Making the profile images
This step will produce a series of unfocused raw images with your camera, one for each ISO setting.
Note: You could use gphoto2 to produce the images in this step. I can’t help you with that. I made the images manually.
Each image must contain an under exposed and an over exposed area. I did that with the setup shown here, consisting of:
- My camera on a tripod, with a cable release. (Do not jiggle the camera by pushing the shutter button. If you don’t have a cable release, you could use the camera’s internal timer, but that’s a pain. Do yourself a favor. You needed a cable release anyway, right?)
- A table lamp to provide the over exposed part of the image
- Something to provide the under exposed part of the image. Anything that’s more or less flat black will work. I used the bottom of an old PC keyboard.
I moved the camera close enough to the lamp, so it couldn’t focus. The images all must be de-focused. They cannot be sharp.
I set the camera to produce RAW images, in Aperture priority mode, so that the shutter speed would vary based on the camera’s internal light meter, and set the ISO value to the lowest available manual setting. ISO must not be set to Auto! F11 worked just fine for the constant lens aperture. The lens aperture must be the same for each image.
I adjusted the camera so it approximately split the image between the lamp and the black keyboard, paying attention to the histogram, so the curve had peaks on both the left (under exposed) and right (over exposed) edges. The edge of the keyboard ended up being about in the center of the lens. You could adjust the exposure value offset control up or down if needed in order to get the optimal histogram. I found that the camera just naturally wanted to do the right thing here.
I made one exposure for each available manual ISO setting (trigger the cable release, increase the ISO setting one click, trigger the cable release) until I had an image at each ISO setting.