Sigma 150, not opening to 2.8

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Archive 2009 · Sigma 150, not opening to 2.8

olyacme
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p.2 #1 · Sigma 150, not opening to 2.8

biotar wrote:
Making up a effective aperture is faulty...
Or I could be wrong of course, someone care to explain please?

No, you're as right, as is the converse opinion. But it's worse than you think. As Mawz suggested, most internal focusing lenses make a muddle of the f-ratio by reducing their focal length as they focus closer. Besides making for a more compact lens, this also offsets the speed that would be lost as a conventional macro's exit pupil moves further from the camera. But now it's no longer obvious how to report the working stop. Most unambiguously, the lens would report its focal length, magnification, and apparent pupil diameter, leaving it to the photographer work out what what it all means. But that's not really ideal either, is it?

/Acme

Nov 01, 2009 at 10:52 AM

biotar
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p.2 #2 · Sigma 150, not opening to 2.8

Oh wow. Now I know I'm confused. In the case of a simple 135 mounted on extention rings, the behaviour and optics shouldn't change, only the distance between the rear element and the sensor increases as it does with normal focusing. In such a setup nothing changes, only some light is lost right?

But if I understand correctly modern lenses (IF) change their optical alignment in macro territory. Very much like my C/Y 35-70 does right? Now it's unclear what the implication is of all these variables?

Thanks for clearing things up olyacme. I fear however that I need some time to digest this

Nov 01, 2009 at 11:47 AM

mawz
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p.2 #3 · Sigma 150, not opening to 2.8

biotar wrote:
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?

The thing is that DoF calculations are only a secondary concern compared to exposure (especially since DoF is relative to print size anyways), and by reporting effective aperture you'll get the exposure correct. So reporting effective aperture is correct for most uses.

The T value does not change, it's the transmission value of the lens itself. What's being changed is a third factor, the bellows factor, which needs to be added to the lens's T value to get the actual proper exposure.

Nov 01, 2009 at 01:04 PM

mawz
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p.2 #4 · Sigma 150, not opening to 2.8

biotar wrote:
Oh wow. Now I know I'm confused. In the case of a simple 135 mounted on extention rings, the behaviour and optics shouldn't change, only the distance between the rear element and the sensor increases as it does with normal focusing. In such a setup nothing changes, only some light is lost right?

Yes

But if I understand correctly modern lenses (IF) change their optical alignment in macro territory. Very much like my C/Y 35-70 does right? Now it's unclear what the implication is of all these variables?

Thanks for clearing things up olyacme. I fear however that I need some time to digest this

The IF lenses work the same way, but the bellows factor is specific to the lens design as the optical alignment changes as you focus closer. So giving the effective aperture rather than the actual f stop is a better idea.

Since IF and zoom is rather closely related (you can think of an IF lens as a limited-range zoom with a fixed nodal point with the zoom ring replacing the focus ring and it focuses closer by zooming wider) many zooms with macro modes work this way as well, even if they aren't actually IF designs.

Nov 01, 2009 at 01:08 PM

biotar
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p.2 #5 · Sigma 150, not opening to 2.8

I think I now get it, since you mention the bellows factor. For product and macro photography I can imagine the use of dof calculations though, but I understand the philosophy now.

Thanks for shedding a light on this subject Mawz and olyacme

Nov 01, 2009 at 03:30 PM

dgenx24
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p.2 #6 · Sigma 150, not opening to 2.8

when I purchased 85L II, it didn't open up to 1.2 as it should..
so I ended up exchanging it..

Nov 01, 2009 at 03:34 PM

theSuede
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p.2 #7 · Sigma 150, not opening to 2.8

Regarding the DoF vs aperture question.
No matter WHAT the bellows factor or the focusing method is, the DoF in your picture is a PURE FUNCTION OF EFFECTIVE APERTURE, IE LIGHT EFFICIENCY. So you get different DoF - with the same nominal F/no! - depending on your effective aperture.

So I really don't see why reporting the TRUE aperture, and thereby indicating TRUE light efficiency/s (exposure) and TRUE depth of field can be wrong. Nominal F/no says NOTHING about your DoF or how much light is really needed to expose correctly when you aproach makro distances. It can be off by as much as a factor of four (2Ev)!

Nov 01, 2009 at 06:14 PM

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