The technical explanation
- Film stocks were less sensitive, so there was less exposure to spend on a wide aperture.
- Maximum apertures were narrower than what is ordinary now.
- Large formats such as 65mm were not easily available.
- Together those pushed shooting towards daylight and towards stopping down, which was also where image quality was best.
The consequence the reply drew from this is the part worth keeping: shallow depth of field and low-light capability arrived as new capability rather than as the default. Something that reads today as a stylistic choice was, for a long stretch, the harder and more expensive option.
The explanation about audiences
A second reply put the change alongside two others from the same period — cutting got faster and visual effects became more common — and attributed all three to films being made for shorter attention spans. It is a claim about audiences rather than about equipment, and the thread offered it as an opinion.
What the thread did not settle
- Whether the technical constraints or changing taste did more of the work. The first reply said outright that other factors were probably at play.
- Whether the trend is still moving. The question assumed modern cinema is shifting further towards shallow focus, and nobody in the thread tested it.
- Neither reply cited a source, so this is two people's reasoning, not a researched history.