This is one of a series of posts on the Sony a7RIII. You should be able to find all the posts about that camera in the Category List on the right sidebar, below the Articles widget. There’s a drop-down menu there that you can use to get to all the posts in this series; just look for “a7RIII”.
Yesterday, I showed graphs relating to the Nikon D850’s noise and how that relates to the ISO setting in the camera. In my first post today, I posted visual takes on the noise differences from ISO 100 through 500. In the next post, I did the same thing for ISOs 640 through 5000. In the one after that, I extended the series to ISOs 640through 10000. As expected, the a7RIII looked pretty ISOless above ISO 640. In each of the three sets of images, the exposure was the same, and I compensated for the “underexposure” at lower ISO settings by boosting Exposure in Lightroom.
In the preceding post, I looked at what happened to shadow detail at ISO 500 and 640, on both sides of where the camera changes conversion gain, at exposures that compensated for the ISO differences, with a fairly aggressive push, and concluded that the theoretical shadow detail enhancements of the higher conversion gain at ISO 400 were not material.
Now I’m going to do the same thing with a more aggressive push. I made two images of my standard subject for this test all with the same shutter speed, using an Otus 55/1.4 set to f/4. I used an ETTR exposure appropriate for ISO 10000 for the ISO 640 shot, and one appropriate for ISO 8000 for the ISO 500 shot. I processed them all in Lightroom with default settings except for a custom white balance that assured that the WB wouldn’t change from image to image. Here are the full-frame images.
Here they are after a 4-stop push in Lightroom.
Here are some crops, magnified to about 200%.
Now the lower read noise in the ISO 640 case more than compensates for the greater photon count in the ISO 500 one. But the differences are certainly not night and day.