What does f/1.8 vs f/4 actually mean?
The f-number is a ratio of the lens focal length to the diameter of the aperture opening. f/1.8 has a much larger physical opening than f/4, letting in roughly 5 times more light and producing significantly shallower depth of field — more background blur.
The light difference
f/1.8 lets in approximately 5 times more light than f/4. In practical terms, this means at f/1.8 you can use a shutter speed 5x faster, or an ISO 5x lower, than at f/4.
In a dimly lit room where f/4 forces you to ISO 6400, an f/1.8 lens lets you drop to ISO 1250 — dramatically cleaner images.
This light advantage is why photographers call f/1.8 and wider lenses fast lenses — they enable faster shutter speeds.
The depth of field difference
At f/1.8, only a very thin slice is in sharp focus. In a portrait, the eyes might be sharp while the ears are already slightly soft. The background becomes a smooth, creamy blur.
At f/4, depth of field is much deeper. More of the subject is in focus, and the background, while still blurred, is more recognizable.
The strength of background blur depends on aperture, focal length, and subject distance combined — but aperture is the most directly controllable factor.
f/1.8 and faster lenses at Camera Shop Egypt
Is faster always better?
Not necessarily. Wider apertures mean shallower depth of field, which can be a problem — at f/1.4 in a group photo, only the front row will be sharp.
Wider aperture lenses are larger, heavier, and more expensive. An f/1.4 version can cost 2-3x more than the f/1.8 version.
For many situations — landscapes, architecture, product photography — you stop down to f/5.6 or f/8 anyway, making maximum aperture irrelevant.
The f/1.8 versions of prime lenses (50mm f/1.8, 85mm f/1.8) offer the best value in photography. Affordable, sharp, great in low light, and beautiful background blur.