topic Re: ISO ON 1DMK III in EOS DSLR & Mirrorless Cameras

EOS-1D Mark III Unable to set ISO below 200

harryset — Tue, 09 Sep 2025 17:56:54 GMT

Canon 1D mark III, canon EF 24-105 IS, 1:4 L lens. I am unable to lower the iso below 200 even with the CFN settings set Max H, Min 100.

Re: ISO ON 1DMK III

Waddizzle — Sun, 07 Sep 2025 17:40:52 GMT

Do you shoot RAW? Which noise or image correction settings are enabled. I think that one of the correction settings restricts the ISO range at the low end.

I shoot RAW and keep all correction settings disabled.

Re: ISO ON 1DMK III

deebatman316 — Sun, 07 Sep 2025 18:11:20 GMT

The OP most likely had “Highlight Tone Priority” enabled. When this is enabled the camera underexposes the shot by a stop. Therefore ISO 100 cannot be set.

Re: ISO ON 1DMK III

Waddizzle — Sun, 07 Sep 2025 23:54:22 GMT

I don’t know if that older camera has that setting. I have one, too. It’s turned into a paperweight with an uncharged battery.

Re: ISO ON 1DMK III

deebatman316 — Mon, 08 Sep 2025 02:44:55 GMT

That camera has that setting so does my EOS 40D those cameras were both released in 2007. But a couple months apart. The EOS 40D was released in August of 2007. A lot features in the EOS-1D Mark III were in the EOS 40D. Such as the External Speedlite control menu.

Re: ISO ON 1DMK III

Peter — Mon, 08 Sep 2025 06:46:58 GMT

It sounds like HTP, as already mentioned. There's no benefits to using that if you shoot raw.

Re: ISO ON 1DMK III

p4pictures — Mon, 08 Sep 2025 09:52:55 GMT

There may be little benefit, but it does change how the image data is written to the RAW file. In effect the camera uses a different "curve" when digitising the data to convert the signal level to a stored value in the RAW file. Shooting the same scene in RAW with HTP on and off gives slightly different results.

Re: ISO ON 1DMK III

Peter — Mon, 08 Sep 2025 10:26:03 GMT

ISO 100 in the metadata is changed to ISO 200
The metering is changed to underexpose 1 stop
The MakerNote tag Highlight Tone Priority is changed from 0 to 1 or 2
The embedded JPEG will be brighter compared to the embedded JPEG at ISO 100.

For the raw file itself, I see no reason to shoot HTP.

ISO 100

HTP ISO 200

DPP4 reads the MakerNote tag Highlight Tone Priority and will add a different tone curve for 0, 1 or 2.

RawTherapee will try to copy the tone curve from the embedded JPEG, so we can actually see what is happening in the post processing.

ISO 100

HTP ISO 200