GoKawiilThis is one of my favorite lenses: the Helios 44. It has an "aperture" of 29mm, meaning that if you look straight through it, the circle that you see has a diameter of 29 millimiters. Why do we care, you might ask?
You probably know that pictures can be out of focus. But why does "out of focus exist"?
Well, without getting too technical, the lens takes the rays of light emitted by an object and then converges them into a single point; if you place the camera sensor in that point, you'll be able to see the object. However, if you place the sensor closer or further away from where the rays of light intersect, then you'll get a circle and the image will be out of focus.
Now, here's the kicker: the bigger the focusing lens is, the larger the cone of light rays is, meaning the the out of focus parts of the image will be more out of focus:
This is (partly) why phone cameras can't naturally reach the same levels of background-out-of-focus that camera can reach: they have such small lenses!
This is the camera lens that I do most of my work in. As you can see, this lens is much bigger; indeed, its (maximum) aperture is more than 53mm, almost double what the Helios can offer, which allows for a very strong separation between the subjects and the background.
The question now is: how big can we go? How much blurriness can we get out of a lens?
Well, assuming we're okay with carrying around a lot of weight, we can go pretty big. As an example, take the Sigma 135mm f1.4; this lovely lens (which I sadly don't own, yet) has a majestic aperture of 96mm,
and it can deliver pictures like the following one. There's a catch, though: due to physical constraints, all of these large lenses are also always very "zoomed in"! Take your phone camera and set the zoom to 4x: that's roughly what you're going to get with this Sigma lens. This can be great for some scenarios (and it's my favorite focal length indeed) but what if we want to keep the same giant aperture but with a wider field of view too?
Well, we can't.
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