the overall image (midtones, shadows, etc.) are too dark. So you push the midtones and shadows without blowing the highlights (e.g. by using a curve). This makes the increased noise of a lower ISO setting visible (you wouldn't see the difference if you left the overal image darker). That is what HTP does.

[Johnny Rasmussen]

Agree. I also think it's a combination of factors. I also think HTP is a nice feature but you have to use it when the situation calls for it.

[canoli]

This makes the increased noise of a lower ISO setting visible (you wouldn't see the difference if you left the overal image darker). That is what HTP does.

The manuals always say to beware of HTP because it'll increase shadow noise. I guess the authors assume we'll want to open up the shadows (and 1/4 tones) in PP, thereby "increasing" the noise.


Seems like HTP is a mixed bag - gain HL range but then you either live with a darker (and flatter) image or reveal that nasty pattern noise in the shadows...