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 p.38 #6 · Pre-order: Fujifilm GFX 50S Medium Format body ($6,499) | |
gdanmitchell wrote:
I don't believe it "boosts the ISO." It uses what the ISO is and alters the exposure and then the curve applied to it.
the purpose of these modes is not to give you "higher ISO," but rather to retain detail near the ends of the usable dynamic range. In essence, the camera is doing the sort of thing that a careful photographer, previsualizing the output, might do at the time of exposure if faced with a similar issue.
Let's say that I'm shooting at what ever ISO I'm using — let's make it ISO 200. I'm photographing a scene with a relatively conservative dynamic range and no particularly challenges at the dark or bright ends of the luminosity scale. I probably expose for the average of the scene, so say it in a simple way, without too much concern about the extremes.
Let's say that I'm shooting a different scene, also at ISO 200. But this time there are a lot of very bright values, and I want to retain the subtle differentiation among them in my final image. I may reduce the exposure just a bit in order to "protect the highlights" and provide a bit more headroom for adjustments in post, where I may steepen the luminosity curve in the brightest tones.
Let's say I'm shooting a third scene, also at ISO 200. This time let's say that the dynamic range is extremely wide, and that the scene includes interesting details in both the shadows and in the very bright areas. I'll likely take care to avoid blowing the highlights, with the expectation that I'll need to do some work in post both to increase the curve steepness in the bright tones and bring up the shadow low luminosity areas in post. Here, again, I'll possibly "under-expose" a bit, knowing that this will give me the best balance of scene detail to work with in post.
When shooting raw with these methods in mind, I can do some very sophisticated things in post, including planning to adjust different areas of the image in different ways — perhaps applying some processes only to bright areas and others only to dark areas, while keeping them away from middle tones.
When you choose one of the modes you mention on the x-trans Fujifilm cameras, the camera is attempting (with surprising success) to automate the process of doing something similar in-camera rather than waiting for you to do it in post. Again, to the best of my knowledge, the ISO does not change here either. Instead, exposure is altered, and different curves are applied to the conversion in order to expand the highlight (and probably shadow) details at the potential expense of some mid-tone values, though I expect that the camera's algorithms make some beneficial adjustments even there.
Dan...Show more →
It is trying to protect highlights by underexposing. I think they expect the typical use case to be someone who has DR and ISO both set to auto and it tries to detect high DR scenes and protect the highlights by underexposing one or two stops.
If you explicitly choose the ISO I'm not sure that it does anything, I just tried and it set it to DR100 which I think is just normal. But if you have it in one of the auto ISO modes it will bump the ISO one stop for DR200 and 2 stops for DR400, I think it's also applying some sophisticated processing to the jpeg's as well. You can try it yourself we both have the XP2, put ISO to A and set DR explicitly to 200% and the lowest ISO value it will choose for the exposure will be 400 with the XP2, if you set DR to 400% it will choose not lower than 800. The GFX camera in the picture appears to have the ISO dial set to A so I think this is the reason it's ISO was at 200.
If you have the ISO manually set to 200 it will turn off the DR mode (set to 100) and gray out the options for selecting DR200 or DR400. So I'm not sure it does anything out of the ordinary at DR100 or ISO 200.
Here's another way to describe the behavior of DR mode, ISO and auto ISO.
If you have auto ISO set to a range of 200 - 1600.
1) If DR is set to 100% your ISO range will be 200 - 1600, normal.
2) If DR is set to 200% your available ISO range will be 400 - 1600.
3) If DR is set to 400% your available ISO range will be 800 - 1600.
4) If DR is on auto it will try and detect the setting on it's own.
If ISO is manually set to 200, DR200 and DR400 are disabled.
If ISO is manually set to 400, DR200 is available and DR400 is disabled.
If ISO is manually set to 800, DR200 and DR400 are both available.
This came up on another forum before and it's fairly non obvious behavior that I don't think gets described anywhere very well.
EDIT: I think these two articles help to explain how this feature is protecting highlights, why it seems so counterintuitive and what the effect is on both raw files and jpeg's.
http://adambonn.com/my-love-affair-with-the-fujifilm-x-pro1/xp1-eleven/
http://www.clarkvision.com/articles/iso/
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It was hard to find info on exactly what it was doing