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12-31-2008, 12:31 PM   #1
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An observation regarding minimum aperture.

I have a K200D+18-55 mm+55-300 mm. Now, for example, when using the 18-55mm in Aperture Mode, I sometimes had trouble in setting the aperture to f/3.5. Similarly for the 55-300mm I couldn't always set the aperture to f/4. But just right now I made an observation. The camera is in Aperture Mode, I set the focal length of the 18-55mm to 18 mm and the minimum f-stop I get is f/3.5. But as I increase the focal length, the f-stop starts increasing and I can't bring it back to f/3.5. Similarly with the 55-300 mm, at 55mm the minimum f-stop is f/4. But as I increase the focal length, the f-stop starts increasing by itself. I've noted down the focal lengths at which the f-stops change. They are as following:-

18mm f/3.5
20mm f/4.0
35mm f/4.5
50mm f/5.6

55mm f/4.0
120mm f/4.5
210mm f/5.6
300mm f/5.8


Why is this happening?

Can I stop it?

12-31-2008, 12:36 PM   #2
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The lenses are doing exactly what they are supposed to. You have two variable max aperture lenses. Look on the front of the lenses and read the f number specs. The Pentax 18-55 is a f/3.5-5.6 max aperture. That means 3.5 is the max aperture on the wide end and 5.6 on the long end. The variable max aperture allows those lenses to be smaller, and less expensive.
12-31-2008, 12:39 PM   #3
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I know the specs. But can I use it without the camera changing the f-stops?

Can I use f/3.5 at 55mm and f/4 at 300mm?

If not then why?
12-31-2008, 12:39 PM   #4
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This is normal!

The f-number is the ratio between the focal length of the lens and the actual size of the aperture. So as you zoom, the focal length gets bigger, and assuming nothing at all happens to the aperture (it stays the same actual size), the f-number will increase.

There are some zoom lenses that maintain the same f-number as you zoom, but these are the exception, and they are the result of some complex engineering to make this happen.

The 18-55mm kit lens is identified by Pentax as "f/3.5-5.6" -- these are the maximum apertures (lowest f-numbers) at the widest and longest zoom positions of the lens, respectively.

This is driven by the lens design, and it isn't something you can override, I'm afraid. To override it (e.g. f/3.5 at 55mm) would require making the aperture larger than it is physically capable of being. At 5.6 it's already as big as it can get.

12-31-2008, 12:41 PM   #5
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I see. Thank you for the replies!
12-31-2008, 12:45 PM   #6
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If you want a constant max aperture lens, you need something like the Pentax 17-70 f/4, the Tamron 17-50 f/2.8, the Pentax DA*50-135 f/2.8, etc. etc.
12-31-2008, 02:08 PM   #7
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This is absolutely normal for non-constant aperture zooms. If the kit lens could be set at f3.5 throughtout it's zoom range, it would be called a f3.5 not 3.5-5.6. Plus, to get the sharpest pictures out of the lenses you would have to step them down a couple stops anyway...
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