What is HIGHLIGHT TONE PRIORITY and why do i lose ISO 100, 200, 300 when I enable it?
- Thread starter Lucas 7D
- Start date
From what I understand in highlight priority it essentially under-exposes by one stop to the normal exposure, and then applies a different tone curve so the image looks like a normally exposed one. By doing this it gives the highlights more protection from blowing. The cost is more shadow noise. Everything I am saying is from a stills photography perspective. Personally I have not found it particularly useful with my stills photography and I prefer to just keep an eye on the histogram and under-expose by 1/3 or 2/3 of a stop if necessary and adjust in post. However, I always shoot in RAW and it might be different using video.
Sensors don't have an inherent correct exposure and all the image information collected by the sensor then has a tone curve applied to it by the camera to produce what we would regard as a correctly exposed image. However, sensors do have limits to the visual information they can record. At the extremes of light and dark the sensor cannot record any more detail and the image information is clipped. This is what leads to "blown highlights" i.e. white patches containing no detail, and no recoverable detail. Or the opposite effect of "crushed blacks" where the shadows contain no detail and are just black.
The ISO of the sensor is therefore created by the tone curve applied to it and is not a property of the sensor itself. If you were to set the exposure compensation to under-expose by one stop with the camera set to ISO 100, the effective ISO would be ISO 200 - in terms of the shutter speed and apeture combination indicated as correct by the camera in a given amount of light on a scene. Hence the different ISO range in highlight priority mode.
By effectively under or over exposing what the camera's meter would indicate as the correct exposure you can create different image properties. So in very flat light with little contrast, and where there are no very bright highlights, I often over-expose by a stop and adjust in post - this results in much lower noise. However, you can only do this when the scene highlights are a long way from blowing. In very harsh and contrasty light such as direct sunlight the highlights are often outside the sensors dynamic range when the mid-tones are correctly exposed, and it is in these type of conditions that the highlights need to be protected by under-exposing. The expense of under-exposing is more image noise and possible clipping in the shadow regions - crushed blacks.
The tone curve a camera applies to the information it collects is based on what is considered average lighting. Naturally lit scenes vary greatly in their dynamic range. In strong direct sunlight there can be a huge dynamic range, beyond the capability of any sensor to capture the full range of tones from highlights to shadow detail. Whereas in very dull flat lighting the scene has much less dynamic range and so the whole range of tones is easily captured by any image sensor - on a histogram everything would be around the centre with a big gap between the shadow and highlight ends. So in reality there is no generally correct tone curve and this is why corrections have to be applied to scenes that have a dynamic range different to the average scene the normal tone curve is meant for. Blown highlights normally look much worse than a lack of shadow detail, and so that is the reason for the highlight priority mode.
Sensors don't have an inherent correct exposure and all the image information collected by the sensor then has a tone curve applied to it by the camera to produce what we would regard as a correctly exposed image. However, sensors do have limits to the visual information they can record. At the extremes of light and dark the sensor cannot record any more detail and the image information is clipped. This is what leads to "blown highlights" i.e. white patches containing no detail, and no recoverable detail. Or the opposite effect of "crushed blacks" where the shadows contain no detail and are just black.
The ISO of the sensor is therefore created by the tone curve applied to it and is not a property of the sensor itself. If you were to set the exposure compensation to under-expose by one stop with the camera set to ISO 100, the effective ISO would be ISO 200 - in terms of the shutter speed and apeture combination indicated as correct by the camera in a given amount of light on a scene. Hence the different ISO range in highlight priority mode.
By effectively under or over exposing what the camera's meter would indicate as the correct exposure you can create different image properties. So in very flat light with little contrast, and where there are no very bright highlights, I often over-expose by a stop and adjust in post - this results in much lower noise. However, you can only do this when the scene highlights are a long way from blowing. In very harsh and contrasty light such as direct sunlight the highlights are often outside the sensors dynamic range when the mid-tones are correctly exposed, and it is in these type of conditions that the highlights need to be protected by under-exposing. The expense of under-exposing is more image noise and possible clipping in the shadow regions - crushed blacks.
