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09-20-2019, 02:49 AM   #1
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SMC F-1.7x converter odd behaviour

I was playing around with my smc FA 20mm lens and tried using the AF 1.7x converter with it. Normally the minimum focus distance is 0.25m

then I was thinking about trying something a little different with the lens today , so I pulled out the Pentax F 1.7x teleconverter and noticed that the close focusing distance was even closer than normal. I tried a little experiment at lunchtime
smc FA 20mm at MFD

smc FA 20mm at MFD with 1.7 AF converter from same position of the camera

Then I tried moving the camera and lens forward to the closest distance I could with the combo

This brought the MFD down to about 0.1m
I guess that not only is the 1.7 converter able to use a little of it's own autofocusing range to focus a little closer than normal, however the extra range seems to be more than the AF range in the converter, I guess that the converter is also acting a little like an extension tube as well making it more into a macro type of situation, any thoughts?

09-20-2019, 06:07 AM - 1 Like   #2
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There typical effect of a converter is to magnify the image since the focal length increases and minimum focus distance (mfd) is held constant. Clearly in this case the mfd was reduced as well resulting in what you saw.
09-20-2019, 06:37 AM - 1 Like   #3
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Yes, the AF in the AF 1.7x converter does act like tiny motor-controlled extension tube.

Wide angle lenses show especially large changes in MFD (and image magnification) with small amounts of extension. That change in MF would be much much less with a longer lens.

Enjoy this cool gadget!
09-20-2019, 11:58 AM - 2 Likes   #4

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The converter can also find focus at different positions (it has its own moving part) depending on where it starts and how far off the focus on the lens is. So the MFD when using the converter is actually not constant (with a given lens) because it is finding equilibrium between two focusing elements (the base lens and itself) and the starting position of each matters.

09-20-2019, 05:03 PM - 1 Like   #5
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You need to remember that the AF adaptor is used on a wide variety of lenses, and gives a portion of AF range to long Tele photos. In total and I Have not taken time to look at the difference in focal range, but a 20mm lens only needs about 3 mm focus throw to go from minimum to infinity.

The change in minimum focus distance will be there on any lens but while a few mm does very little for a 300 mm lens, it is a lot for a 20mm

It is not an extension tube as such it is just its range of focus likely exceeds the full range of the 20 mm lens

Last edited by Lowell Goudge; 09-20-2019 at 07:02 PM.
09-20-2019, 05:15 PM   #6
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Thanks for the thoughts, I will try a couple of my other teleconverters ( non AF) and see what happens.

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