Sigma 105mm f2.8 Macro question

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Chris
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Is it normal for this lens to change aperture as you focus closer?
Mine arrived this morning and is doing exactly that on my D750. It will only shoot wide open when focused to infinity.
My first experience with macro photography so unsure if that's the norm or not.
 
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Yes.

If you Google this you'll probably find long, confusing and baffling explanations and when you've read them all you might think you'd have been better off just accepting that it happens and it's normal.

I'll take your word for it and just accept it then haha. Thanks for the reassurance that all's normal
 
Had a look at the manual and it seems to refer to focus distances combined with magnification ratios.

So, I can surmise that at min focus distance & 1:1 magnification the lens is programmed to stop down for as much DoF, as macro needs good DoF to ensure as much of the tiny subject is in focus!
 
Yeah after using it for an hour in the garden this morning I can understand why it does it. Even at f16 it's DOF is tiny. Lovely and sharp though.
Definitely underestimated how difficult macro photography is though
 
It's a very good lens, I am using this lens currently and would agree macro is more difficult than it looks.
 
To the best of my knowledge, this doesn't happen on Canon camera's.
Nikon camera's show the effective aperture.
 
Had a look at the manual and it seems to refer to focus distances combined with magnification ratios.

So, I can surmise that at min focus distance & 1:1 magnification the lens is programmed to stop down for as much DoF, as macro needs good DoF to ensure as much of the tiny subject is in focus!

I could be wrong but I don't think it's stopping down to achieve greater dof as such as that surely is a decision for the photographer to make. I think it's more the physics of the thing. The actual size of the aperture doesn't change but the lens sends focus distance information to the camera and the camera then displays the relative aperture for that distance. I believe that Nikons do this but Canons don't. I've no idea what other makes do.

I've only ever had 2 macro lenses, one in Canon fit and the one I have now which is completely manual so I've never seen the reported aperture change with distance in the VF.

There's a relatively easy to read piece here with further links you can follow.

https://photo.stackexchange.com/questions/99848/can-i-force-the-aperture-to-stay-open-on-macro-lens

"So from those equations, you can see that the effective aperture is equal to the real aperture (the focal length of the lens divided by the entrance pupil diameter) only when the lens is focused at infinity. At infinity focus (or even just very large subject distances, such that do ≫ ƒ), the magnification is basically zero, so the effective aperture E equals N."
 
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I could be wrong but I don't think it's stopping down to achieve greater dof as such as that surely is a decision for the photographer to make. I think it's more the physics of the thing. The actual size of the aperture doesn't change but the lens sends focus distance information to the camera and the camera then displays the relative aperture for that distance. I believe that Nikons do this but Canons don't. I've no idea what other makes do.

I've only ever had 2 macro lenses, one in Canon fit and the one I have now which is completely manual so I've never seen the reported aperture change with distance in the VF.

There's a relatively easy to read piece here with further links you can follow.

https://photo.stackexchange.com/questions/99848/can-i-force-the-aperture-to-stay-open-on-macro-lens

"So from those equations, you can see that the effective aperture is equal to the real aperture (the focal length of the lens divided by the entrance pupil diameter) only when the lens is focused at infinity. At infinity focus (or even just very large subject distances, such that do ≫ ƒ), the magnification is basically zero, so the effective aperture E equals N."

Interesting read, now makes sense.
 
To the best of my knowledge, this doesn't happen on Canon camera's.
Nikon camera's show the effective aperture.

Have to second that, been using one for many years attached to my Canon and never seen this issue
 
F=f/d
This remains true for all lens and camera fittings.
However the "spec" of the lens is always and can only be stated at infinity focus. I.e. 105mm F2.8 (@ infinity).
As you move the lens away from infinity focus the focal length (f) changes AKA "focus breathing". The focus throw on a macro lens is very long so the shift in focal length is sufficient to affect the amount of light reaching the film. As the f in F=f/d has changed then the apature (F) number must also change.
Mostly the camera and lens combo don't reflect this to avoid confusion. Clearly the Sigma 105mm on Nikon's does. However if you have this lens or pretty much any macro lens on a Canon with the asa set manually, not auto ISO, you will see your shutter speed fall away from infinity to min distance. Or the exposure meter will show under in manual.
While macro lenses are most prone to this you may notice shutter speeds falling or under exposure to a lesser extent on other lenses.
 
F=f/d
This remains true for all lens and camera fittings.
However the "spec" of the lens is always and can only be stated at infinity focus. I.e. 105mm F2.8 (@ infinity).
As you move the lens away from infinity focus the focal length (f) changes AKA "focus breathing". The focus throw on a macro lens is very long so the shift in focal length is sufficient to affect the amount of light reaching the film. As the f in F=f/d has changed then the apature (F) number must also change.
Mostly the camera and lens combo don't reflect this to avoid confusion. Clearly the Sigma 105mm on Nikon's does. However if you have this lens or pretty much any macro lens on a Canon with the asa set manually, not auto ISO, you will see your shutter speed fall away from infinity to min distance. Or the exposure meter will show under in manual.
While macro lenses are most prone to this you may notice shutter speeds falling or under exposure to a lesser extent on other lenses.
Thanks for the explanation, it makes sense :)
 
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