problems with sigma 180 macro...

mark richards

mark richards

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Hi all , i bought a kenko 1.4 converter for this lens, but it won't focus lock..it hunts but won't lock on.. it just zooms in & out, the conv works ok on nikon 80 200 2.8 & it also works on my 150 500 sigma os...but the lens that i wanted it for, it won't work.. yet if you look at the sigma lens guide it says that this lens is ok with the 1.4 converter ... or is it they'er own make?
 
mark richards said:
Hi all , i bought a kenko 1.4 converter for this lens, but it won't focus lock..it hunts but won't lock on.. it just zooms in & out, the conv works ok on nikon 80 200 2.8 & it also works on my 150 500 sigma os...but the lens that i wanted it for, it won't work.. yet if you look at the sigma lens guide it says that this lens is ok with the 1.4 converter ... or is it they'er own make?
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They refer to their own make IIRC.
 
Are you trying to take macro shots or just use the lens as a normal 180mm + 1.4x?
 
I'm guessing that when you're focusing close you are losing aperture, which is entirely normal, but it takes the effective f/number beyond the AF threshold. Which I think is around f/5.6 with the D300?

It's an f/3.5 lens so when you get down to 1:1 that becomes two stops less, ie almost f/8. Then you add the 1.4x and lose another stop putting you close to f/11.

If this is the problem then it should AF on distant subjects, but as you get gradually closer at macro range at some point it will stop auto focusing.

Give it a try and please post your findings. I'll be interested to know if this is what's happening.
 
HoppyUK said:
I'm guessing that when you're focusing close you are losing aperture, which is entirely normal, but it takes the effective f/number beyond the AF threshold. Which I think is around f/5.6 with the D300?

It's an f/3.5 lens so when you get down to 1:1 that becomes two stops less, ie almost f/8. Then you add the 1.4x and lose another stop putting you close to f/11.

If this is the problem then it should AF on distant subjects, but as you get gradually closer at macro range at some point it will stop auto focusing.

Give it a try and please post your findings. I'll be interested to know if this is what's happening.
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no it hunts no matter what settings?
 
mark richards said:
no it hunts no matter what settings?
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Well, my understanding is that it should work. Is it a Kenko Pro 300 1.4x, with full lens-to-camera couplings? Maybe try cleaning the contacts carefully, with a rubber :thinking:
 
HoppyUK said:
It's an f/3.5 lens so when you get down to 1:1 that becomes two stops less, ie almost f/8.
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What's the formula for that? Does it depend on extension or change in focal length or ???
 
pxl8 said:
What's the formula for that? Does it depend on extension or change in focal length or ???
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It's the magnification. F/numbers only hold absolutely true for infinity focus, though in practise it makes no odds down to quite close 'normal' focusing distance (and TTL metering takes care of any slight shifts without us even noticing).

But when you get real close, and regardless of lens or camera, at half life size 1:2 magnification, exposure needs to to be increased by one stop. At 1:1 life size it is two stops. It's the inverse square law.
 
So, if I focus the 180mm at a wall 10ft away and get a reading of say 1/500s @ f/4 and then refocus at min. focus distance (but not change position so the shot if OOF) the camera will now say 1/125s @ f/4?
 
pxl8 said:
So, if I focus the 180mm at a wall 10ft away and get a reading of say 1/500s @ f/4 and then refocus at min. focus distance (but not change position so the shot if OOF) the camera will now say 1/125s @ f/4?
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Why would you want to do that? We're talking macro, not a wall ten feet away.

All sorts of things change with focus, including focal length (sometimes a lot with IF zooms) but you can't escape the fact that exposure needs to be increased at macro range, as outlined above. It's basic physics.
 
HoppyUK said:
Why would you want to do that? We're talking macro, not a wall ten feet away.
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To see the effect in action. I want to understand where the light loss occurs and why. You said the lens loses 2 stops of light at MFD and to my somewhat simple mind it doesn't really make much sense that turning the focus ring will do that so I was hoping to learn a bit about it...
 
pxl8 said:
To see the effect in action. I want to understand where the light loss occurs and why. You said the lens loses 2 stops of light at MFD and to my somewhat simple mind it doesn't really make much sense that turning the focus ring will do that so I was hoping to learn a bit about it...
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The light loss only occurs at macro focusing range. The image is effectively enlarged as the lens is moved forward (either physically, or by internal movements). It's the same effect as moving a slide projector back to increase the image size, so the brightness drops. Double the distance is double the width, which is four times the area, which is two stops lower intensity.

It's the Inverse Square Law at work. Same thing as a teleconverter - 1.4x is one stop, 2x is two stops.
 
mark richards said:
yes i tried that , i think that it must be a compatibility problem...it just wont stop trying to focus...:bang:
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Yes, I guess that's it. In the absence of any fundamental reason why it should not work, I'm sure that Sigma can fix it with a re-chip or something. I think they are used to resolving AF issues... ;)
 
Mike.P said:
Talking rubbish, ignore me :)
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Well, if anbody was post this information, hypothetically of course ;) it would not be rubbish at all.

From Sigmas compatibility chart:
APO MACRO 180mm F3.5 EX IF HSM AF/MF
APO MACRO 180mm F3.5 EX DG HSM AF/MF

With 1.4x APO Tele Converter, AF can operate between infinity and 1.2m. If the subject is closer than 1.2m, only MF is available.

I think this is what I was referring to in an earlier post, in that the lens should AF fine at distance, but when the effective f/number drops below a certain value at close macro distances, AF will stop. But in the case of the OP, I think we've concluded that he's got one of Sigma's famous electronic incompatability problems.
 
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