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01-12-2014, 09:11 PM - 1 Like   #1
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Magnification Factors for several Kenko Teleconverters on K5

I recently acquired a Kenko 2X MC7 teleconverter to go with my Kenko 2X MC4 TC and Kenko 1.5X SHQ TC.

I stacked various combinations of these TCs with my DA* 300mm F4 lens and my K5 while taking pictures of the sun, trying to get some nice sunspot images. The various combinations allowed me to measure the change in angular scale for each of the three different converters. In no case, does the converter provide the claimed increase in scale!

The sun is so bright that even with 11 stops or so of filtration (an ND 3 filter (factor of a thousand, or just about 10 stops) plus a polarizer (good for 1+ stop)), the exposures can be as short as 1/1000 second (at f/13 commanded lens setting and ISO 200) when all three converters are stacked. At this shutter speed, though, you still need a very solid support and should use the 2 second shutter delay so that the mirror goes up 2 seconds before the shutter goes off. With all three converters stacked, the sharpness is not too bad, and the solar image is BIG: around 80% of the frame height. All three converters are autofocus models but I used manual focus.

I measured the width of the solar image on various frames in pixels, and used the pixel values to compare the multiplication factor for the teleconverters. Given the distance to the sun, these factors are very accurately the scale expansion factors for the TCs for objects at infinity.

When I derive the angular scale multiplication factors for these teleconverters, I find

Kenko 1.5X SHQ ------ a factor of 1.39 +/- 0.01 - - - - nowhere near 1.5X !

Kenko 2X MC4 -------- a factor of about 1.84 - - - - - - not exactly 2X

Kenko 2X MC7 -------- a factor of about 1.83 - - - - - - not exactly 2X either

It seems to me that the Kenko 1.5X is really a more standard 1.4X teleconverter. The 2Xs match closely, but fall rather short of the claimed 2X.

By the way, for various order of stacking all three TCs, I did not see any obvious difference in image quality.

PS If you try taking pictures of the sun, be very careful - of both your eyes and your camera. Adequate filtration - at the front end of your optical system - is absolutely necessary to avoid damage and even destruction!

01-12-2014, 09:13 PM   #2
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I've compared the Kenko 1.5x side-by-side with the Tamron-F 1.4x. They are obviously the same thing, except coatings seem to be different...
01-13-2014, 05:35 AM   #3
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QuoteOriginally posted by AstroDave Quote
It seems to me that the Kenko 1.5X is really a more standard 1.4X teleconverter.
QuoteOriginally posted by vonBaloney Quote
I've compared the Kenko 1.5x side-by-side with the Tamron-F 1.4x. They are obviously the same thing, except coatings seem to be different
I agree. I've also meaured out my "PzAf 1.5X SHQ Kenko" against known targets, and I consitently find a value of 1.4X.
01-13-2014, 11:06 AM   #4
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QuoteOriginally posted by Stone G. Quote
I agree. I've also meaured out my "PzAf 1.5X SHQ Kenko" against known targets, and I consitently find a value of 1.4X.
Yes. What I meant was it is not only the same value magnification value, but the exact same TC with a different name on it. (Kenko 1.5x SQH = Tamron-F 1.4x Pz-Af) However, the colors were noticeably different when comparing side-to-side images so I suspect different coatings.

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