@dhruvasambrani Vertical lines generally are *not* parallel to each other, simply due to perspective. However, since we mostly exist at ground level, our mental model does not account for that, and when we see a photograph — which simply reflects the actual perspective, as it can do nothing else. This is one of the reasons looking up from the base of a skyscraper can induce vertigo — your brain wasn't expecting that!
@dhruvasambrani Historically, a bellows or tilt-shift lens was used to shift the perspective to create an images with parallel verticals. Nowadays, it's a single-click automatic operation in a RAW processing program. (In other words, the filter counteracts the natural effect rather than the other way around.)
I'm poking fun because, while I do expect profesional architectural and real-estate photos to pay attention to details — including this — it's really a trivial thing. I'd say the more import concern of a skilled photographer (pro or not!) is *lighting*. And actually the examples he was so excited about did a poor job on that front.
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