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05-08-2015, 05:02 AM   #1
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Easy question about 18-135mm

So I got a K-50 with the 18-135mm and its great, but I have a question. I am sure its easy and just missing the answer somewhere.
Sometimes I can get 3.5 out of it and others 5.6, whats causing this? Shutterspeed?

05-08-2015, 05:13 AM - 1 Like   #2
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Focal length is causing it.
05-08-2015, 05:19 AM - 1 Like   #3
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It's what's known as a variable aperture zoom. It's f/3.5 at the wide end and f/5.6 at the long end.

05-08-2015, 05:41 AM - 1 Like   #4
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As Anders has said: It's a variable aperture zoom, and take a look at the front of your lens. It says: "1:3.5-5.6 18-135mm ......"

While this is pretty normal, some zooms have constant aperture ratios throughourt their focal range. The cost you pay for that is more glass, increased weight and a higher price.

05-08-2015, 05:41 AM - 1 Like   #5
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it has to do with optics and the focal length the bigger the focal length the wider the front / filter element needs to be.
and more glass needs to be used to achieve the wanted magnification there is a point around 40mm where the front element needs to get bigger again to achieve the FOV. see below

in case of zooms the manufacturer has to find the balance between weight and size of the resulting lens.
Using a variable aperture allows to make compact lenses as in the case of the 18-135.
Take a look in the section "lenses" and look at the primes. The longer the focal length, and especially the smaller the f-number the bigger and heavier the lens and the price...:-)

A* 400 2.8 front filter 145
A* 300 2.8 font filter 112 mm
DA* 200 2.8 front filter 77 mm
A 135 2.8 font filter 52 mm
DA 40 2.8 front filter 49 mm
A 28 2.8 font filter 49
A 20 2.8 font filter 67

Last edited by max_pyne; 05-08-2015 at 06:37 AM.
05-08-2015, 06:54 AM   #6
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Thanks everyone Very helpful, appreciate it. Knew it was something I was overlooking. I googled the heck out of that and did not get anywhere.

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