ND filters - why use them?

Maris said:

I can do it when I screw a +3 dioptre close-up lens on the front of my standard 8x10 lens; a Fujinon-W 300mm f5.6. The combination becomes (approximately) a 150mm f2.8 lens that covers 4x5. That's two stops gained and a lot of image quality lost.

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Dr Croubie said:

Sounds kind of like those Speed Boosters. Those ones are for mounting a 35mm lens on a 24mm sensor. Imagine what you could do with a string of similar adapters between an 8x10 lens and a 35mm film body...

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