Hi Steve - I tried the DFA 100 and the 1.7x converter in the field with no prior tests or measurements, but I just did a crude test. Without the converter on the lens, I set it to 1:1 (minimum focusing distance) and focused on a ruler. I could see 23mm marked on the ruler through the finder. The measured distance from the front of the lens was ~5 inches. I then put on the 1.7x TC and moved the camera around till it was focused on the ruler and again showed 23 mm. Now the working distance was ~8 inches. (In both cases I measured the working distance as from the front of the lens to the subject - no hood.)
In theory, the working distance should have been 8.5 inches with the TC on. The converter itself adds about an inch to the length of the lens though, and 1/2 inch is probably within the margin of error for my measurements. So, it looks like the converter is increasing the working distance for by 1.7x, as it should, but is taking back the inch it adds to the length of the lens. As for the focus position of the elements - I had not considered that. I just did a quick test on that too, though. I coaxed the converter to its farthest setting (infinity focus) and then set the lens to 1.5x lifesized and measured the focus distance. I then repeated with the converter at its nearest setting. There seemed to be about 1 inch difference in the focus distance between the two settings. But doing that - I realize that you would really have to control for magnification. My guess is that is all comes out in the wash - with the converter set to its closest focus you can get closer, but get greater magnification. thinking about it - both the converter and the lens add extension to focus closer. Extension added by the converter is added behind the converter. Extension added by the lens is added before the converter. I have a book here that talks about adding tubes in front of or behind a converter and the difference between them. That's the same issue. I'll look that up... but not tonight. - MCC ----- Original Message ---- From: steve harley <[email protected]> To: Pentax-Discuss Mail List <[email protected]> Sent: Tue, June 14, 2011 9:11:45 PM Subject: Re: D-FA 100mm Macro Plus Teleconverter On 2011-06-14 05:52 , Mark Cassino wrote: > I'm pretty satisfied with the 1.7x telelconverter - though I still hope to hear > from Pentax that they can fixed the lens! very nice practical experiment; i also have a 1.7x AF TC; i tried it with my Sigma 105mm, and found the quality pretty good, and if i stick to 1:1 it seems to somewhat increase the working distance, but at maximum magnification (nominally 1.7:1) the working distance is back down to the 4" or so of the 105mm alone; did you get better working distance with your test? (wasn't that one of your main concerns?) i also noticed the 1.7x leaves it's optics "focused" to wherever one left it when AF was turned off; my guess would be that for macro one should prefocus it to it's shortest focus then switch to MF, but i wasn't in a position to test this carefully -- any thoughts? -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
