Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
bobn2
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Re: Exposure basics, lesson two point one (& ISO)
In reply to texinwien, Mar 19, 2013

Firstly, good idea starting a thread on a sub-theme from the last discussion. This question of the effect of REI and multipattern metering is really an interesting one, and deserves to be discussed in more detail. With luck, a thread that can result in new knowledge.

texinwien wrote:

Continued from here .

bobn2 wrote:

texinwien wrote:

Interesting. Some posters think that the discrepancy between the E-M5 Camera ISO Settings and the DxO "Measured ISO" or saturation-based sensitivity measures would somehow allow Olympus to make it appear as though the E-M5 has better noise performance at any given camera ISO Setting than it actually has at that setting.

I think that's bunk. But it's a bit of an aside, anyway.

I think it's bunk also, at least if you take just that factor. However, one wonders why olympus has left so much headroom, instead of putting in a lower ISO setting that many would have appreciated. As I suggested, one good stratagem to make low light performance of a camera appear better than it really is is to make the multi-pattern metering recognise low light situations and dial in more exposure. If one were doing that, it would pay to have a bit of extra headroom.

I'm working to wrap my pea brain around the information here, so I hope you'll bear with me while I ask some questions and do a bit of reasoning out loud.

As I say, worthwhile doing this thinking in the open, because maybe people will join in who have hard information on the topic, and we will all learn.

First, when you say 'dial in more exposure,' I take it to mean that the camera would lower the shutter speed. Assuming that's what you mean, is there any evidence that would suggest that the E-M5 does, indeed, do this?

That evidence would require a systematic investigation. If someone with an E-M5 wants to do it, then it would be most interesting to see the results. I would think you'd want to calibrate the E-M5's meter against a control meter of the integrating variety, and then go through a variety of scene types noting the exposures given by the control meter and the E-M5.

Also, in your good example stratagem, the camera would be programmed to only lower the shutter speed when it recognized low light situations. With that in mind, I have a couple of questions:

What would you expect to happen in well-lit situations? I assume that you would expect to see 'correct' metering in these (correct based on the camera ISO setting).

Well, remember that the point of multi-segment metering is to find a 'better' exposure than you get by simply using an integrating meter. On the one hand, one might say that the optimum exposure (from the output brightness point of view) is that which you'd get from an incident light meter, so you'd think perhaps that a multi-pattern meter should try to compute and replicate the incident light meter result. But another way of looking at it is that it should just give a more 'pleasing' result. So far as I know, no manufacturer reveals the algorithms behind their multi-segment metering, so we don't know in practice. But I would think they might do things such as lowering the exposure in high contrast scenes to try to avoid blown highlights, increasing the exposure where there seems to be a preponderance of dark to avoid noisy shadows and so on. When you start to think how multi-pattern metering would work, it seems pretty natural to think it's going to give more exposure in dark scenes (such as night shots) and less in bright high contrast (snowscapes and beaches, etc) because that is what most photographers would do.

Do you think most ISO test scenes used by DPReview, imaging-resource.com and other well-known testing outfits would count as low-light scenes in such a stratagem?

Mostly they look like mixed contrast scenes. Since DPReview, IR and the like don't use the camera metering for their tests (which invalidates the ISO rating, really) any meter effect would not affect them. However, I would not be at all surprised if the metering didn't get gamed for test scenes. There was a piece on the radio yesterday in which it was reported that motor manufactures are setting up their ECUs to recognise being driven to the statutory test procedures and lean out the engines to achieve apparently lower carbon emissions, wile in normal driving the emissions are unchanged.

If we saw 'correct' metering in well-lit scenes (correct based on the camera ISO setting), we would see more noise in them than we would have seen had the E-M5's ISO settings been more closely aligned with its saturation sensitivity. Would you expect consumers not to notice this excess of noise in well-lit images?

Most modern cameras have an excess of SNR (for most photographers) in well lit situations. Generally, an advanced photographer will trade a bit of that SNR for highlight protection, and I would expect that is what multi-pattern metering will do to.

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Bob

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