The tone curve a camera applies to the information it collects is based on what is considered average lighting. Naturally lit scenes vary greatly in their dynamic range. In strong direct sunlight there can be a huge dynamic range, beyond the capability of any sensor to capture the full range of tones from highlights to shadow detail. Whereas in very dull flat lighting the scene has much less dynamic range and so the whole range of tones is easily captured by any image sensor - on a histogram everything would be around the centre with a big gap between the shadow and highlight ends. So in reality there is no generally correct tone curve and this is why corrections have to be applied to scenes that have a dynamic range different to the average scene the normal tone curve is meant for. Blown highlights normally look much worse than a lack of shadow detail, and so that is the reason for the highlight priority mode.
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Ethan Lane
Active member
Just make sure you color grade your footage. Using HTP actually makes the footage coming off the camera look worse (much less contrast) but gives you much better results after grading.
Ian-T
Veteran
I agree. That's what some folks don't understand about features such as this or custom color curves. They tend to get a little noisier at times but seems to give a lot more flexibility in post production (IMO).
PaPa
Major Contributor
I would agree with that. Definitely avoid using it indoors as you're asking for low exposure artifact hell. Outside where light is sufficient it would be much better.
I'm sorry for my long explanation because as always it turns out harder to explain something you understand yourself, than you imagined. However I was just trying to explain what HTP is and what it does. I've no experience from the video side or on grading video footage. What I will add though is what the different tonal curve that HTP applies results in. The shadows are effectively under-exposed by a stop and then brightened to look normal with this different tonal curve, which could result in the banding referred to. You should be able to replicate the type of tonal curve HTP applies in the Canon Picture Styles Editor, and to maybe adjust it a bit better for your requirements i.e so it doesn't necessarily under-expose by a whole stop. This could be important if your scene contains a lot of shadow detail that will look messy if under-exposed too much.
The basic point I was trying to make is that in my experience under-exposing by a whole stop (which is what HTP does) is probably unnecessary in a lot of lighting situations for just preserving the highlights. I don't think that there is any other magic with HTP apart from the different tonal curve it applies to the image information coming from the sensor. It should be possible to create your own tonal curve in the Picture Styles editor for a flat low contrast look. These are only suggestions to help and I always keep to what I know, and more importantly I know what I don't know - so I can't really comment on what is best for grading video footage. All I would say is that in my experience it is very difficult to create your own general custom tonal curve that would suit all lighting conditions and you really need quite a few different ones for various lighting conditions. What works with one type of lighting doesn't work with another.
The basic point I was trying to make is that in my experience under-exposing by a whole stop (which is what HTP does) is probably unnecessary in a lot of lighting situations for just preserving the highlights. I don't think that there is any other magic with HTP apart from the different tonal curve it applies to the image information coming from the sensor. It should be possible to create your own tonal curve in the Picture Styles editor for a flat low contrast look. These are only suggestions to help and I always keep to what I know, and more importantly I know what I don't know - so I can't really comment on what is best for grading video footage. All I would say is that in my experience it is very difficult to create your own general custom tonal curve that would suit all lighting conditions and you really need quite a few different ones for various lighting conditions. What works with one type of lighting doesn't work with another.
Richard J. Johnson
Joystick Member
I was doing some experimentation with HTP on and off and I could see any difference. But I really don't know what I'm looking for or what lighting would allow me actually seeing a difference.
PaPa
Major Contributor
If you want to see HTP in action, point your camera at a subject/area that goes from bright to overexposed, such as the light fall off from a light bulb against the wall. Keep it pointed at the bulb and examine the light hitting the wall when you switch it on and off, you will notice a big difference right away.
Richard J. Johnson
Joystick Member
Very cool I am going to try that right now. I have a shoot indoors tonight and I have ample light. So it seems HTP should be turned off.