To ETTR or not to ETTR...?

Started 3 months ago | Discussions thread
XRF Forum Member • Posts: 78
Re: To ETTR or not to ETTR...?

It seems that the number of actual answers to "do you use ETTR" are less than the number of technical arguments, which I myself am guilty of contributing to. Still want to discuss the technical side but first I should address the actual question.

Sometimes. Since ETTR involves taking a more active role in selecting your exposure, it takes more time. Therefore, I use ETTR mainly when I am shooting in situations where I have a large amount of time to take the shot such as landscape. If my shutter speed is high enough and there is plenty of light, I may use a slight increase in exposure compensation (normally +1 EV) for dynamic subject like wildlife, but I won't fully ETTR in case they move into a situation where overexposure is risked. I do not bother with ETTR at ISO values other than base ISO.

Now as to why. Given a perfect sensor, the noise level in any area of a photo is the square root of the number of photons hitting the sensor during the exposure. Quadruple the amount of light and you double the signal to noise ratio (SNR). Electronic image sensors have a hard cut-off to the number of photons they can record before the electron wells are full and you get clipped highlights. The best possible SNR occurs just before this happens. Since the data from an electronic sensor needs to be decoded and processed before it is displayed, the number of photons corresponding to a particular output lightness value can be practically anything the processor or camera company wants it to be.

When a camera meters for a photo it has no knowledge of what the actual incident light is, so it makes an estimate. First the camera assigns a weight to each region of the sensor depending on the metering mode, and possibly adds a vignetting compensation factor for those regions. Next the camera averages the incoming light, and selects the exposure based on the assumptions that (1.) the scene is evenly lit, and (2.) the average amount of reflected light in the scene is a set value, normally 15% or 18%. The second assumption is the reason you get underexposures in scenes filled with reflective materials such as snow, and overexposures in scenes filled with non-reflective materials. Some manufactures may also be implementing smart exposure functions now such as ignoring sections of the image that are much brighter or darker than the rest.

Where ETTR becomes useful is the relation between sensor saturation and what the camera company sets as corresponding to middle grey. The ISO standard for digital camera sensor offers three methods of determining what the base ISO of a sensor is. Of those, one is based on SNR, one is based on sensor saturation, and the last basically allows the company to do whatever they want. The saturation method is the only one that actually ties the amount incoming light to the ISO value. The saturation method states that the sensor should saturate at 3 1/2 stops above middle grey, that gives 2 1/2 stops between middle grey and white, and 1 stop extra for specular reflections and emitting light sources. It also states that at 100 ISO a sensor should saturate at 0.78 lux·s. Now take a look at

http://www.photonstophotos.net/Charts/PDR.htm#Ideal%204/3,Olympus%20OM-D%20E-M1%20Mark%20II

notice how the E-M1 mk.II has better than ideal performance. That is not because the senor is perfect, far from it, remember the CFA is removing a good portion of the light, it is because camera manufacturers do not use the saturation definition of ISO. By the saturation definition 200 ISO may actually be 100 ISO or 50 ISO, the camera just sets middle grey further from saturation. The extra dynamic range comes from extra highlight headroom, not better performance in the shadows. To take full advantage of the dynamic range, you must bring the sensor as close as possible to saturation as you can. Both ETTR and expanded low ISO ranges help to do this. The advantage of ISO based ETTR is that even though you are loosing dynamic range, you are gaining slightly in shadow performance, and if you are not clipping highlights, you used dynamic range is increased.

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