It's interesting that the Canon 100/2.8 macro loses 1.3 stops instead of
1 stop at 1:2. There must be some internal inefficiency that is not
evidenced at 1:1.
John Shaw's explanation for the different working distances and amounts
of light lost in getting to 1:1 with different combinations of tubes and
teleconverter was that the focal length was not constant. 50mm in one
case, 100mm in another and somewhere in between on yet another
combination. And note that the Canon 100/2.8 macro does internal
focusing and its focal length grows shorter as magnification increases.
Chuck Norcutt
usher99@xxxxxxx wrote:
> Dr. No Flash states:
> "I agree with Wayne. It doesn't matter how the magnification is
> achieved,
> the inverse square law applies."
>
> Moose
>
>
> Hmmm.
> Sounds quite sure and both Wayne and Moose are smarter than the average
> bear.
>
> Looked at John Shaw's second macro book again to confirm and clearly
> says that the light
> light loss with floating element lenses might be less than with
> standard helicoid.
> I read a post on Pnet claiming that if one takes a eg. Canon 100mm 2.8
> lens with no IF and adds extension to reach 1:2 keeping it at infinity
> and use a macro lens of the same type with IF, the IF lens will lose
> slighly less light.
> Given that N[eff] = N (1+M)---there are no other variables in this, now
> I think magnification may be the only relevant factor and the pupilary
> magnification is a side issue . Probably the FL of standard helicoid
> lenses changes a bit as well as the mag increases.
> Seems there are references regarding discussions about this in the past
> when it mattered with external metering , but no one cares much anymore
> with TTL metering and digital.
>
>
> There is only one standard helicoid lens here but no clear difference
> from the standard bellows factor.
>
> http://www.dpreview.com/lensreviews/panasonic_45_2p8_o20/page3.asp
>
> (look down a bit)
>
> The practical answer is clear. Whether in theory , there is even a
> slight inconsequential difference is uncertain. There is at least
> some confusion. Error in JS macro book??? gasp.
>
> An Owner of an "I agree with Moose" T-shirt who ignores it at his peril,
> Mike
>
>
>
>
>
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