Thursday, 16 April 2015

Tutorial - Expose To The Right (ETTR)

At wilson photographics we sometimes use a technique know as ETTR or Expose To The Right when shooting landscapes and seascapes in Northumberland. You may have heard this term and wondered what it was ..... read on, dear reader, read on ....

As with all my articles I will keep it simple and adopt a "just gimme the facts" approach, only introducing as little brain-melt as possible.

The Problem ...


A common held philosophy followed by photographers for a long time has been the technique of setting your exposure for the highlights in your scene and taking an, effectively, underexposed shot with a view to raising the shadows later in post-processing to reveal the detail in those areas. The main problem with this technique is, due to the lack of information in the shadow areas of the image, increasing the exposure to recover these areas will introduce noise into the shadow areas resulting in a poor end-image.

I'll try to explain ...

ETTR - Background info ...


The recording of the scene by the photo-cells on your cameras sensor depends on the sensors linear response to the light striking it. Generally speaking, it is not a linear response so if your camera's sensor has a dynamic range of, say, 5 stops then the brightest stop (at the far right of the histogram) will record one half of the available tonal values, the next stop down will record one half of the remaining tonal values, and so on, halving all the way to the darkest stop (at the far left of the histogram). This explains the halving or doubling of light entering the camera when you stop-down or stop-up.

ETTR - The Skinny ...


This may seem somewhat complex to take in so, in a nutshell, just accept that much more information is recorded at the far right of the histogram than at the far left. You can always read up why at a later date (if you need to).

So, to follow on from that little bombshell, it would seem common sense that, in order to record the maximum possible information about a scene to produce the best possible image you would do well to overexpose the image with a view to lowering the exposure later in post-processing. This is, indeed, a valid technique - welcome to ETTR.

The below image, of a chain in a harbour somewhere in Northumberland, was taken using ettr ...

 

[caption id="attachment_1335" align="aligncenter" width="300"]ETTR Before Processing ETTR Before Processing[/caption]

... the histogram clearly show an over-exposed image, but with adjustment to exposure, contrast and saturation in post-processing it can be recovered ...

[caption id="attachment_1336" align="aligncenter" width="300"]ETTR After Processing ETTR After Processing[/caption]

... the histogram now shows a much more balanced image with detail covering the full range of tonal values.

Importantly no noise was introduced into the shadow areas, which would almost certainly have been the case if the shot had been under-exposed and then lightened in post.

All-in-all, ettr is a valuable technique to have in your arsenal (just to be clear, you may still have to use filters if the range of the scene is too great). It will take a little practice and experimentation to work out how much your equipment will let you get away with because, bear in mind, the histogram you see is not based based on the RAW file but is based on an in-camera jpeg representation of the RAW file. With this in mind your histogram may show a little clipping when that is not the case, you will need to try a few images to work this out for your own gear.

It took a few test images to work out how far we could push our own gear, when out shooting landscapes and seascapes in Northumberland, and we found we could allow the histogram to venture out of the top stop a little before it affected the end result.

ETTR - The Caveats ...



  • You must shoot in RAW, to enable you to get the best results from post-processing later.

  • You cannot allow the shot to leave the right hand side of the histogram, this would result in lost information and clipping, the skill is in exposing as far right as your particular camera will allow without clipping.


Relevant Links ...

Wikipedia - http://en.wikipedia.org/wiki/Exposing_to_the_right

 

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