Easy question about 18-135mm

SininStyle · 05-08-2015, 05:02 AM

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?
Thanks

focusr3 · 05-08-2015, 05:13 AM - **1 Like**

Focal length is causing it.

asp1880 · 05-08-2015, 05:19 AM - **1 Like**

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.

Regards,
--Anders.

Stone G. · 05-08-2015, 05:41 AM - **1 Like**

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.

max_pyne · 05-08-2015, 05:41 AM - **1 Like**

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

SininStyle · 05-08-2015, 06:54 AM

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.

05-08-2015, 05:02 AM	#1
SininStyle Senior Member Join Date: Mar 2015 Photos: Albums Posts: 138	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? Thanks

05-08-2015, 05:41 AM - 1 Like	#5
max_pyne Pentaxian Join Date: Aug 2010 Location: zurich Photos: Gallery \| Albums Posts: 409	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.

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05-08-2015, 05:13 AM - 1 Like	#2
focusr3 Senior Member Join Date: Aug 2010 Location: Baltimore,MD Photos: Gallery Posts: 107	Focal length is causing it.

05-08-2015, 05:19 AM - 1 Like	#3
asp1880 Veteran Member Join Date: Jul 2010 Location: Lyngby, Copenhagen Photos: Albums Posts: 742	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. Regards, --Anders.

05-08-2015, 05:41 AM - 1 Like	#4
Stone G. Pentaxian Join Date: Jun 2010 Location: North Zealand, Denmark Photos: Gallery \| Albums Posts: 1,516	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, 06:54 AM	#6
SininStyle Senior Member Join Date: Mar 2015 Photos: Albums Posts: 138 Original Poster	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.