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12-12-2011, 05:59 PM   #1
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Macro test

I de-glassed a Quantary AF 2x converter to try and use for a macro extension tube. Here's a couple of test shots. These were taken on my desk with a 40w desk lamp. The first one was at ISO 1000, Pentax DA 18-55, set at 55mm, f5.6 @ 1/25 sec. The second one was at ISO 500, Pentax A 50mm 1.7, f1.7 @ 1/100 sec. I hope to try this out in the field if the sun will come out this week.


Last edited by bluestringer; 12-14-2011 at 08:59 AM.
12-12-2011, 08:18 PM   #2
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And, no surprise, the wider aperture of the A50 gives an image with thinner DOF. Try stopping them both down to f/8 (in good light, of course) and see if either has significantly better IQ. If your TC is 25mm thick then at infinity focus the magnification 25/50= 1:2 with the A50. The kit lens magnification would range from 25/18= ~1.4:1 @18mm to 25/55= 1:2.2 @55mm. That's an interesting range to shoot in. Have fun!
12-12-2011, 09:08 PM   #3
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Thanks. Yea, I want to try them out in good outdoor light. It has been cold, windy, and overcast here for 3 days. Hoping that changes tomorrow. Maybe I can find some bugs that will cooperate. Funny thing with the kit lens, I couldn't get it to focus at 18mm. I was using manual focus with it, and I used catch in focus with the manual 50, moving the camera instead of turning the focus ring.
12-12-2011, 10:06 PM   #4
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QuoteOriginally posted by bluestringer Quote
Funny thing with the kit lens, I couldn't get it to focus at 18mm.
It could be that the TC's extension puts the CFD (close-focus distance) WITHIN the lens. Let me try that... Yup, no focus at 18mm with a 2.5cm-thick tube behind the lens. The shortest focal length that will focus is about 25mm, with the lens at infinity focus, and that's with the subject almost touching the objective. There's a rule: No lens can focus closer (from its optical center) than its focal length, no matter how little or much extension is behind the lens. If I put another 2.5cm-thick tube behind the lens, I'd get more magnification but the same CFD. Apparently at 18mm the optical center is more than 18mm behind the objective. Such is life.


Last edited by RioRico; 12-12-2011 at 10:11 PM.
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