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01-31-2009, 06:07 AM   #1
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Lens confusion

First off it's great having a place like this to come to for those of us new to digital photography. Thank you
My question:
I'm looking to add and get a better understanding of lens for my camera as I have only the 18-55 kit lens that came with my K200.
In researching what's out there and price ranges I found that lens with "2.8" are very expensive, why is this?
What makes make's a 16-55 2.8 better than the 18-55 kit?

01-31-2009, 07:20 AM   #2
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The "3.5 -5.6" in your kit's lens denomination indicates the maximum open aperture the lens can reach. So at 18mm (the wide end) the max. aperture will be 3.5 and near its longer focal length of 50mm the max. aperture will go down to 5.6.

A lens with a "2.8" max. aperture lets through more light. The 2.8 setting indicates an advantage of 1/2 aperture stop (or f-stop, for short) compared to the 3.5 of you kit lens in the 18mm position or even 2 full f-stops, compared to the longer setting of your kit lens at around 50mm with f-stop 5.6

Larger maximum aperture (smaller aperture number) means:
- the lens is "faster", lets in more light, because the lens diameters are bigger
- you can use shorter shutter speeds, stopping motion blurr and allowing to use the lens in dimmer light without flash
- the bigger the aperture (the smaller the aperture number/f-number, for short), the more pronounced the sharpness can be placed.. I.e. with a 2.8 lens of 50mm focal length, you can make a portrait of a person, where they eyes really stand-out nice and sharp, whereas the ears are already lost in the unsharp background and the background detail is completely blurred out. --> the so called "depth of field" is smaller, the wider the maximum aperture. With a 5.6 lens you will have most of the head in this portrait sharp and still will recognize loads of details in the background.

Larger max. aperture also means:
- bigger glas lenses = more expensive and heavier
- in many cases the optical quality is also better, but not necessarily so.

So we all have to make our own choices, which lens, a "fast" lens or a "slower" modell, suite our needs best. In some cases a fast lens is the right one (if you take photographs in low light or need the shallow depth of field) and in other situations, the slower, but lighter and smaller lens might be better (for walking...)


Last edited by Ben_Edict; 01-31-2009 at 12:31 PM.
01-31-2009, 08:58 AM   #3
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Ben covered it well. Faster lenses (the smaller f/number) allow you to shoot in lower light at a faster shutter speed. They also tend to cost more as the manufacturing and machining tolerances are more critical and the glass itself is bigger. Whether the lens is actually better is a matter of opinion. Pentax lenses are all very good. The kit lens that came with your camera is an excellent lens and rated as the best kit lens in the industry right now. Whether you need the faster lens will depend on the type of photography you do.
01-31-2009, 10:42 AM   #4
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Original Poster
Thank you for the information. It's a lot to ingest, but now that I have an excellent explanation I can look at lens information with a lot better understanding.

It will help greatly with knowing what to buy in the future.

02-06-2009, 09:03 AM   #5
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A little bit off topic may be -

You can buy used manual focus primes with 1.4/1.7/1.8 f values cheap these days. Once you try one of these lenses on your camera and start taking pictures, you will understand the subject more i.e. why photographers go after fast lenses.

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