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12-03-2010, 12:54 AM   #1
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What the hell is 18-55mm?

I've seen this a lot on photo websites, forums and brochures and probably chose to ignore it until now.What the heck does 18-55mm mean? What is in the range of 18-55 mm?


Last edited by Lzel; 12-03-2010 at 01:57 AM.
12-03-2010, 01:29 AM   #2
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Hi,
18-55mm refers (generally) to the kit zoom lens you get with your Pentax camera - it is the minimum and maximum focal length of the lens. This is a typical way of referring to a lens instead of its full title - SMC Pentax-DA 18-55mm F3.5-5.6 AL II - which is a mouthful!
Hope this helps.
12-03-2010, 01:39 AM   #3
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The focal length range 18-55mm means that the lens can be used :

* from a wide angle (18mm) very useful for landscape,

* to a focal length (55 mm) that is typical to shooting portrait.

With a P&S camera, a 18-55mm would correspond to a 3 times optical zoom. Does it make sense

Hope that the comment will help.

Last edited by hcc; 12-03-2010 at 01:56 AM.
12-03-2010, 03:08 AM   #4
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I am not completely sure what you are asking - but I will give it a shot.

18-55 is the range of focal lenght of your Pentax kit lens. Focal lenght determines your picture fram at a given distance from your motive. So, if you are standing a short distance from a large building, a _small_ focal lenght (=wide angle) will allow you to capture the whole building in one foto, while a large focal lenght (=tele) will allow you to fill out the frame with just a detail from the building.

Moving from a small focal lenght to a bigger one is called zooming. So, your 18-55mm is a zoom lens covering the focal lenghts from 18mm to 55mm, corresponding to a small wide angle to a small tele.

I hope that was helpful. It's the low-tech version of the answer. You would definitely benefit from this wikipedia-article on focal lenghts if you want to know more:

Focal length - Wikipedia, the free encyclopedia

12-03-2010, 03:09 AM   #5
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EDIT: Oops, that is what you get for going to a meeting half way through your reply. Double ninjaed! :-)

(And double oops, that's what you get for writing your edit in the wrong window! )
12-03-2010, 11:59 AM   #6
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Hi Lzel and welcome to the forums. As others have said 18-55mm refers to the focal lengths of the kit lens that comes with most cameras. BTW as far as I know this focal length range is somewhat of an industry standard, as Canon and Nikon in addition to Pentax have this focal range for their standard "Kit" lens. The name "Kit lens" refers to the fact that the camera body and the lens are bundled together as a kit.

NaCl( goes from wide angle thru to short telephoto)H2O
12-03-2010, 12:08 PM   #7
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Maybe this helps to understand

Focal length comparison tool, Tamron USA
12-05-2010, 04:18 AM   #8
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QuoteOriginally posted by hcc Quote
The focal length range 18-55mm means that the lens can be used : * from a wide angle (18mm) very useful for landscape, * to a focal length (55 mm) that is typical to shooting portrait. With a P&S camera, a 18-55mm would correspond to a 3 times optical zoom. Does it make sense Hope that the comment will help.
How much distance would that be in feet?

12-05-2010, 04:26 AM   #9
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QuoteOriginally posted by Lzel Quote
How much distance would that be in feet?
18 mm is .059 feet (almost 3/4 inch).

55 mm is .18 feet (a little over 2 inches).
12-05-2010, 04:31 AM   #10
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QuoteOriginally posted by Lzel Quote
How much distance would that be in feet?
Not very many.
From Google:
55 millimeters = 2.16535433 inches

An 18-55mm lens is a zoom lens.
An 18mm (or a 55mm lens) lens is a fixed lens.
12-05-2010, 04:44 AM   #11
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I think that you don't get it yet Maybe the below calculator helps

http://www.tawbaware.com/maxlyons/calc.htm ; scroll down to Dimensional Field of View Calculator

Fill in the numbers (use 1.5 for the multiplier)
At 18mm and 10 meter distance, a building of about 13 meters wide and 9 meters high will fit in the image
At 55mm you need to be about 30 meters away to get the same building in the picture

And this is another calculator: Lens focal length calculator
The sensor of the Pentax dSLR cameras is 24x16mm

Fill in the size of the subject (e.g. adult person is about 1.80 meters high), camera in portrait position (so sensor is 24mm high), object distance e.g. 5 meters and the program will calculate that you need a 65mm lens to take that shot and have the adult person fill the image (vertically).

Hope this helps

PS sorry, I don't think in feet when it comes to distances
12-05-2010, 10:33 AM   #12
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QuoteOriginally posted by Lzel Quote
How much distance would that be in feet?
If you are thinking of distance to the subject, that's another issue entirely. The 18-55 DAL kit lens can focus on any distance from roughly 1 foot to infinity. Most lenses go to infinity; their shortest focus distance varies, but it doesn't much depend on their focal length.

The focal length determines the magnification of the lens, and its field of view. Those are related inversely. When you make a subject look twice as tall, only half as much of it will fit into the picture. So you tend to want a long focal length with small and/or distance subjects, to make them big enough to see, and a short focal length with large and/or close subjects, to fit them all in.

You'll become familiar with what the numbers mean over time. As a rule of thumb, 30mm is about neutral, neither artificially wide field of view nor artificially magnified size. The magnification is directly proportional to the focal length. 300mm will make the subject look 10 times as tall as 30mm, and roughly corresponds to what you'd see with a pair of 10x binoculars. 15mm will make the subject look half as tall, but you'll include correspondingly more of it in the picture.

18-55mm thus includes the middle, 30mm; and zooms in to 55mm which is nearly x2, ie subject nearly twice as tall; and out to 18mm which is nearly x0.5, ie fitting nearly twice as much of the scene into the picture. It's a useful, every day range.
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