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grnzbra

Springfield, US
393 posts

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"RE: Where is the focus?"

grnzbra Silver Member Nikonian since 28th Sep 2011
Sat 25-Aug-12 01:11 AM

>I think that the real issue is that the 18-200 3.5-5.6 G DX
>is not a very sharp lens. My experience with that lens is
>that is not capable of the critical detail that you seem to be
>looking for.
>

Well that really sucks considering it's an eight hundred plus dollar lens.

>
>I have been very disappointed with my 18-200 at 18 and 200mm,
>and especially macro at 200mm. The in-between focal lengths
>and distances can be ok especially at f/8. I read a post by a
>Nikonian team member a few months ago that the 18-200 NIKKOR
>lens was not as good optically as the 18-55 or 55-200
>"kit lenses" so I tried some macro shots of a cone
>flower with the 18-55 and had fine detail in the flower that
>the 18-200 only had fuzzy edges with. I used manual and auto
>focus with both lenses on a D3100.
>
>Try those same shots you had with the 18-55 "kit
>lens". The 18-55 focuses very close and will reveal
>detail in the photo that the 18-200 can't come close to.
>
>
>I hope this helps.
>
>Well Wishes,
>
>Ken

Your right! This pic ( http://images.nikonians.org/galleries/showphoto.php/photo/388947/size/big/cat/22196 ) was taken with the 18-55 kit lens. I was focused on the tip of the blue plant and expected to see something an inch down from the top in focus. But that tip is really there.

But why? As I understand it, the light comes in the lens and most of it is reflected up to the eyepiece. The rest of it, since the mirror lets some of the light through, goes to anoter mirror behind the main mirro and at a 90 degree angle and is reflected down to the focusing sensor on the floor of the camera. When the picture if focused, the sensor tells the lens to stop focusing. If the distance the light travels between the back of the lens and the focusing sensor is not exactly the same as the distance it travels between the back of the lense and the imaging sensor, the focus will be wrong. If there is something wrong, how is changing the lens going to affect this. If the focus is right, how is a different lens going to mess it up? What does the lens have to do with any difference between these two distances?

Visit my Nikonians gallery.

A general, generic topic Where is the focus? [View all] , grnzbra Silver Member, Tue 21-Aug-12 12:22 PM
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