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12-02-2013, 06:32 AM   #1
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ISO cannot be reduced below 4.0

Hi Snappers,

I am using a Pentax KR with 18-55mm Kit lens. I find in lens description 1:3.5-5.6 AL. If the 1:3.5 explains the minimum aperture which my lens can attain in Av mode my camera uses the below settings.

Shutter speed 1/160
ISO400
F4.0
Exposure compunsation "0". File format RAW.

The aperture "F" never gets below 4.0 (I have never managed to hit 3.5 in all lighting conditions). Kindly advise.

Thanks in advance.

Regards,
Vivek

12-02-2013, 07:06 AM   #2
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This lens has a variable aperture dependent on focal length. At 18mm the maximum aperture is f/3.5 and at 55mm the maximum aperture is f/5.6.

If you set the lens to 18mm and use aperture priority or manual exposure, you should be able to achieve f/3.5.
12-02-2013, 07:11 AM   #3
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Variable Aperture?

Hello,Vivek, Welcome to the forum!
That lens has what's called a 'variable aperture', meaning it will only open to f/3.5 at the shortest focal length (zoom) setting, in this case, 18mm.
Once you zoom to a longer focal length, the aperture starts closing down, eventually reaching f/5.6 at 55mm.
Make sure you're trying to set f/3.5 at 18mm. If that doesn't work, post here and we'll try to help!
Ron
12-02-2013, 07:24 AM - 1 Like   #4
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Man, I wish my ISO went to 4.

(of course you meant aperture)

12-02-2013, 07:27 AM   #5
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QuoteOriginally posted by ElJamoquio Quote
Man, I wish my ISO went to 4.

(of course you meant aperture)
Imagine the long exposures...
12-02-2013, 08:01 AM   #6
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QuoteOriginally posted by Eulogy Quote
Imagine the long exposures...
Imagine the crispness of that image though... and virtually no noise... not even "real life noise"...
12-02-2013, 09:54 AM   #7
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QuoteOriginally posted by ElJamoquio Quote
Man, I wish my ISO went to 4. (of course you meant aperture)
I wish my ISO went to 4. I remember the good old days of ISO 25 film. You'd take a picture of a model in the studio and a reverse silhouette would be burned into the backdrop.
12-02-2013, 11:24 PM   #8
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QuoteOriginally posted by JimJohnson Quote
This lens has a variable aperture dependent on focal length. At 18mm the maximum aperture is f/3.5 and at 55mm the maximum aperture is f/5.6.

If you set the lens to 18mm and use aperture priority or manual exposure, you should be able to achieve f/3.5.
Hi Jim,

Yes, at 18mm I was able to get F3.5. But I can't understand the logic behind this ! Why a lens needs variable aperture?

Thanks.

12-02-2013, 11:28 PM   #9
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QuoteOriginally posted by rbefly Quote
Hello,Vivek, Welcome to the forum!
That lens has what's called a 'variable aperture', meaning it will only open to f/3.5 at the shortest focal length (zoom) setting, in this case, 18mm.
Once you zoom to a longer focal length, the aperture starts closing down, eventually reaching f/5.6 at 55mm.
Make sure you're trying to set f/3.5 at 18mm. If that doesn't work, post here and we'll try to help!
Ron
Hi Ron,

Yes, at 18mm I was able to get F3.5. Can you explain why a lens needs to have variable aperture ?

Thanks.
12-02-2013, 11:33 PM   #10
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QuoteOriginally posted by ElJamoquio Quote
Man, I wish my ISO went to 4.

(of course you meant aperture)
Yeah dude. Thanks for bringing this up. I also wanted to ask about ISO. Pentax K-r's minimum is 200. Can't I bring it down to 100, like in other Nikon or Cannon ?
12-03-2013, 04:28 AM   #11
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QuoteOriginally posted by sanvic15dj Quote
Hi Jim,
Yes, at 18mm I was able to get F3.5. But I can't understand the logic behind this ! Why a lens needs variable aperture?
Thanks.
It doesn't need it. It's just designed like that. It's an easier design and hence cheaper.

