I'm just wondering as i have a 6 speed 105 rear derailleur (rd 1050) and a 7 speed wheelset. If I grab a set of 7 speed indexed shifters will the derailleur shift as 7 speed indexed or is the only way to run this as 7 speed would be with friction shifters?

edit: idk if this should HO here or in the mechanics section, if it's more appropriate there please move the threads mods. Thanks!

It'll work. The indexing is in the shifter, not the derailleur, and all 105/600 RDs have the same actuation ratio. Just don't put a 7s DuraAce shifter with it and you'll be fine

Interesting. So it will index perfectly as if it was a 7 speed derailleur?

If that's the case what's the difference between the 1050 and the rd 1051 model (105 7 spd rd). Am I correct to assume that the 1051 would be capable of indexing up to 8 speeds?

Either will index up to 10 speed (maybe even the new 11, on that I'm not sure). The Shimano derailleurs (ignore 6/7/8s DA) from the first indexing to today's differ mostly in the width of the pulleys/ cage/ tooth count. Other than that it's cosmetics. (yea, I'm probably discounting other improvements)

No difference except minor finish color on the 1051, mechanically they are the same.

As Ex-Pres says, any indexing non-DA 740X series Shimano road (and many MTB) RD will work with any Shimano shifter.

Except maybe 11 speed, but 'whatever' on that.

That's cool to know. Any suggestion on 7 speed indexed downtube shifters? Seems like sl 1051 may be pretty hard to find although preferred....

I've used 7 speed Shimano RX100 and 6400 (tricolor 600 7 speed) downtube shifters to convert from 6 speed to 7 speed. Just avoid the DA stuff like the others have said. Any other Shimano 7 speed shifter will work.

I've used SIS 6-speed derailers with 8s cassettes, no problem and the indexing was identical as with an 8-speed derailer.

With 11 speed, Shimano changed to a longer cable movement, but I have yet to see published what the actuation ratio is, and whether it might be a good substitute for an earlier Campag rear derailer from the 8-9-speed era. The very small cable movements of increased number of cogs caused the traditional SIS actuation ratio to max out with 10-speed systems, so for 11-speed Shimano increased the cable travel to restore derailer movement accuracy.
I would like to at least get my hands on one of these Ultegr 11s derailers, to test it out with older Campagnolo Ergo shifters. They're about $100, but there is a swap-meet coming up...

DA shifters work great with other non-DA rear mechs if you use the alternate cable routing. See Sheldon's site for info.