Backfocus - no error with one lens; error with another lens?

Archived from groups: rec.photo.digital (More info?)

I tested the backfocus of the Maxxum 7D. Links below.

For the 100mm lens, all seems well.

For the 50mm lens, it seems to focus camera side of the target.

Any ideas?

Cheers,
Alan



Tested:

100 f/2.8 macro against two targets on an oblique

50 f/1.7 against one target on an oblique.

--Max7D + lenses above
--Tripiod
--cable release
--mirror lockup
--angle finder at 2X for manual focus
--A/S off
--ambient light

100mm results: perfect

50mm results: appears to focus slightly close to the camera v. target
(2 samples of each from bunch follow).

100mm f/2.8:
http://www.aliasimages.com/images/ [...] T0710a.JPG
http://www.aliasimages.com/images/ [...] T0721a.jpg

50mm f/1.7:
http://www.aliasimages.com/images/ [...] T0717b.JPG
http://www.aliasimages.com/images/ [...] T0718b.jpg

note: on the paper target, the focus point is marked with bars with
ambient light from window.

ruler target, the focus point is the 15 cm line with
ambient light from softbox on farside of ruler.

Comments on why the 100mm shows in-focus at the selected focus point and
the 50mm does not?

Cheers,
Alan

Archived from groups: rec.photo.digital (More info?)

Roland Karlsson wrote:

> Alan Browne <alan.browne@FreeLunchVideotron.ca> wrote in news:GCmZd.54121
> $WM1.894295@wagner.videotron.net:
>
>>Comments on why the 100mm shows in-focus at the selected focus point and
>>the 50mm does not?
>>
> I read an article that showed that the cover glass on the
> sensor made that some lenses did not focus correctly.
> Non digital lenses are corrected for having air between the
> lens and the film.

Very odd. Do you have this article online? Or what magazine was it?

In any case, none of my lenses are "digital".

Cheers,
Alan.
--
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-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.

Archived from groups: rec.photo.digital (More info?)

Alan Browne wrote:
> I tested the backfocus of the Maxxum 7D. Links below.
>
> For the 100mm lens, all seems well.
>
> For the 50mm lens, it seems to focus camera side of the target.
>
> Any ideas?
>
> Cheers,
> Alan

Pity you can't focus through the taking sensor!

David