Since we're probably never gonna get film-to-digital camera conversion kit, I'll continue to use old lenses with digital camera but I'd like to avoid the 2 stops of loss caused by the lens adapter.
Is there a way to achieve that?
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The FD to EOS adapter needs a lens to correct for the difference in flange distance, and make the lens focus correctly at infinity. The optical design and the inherent light loss cannot be avoided. If you have a significant investment in such lenses (or an emotional attachment to them), maybe the best way to deal with the light loss is to get a camera that allows you to use a high ISO (most recent cameras allow you to work at very high sensitivity with very little noise).
An alternative is use those lenses on non canon cameras with a short flange distance. That would allow you to use a glass-less adapter. Sony cameras with E mount would be well suited for this.
The way these with-glass-element adapters work to help you regain focus to infinity even though you're holding the lens farther away from the image plane than the original mount did is by increasing the len's focal length. In essence, your adapter is a teleconverter.
Teleconverting increases the focal length, but the aperture size remains the same. You do this, and you decrease the lens's maximum aperture.
However, a loss of two stops seems extreme, as that would require a 2x tc, and most of this type of adapting can be accomplished with a 1.4x tc, which would be a loss of only one stop.
The easiest way to use the lenses at the designed maximum aperture, would be to swap from adapting to EOS to adapting for any of the mirrorless mounts (micro four-thirds, Fuji X, Sony E, Canon EOS M, etc.), which are shallower than Canon FD/FL. This would then no longer require a teleconverter, but only a simple ring adapter to achieve focus to infinity, and would not reduce the maximum aperture of the lens.