Exposing to the Right - Part II

[Originally posted on 12/25/2018]

Continue on from Part I. Here is the quick summary first.

1. Histogram is our good friend to check if the exposure of the picture is good(correct).
2. The curve on the histogram should be complete, no clipping (cut-off on right or left end).

Before we continue, let’s use the well exposed day light picture as an example. The histogram for these kind of picture should have a curve close to the normal distribution (bell shape) and the peak of the curve should be close to the middle.

A histogram for a well exposed picture. The curve is complete and the peak is close to the middle.

We know clipping means losing details so it’s bad. The less worse situation is, although there is no clipping, the curve is near towards two ends. When the complete curve is close to the left hand side of the histogram, that’s when your picture is under exposed. If the curve is close to the right hand side, the picture is over exposed. With these two situations, the post processing can help to shift the curve to the middle and your picture will look correctly exposed. You either shift the curve to the right or left. Here we finally come to the title of this notes - exposing to the right.

For digital photography, if you need to shift the curve, you have more advantage to shift it from an over exposed picture then the under exposed one. The simplified reason is because for digital cameras, the bright area records more details so when you shoot towards right end of the histogram, you preserve more details and when you shift the curve from right to the middle (i.e. shift to the left), the picture still looks good. On the other hand, if you shift the curve from left to the middle (i.e. shift to the right), the dark area is already has less details, whatever detail is not captured by the sensor, it will be filled with the noise. When you shift the curve, the picture becomes brighter and that exposes the noise more, the picture will look very coarse. This is one of the biggest difference between film and digital photography. On film, the chemical particles are equally distributed on the film surface but the photodiodes (the photons collector) on the digital sensor are not. That’s why in digital era, you hear “exposing to the right” (ETTR) a lot, mainly is for post processing advantage.

There is There is more details on this but I just tried to keep it simple. Ask me questions if this notes confuses you more.

Merry Christmas!!