speaking of silly qestions

[bradzo66]

I was advised in previous post to drop f/stop to 4 to try to get a clear shot of the moon rising , it would go as low as 2.8 with 50mm lens ,then when I put the tele on it would only go to five now with the 2 x converter on it wont go lower than 13 , I re read the manual that came with my 40d and couldnt find an answer there .........am I missing something , any direction appreciated .Dont even know if Im looking at the right numbers now ......

[OsmosisStudios]

There are explanations to both your problems.

I'm assuming the "tele" you're referring to is your 150-500. I would also hazard a guess that you're at the very end of that (ie 500mm). The lens likely has a maximum aperture of f/5 at the point (which is odd: it's likely f/5.6). This explains why you cant get any lower than that with that lens: its a physical limitation of the lens.

When your camera automatically stops down to f/13 (which, again, is very low), it's because of the teleconverter. A 2x converter adds 2 stops to the maximum aperture: so f/5.6 goes to f/8. Why it's going to f/13 is beyond me, but if you've stopped down to f/8 before adding the teleconverter, it could be doing the math by itself. f/8 + 1 = f/11, and f/11 +1 = f/16.

Hopefully this answers your question.

[sk66]

f6.3 is the max aperture at max zoom for the 150-500...f 6.3x2 (2x converter)= f12.6 or f13

[OsmosisStudios]

Originally Posted by sk66

f6.3 is the max aperture at max zoom for the 150-500...f 6.3x2 (2x converter)= f12.6 or f13

Bingo. My numbers were based off an assumption, but thanks.

[sk66]

here's the fstop scale (in full stops):

f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22

[sk66]

Originally Posted by OsmosisStudios

Bingo. My numbers were based off an assumption, but thanks.

Yep, but f 5.6 would go to f12, not f8

[bradzo66]

Thank you all so very much you've saved me losing my mind !! lol .Much appreciated , back to the drawing board .