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Question About Lens Features
I tried a bit of searching, but it's getting quite late, I'm looking to buy tomorrow, and I'm not even sure what keywords to look for, so I hope this hasn't been asked (too often  ) before:.

How important are features like aspherical lenses, low-dispersion or extra-refractive index glass, etc in a lens? How much difference do these things make in image quality, and how much more is it worth paying for something that has these features vs. something that doesn't?.

Also, if a lens has internal focusing, would that eliminate the option of manually focusing if desired?.

If it helps, I'm specifically comparing looking at:Tamron AF 18-200/3.5-6.3 (XR Di II LD ASPHERICAL IF)Sigma 28-200mm f/3.5-5.6 (DG ASPHERICAL IF)SIGMA AF DC 18-200/3.5-6.3 (LD ASPHERICAL IF)(all for Pentax).

I haven't had a chance to look into them incredibly closely - which is why I'm not buying tonight. I understand that I'd find the *best* quality in more limited-range, specialized lenses, but I've only ever worked with one lens, so if I can start out with something that will cover most of my needs, I think that'll end up being my best bet. So I'm mostly wondering how much weight to give all those extra letters they like to tag onto the names to make them sound impressive ..

Thanks in advance for any help or thoughts anyone can give me...

Comments (5)

Blinkjona wrote:.

I tried a bit of searching, but it's getting quite late, I'm lookingto buy tomorrow, and I'm not even sure what keywords to look for, soI hope this hasn't been asked (too often  ) before:.

How important are features like aspherical lenses, low-dispersion orextra-refractive index glass, etc in a lens? How much difference dothese things make in image quality, and how much more is it worthpaying for something that has these features vs. something thatdoesn't?.

If you're asking those questions, it probably won't matter to you..

Also, if a lens has internal focusing, would that eliminate theoption of manually focusing if desired?.

No. It means that the lens changes focal length without changing physical length. For some, that's important. Others are not concerned..

If it helps, I'm specifically comparing looking at:Tamron AF 18-200/3.5-6.3 (XR Di II LD ASPHERICAL IF)Sigma 28-200mm f/3.5-5.6 (DG ASPHERICAL IF)SIGMA AF DC 18-200/3.5-6.3 (LD ASPHERICAL IF)(all for Pentax).

I haven't had a chance to look into them incredibly closely - whichis why I'm not buying tonight. I understand that I'd find the *best*quality in more limited-range, specialized lenses, but I've only everworked with one lens, so if I can start out with something that willcover most of my needs, I think that'll end up being my best bet. SoI'm mostly wondering how much weight to give all those extra lettersthey like to tag onto the names to make them sound impressive ..

Toss out the 28-200 right away. It doesn't go wide enough. Tamron vs. Sigma? Your call, really. You'll find equal satifaction with either. They're good all-around, do-everything lenses that will get you started..

At some point, you may begin to notice their shortcomings. When you recognize them, you'll have an idea of what you want to buy to replace/supplement the 18-200...

Comment #1

Each technology is very important under certain circumstances. Combining them is the reason that high ratio zoom lenses work quite well, providing excellent enlargements up to sizes that are beyond the needs of most consumer photography. They also provide a pretty good macro function for non critical small object photography. Check their minimum object distance specs..

In each of the technologies you are asking bout provide just microns of adjustment gained at the focal plane as the light falls on the recording media. But when enlarged, those microns enhance image quality greatly. The greater the enlargement, the greater the value..

I'll try to help you understand each......

Blinkjona wrote:.

How important are features like aspherical lenses,.

Aspherical lens elements have been a huge breakthrough in permitting smaller lighter, higher image quality and more user friendly lenses. In using ONE element of aspherical design, lens makers can replace TWO larger elements (usually bonded together to create "compound elements") AND improve image quality. This means that the size and weight of the combined element is smaller, It also means that the designers can correct for fall off in the corners and contrast and sharpness of the overall image plane. it will go down with LD and true multi-coating as the "big three" lens design technology advances of the last 50 years..

Low-dispersion.

