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01-19-2008, 08:29 AM   #1
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Lenses and what do the numbers mean?

Ok i have the Pentax k100D kit that comes with the DA 18-55mm lens. What exactly do those numbers represent? I want a zoom lens that would be sorta like looking into binoculars..you know, that right in your face zoom affect. So what numbers do I look for on the lens? I just don't understand what the millimeters represent when you are looking thru a zoom lens at your subject that is 20 or 30 feet away. I hope you understand my question. Thanks in advance!!!

01-19-2008, 08:35 AM   #2
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18-55mm refers to the two ends of the zoom range. 18mm is a moderate wide angle and 55mm is a moderate telephoto. It sounds like you might also be interested in the DA 50-200mm lens, which might give you more of the telephoto range you are looking for.
01-19-2008, 08:53 AM   #3
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okay so this lens has 2 sets of numbers

the ones at the bottom represent whats known as the FOCAL LENGTH

focal length in laymens terms is the distance between the lens element (or one of the elements depending on the lens) and the digital sensor (or film)

the effect of focal length on your "magnification" is relativly simple, the higher the number, the more zoom you get.

this crappy graph sort of shows you that (note the darkened triangles)




if you want a no headache answer, get the Pentax DA 50-200mm lens as was suggested.

however you can save more money if you get yourself an older lens, but it would require manual focusing, but it would be dirt cheap.
01-19-2008, 09:57 AM   #4
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I believe that question about the numbers has been answered...
to the "sorta binocular" thing...
If you want more zoom power, try Tamron 70-300. It has slightly bigger reach than above recommended Pentax DA 50-200.
If you want all in one lens, try 18-250 either by Pentax or Tamron...

01-19-2008, 10:04 AM   #5
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Hey everyone, Thanks! I think I understand. Great knowledge on this forum!
01-19-2008, 12:06 PM   #6
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Just to add a little more to this. 43mm is considered the same as what the eye can see on a Pentax camera ("normal" field of view). It's is slightly different from one brand to another due to the distance from the back of the last element (peice of glass) in the lens to the film or sensor. In a Pentax the distance is 45.46 mm. In a Nikon for example it is about 49.5mm

So if 43mm is considered normal, then a 300mm lens is 6.98 x magnification (300/43 = 6.976)

For 35mm cameras
Ultra-wide 20mm
Wide-angle 28mm
Standard 50mm (or 43mm)
Short-telephoto 90mm
Medium-telephoto 200mm
Long-telephoto 300mm+

For DSLR's
Ultra-wide 12mm
Wide-angle 20mm
Standard 31mm
Short-telephoto 50-70mm
Medium-telephoto 135-200mm
Long-telephoto 300mm +

The focal length of a lens establishes the field of view (FOV) (sometimes called the Field of Vision) of the camera. The shorter the focal length is, the larger the FOV. The magnification factor of the picture of an object and the object's actual size can be found by dividing the focal length of the camera lens used by the focal length of a standard lens. A telephoto lens magnifies the subject while at the same time narrowing the FOV. These lenses create an image that looks flatter than that produced by a standard lens.

To confuse the subject further. The FOV of a digital camera is different than a film camera. The digital sensor is smaller than 35mm film. The ratio is 1.5 So a digital camera (DSLR) will have a narrower FOV. A 300mm lens appears to have the same FOV as a 450mm lens on a 35mm camera. The magnification is the same on both cameras but the FOV is smaller on a DSLR.

Another thing to consider on a lens when buying, is the aperture. If you look inside the lens, you'll see a mechanical iris that functions just the same as the iris in your eyes. It opens and closes the same as your eye does to allow more light or restrict the amount of light. Not always true but generally the larger the opening at maximum will tell you it is a better lens optically and it will be faster in low light. The smaller the number the more light the lens can let in. So you will se on the box and printed on the lens a number like: f4-5.6 or on a prime lens (fixed length that does not zoom) f2.8 or f1.4. Just remember the smaller the number the more light that comes in.

I would strongly recommend you pick up the book "Understanding Exposure" by Byran Peterson 2004. It explains this and much more in great detail. There is a lot more to a camera and lens than mets the eye at first.

