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next PZ lens test report: Sigma AF 105mm f/2.8 EX HSM DG OS macro
#1
Slightly better than the Canon L IS ...

[url="http://www.opticallimits.com/canon_eos_ff/743-sigma105f28oseosff"]http://www.opticallimits.com/canon_eos_ff/743-sigma105f28oseosff[/url]
#2
[quote name='Klaus' timestamp='1339226667' post='18798']

Slightly better than the Canon L IS ...

[url="http://www.opticallimits.com/canon_eos_ff/743-sigma105f28oseosff"]http://www.opticallimits.com/canon_eos_ff/743-sigma105f28oseosff[/url]

[/quote]



Kinda expected better from a new design.



I keep my 70/2.8 Macro for now <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />
#3
Does the fact that it gets longer (not IF design) mean that the FL stays 100mm at 1:1 and not reduced like the Canon?
#4
Am I missing something here? It is inner focus.
<a class="bbc_url" href="http://snowporing.deviantart.com/">dA</a> Canon 7D2, 7D, 5D2, 600D, 450D, 300D IR modified, 1D, EF-S 10-18, 15-85, EF 35/2, 85/1.8, 135/2, 70-300L, 100-400L, MP-E65, Zeiss 2/50, Sigma 150 macro, 120-300/2.8, Samyang 8mm fisheye, Olympus E-P1, Panasonic 20/1.7, Sony HX9V, Fuji X100.
#5
Look at the picture. It gets longer as you get closer to 1:1.
#6
Which picture? The one in the review shows the lens by itself, and with the two part lens hood arrangement. Sigma's own page here explicitly describes it as inner focusing.
<a class="bbc_url" href="http://snowporing.deviantart.com/">dA</a> Canon 7D2, 7D, 5D2, 600D, 450D, 300D IR modified, 1D, EF-S 10-18, 15-85, EF 35/2, 85/1.8, 135/2, 70-300L, 100-400L, MP-E65, Zeiss 2/50, Sigma 150 macro, 120-300/2.8, Samyang 8mm fisheye, Olympus E-P1, Panasonic 20/1.7, Sony HX9V, Fuji X100.
#7
Oh..... <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/ohmy.gif' class='bbc_emoticon' alt=':o' /> <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/huh.gif' class='bbc_emoticon' alt='Huh' />
#8
And since it focuses down to 31.2.cm, that means at 1:1 its FL is 78 mm <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />.



Kind regards, Wim
Gear: Canon EOS R with 3 primes and 1 zoom, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
#9
Lenses that change length during focussing do change focal length too at different distances. Kind of a strange myth that changing focal length has to do with internal focussing.



Some simple proof:



Lens / MDF:

Tamron 90mm f2.8 [color="#FF0000"](NOT IF)[/color] / 29cm

Canon 100mm f2.8 L USM[color="#2E8B57"] (IF)[/color]/ 30cm

Sigma 105mm f2.8 HSM OS [color="#2E8B57"](IF)[/color] / 31.2cm

Signa 105mm f2.8 [color="#FF0000"](NOT IF) [/color]/ 31.3cm

Tokina 100mm f2.8 [color="#FF0000"](NOT IF) [/color]/ 30cm

Canon 100mm f2.8 USM [color="#2E8B57"](IF)[/color] / 31cm

Nikon 105mm f2.8 [color="#FF0000"](NOT IF)[/color] / 31cm

Nikon 105mm f2.8 VR [color="#2E8B57"](IF)[/color] / 31cm

Pentax SMC-D FA 100mm f2.8 [color="#FF0000"](NOT IF)[/color] / 30cm

Pentax SMC FA 100mm f2.8 [color="#FF0000"](NOT IF)[/color] / 31cm



Or rather, I should say it differently.



Since focal length is calculated with the distance to the front element which does change depending on lens length... The thing that matters, field of view, does NOT depend on if a lens is IF or not IF. The field of view changes both with IF and non-IF lenses.

The focal length with the non-IF lenses will change more (due to its changing position of the front element), but as we can see above, the field of view remains the same to lenses with IF.



The problem with "focal length" definition is that it is not mapped 1:1 to field of view. It can be that a lens may lose some focal length when focussing close by, but that the field of view gets more narrow. We do tend to think of focal length saying everything about field of view, but it does not.



With my Canon 70-200mm f4 L USM (IF) for instance, it loses calculated focal length at MFD, but its field of view gets noticably more narrow.

My Sigma 18-50mm f2.8 (IF) loses calculated focal length too (more) at MFD, and its field of view gets noticably wider.



Basically, what we should be looking at is view angle or field of view, not focal length (as that is quite an abstract figure which we do not see in photos).



To summarize:

* Focal length and field of view are not linked 1:1.

* Non-IF lenses can have MORE focal length change than IF lenses.

* Non-IF lenses can have the SAME field of view at MFD as IF lenses.
#10
[quote name='wim' timestamp='1339358189' post='18830']

And since it focuses down to 31.2.cm, that means at 1:1 its FL is 78 mm <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />.



Kind regards, Wim

[/quote]

Different sources give different figures on how to calculate focal length, but theory says to use front nodal point.



Focal lengths are defined by front principal point (nodal point) distance to the subject. And this is not easy to determine at all, so usually the distance to the front element is used as a crude approximation.

Since the Sigma has a quite exposed front element, we can use the working distance given from the review (0.142m).

And then I get a focal length of 71mm, using

http://www.giangrandi.ch/optics/lenses/focalcalc.html



How did your calculator come to 78mm, only knowing the MFD and not the front nodal point or the working distance?
  


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