Another of the "big three".... Low dispersion glass is of special importance in telephoto applications as it reduces color separation magnified by the bending the light to gain enlargement of the telephoto function (think of it as spray induced by the physics of the the glass itself) and aids in defining clear changes in tonal values (edge sharpness)..

Or.

Extra-refractive index glass,.

Extra (or very high) refraction index glass bends light faster than low or normal refraction index glass. This permits thinner elements, which keeps cost, size and weight down. Employed properly, it requires control of another kind of aberration so that a different kind of light deformity is managed to provide image quality. So far as I know, only Canon and Tamron are using this technology yet..

Also, if a lens has internal focusing, would that eliminate theoption of manually focusing if desired?.

Not at all, in and of, itself.

While I agree with another poster that the 28-200 is NOT wide enough for most people, I try not to generalize. At 7X zoom ration, it covers many situations well. It is limited in wide angle use, therefore, I personally would agree with the recommendation of the 18-200 from your group..

I note that of the two companies you listed, Tamron established and generally is considered to lead this lens category and has won every design award over and over to prove it..

If it helps, I'm specifically comparing looking at:Tamron AF 18-200/3.5-6.3 (XR Di II LD ASPHERICAL IF)Sigma 28-200mm f/3.5-5.6 (DG ASPHERICAL IF)SIGMA AF DC 18-200/3.5-6.3 (LD ASPHERICAL IF)(all for Pentax).

Van..

Comment #2

All that really matters is the quality of the image: how that is achieved might be of academic interest to an engineer - but as long as the lens delivers, that's what matters. Have a look at the reviews of the lenses you mention onhttp://www.photozone.de.

BTW - the newer Tamron 18-250 is generally reckoned to be better than the older 18-200, and has attracted a lot of favourable comments from users in this forum..

Best wishesMike..

Comment #3

Thanks so much for your help, everyone. I'm lens-hunting now, and your explanations and advice have been a great help.  ..

Comment #4

Hi Mike,.

This is not an attempt to start an argument with you at all, but a serious discussion could happen..

I agree that the image quality is ultimately the first thing that matters.But how do you think it is achieved if is not through these technologies?Engineers will debate the numbers, not generalities and overall trade offs.The question was asked as to what them meant. That was what was answered..

I suggest that: use-ability, expanding the range of imaging options into new areas (extended focal range, extended macro range, etc.), portability, and build quality are also key issues. And the ability to expand image making opportunities for everyone is a very good thing..

As to image quality, you can't maintain it over time without very good build quality. And without the other desirable use features, fewer photographers will be excited to use them or will carry them..

I also have a personal peeve (not with you but with urban legends about lenses bespoke by amateur evaluators). While I categorically state that often zoom lenses sacrifice quality of image at a specific focal length for expanded use range, I am amazed that nobody takes the time to look at what that means in practical terms..

Whether it is NIKON or Canon or Olympus or Sigma, Sony, Tokina or Tamron; often the image quality loss is INSIGNIFICANT in output. By that I mean that the small loss in image acutance is basically un-noticeable up to a specific enlargement size (and that often that size exceeds the practical production demands of the vast majority of photographers)..

Until people start to condemn every brand of zooms to the same compay's primes, in terms of the terminal limits of enlarge-ability (and where the primes present better viewable images) I will be OK with it. Until then, it is just perpetuating urban legends..

POP Photo's test labs recognized this when they instituted their testing format..

I note that the Tamron 18-250 you identify below will provide superb to excellent image quality up to 11x14 at ALL apertures and very good to excellent to superb up to 16x20. If that is considered a 'sacrifice' for photographers, I want to meet the standard maker that says so. Particularly, once viewing distance is included the image qulaity is of a professional level. This is why it is generally reckoned to be better than a whole lot of lenses..

John.

Mike703 wrote:.

All that really matters is the quality of the image: how that isachieved might be of academic interest to an engineer - but as longas the lens delivers, that's what matters. Have a look at thereviews of the lenses you mention onhttp://www.photozone.de.

BTW - the newer Tamron 18-250 is generally reckoned to be better thanthe older 18-200, and has attracted a lot of favourable comments fromusers in this forum..

Best wishesMike.

Van..

Comment #5

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