Hi--

>Hope all are well with you. I have planned to buy a close up filter, for
>my Nikkor 105/2.5. According to the lens' datasheet if I use a Nikon
>close up filter no. 2 the magnification will go down to 12.2. It is
>strange that the datasheet doesn't mention the use of filter 4T.

The lens may pre-date the fairly recent "T" series...

>I know
>the diopters of the 4T is 2.9. So the magnificaion will be 2.9 x 0.105 =
>0.3, hence 13.3, am I right? Nikon once stated that their close up
>filter no.1 and no.2 is for lens focal length up to 55mm. Why they put
>filter no.1 and no.2 in the 105/2.5 datasheet but not the 3T and 4T? How
>is the quality of the no.2 anyway?

Good, but it is a single-element lens (which, BTW, works beautifully
on some video cameras, alowing infinity focus at the short end of the zoom range...).

>Actually my desired magnification is around 11 to 12. Can I use B+W
>+10 filter for my 105/2.5? How is the quality of this filter? Is it
>one-element or achromatic? The mount of this filter is somewhat bigger
>than other filter. Will it cause vignett?

No vignette, even with much smaller glass (you may be able to use
a reversed 50mm f1.8 E-series lens on the 105...). Expect poor performance
with a number 10.

>Other people suggest using extension tube instead of close up filter. If
>I use extension for my 105/2.5 for 11 I need an extention of 105mm, am
>I right?

Yes, but DO NOT do it!!! The 105 image quality is relatively poor
even at normal closest focus, and will get worse with extension tubes.
Sometimes a combination of tubes and achromat works well, as does a converter + achromat + tubes - hard to tell without trying it.

>If I am right, that will be impractical. What about using an
>extension tube of 27.5mm (available from Nikon?) on my 35/f2 and yeild
>an magnification of 0.78? Will the focusing distance be too close for
>practical work?

This will not be sharp, and will probably focus inside the lens...

>What will be the light fall off and 'stop loss' for this
>combination? Is the effective focal length of this combination
>increased?
>
>This leads to another issue. The other function of the extension ring is
>to increase the focal length of a lens. Am I right? If so, how can I
>change my 105/2.5 to 135/xx? (Wehh, I am getting another portrait lens?)

No - it has no effect on FL - just close focus.

>Going back to the 11 magnification, is it justify to use a 4T together
>with a 27.5mm extention tube?

Yes - it MAY work very well! (And may not...) Try f11-16 and see if
it is sharp enough. TTL flash helps a LOT here... (see the bug photos
on my web page - some are at 3X "hand-held"...;-).

>Sorry for so many Qs from a newbie of close-up -- some of them may sound
>silly to you expert. I'd appreciated if you could shed light on me.
"Ng, Stanley"

We try...;-)