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01-30-2009, 01:38 AM   #1
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Pentax 55-300mm F4-5.8 - wide open?

can you give me info about max wide open for Pentax 55-300mm f4-5.8?

at 55mm => f4
at 100mm => ?
at 150mm => ?
at 200mm => ?
at 250mm => ?
at 300mm => f5.8


thanks

01-30-2009, 02:24 AM   #2
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sorry, this isn't the goof forum for this answer.
01-30-2009, 03:25 AM   #3
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I did this last night before seeing your email. My DA 55-300/F4-5.8 arrived on Wednesday and I'm going to use it tomorrow.

I've written down where the F-stop changes occurs in Av mode as you zoom in:

55 - 4
120 - 4.5
210 - 5.6
300 - 5.8

As soon as the change in F-stop occurred, I took a shot and then looked at the recorded FL.

This is as much you can get with 1/2 EV steps. I believe there are physical contacts inside the lens at specific locations because these changes are specific & repeatable.

Dan.

Last edited by dosdan; 01-30-2009 at 04:48 AM.
01-30-2009, 03:33 AM   #4
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hello dosdan and thansk for info

i read
200 - 4.5

so, it is possibile?

01-30-2009, 03:51 AM   #5
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Not exactly 200mm. I get F4.5 @ 190mm & F5.6 at 210mm, but I could not get exactly 200mm as a recorded FL, no matter how much I tried. So there is some granularity in the recorded FLs.

Dan.
01-30-2009, 04:09 AM   #6
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so

190 - 4.5

is possibile.

dosdan, very thanks :-)
01-30-2009, 04:18 AM   #7
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I thought there would be some gradual drop off in EV that the the granularity of 1/2 EV F-stops was hiding so I tried centre-weighted exposures of a bright wall with the camera on a tripod and M-mode with the same shutter speed at the same reported aperture. I compared 120mm vs. 190mm. As expected the peak of the histogram moved a little to the left at 120mm so there a bit of EV drop. If at 190mm the white peak was at 40% from the L, at 120mm it was at 25%.

So, for example, 120mm might actual be F4.3 and 190mm might be F5.0, but both would show as F4.5.

Also the aperture value is reported by the lens to the camera, not measured by the camera. For example, the next 0.5 EV step above F5.6 is F7.1, but at 300mm the camera shows F5.8.

Dan.

Last edited by dosdan; 01-30-2009 at 04:57 AM.
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