The iris diaphragm made its appearance as an adjustable lens stop in the 1880s, andi t became the standard adjustable stop about 1900.
2.
Although the viewfinder does not blackout, it can however go very dim as the lens stops down to the taking aperture, so this advantage is greater at wider apertures.
3.
Having a thin tear film may prevent one's ability to wear contact lenses, as the amount of oxygen needed is higher than normal, and contact lenses stop oxygen from entering the eye.
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