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08-02-2017, 04:07 PM - 1 Like   #1
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figuring f-stop with green button exposure on K-50

Getting ready for the upcoming eclipse, I've been doing some preparations and practice sessions, and a question arises as I try to record the successful settings.

Normally I am using modern lenses that communicate with the camera, so I haven't really had to use the infamous green button exposure setting. But this time, I have added a new wrinkle. In between the K-50 and my trusty 70-300 Promaster, I put in an old (pre-1983, came with the wife and her ME Super...) Zykkor brand 2x TC, which is totally manual. It gives me a good enough reach, of course, but... Figuring the exposure has me a bit puzzled. Yes, I am using a solar filter, one of the inexpensive cardboard and mylar ones from B&H.

First, the lens: If I take it off the 'A' setting and put it at f/11, that will mechanically set the internal mechanism to the right opening size and leave it there, yes? It does when looking thru the lens off-camera at any rate.

Second, the green button: When I press it (Manual mode, with ISO 100, 1/500s), the shutter speed was reset to 0.5s, and the sun came out reasonably well exposed. No information on what the f-stop is though.

I've read a lot of green-button questions here over the years (tho not recently), and it boils down to 'shooting with the lens stopped down' as I recall. So that would be f/32 for that lens -
--- does a 2x TC take it two stops to f/16? Or am I completely confused?
--- with a manual TC in there, howintheHelsinki does it do that?

this image was also done with almost the same setup, just a different celestial body so no filter



08-02-2017, 04:25 PM   #2
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The f stop is whatever you set on the lens without a TC, 1 stop less with a 1.4x, or 2 stops less with a 2x TC.

So F11 would become F16 at 1.4x or F22 at 2x.

Adam
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08-02-2017, 04:35 PM   #3
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Thanks, Adam... Confusion, it ought to be my middle name. <sigh> I'll keep practicing
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