Just purchased a Sigma 150 macro and cannot get it to shoot wide open at 2.8. It will only go to f / 3. Anyone know if this is a common issue or it's 'operator error' ?

Shooting off a d90 btw.

JC, how about none of the above! I forget the details, but I learned this lesson with my Nikon 105 macro. There's something about the way these lenses work that won't allow them to reach the 2.8 all the time. I'm sure someone will follow along later with the more detailed reason. Your lens is working as it should. I think the aperature changes with distance on the macro.

Thanks Logger. I didn't let it detract from enjoying the few shots I was able to get with it, but it was just something that had me concerned that my lens might have an issue. I'll try shooting with some different settings this weekend.

I believe loggerhead is indeed correct. I had that lens in EOS mount , and now own the Sigma 180 Macro in Nikon mount. They both worked that way. The 180 seems to have its F3.5 at inf. , or slightly less it goes to about F3.8.

Good discussion of that phenomenon here:
http://forums.steves-digicams.com/nikon-lenses/139137-macro-lens-question.html

But the 180 Sigma is an f/3.5 lens. My Sigma 150/2.8 does go to f/2.8 on my Canon.

No it doesn't, the Canon system is just to stupid to correct for the purely optical phenomenon. Canon says F/2.8 when true F/no is really higher. This is not just 3d party lenses, same goes for Canon's own lenses. As an example, the EF 50F/1.4, a pure "unit focusing lens", should (correctly reported) be something like 60F/2.0 at MFD, if memory serves me right.

Aperture number (true aperture size relative to focal length) DOES increase if you focus closer, minimum F/no is only reached at infinity (or at least "long" distance focus). This is a physical property that Canon just choose to compensate for in another way (by using longer shuttertimes) - without telling you so.

If you'd care to check, grab a frame of a big white surface (evenly and constantly lit), at maximum aperture and 3-5m focus distance. Then do the same at MFD. Your shot taken at MFD will have a longer shuttertime, but will be exactly as bright.

It is only 2.8 at infinity. As you focus closer the aperture remains the same but the effective aperture closes down because it loses light. Canon doesn't show it on their bodies but it still happens. Nikon does this so that the scene will meter correctly.

Yes, many lenses, especially those that change in physical length have an aperture that changes as you get closer. But, I thought someone asked why the Sigma 150/2.8 never can be set to f/2.8, even at infinity.

Thanks all. I didn't even try to shoot at infinity that's why I wasn't opening to 2.8. . Just did so and all is ok with the lens! Interesting about the Canon bodies though.

It should still show f/2.8 on the body at any range, even though the effective aperture is a stop or two less at close focusing distance (like any macro lens).

It "should" show the wrong value because...?

Eric Schwab wrote:
As you focus closer the aperture remains the same but the effective aperture closes down because it loses light. Canon doesn't show it on their bodies but it still happens. Nikon does this so that the scene will meter correctly.

I don't see how the metering is affected by the aperture number that the camera displays. When there is light loss due to extension, or an ND filter or pola filter, or when you leave the lens cap on the lens, the metering system will detect less light and will calculate the right exposure.

I know that one of the old versions of the micro-nikkor 55/3.5 had a "compensating aperture", meaning that the effective aperture was mechanically "reported" to the camera (so that the photographer wouldn't have to calculate/estimate the EC needed due to extension).

Maybe I'm missing something here; are macro lenses not metered correctly on Canon bodies because of extension?

theSuede wrote:
It "should" show the wrong value because...?

It's not the "wrong" value unless the physical size of the aperture has actually changed.

If I put a 3 stop ND filter on, should the camera say f8?

AhamB wrote:
Maybe I'm missing something here; are macro lenses not metered correctly on Canon bodies because of extension?

They meter just fine...

The "physical" size of the aperture doesn't matter at all. There's two parts in the "F/no" definition, true focal length (as a function of viewing angle) and the apparent size of the aperture as viewed from the predetermined focal distance. This size actually goes DOWN as you move your vantage point closer to the lens first surface (focus closer).

This, combined with the fact that FL as a function of viewing angle (and this is what is important!) increases the closer you focus makes the F/no increase. This is a very real, and totally logical effect. FL increases, apparent aperture diameter decreases > the ratio of F/no increases.

Canon "solve" this by increasing your shutter times, without telling you why.

theSuede wrote:
the apparent size of the aperture as viewed from the predetermined focal distance. This size actually goes DOWN as you move your vantage point closer

It's heartening that Sigma doesn't hide this "bellows effect" from the photographer. Below 1:4 one can be blissfully ignorant of it, but at 1:1 and beyond it's critical to be aware of it both for exposure and also for diffraction.

/Acme

Typical Nikon-mount macros run from f/2.8-5 at close focus.

EBH

Nikon's show effective aperture on Macro lenses. So your f2.8 lens will only show f2.8 at infinity, and closer it the displayed aperture will be f2.8 + the Bellows Factor. This allows easier use of manual flash when shooting macro as you don't have to calculate effective aperture for an IF, RF or FREE macro lens (which is a pain as they do not have the easily calculated bellows factor of a pure extension focused macro). Bellows Factor being the compensation to exposure necessary to compensate for extension-based light losses, this is 1 stop at 1:2 and 2 stops at 1:1 for pure extension-focusing macro lenses.

Canon reports physical aperture only, not effective aperture.

IIRC Sigma works Nikon-fashion even on Canon bodies. Not 100% sure about that though.

I'm sorry but I just don't buy the whole effective aperture thing. Shouldn't they report it as a T value, since the real aperture stays the same.

Making up a effective aperture is faulty since the depth of field regards to real aperture only. If nikon suddenly shows a higher aperture value just to compensate for the light, then that does seem like a stupid workaround doesn't it? Since it's only the T value that changes, and dof-calculations will be faulty.

Or I could be wrong of course, someone care to explain please?