output impedance of led?

I was playing around with my opamp and an led being used as a voltage soure for my op amp, and I was just curious what the output impedance of the led would be, under bright light there's no problem with it but when its dim, my multimeter was too much load, and then I realized my oscope was actually acting as a pulldown reistor, right now I have a 2M resistor as a pulldown so it doesn't stay full up
is there anyway to calculate this for certain light levels? And do they make like 10M potentiometers?

Also a quick question does it matter when there are two amp stages wnich goes first? I have a 5x first then a 10x, I figure it'll pickup less noise but no reason really behind it,

is there anyway to calculate this for certain light levels?

No.

do they make like 10M potentiometers?

Well 5M is the biggest I have seen:-
http://uk.farnell.com/jsp/search/browse.jsp?N=2031+204275+110146310&Ntk=gensearch&Ntt=Pot&Ntx=mode+matchallpartial&No=0&getResults=true&appliedparametrics=true&locale=en_UK&divisionLocale=en_UK&catalogId=&skipManufacturer=false&skipParametricAttributeId=&prevNValues=2031+204275&mm=1002553||,&filtersHidden=false&appliedHidden=false&autoApply=true&originalQueryURL=%2Fjsp%2Fsearch%2Fbrowse.jsp%3FN%3D2031%2B204275%26Ntk%3Dgensearch%26Ntt%3DPot%26Ntx%3Dmode%2Bmatchallpartial%26No%3D0%26getResults%3Dtrue%26appliedparametrics%3Dtrue%26locale%3Den_UK%26divisionLocale%3Den_UK%26catalogId%3D%26skipManufacturer%3Dfalse%26skipParametricAttributeId%3D%26prevNValues%3D2031%2B204275

I have a 5x first then a 10x, I figure it’ll pickup less noise but no reason really behind it,

The smaller gain might have a smaller input impedance and so pick up less noise, but it depends on the configuration.

I figured there's no calculation, so maybe ill look into those 5M pots,
its configured in the regular non inverting way, a 100k and a 470k resistor I think I used,
Which configuration has the least noise?

Which configuration has the least noise?

Ones with lower input impedances, but don't fuss the difference is very marginal.

Is there a way to dynamically control the gain of an opamp digitally? I don't believe digipots go up that high, and I think I need gain adjustment from .5x to maybe 100x
hopefully to adjust to sense a light across the room to sense a light right in front

You can get digitally controlled op amps
http://www.linear.com/product/LTC6910