Last edited by Peter Zack; 01-20-2008 at 09:02 AM.
01-19-2008, 01:04 PM   #7
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I think Peter Zack might be wrong by saying 43mm is the standard lens for Pentax
digital cameras. With a crop factor of 1.5X, and the standard focal lenght for 35mm cameras being 50mm, then the standard lenght for APS-C sensor on digital cameras would be 33mm.
01-19-2008, 01:38 PM   #8
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QuoteOriginally posted by flyer Quote
I think Peter Zack might be wrong by saying 43mm is the standard lens for Pentax
digital cameras. With a crop factor of 1.5X, and the standard focal lenght for 35mm cameras being 50mm, then the standard lenght for APS-C sensor on digital cameras would be 33mm.
Yep. "Normal" means the diagonal of the frame (or sensor) size, which gives the most natural perspective. So on digital, it is a shorter focal length than on film.

01-19-2008, 03:03 PM   #9
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OK now I'm lost! LOL! I think I will just keep playing with the lens that came with the camera for a while longer
01-19-2008, 04:04 PM   #10
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Sorry Flyer and Finn I misspoke myself. Pentax considered the 43mm "normal" for the film bodies and it would have been 31mm for the digitals. (45.46/1.5= 30.31) That's most likely why Pentax has the 31mm lens. which BTW is one of, if not the highest regarded lens they make.

Leana, What is it that has you lost? If you look at the Standard length as 31mm for a digital body (what your eye would see from corner to corner clearly) then anything longer is telephoto and anything shorter is wide. Just take the math above for a digital camera and then you have an idea of the magnification.
So for a DSLR 300mm / 31mm = 9.68 In other words a 300mm lens will magnify the subject about 9.7 times as much as a 31mm lens would.

If there's something there that doesn't make sense, one of members will surely clarify more. Just post the question(s).
01-19-2008, 06:50 PM   #11
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QuoteOriginally posted by Peter Zack Quote

I would strongly recommend you pick up the book "Understanding Exposure" by Byran Peterson 2004. It explains this and much more in great detail. There is a lot more to a camera and lens than mets the eye at first.
Couldn't agree more with this last line. My copy will be here on Monday
01-19-2008, 07:22 PM   #12
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QuoteOriginally posted by Leana Quote
OK now I'm lost! LOL! I think I will just keep playing with the lens that came with the camera for a while longer

LOL. Confusing isn't it, Leana? Anyway, I think you're seeking is a way to relate those numbers (18-55mm) to the numbers (8x40, etc) used for binoculars. Sadly, because of the factors involved, there is no reliable way to make direct comparisions. Instead, we can only offer generalities. For example, a lens in the range of 30mm to 40mm provides a field of view roughly similar to what you might see with your own eyes (field of view = area seen left, right, up, down when looking at a subject without moving your eyes). A lens with a smaller number (called a "wide angle") would provide a wider view (shrinking the subject to add more to the image), while a lens with a higher number (called a "telephoto") provides a more narrow view (enlarging the subject in the image).

Of course, zoom lenses provide more coverage than just a lens with a single focal length (single number, such as 18mm). Instead, zoom lenses often allow one to zoom out to a wider image (18mm with your lens) or zoom into a telephoto image (55mm with your lens). And there are obviously zoom lenses providing even greater wide angle images and even greater telephoto images (just as there are fixed, single focal length, lenses offering the same).

Does that help somewhat? Feel free to ask about anything still on your mind. I'll try to keep the answers as simply and clear as possible, without a lot of unexplained technical jargon.

stewart
01-19-2008, 07:26 PM   #13
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Go out and play and experiment by yourself. It's a waste not to use it to learn.
01-20-2008, 02:58 AM   #14
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QuoteOriginally posted by Leana Quote
Ok i have the Pentax k100D kit that comes with the DA 18-55mm lens. What exactly do those numbers represent? I want a zoom lens that would be sorta like looking into binoculars..you know, that right in your face zoom affect. So what numbers do I look for on the lens? I just don't understand what the millimeters represent when you are looking thru a zoom lens at your subject that is 20 or 30 feet away. I hope you understand my question. Thanks in advance!!!
The best way to think about it, using your reference to binoculars, is this:

35mm = no magnification
70mm = 2x
140mm = 4x
200mm = 6x magnification etc etc

The above examples are approximate but very close. They apply to cameras with the APC size sensor such as the K100D and K10D. Once you get used to the concept you no longer think about magnification as such, just millimetres.

Hope this helps
01-20-2008, 05:57 AM   #15
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Just remember! The bigger the number, the bigger the magnification.
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