In photography, 200mm is the focal length of the lens. f/X is the focal ratio, with X being some number/ So a 600mm f/4 lens means it's got a 600mm focal length and 600/4 = 150mm aperture diameter. The reason for this is that camera lenses have an "iris" that reduces the aperture; that blocks some light but increases the depth of field. This means that unlike a telescope a camera lens's aperture can vary, so it doesn't make much sense to specify the absolute aperture for most situations.
It's also because the focal ratio tends to be a more accurate way to estimate how bright an image will be than the absolute aperture. A 35mm f/4 lens will take an image with just about the same brightness (LV) as a 600mm f/4 lens, even though the 35mm f/4 is only 8.75mm in diameter compared to the 150mm diameter of the 600mm f/4. The shorter focal length means light gets gathered onto the sensor from a wider angle, which exactly compensates for the decreased diameter. Since photographers typically can't take hours-long exposures (or stack hundreds of photos to get the same effect) the way astronomers can, this system works better for photography.
It's also because the focal ratio tends to be a more accurate way to estimate how bright an image will be than the absolute aperture. A 35mm f/4 lens will take an image with just about the same brightness (LV) as a 600mm f/4 lens, even though the 35mm f/4 is only 8.75mm in diameter compared to the 150mm diameter of the 600mm f/4. The shorter focal length means light gets gathered onto the sensor from a wider angle, which exactly compensates for the decreased diameter. Since photographers typically can't take hours-long exposures (or stack hundreds of photos to get the same effect) the way astronomers can, this system works better for photography.