There are two classes of shutters: focal and aperture. Focal shutters
are located near the plane of the film and light the entire frame of film. Aperture shutters are located near or inside the lens. Such a shutter blocks the light, being closet to the lens than the film. This shutter can have be smaller, comparable to the size of the lens glass. That is why such a shutter can be much cheaper than
a focal shutter.

The most commonly used aperture shutter option is the central shutter. It blocks the light with the help of 3 or 5 movable leaves, which, moving during operation, first open and then close the round hole. Central shutters were invented more than a century ago. This is one of the first types of shutters used on early cameras.

Such a shutter has one big minus – the insufficient value of the fastest shutter speed. Opening and closing of the leaves (they have inertia) takes time. The leaves start moving from the closed position, then brake to a complete stop in the open position, and then start moving in the opposite direction.

It seems to be a simple process, but the fastest shutter speed of such a shutter is 1/250. This value of the fastest shutter speed was normal for photographic materials of 100 years ago. Today’s photographic materials have higher light sensitivity, and today we need a shutter with a faster shutter speed. Also, the central shutter has to be installed inside or directly behind the lens. Only in this case the exposure across the frame will be even. If we move the central shutter to the film plane, the frame
will be exposed unevenly: the centre of the frame will be exposed more than the edges.

But we are making a camera for a wide range of lenses! And we don’t know exactly where the rear lens of a particular lens will be. So, the aperture shutter is not suitable for our camera. In general, it is impossible to apply it. There is only one option left for our universal camera – a focal plane shutter placed directly in front of the film.