Fake ISOs on the a7RII

Started Aug 25, 2015 | Discussions thread
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JimKasson Forum Pro • Posts: 44,436
Fake ISOs on the a7RII

I've been asked about the dynamic range of the a7RII at the ISOs below the base ISO. I'd not looked into that with any of the alpha 7 cameras, but I decided to do so with the a7RII.

The take home lesson from many of my photographic tests is sometimes complicated and difficult to explain. This one is not: if you shoot raw, don't use the fake ISO settings.

Let's back up a minute.

If you're a camera manufacturer, there are at least two approaches possible for providing ISOs below the base ISO.

The zeroth is for the engineers to say to the product managers: "No, we're not going to do fake ISOs on this camera. If you want it done, get somebody else to design this sucker." That's what I wish would happen with every camera.

The first is manipulating the raw data so that the images exposed at the fake ISOs are darker than they would otherwise be. This lowers the saturation point of the resultant raw file, and causes the dynamic range calculations to be wrong if this is not compensated for.

The second is to set the ISO to a higher value than indicated by the knob, record the result in the raw file, but change the processing of the JPEG preview image to make it darker than it would be if it were a straight rendition of the raw file. This is what the a7RII does.

You can prove the to yourself by making a set of raw and JPEG images of the same subject at ISOs 100, 80, 64, and 50, opening up the lens a third of a stop each time. If you look at the JPEG images, you'll see that they look like the all have the same brightness. However, if you look at the raw images in RawDigger, you'll see that the ISO 80, 64, and 50 images are all almost a stop overexposed from the ISO 100 image.

In fact, except for the preview JPEGs, the ISO 80 raw image looks like an ISO 160 raw image at the same exposure. An ISO ISO 64 raw image looks like an ISO 125 raw image at the same exposure. And an ISO 50 raw image looks like an ISO 100 raw image at the same exposure.

However, when you bring the images into Lightroom, the fake ISO images will look darker than the raw values would suggest. The camera must pass some information to Lr to get it to do that.

Anybody know the details on that?

If you are unwary and use the fake ISO's, you'll lose headroom and never know why your highlights are flat.

I cannot think of any reason why the fake ISOs are useful to a raw shooter.

Details here:



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