"Compress without losing quality" sounds like a contradiction — surely making a file smaller means throwing something away. In a strict technical sense, that's true, but the more useful question is what gets thrown away, and whether it's something you'd ever actually notice.
A photo file, in its raw form, often stores far more color and detail information than the human eye can actually distinguish, especially in areas of similar color or subtle gradients. Compression algorithms are built specifically to identify and discard this redundant information first, before touching anything that would be visibly noticeable.
Compressing an image by 60-70% typically removes almost entirely imperceptible data — side-by-side comparisons at normal viewing sizes usually show no visible difference. Push compression much further, into extreme territory, and you start seeing real artifacts — blotchy color transitions, blurred fine detail — which is where "lost quality" actually becomes visible.
An image compressed enough to look slightly soft at full-screen, pixel-level zoom might look completely fine as a normal-sized photo in a product listing or a social media post. Most real-world use cases don't involve anyone zooming in to inspect pixel-level detail, which gives you more compression headroom than you might assume.
Some image formats compress more efficiently than others for the same visual result — modern formats can often achieve significantly smaller file sizes than older ones at equivalent visual quality. If file size is a priority, choosing an efficient format is worth as much attention as the compression level itself.
For everyday use — website images, social media, sharing over WhatsApp or email — moderate compression settings almost always produce a result that's visually indistinguishable from the original at normal viewing sizes, while cutting file size dramatically. Extreme compression is really only a concern if you're already pushing file size limits aggressively.
Compression isn't an all-or-nothing tradeoff between size and quality — for most everyday photos, there's a wide range where file size drops significantly and quality loss stays completely invisible to the eye.