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05-22-2008, 03:56 PM   #1
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General lense question?

Hi,

I am using a mix of old and new lenses. Didn't pay much for any of them but i will soon be getting the wallet out to get some new shiny stuff. I need a bit of help in understanding what it is that you pay for (or are looking for) in an expensive lense.

The kit lense that came with my K100d has a short focal length and very limited aperture, 18-55 4 - 5.6

I also have an 28 - 80mm with aperture from 2.8 - 32. And a 70-300 that goes from 4 - 222. *** some of those may not be the exact values asa the gear is not in front of me now.

All of the new lenses i look (online) seem to have a very linited aperture, 4 - 5.6. Why is this?

Can some one explain the real life benefits of a lense with aperture from 2.8 - 32 vs a lense with 4 - 5.6?

thanks
mike

05-22-2008, 04:06 PM   #2
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QuoteOriginally posted by schmik Quote
4 - 5.6
This means that the maximum aperture of the lens (sic) is F4 at the wide end, and F5.6 at the long end.

You can still set them to smaller apertures, just as with all the lenses you're used to.

The optical design is just such that when the focal length is longer (at the long extent of its zoom), and the size of the hole in the front is unchanged, the F number wide open has to be a bit larger.

Constant aperture zooms (where the maximum aperture is the same regardless of focal length) are much rarer, and usually a lot more expensive.
05-22-2008, 04:15 PM   #3
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Original Poster
Thanks Chris.
If I may have one more question: what is it that makes one lense 'better' than another. I understand that higher quality glass is used and that the AF is probably quicker and quieter but in terms of Fstops, shutter speed???

Essentially what is one paying for in an expensive lense. In particular I am looking for a wide lense for landscapes.

cheers
mike
05-22-2008, 04:25 PM   #4
osv
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you want the maximum aperture size for low light situations, and creative control over the focus area of the picture.

take a look at the focal lengths of your pictures, and think about investing in prime glass instead of zooms, for the pics you shoot the most... there is still some killer used glass out there.

if i had a gun at my back, and i had to buy a zoom, for weddings and such, i'd be looking at full-frame zooms, not aps-c zooms... simply because i think that pentax will eventually be forced to embrace the dark side, and give us a full-frame digital body.

camera bodies are disposable, the glass is where you should spend the money.

05-22-2008, 04:26 PM   #5
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Complicated question.

VERY generally, the larger the maximum aperature (lower numbers = larger opening), the less available light is needed to create an image through the lens, the more glass is needed in the lens, therefore the more expensive the lens is.

Also, the larger the aperature, the faster the light goes through, therefore the faster the image is created, therefore the faster the shutter speed that can be used.

Like I said...VERY generally. Quality, brand name, and rarity are other big things that you might not be able to quantify to a dollar amount.
05-22-2008, 04:36 PM   #6
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QuoteOriginally posted by osv Quote
you want the maximum aperture size for low light situations, and creative control over the focus area of the picture.

take a look at the focal lengths of your pictures, and think about investing in prime glass instead of zooms, for the pics you shoot the most... there is still some killer used glass out there.

if i had a gun at my back, and i had to buy a zoom, for weddings and such, i'd be looking at full-frame zooms, not aps-c zooms... simply because i think that pentax will eventually be forced to embrace the dark side, and give us a full-frame digital body.

camera bodies are disposable, the glass is where you should spend the money.

Thanks again guys. I agree about the body being disposable.
Why go prime glass over a zoom?
"full-frame zooms, not aps-c zooms" this is the size of the sensor? The full-frame being larger?
How does a full frame lense work with an aps-c sensor? Does the focal length of the lense get 'shifted'?

mike
05-22-2008, 06:31 PM   #7
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Prime over zoom. Usually with a prime you will get a better image quality compared to a zoom at the same focal length. You can also get wider apertures. My understanding is basic, the only glass that has to move is the focus, so you don't have to compromise on the rest of the lens.

Yes, APS-C does refer to the size of the sensor. "Full-frame" is the same size as 35mm film, which is larger. APS-C is 1.5 times smaller than full-frame. Full-Frame lenses cast a circle that will enclose one cell of 35mm film (or the sensor) and since APS-C is smaller it will also be inside the circle. But, it will represent a crop of the image. So if you use a 35mm lens on an APS-C camera the image would be the same as a using a 53mm lens on a full frame camera (assuming a 1.5 crop factor). Lenses marked as being made for APS-C sensors will work with full-frame senors, but usually the pictures will have very strong vignetting (dark edges).

For myself I buy APS-C zooms and full frame primes (with one exception). Most of the better Pentax lens have been (or are) primes and are full frame compatible since they were made for 35mm SLR bodies.

And as other commenters mentioned buy glass not the body. I have some glass that is older than me (30+) and they work great, wonderful pictures. I only expect my K10D to last 5-8 years.

That's my basic understanding. Hope that helps.
05-23-2008, 01:06 PM   #8
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The benefit of "fast" lenses - apertures say f2.8 or wider (lower numbers) over "slower" lenses (say max f4.5 or wider/larger numbers) are in the type of photos you take, for example:

Want to photograph something moving quickly, be that a bird in motion or your kid playing football? You need a very short shutter speed so that the thing doesn't move much whilst the picture is taken. Therefore, to get enough light for the photo in such a short time, you need a wider aperture.

Want to photo in darker conditions, or indoors without using a flash? Then you need a wider aperture to let in more light (or else you have to use a longer shutter speed to let in more like, which works but makes blurred photos much more likely).

"fast" lenses cost more than "slow" lenses, and seem to cost more at the more extreme zoom ranges than in the middle (ie fast extreme telephotos and fast super wide angles are hard to make and expensive, whereas fast midrange designs, especially older ones, seem to be more common).

Prime glass over zoom: It is easier to design a prime lens than a zoom, therefore (basically) your money goes further. You are more likely to get a better and/or faster lens on a given budget if its a prime. Especially given the large number of older pentax lenses around. The trick is to identify the focal length that is right for you, then get the best you can. Also consider whether you can/want to focus manually or not.

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