What I learned from Gollywop -- and what I wonder

Started Mar 26, 2013 | Discussions thread
texinwien
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Re: What I learned from Gollywop -- and what I wonder
In reply to bowportes, Mar 26, 2013

bowportes wrote:

Rather than accepting the camera's exposure, I should overexpose images just (but not quite) to the point of clipping highlights at base ISO (ETTR). ... My raw-processed image will be better for it, in spite of the fact that it doesn't look as good in the EVF at +2/3 exposure as when it's not set to overexpose.

Correct

Something that's not as clear to me though. If ISO is at base (160 for most Pannys and 200 for Olympus), the lens is at maximum aperture, and shutter-speed can't be set slower, but the histogram still sits right in the middle -- not overexposed at all -- if I understand correctly, I might get some improvement in image quality by raising the ISO to 400 or 800 in order to overexpose the image, as long as I'm careful not to overexpose to the point where I'm clipping highlights. I'd like confirmation that this is correct.

That's correct.

You'll want to look at sensorgen.info and DxOMark for data on your camera's sensor to figure out which ISOs make sense for you to use. The basic story is that going up in ISO always(?) lowers dynamic range, tonal range and color sensitivity (bad) but sometimes lowers read noise (good). Your task is to balance those positives and negatives out. You could do this in a per-scene basis, but that might be a lot of work, so basic guidelines might be more helpful and easier to remember.

For instance, on the G3 you should avoid ISO 200 - you'll have lower dynamic range, tonal range and color sensitivity than at ISO 160, and you'll have HIGHER read noise. E-M5 users should avoid ISO 250 and 320 for similar reasons.

E-M5 rule of thumb: Use ISO 200, where possible. Skip 250 and 320. ISO 400 and 800 will net you a reasonable reduction in read noise. ISO 1600 will net you a tiny reduction in read noise. After that, the sensor is pretty much ISO-less, and it won't pay off to use higher ISOs. Short rule: 200 when you can, 400 or 800 are ok if you can't ETTR at 200, 1600 isn't all bad. Anything higher than that should be avoided.

G3 rule of thumb: Very similar to the E-M5 - 160 when you can. Avoid 200. 400 and 800 are ok if you can't ETTR at 160. 1600 isn't all bad. Anything higher than that should be avoided.

These rules of thumb also apply in cases where things are really dark. It can make more sense to 'underexpose' at ISO 800 than bump up your ISO to 3200 or 6400. I shoot like this some times, using a maximum ISO of 800 or 1600 and shutter speed priority to freeze motion at parties or indoor events. Here's one I 'underexposed' by a couple of stops at ISO 1600, then brightened in post:

Available light. Underexposed by a couple of stops at ISO 1600. Brightened in post.

But now, if I understand the Gollywop thread correctly, I should be pleased with the overexposure and leave the aperture wide open, as long as it doesn't clip highlights.

Correct - as long as you're not blowing important highlights, you're in good shape.

So my understanding is that the point is to fully charge the sensor. Rather than thinking, as I used to, in terms of proper exposure (as my G5's visual indicators still recommend), I should instead think of full exposure, giving the sensor as much light as it can take (without clipping).

Right.

I need to stop thinking in film terms; they would never have led me to these conclusions.

Mostly right, I think. If you were developing your own film, however, and knew its characteristics, it could have also made sense to use a similar strategy - get as much exposure as you can without blowing highlights and process accordingly when you develop.

For the most part, though, I think film-shooters were primarily concerned about protecting highlights, especially if they weren't developing them film, themselves.

tex

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