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Old Feb 21, 2007, 2:04 PM   #1
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I've been playing with a Kenko 2x doubler on my *ist DL with my old Vivitar 75-260 f4.5 screw-mount lens and my Tamron 28-300 f3.5 lenses. I was getting inconsistent results with handheld shots. So I ust ran a test with the tripod. Used the old vivitar lens first, thenthe Tamron

Now the Tamron claims to go down tof3.5, while the Vivitar claimsf4.5 ... and yet to use the same shutter speeds and aperture out in the ~250mm range on theTamron I had to use the 800 speed settingto get the same exposure I got with the Vivitar at 260 and a 200 speed setting.

So I don't know what's up with that unless the Vivitar is designed such that the minimum aperture drops off more slowly than with the Tamron. The Vivitar is a much longer lens, physically. And it has perhaps a bit larger piece of glass at the end than the Tamron, but we're talking about a very few milimeters at best.

At any rate .... the results for the Tamron were much better. There was much more CA with the old Vivitar lens, and hardly any to no noticible CA with the Tamron, even extended out to the full 300mm.

One thing about the old Vivitar -- it is very heavy and it took longer to wait out the larger oscillations on the tripod with it than with the lighter Tamron (the camera shook on the tripod after you let go of it).

So I'm wondering what this means. The Tamron claims to be a slower lens, and yet in practice this turns out to be far from the case. Even so, the image quality from the Tamron is better once you get your exposure worked out.

-P
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Old Feb 21, 2007, 10:40 PM   #2
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Well even with the IS of the K10D there is a HUGE difference when you zoom from the 200mm (even 250) range to full 300... and if not a constant F high end lens loss of light exposure as well, likely by a stop or more. Definately needs much more steadying. at 300 than 200.
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Old Feb 22, 2007, 9:34 AM   #3
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Is a jump from200 ASA to 800 film speed equivalent to two stops?

In the first test I had the Tamron set to about excactly between 200 and 300 to approximate the 260 max of the Vivitar. Since the scale isn't linear, it wouldn't be excact but I'd expect it'd be ... well, actually less than 250. The farther you turn it "up", the more compressed the scale would get. So halfway (physically)should be less than halfway (actual focal length), if you follow me.

I could try them both at 200mm and see what the difference is.

Ultimately, I'm thinking of buying a 500mm fixed lens. I'm definitely an amateur -- I'm not going to spend hundreds of dollars on one. Well, MAYBE $250-300 (and I'm not afraid of used ones, even screw-mounts) if somebody can reccomend something. If I'm going to do just as well with the Tamron and the doubler I'd be inclined to skip buying one altogether (that gives me a 600mm max).

Well, except for that temptation to try the magnification of a 1000mm (using the doubler). I have some Tim Allen in me, I guess. :G I know, next up, a gyroscope.
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Old Feb 22, 2007, 3:28 PM   #4
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Yes in theory ISO 200 to 800 would be two stops equivalent exposure or 4x shutter speed Or one stop and 2x shutter, etc..
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