On the Sony A7, ISO settings below 100 are generally not “true” lower-gain ISO values. They behave more like the camera exposing about 1 stop brighter and then scaling the result back down.

That means they typically do not increase noise the way high ISO does. In fact, noise can appear slightly lower. The main tradeoff is reduced highlight headroom: bright areas are more likely to clip because the sensor is effectively being overexposed relative to ISO 100.

So for RAW shooting, there is usually little benefit to using ISO below 100 unless you specifically want the metering/shutter-speed shift. If your goal is simply longer exposures in bright light, an ND filter is often the better solution because it preserves highlight range.

In short: sub-100 ISO on the A7 usually won’t add more noise, but it can cost you highlight dynamic range, so ISO 100 is generally the safer choice.