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06-17-2010, 01:50 AM   #1
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Lens explanations

Hello,

i was trying to find some basic information on lenses but i couldn't find online anywhere.

Basically for a 15-55 lens, i understand it has a zoom capability...

When we are talking about 20mm or 15mm or 55mm, we mean that they don't have zoom capabilities?

Thanks, and sorry for this....elementary school question

06-17-2010, 02:33 AM   #2
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There are two types of lenses.

Zoom lenses and prime lenses.

A prime lens has a single focal length, for example the DA 40 mm limited has a focal length of 40 mm.

A zoom lens has a range of focal lengths, for instance the DA 18-55 mm has a range at the wide end of 18 mm and at the long end of 55 mm.

Hope that helps.
06-17-2010, 02:33 AM   #3
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That correct. Focal length ranges typically indicated zoom lenses, while a single number indicates a fixed focal length, or 'prime' lens. Zoom lenses typically offer more flexibility, but prime lenses are often faster and have better image quality.

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06-17-2010, 02:36 AM   #4
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And primes are most often smaller en lighter!

06-17-2010, 02:37 AM   #5
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ok thanks great!

So i suppose for a general use for travelling, would it be wiser to take a 15-55 lense, or a single focal length that would be of smaller size?

For general day-family-night photos what would be suggested?
06-17-2010, 04:13 AM   #6
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First of the 15-55 lens you mention probably is the DA 18-55 kit lens; I haven't heard of a 15-55 lens yet. But the advice most people in your situation get and it's a sound advice; is to start of with a DA 18-55 II kit lens. When you buy it with a camera it's a very cheap lens that optically is very good in relation to the price you pay for it. It has it's limitations and when you run into them you will have a better idea of what second lens you want. Good glass can be rather expensive and it's better to choose after you find out what you really need!

Good luck!
06-17-2010, 06:18 AM   #7
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Yup, it is best to start with a (less costly) versatile lens ... like the kit lens 18-55 mentioned ... you have the flexibility to try out different focal lengths and be creative without over-complicating the process ... once you start liking certain focal lengths then you may want to (but not necessarily) consider buying some fixed focal length lenses ...

For travelling, keep a zoom, the 18-55, or maybe even an 18-250 ... for family shots, these zooms will be good in good light as well ... but if you start taking a lot of pictures indoors, in lower light, then you need to consider "faster" lenses ... where the aperture can go till 2.8 (on zooms, like several reasonable offerings from Tamron & Sigma) or better (on fixed focal length lenses, also called 'primes' ... you can go till 1.4) ...

"Faster" lenses cost more, good quality 'primes' are expensive as well... which is why, before making an investment, playing with some more affordable zoom lenses is a good idea... and depending upon the camera body you have, for low light you can increase the ISO ...

For some general reading on terms/concepts, you could take a look at these links ...
Understanding Camera Lenses
How to Choose the Best Digital SLR Lens
06-17-2010, 12:32 PM   #8
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QuoteOriginally posted by fekish Quote
So i suppose for a general use for travelling, would it be wiser to take a 15-55 lense, or a single focal length that would be of smaller size?
Personal preference. Some prefer zooms, others primes. That's why both exist. But there's no point buying a prime until you know which specific focal lengths you personally find most useful, so it pretty much always makes sense to start with the 18-55 to help you learn that.

06-18-2010, 09:13 AM   #9
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Try this on your 18mm-55mm zoom lens.

Set focal length to around 35mm. Look at a subject in the viewfinder. Compare that view with what you see with the naked eye. The subject size is almost the same, right? (On old 35mm film cameras, this focal length corresponds to about 50mm. On point & shoots, it's a much smaller number and it varies.)

Focal lengths below this (shorter FL) are considered to be wide angled. FL above this (longer FL) are considered to be telephoto.
06-18-2010, 10:01 AM   #10
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QuoteOriginally posted by grillfire Quote
Set focal length to around 35mm. Look at a subject in the viewfinder. Compare that view with what you see with the naked eye. The subject size is almost the same, right?
No, not even close. On Pentax DLSR's whose viewfinders provide ~95% magnification (the KXXD & K-7, plus the *ist D & DS), you get the subject at life size with about a 55mm lens. On the cameras with ~85% magnification (KXXXD, K-x, K-m/K2000, *ist DL), you need about a 60-70mm lens to get the subject to look life size. On either camera, a 35mm lens will render the subject in the viewfinder *much* smaller than life.

It's true that 35mm is more or less "normal" on Pentax DSLR's, but that does *not* mean it makes the subject look life size in the viewfinder. It happened to work out that way on some (but by no means not all) film cameras, but the actual definition of "normal" has nothing to do with how big something looks in the viewfinder. The actual definition is the subject of about a thousand or more posts in other threads arguing back and forth about the finer points of perspective distortion and field of view of the non-peripheral vision.
06-18-2010, 11:43 AM   #11
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okay, my bad. I was using wrong terminology. I meant perspective. Here are a few picture to explain it way better than I can say in words.

Focal length and Perspective
06-18-2010, 04:32 PM   #12
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Sorry, that still isn't right. Perspective never changes, regardless of focal length. The only thing that changes perspective is changing your position. That's why many people have the mistaken impression that longer focal lengths gave you a "compressed" or "flattened" perspective - because with a longer focal length, you are more likely to back further away from your subject, and the backing up is what actually changes perspective. That's exactly what's going on in the link you posted. Despite it's claim, it's not a demonstration of the effect of *focal length* on perspective, it's an demonstration of the effect of *distance* on perspective. The reason the 100mm picture looks the way it does is that it taken from much further away than the 17mm picture. Had he stood in that same spot and used the 17mm, the perspective would have been *exactly* the same as with the 100mm lens. You literally would not be able to tell difference between the shot taken with the 100 and a corresponding crop from the picture taken with the 17. His statement that the 50 represents the perspective "as you can see it with your naked eye" is just malarkey. *All* of them represent the perspective as you can see with your naked eye, providing you are standing in the same spot each of those pictures was taken. They were taken from different places, so they have different perspectives - and in each cas,e the lens sees *exactly* the same perspective as the eye. Well, the lens is a couple of inches lower and closer to the subject than your eye, and that might make a slight difference in certain cases, but that's not really relevant to that demonstration (I mention it only because if I don't, someone will call me on it).

As I said, there are tons of exisitng posts getting into what the term "normal" actually means, but with respect to "perspective", what is relevant is the fact that if you *print* a picture taken with a normal lens, then view the print from a "typical" distance for that size print, the sizes and position of objects in the print will appear to match how they appeared in real life. However, that's not something you can really evaulate just by looking through the viewfinder - you really have to do the print in order to see this effect.
06-24-2010, 08:19 PM   #13
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This is a great thread. I've had the same question as fekish! Thanks.
06-24-2010, 08:26 PM   #14
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QuoteOriginally posted by Marc Sabatella Quote
Perspective never changes, regardless of focal length.
Ditto
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