A fellow photographer said that they use the Highlight Tone Priority (HTP) mode on their Canon dSLR as it helped to prevent clipping of highlights. I never blindly follow anyone’s advice these days but decided to do some digging around on the Internet to see what all the fuss was about.
If you search ‘highlight tone priority’ on the web you get a whole raft of articles (see the references section below for a few examples) which explain the theory behind this mode. Some things were clear from reading these. Firstly, it does not magically increase the dynamic range of your sensor and secondly, it works differently depending on whether you shoot JPEG or RAW (even which RAW converter you use). So, it was time to do some experimentation with my own kit and software to find out what can be achieved.
Before reading on please be aware that HTP is part of the camera’s internal image processing algorithms that take the data from the sensor and transpose it into the digital file on your memory card. There are many Canon cameras with this feature and I don’t know how similar the implementations are across the product range.
Note: For Nikon owners, the equivalent technology is referred to as Active D-Lighting.
My objectives were quite simple:
- Asses the value of the mode when shooting JPEG and RAW
- Assess whether the choice of RAW conversion software matters
- Prove/disprove the commonly held notion that enabling HTP is equivalent to underexposing by 1 stop
My test kit was a 1D Mk IV with a 100mm macro lens. I set up a very simple still life scene (from what was readily available; namely a tea cup, tea pot and a remote trigger). The items were assembled in the studio, with controlled side-lighting.
I set the ISO to 200 and shot in aperture mode at f/8. The camera was tripod mounted as exposures were under a second.
I captured three images, each being saved as JPEG (‘faithful’ mode) and RAW, giving six image files in total.
- HTP disabled – the control
- HTP enabled
- 1 stop underexposed
Results – JPEG
Images and histograms from JPEGs shown via the ‘Levels’ control in CS5
When HTP is turned on note how the brighter areas (the china of the mug) are rendered darker. The highlight area of the histogram has been being shifted to the left, whilst the shadow area is largely the same as with HTP off. So, pretty much as claimed by Canon.
Now let’s look at the image shot 1 stop underexposed. This looks very different to either of the first two shots and to my eye just looks like a slightly underexposed image – certainly anyone who regularly ‘shoots to the right’ would say so.
Results – RAW
For my raw processing I use Lightroom 3 exclusively, so I installed V3.9.3 of DPP to complete the experiment.
There are claims that Canon’s own DPP software will interpret the RAW file ‘properly’ and the HTP effects that are seen in JPEG mode will be evident.
The images you see below were loaded into Lightroom and the histograms taken directly from there with the default settings I use with my 1D camera. The corresponding histogram from DPP is shown below.
Comparison of the two histograms shows that DPP brings the highlights safely back from being clipped, whereas Lightroom does not.
The 1 stop underexposed image just looks like its JPEG counterpart – underexposed.
The biggest thing to notice is that if anything, the one taken with HTP on actually looks more overexposed that when it is turned off! This may indeed be due to Lightroom not being able to interpret the HTP nuances in the RAW file.
My findings are as follows:
1. For JPEG shooters
HTP appears to protect the highlights with little or no change to the mid or shadow tones. So, if you are a JPEG shooter it may be worth turning HTP mode on if you find that your highlights are being clipped.
2. For RAW shooters
If you use DPP it’s possible to open the file and for the HTP effects to be visible (making it appear similar to what is achieved in JPEG mode).
Using ACR or Lightroom the effect appears to be ‘lost in translation’ and the resultant image/histogram clearly show that highlights are clea still clipped. I’ve been in this game to know that all that glistens isn’t gold and I’d like to find out whether it’s a real gain or just smoke and mirrors.
3. HTP compared to underexposing by 1 stop
It is clearly evident from my findings that the effects of HTP are quite different to what is achieved by underexposing by 1EV, which simply gives an underexposed image.
My initial findings lead me to conclude that HTP can be a useful tool in protecting those important highlights for JEPG shooters. If you are a RAW shooter and you use DPP then you can achieve the same benefit.
My next task is to spend some more time investigating the differences between DPP and ACR/Lightroom to determine whether there is a real world advantage to using DPP.