Need help on Tamron 18-250 lens

Discussion in 'Digital Discussion & Q&A' started by passionphotographer, Nov 4, 2007.

  1. passionphotographer

    passionphotographer TPF Noob!

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    Hi There,
    can somebody shed light on choosing nikkor 18-200 lens vs tamron 18-250 for nikon cameras which is said to boast 13+X zoom. I would like to know the impact on quality of a picture taken using tamron lens and the nikkor lens, which one is worth the money?

    Thanks,
    yours sincerely,
    passion photographer
     
  2. Garbz

    Garbz No longer a newbie, moving up!

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  3. passionphotographer

    passionphotographer TPF Noob!

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    Thank you, I went through the replys in that thread and looks like it is a lens to go for, if you want to compromise quality and that 18-200 VR seems a better buy.

    However considering that fact that I am new to this SLR lenses world, can you help me with this? - DSLRs these days come with default 18-55 lens as a kit and people buy 70-300 VR as a zoom lens on top of this. Some kits contain 18-135 instead of the 18-55 lens.

    So what difference does it make if one buys 18-55 and 55-200 VR OR buy 18-200 VR? cant one get the same quality of the picture shot using 18-55 in the 18-200 VR? and though 105mm is in the 18-200 range, why do people buy it separately as fixed 105mm lens? - Sorry if I sounded stupid.

    Thanks
    yours sincerely,

    passion photographer
     
  4. Garbz

    Garbz No longer a newbie, moving up!

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    That question is only stupid if you make lenses for a living.

    The problem lies in the inherent complexity of lenses. Take a typical fixed lens the cheapest and easiest to make the 50mm f/1.8. It has about 6 elements that take care of taking the image outside projecting it back into the camera, some elements move and are dedicated to focusing, another element may be aspherical (not a standard spherical curve) to compensate for distortion, and some elements may be coated in a special way because different colours of light bend in a different way through glass.

    Sounds complicated right? Well this is the most basic lens to make. Now add the ability to zoom. You need lenses which change the focal length as they are moved, but the need to do so in a way that still uses the most amount of glass or they start letting less light through and may go soft, also there needs to be more correction elements to compensate for the fact that at 18mm a scene would look like a fish bowl compared to 135mm.

    Now the 50mm f/1.8 had 6 elements. The 18-135mm f/3.5-5.6 has 13 elements, the 18-200mm f/3.5-5.6 has 16 elements!
    Ok so you can see that firstly the potential to add elements immediately may cause as many problems as they solve. Many of these elements move too.

    The other thing that may be obvious above is the 50mm even through it is cheaper by far than the other two mentioned has a max aperture of f1.8 This is very wide open and gives you great depth of field. The others however have a varying aperture from 3.5 at 18mm to 5.6 at their long end. This is 2-3 stops less light for your sensor and less depth of field too. And while lenses exists that have f/2.8 over a zooming range like the 70-200mm f/2.8 you can kind of expect it for the $2000 you pay for such a lens.

    So in summary. The larger the zoom range the more quality is sacrificed as the lens becomes hard to design with perfect quality over it's entire range. Check out www.photozone.de and you can see how zoom lenses vary in sharpness and distortion depending on how they are zoomed. Fixed lenses on the other hand have no distortion, are easier to design and thus come with a larger max aperture, but they are fixed and you need to zoom with your feet. They are also nearly a must as you get above 300mm.

    One other thing I saw you mentioned 105mm. Fixed lenses because of their simplicity mean that other special designs can be incorporated. The 105mm you are most likely talking about is the MicroNikkor or Sigma Macro 105mm f/2.8 lenses. These lenses are special in that they are capable of focusing to as near as 38cm from the front of the lens, giving a 1:1 projection. i.e. something that is 1cm wide will be 1cm across the sensor too. Compare that to the 18-200mm which has a 1:4.5 reproduction ratio meaning that it can't focus close enough to make an object that is 1cm appear any larger than 1/4.5th of a cm on the sensor.

    /Edit: On the above site compare the 18-200mm with the 18-135mm to see the massive difference in sharpness and distortion and also take into account the price difference.
     

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