Hypothetical question about lower iso vs built in nd filters

[Unframed Dave]

This question follows a discussion last night / morning with a mate on the other side of the world.

He's an om1 user (bless) and considering the latest version. We were discussing the "built in" ND filters.

It lead me to ask myself why camera manufacturers don't offer lower iso ratings. On the face of it to a simpleton like me, it seems quite plausible.

So why isn't it out there? I'm sure there's a good reason.

 

[FowlersFreeTime]

I'd be curious to know. The a6000-a6600 had a lowest ISO of 100, but now my a6700 has ISO 50. Haven't used that setting yet, might come in handy with this month's long exposure challenge.

 

[Alan Clark]

Perhaps it is impossible to have an ISO much less than 100? When the data are read from the sensor they are amplified electronically by an amount determined by the ISO before being converted from analogue to digital, but the amplification factor cannot be less than x1. (I am guessing that this is the explanation).
The "Built in" ND filter works by taking a series of short exposures (ignoring the light between exposures) and combining them, which gives a different effect from real ND filters, and does not allow more extreme degrees of filtering, so I don't think it is a particularly good idea.

 

[Brownie]

Camera sensors are optimized for dynamic range and a lot of other considerations. It is important to remember that the base ISO comes from the old film numbers (100) and what was considered 'normal' speed film. 200 was fast, and 64 was slow. It is also important to know that while ASA 100 film was ASA 100 (or pretty close to it), not all digital camera's ISO's are the same. One may be higher, another lower (within reason of the rules of the ISO standard).

There is a thread in this forum that discusses the belief that Sony underrates their sensors, thus the need to overexpose in a lot of cases. that would tell me Sony's ISO 100 isn't, it's just what they assign as the native ISO. Setting up the sensor to read a different range at the low end would probably result in sacrifices somewhere else. I don't know if this is based on current technical limitations or physics, but probably a combination of the two. Look at the new A9-OTP. If you've read or watched any reviews of that camera there are clearly limitations on that sensor, with a base ISO of 200, a narrower DR, and (I think I recall) no dual-gain amplifier.

The chart below is the ISO range for the A7 IV. Note the different symbols outside of the normal range. Reading the description at the top, anything outside of the analog range is digitally enhanced to make them acceptable. If you've never tried to shoot an ISO below 100 in bright sunlight, you will find weird colorations, I tend to see a lot of magenta.

I have often wondered why no one includes a digital ND filter setting in their system. I wonder how works, and how it actually affects the sensor.

 

[Richard A]

My Sony RX100 MIII has a built in neutral density filter. I think it’s mainly included for using with video. I can’t remember ever using it. It also has ISO settings below the base ISO which is 125.

My Sony A7R5 has a base ISO of 100 but also has ISO 80, 64 and 50.