If you don't want it, there is (or will be in the future) the Sigma 18-35/1.8 with constant aperture
12-03-2013, 04:43 AM - 1 Like   #12
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QuoteOriginally posted by sanvic15dj Quote
Hi Ron,
Yes, at 18mm I was able to get F3.5. Can you explain why a lens needs to have variable aperture ?
Thanks
Hello Vivek,
As far as I can tell (and I'm not an optical engineer!), the variable aperture feature on zooms is a way to cut down cost and size, while still having decent optical performance.
If you compare the kit 18-55mm f/3.5-5.6 to a similar lens with a constant aperture (usually f/2.8), you will find the f/2.8 lens costing 2 to 4 times as much. It will also be much larger in size and weight.
Most camera makers and 3rd-party lens makers offer at least two lines (some have 3) of popular-sized lenses. A low-priced, budget or 'kit' lens, sometimes called a starter lens, which is often bundled or sold as part of a budget-cost entry kit with the starter-priced camera body. This allows the manufacturer to compete in the new-buyer market.
But, they know the user may later want an upgrade, so they also offer a similar lens in a faster aperture, at a higher price. An option they can't make with the starter kit, because it would make the total price point too high.
So they make another version of the lens (similar, not exactly the same) with better optics, faster aperture (and usually, constant aperture, not variable) for the 'Pro' or advanced market. 3rd party lens makers like Sigma and Tamron do the same thing. It gives us choices, depending upon our budget and needs.
Here's 3 examples to show some of the differences. This isn't an optical review, just size, weight and price.
Pentax DA 18-55mm f/3.5-5.6 AL WR. Price $250, weight 230 gr (8.1 oz), size 68 x 62.5mm, filter 52mm.
Sigma 17-50mm f/2.8 EX DC OS HSM. Price $ 570, weight 565 gr (19.9 oz), size 83.5 x 91.8mm, filter 77mm.
Pentax DA 16-50mm f/2.8 ED AL (IF) SDM. Price $1,000, weight 565 gr (19.9 oz), size 84 x 98mm, filter 77mm.
At one time, Pentax offered a 16-45mm f/4.0 (constant aperture, not variable) that was roughly in the middle in size, weight and price. This lens has been discontinued.
Hope this helps!
Ron

Last edited by rbefly; 12-03-2013 at 04:48 AM.
12-03-2013, 04:58 AM - 1 Like   #13
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QuoteOriginally posted by sanvic15dj Quote
Yeah dude. Thanks for bringing this up. I also wanted to ask about ISO. Pentax K-r's minimum is 200. Can't I bring it down to 100, like in other Nikon or Cannon ?
you have to activate "expended sensitivity" in the camera menus
12-03-2013, 05:03 AM - 1 Like   #14
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QuoteOriginally posted by sanvic15dj Quote
Hi Ron,

Yes, at 18mm I was able to get F3.5. Can you explain why a lens needs to have variable aperture ?

Thanks.
F-stops are a ratio of focal length and size of the aperture. There ARE zoom lens with a fixed rather than variable aperture. However, to accomplish this, the aperture has to change size as the focal length changes. In short, it adds mechanical complexity and therefore cost. Most zoom lens have a variable aperture to constrain engineering and manufacturing costs. Here is a wiki snip defining the ratio:
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12-03-2013, 05:07 AM - 1 Like   #15
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QuoteOriginally posted by sanvic15dj Quote
Yeah dude. Thanks for bringing this up. I also wanted to ask about ISO. Pentax K-r's minimum is 200. Can't I bring it down to 100, like in other Nikon or Cannon ?
As Pablom said, you need to engage expanded sensitity - - you may have to turn off shadow and highlight compensation to do this if you have them active. Those features are selectively using expanded sensitivity behind the scenes.
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