Started Dec 20, 2013 | Questions thread
Ontario Gone
Ontario Gone Veteran Member • Posts: 4,183
Re: Why ETTR?

Heya Ysbrand, some of what has been posted is right, most of it wrong, i will try to summarize and clarify and hopefully this will help.

First we need to clarify how we are ETTR. Technically ISO is not part of exposure, but for this conversation, we will pretend it is. One can either adjust ISO higher to result in overexposure, or adjust SS/aperture. Lets cover both scenarios.

ADJUSTING SS F2.8  SS 1/1000  ISO-800: So in this first scenario, we can change the SS to 1/500 and get a +1, perhaps clipping some highlights, then pull it back down in post. This is the idea. This doesn't help though, because i could simply shoot at 1/500 at ISO-400 and achieve the same result with no need for PP. Same noise, same SS/aperture. If the scene is capable of a 1/500 SS to begin with, it makes no sense to overexpose when you don't have to, may as well just lower ISO to begin with. In fact, some cameras that apply unwanted NR like Pentax are better off shooting at 1600 ISO or less even if it's ETTL, to avoid this NR. The camera will never do a better job than good PP software.

ADJUSTING ISO F2.8  SS 1/1000  ISO-800: This is the scenario that the idea is really based on. The reason is, some sensors have a lower read noise % the higher the ISO setting is set to. As gain increases, read noise is actually less. This thinking however is flawed because of two reasons. First, this only applies to certain sensors, mostly Canon sensors, which are the minority nowadays. Second, read noise comprises such a small % of the visible noise in a photo it is barely worth paying attention to. Low read noise is much more an issue for long exposures for star watching, which is one reason canon is so good for this kind of photography. For most photography, 1/2 sec to 1/8000 SS, it is not noticeable.

So, to explain, lets change that ISO-800 to ISO-1600 to overexpose the image +1. We then pull the shot back down to 0 in post, and supposedly have less noise. Remember though, if you were shooting a sensor like in Canon cameras, the difference would be there but would be so small you wouldn't notice. In newer non Canon sensors, such as the ones Sony makes for just about everybody else, they don't behave this way. The read noise stays almost exactly the same regardless of the ISO setting. This means your ISO setting in camera does exactly the same thing that pulling or pushing in post does: it changes the amplification of the signal. Amplifying a signal darkens or brightens both signal and noise, so there is no advantage to any ISO setting over another in terms of read noise. The noise will be determined by the amount of light hitting the sensor, which has nothing to do with ISO.

Some believe (myself included), it is actually better to ETTL to preserve highlights and pull up later. If you go too far banding can show up so you gotta be careful, but i usually keep my shots underexposed by 1/2 stop or so just in case (always shooting raw). I hope this helps, there is a legitimate theory behind ETTR but the difference is negligible and it is only for a small % of today's sensors. If anybody out there can prove me wrong i would be happy to see a comparison shot.

 Ontario Gone's gear list:Ontario Gone's gear list
Nikon D7000 Canon EOS 70D Nikon AF-S DX Nikkor 35mm F1.8G Nikon AF-S Nikkor 50mm f/1.8G Canon EF 40mm f/2.8 STM
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