D500 - grainy sky even at lower ISOs

Started 1 month ago | Questions thread
FingerPainter Senior Member • Posts: 4,750
Re: Four stops?

Tony Beach wrote:

Thanks for your response. I think I understand where you are coming from now.

FingerPainter wrote:

Looking at the scene, Sunny 16 based on ISO 75 on a D500 applies

Why ISO 75? That is a 1/12 stop between ISO 80 (a 1/3 stop) and ISO 70 (a 1/2 stop). AFAIK, on the D500, one can set ISO 50, 64, 70, 80, 100, 125..., but not ISO 75. But there seems to be no point to set ISO below 100 on this shot, because DR is no higher at the extended ISO's below 100.

Read the link below. In point of fact, the effective ISO could easily have been well below ISO 75 (which technically is half a stop below ISO 100, but you might want to check my math, again).

Ah, it's your maths, plus the fact that DxO measures ISO on a saturation basis at 75 when the D500 is set to a nominal ISO 100. OK.

One half stop below ISO 100 is actually ISO 70.711 which not so coincidentally is 100 x 1/2 x sqrt(2). The nominal setting for this on most cameras is ISO 70, not 75.

...

I just haven't caught on to why we are using ISO 75, or how we skip from that to analysing what OP did at ISO 125. One stop below ISO 125 is ISO 64, not ISO 75.

https://en.wikipedia.org/wiki/Film_speed#Digital_camera_ISO_speed_and_exposure_index

Note that ISO is an "arbitrary" designation, and it is based on the Picture Controls that Nikon (or other companies for that matter, which use different terminology) set as default on the camera. Change a default such as lowering Brightness and the effective ISO changes. I looked up what DxO Mark had for the ISO setting, which is reasonably close to what I typically do when I process NEF files, and that's where I came up with a base ISO of 75 for the D500.

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