Friday, April 7, 2023

ETTR Exposure Meter Update

This post comes with a little twinge of sadness, as it will be the last post I will flag on the DPReview forum, as Amazon, in its infinite wisdom, has decided to shut DPReview down.

Like many photographers, I have got a lot of support from DPReview forums and, in my humble way, I've tried to contribute.

Back to the main purpose of the this post, that is to announce an update to the ETTR meter in my Landscape Bracketing Script, that runs on the M3 and M10 (LBS-M, which is downloadable on the right).

The ETTR meter interface has been simplified and shows the top three (highlight) 1/3 stops, including the % of the histogram in each stop.

For those that are interested, here are the histogram bins that I’ve used, to account for the fact that the CHDK 8-bit liveview histogram data is not a linear representation but a 2.2 gamma one.

for i = 230,255,1 do stop3 = stop3 + histo[i] end
for i = 207,229,1 do stop2 = stop2 + histo[i] end
for i = 186,206,1 do stop1 = stop1 + histo[i] end  

The top 1/3 stops are shown as long as there is no histogram data in the top three histogram bins, ie 253+254+255, implying 100% highlight saturation is occurring. But note, all this is not based on RAW data, so you will have little exposure insurance in hand. In addition, the 8-bit CHDK gamma histogram will not be an exact representation of the scene.

The following screen grabs show the new ETTR meter arrangement in action:

In the above we see an underexposed scene, illustrated by the top three 1/3 stops all showing 0%.

In the above we see the exposure has been slightly adjusted and the Canon histogram switched on, by toggling through the INFO button.

In the above we see the CHDK, log, histogram has been toggled on as well. Both histograms can be on or off independently.

In the above we see that the exposure has been adjusted further and that the highlights are now just saturating. We see this transition is clearly visible, as we now only get feedback on the level of highlight saturation that is occurring.

Finally, in the image below, we see the exposure has been increased even more and we are seeing an indication that about 0.3% of the histogram is saturated.

Clearly, all this information is there to help you set the best ETTR exposure, noting that it is based on non RAW data.

As usual I welcome any comments on this post or any of my posts. 

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