Lense to increase f-stop?

[Donald]

I have found conversion lenses that increase the magnification while still retaining the same aperture, but is there one that keeps the same magnification but increases the amount of light gathered?

[jamison55]

No, it is impossible to build. Even if the extension lens had an aperture that was as wide as a barn, the light would still have to pass through small hole on the lens attached to the camera. In fact, most extension lenses actually reduce the effective aperture, since they tend to cut the amount of light hitting the sensor.

[Donald]

Thankyou jamison for your reply, let me expand furthure. I have a conversion lense that threads onto the end of the lense, it is a 3x with the same f-stop as the lens it's threaded onto. It's outer lense is quite large (120mm or so) and has a much smaller rear lense (40mm) so it gathers enough light to keep the f-stop the same. What I want is a lense with this large outer lense but does't magnify, only condenses light making the image brighter. I have searched the net and havent had any luck. I know this is cheating (why not go and get a f 2.8 lense and be done with it) but I am a tight-wad. heh heh.
My goal is to find an adequate one to thread onto my 300mm lense and take non-flash pictures in an auditorium at my son's graduation.
Thanks
Donald