Sigma 105mm f2.8 will not shoot at 2.8, help!

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AWills

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hi guys. Just bought a new lens from Jessops which I'm very excited about - the sigma 105mm 2.8.

I've got it home and taken it for a few practice shots and despite being on aperture priority mode I cannot get it to shoot at 2.8.

I'm using a D700, can anybody tell me how to fix this?

Thank you
 
AWills said:
hi guys. Just bought a new lens from Jessops which I'm very excited about - the sigma 105mm 2.8.

I've got it home and taken it for a few practice shots and despite being on aperture priority mode I cannot get it to shoot at 2.8.

I'm using a D700, can anybody tell me how to fix this?

Thank you
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Have you tried at non macro distances?

Often macro lenses will reduce the aperture size as the lens extends for closer shots.
 
Yes the afd micro nikor makes my f100 read out F5 at 1:1
 
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My 105mm Nikkor that i purchased recently off this forum does the same. Apparently, it is shooting at 2.8, but the viewfinder is displaying an aperture which will give a correct exposure. It's only what i've read, so don't shoot the messenger. My old Sigma macro displays 2.8 throughout....

p.s. I'd like to know more about this too as i'm unsure myself.
 
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The f/number is only true at infinity focus. At macro distances it reduces and at 1:1 it's exactly two stops* lower. Nikons report this change (technically correct) while Canons do not (prevents confusion). Either way, TTL metering automatically compensates exposure.

It's interesting that most 100mm/2.8 macros typically show f/5 at 1:1 when in theory they should read 5.6. It's still actually correct though, because modern macros also reduce focal length at close distance and at 1:1 a typical 100/2.8 will in fact be more like 85/2.5.
 
Could it have something to do with focallength changing too?
 
So @Hoppy ... on my Nikon, is it physically changing the aperture, or just displaying what a suitable aperture should be in the viewfinder? On the old Siggy, it displays it at 2.8 in the viewfinder regardless whether it's at infinity or macro. Sorry for being such a numpty :D
 
Moey said:
So @Hoppy ... on my Nikon, is it physically changing the aperture, or just displaying what a suitable aperture should be in the viewfinder? On the old Siggy, it displays it at 2.8 in the viewfinder regardless whether it's at infinity or macro. Sorry for being such a numpty :D
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It's lying to you.
 
This is a common issue with the Nikon mount version for some reason. It's shooting f/2.8 but misreporting. Apparently its due to bellow factors during focusing, and way way the Nikon cameras deal with it. I understand Nikon macro lenses do this too, but it's still shooting at f/2.8.
 
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Moey said:
So @Hoppy ... on my Nikon, is it physically changing the aperture, or just displaying what a suitable aperture should be in the viewfinder? On the old Siggy, it displays it at 2.8 in the viewfinder regardless whether it's at infinity or macro. Sorry for being such a numpty :D
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The aperture (the diaphragm blades) is not changing, but as the lens focuses closer the effective aperture reduces and the f/number goes up. At normal shooting distances, the changes are very slight but at macro range they become significant. (Edit: see post below)

It's the 'bellows factor' as Jim says. If you take a 100mm f/2.8 lens, focused on infinity, then attach it to bellows or a set of extension tubes, when the magnification reaches 1:2 the effective aperture will be exactly f/4, and at 1:1 it will be exactly f/5.6. So Nikon cameras are correct when they report the change, but Canons tell a white lie to avoid this confusion. It makes no difference, because the aperture changes in exactly the same way on both systems and TTL metering automatically takes everything into account.

But you won't see exactly f/5.6 at 1:1 with a modern macro focused normally as lens designers cheat a bit (to help keep the lens more compact) and the focal length also reduces at closer distances, and the net result is f/5.
 
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To understand what's going on, try this simple demo of how the image circle changes in size at macro distances and reduces in brightness - effectively raising the f/number for exposure. It's an inverse square law effect and very familiar to large format film photographers, using those huge cameras with big bellows for focusing and a black cloth over your head - they're still used!

Find a plain wall opposite a window. Hold a bare lens with the aperture wide open (easy with Canons, tricky with Nikons ;)) a couple of inches from the wall and you'll see a circle of light with an image of the window - move the lens back and forth so the window is sharp and note the size of the image circle on the wall. Now hold your other hand in front of the lens, say a couple of feet in front and move the lens forward very slightly so the hand is sharp, and note that the size of the image circle hardly changes. Now hold your hand much closer to the front of the lens and you'll need to move the lens quite a bit further forward to get it sharp, and at the same time the image circle will become noticeably larger.

What's happening in terms of exposure is that as the image circle becomes larger, so the light is spread over a wider area and it becomes diluted and less bright. The physical lens aperture has not changed, but the effective f/number goes up.
 
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HoppyUK said:
To understand what's going on, try this simple demo of how the image circle changes in size at macro distances and reduces in brightness - effectively raising the f/number for exposure. It's an inverse square law effect and very familiar to large format film photographers, using those huge cameras with big bellows for focusing and a black cloth over your head - they're still used!

Find a plain wall opposite a window. Hold a bare lens with the aperture wide open (easy with Canons, tricky with Nikons ;)) a couple of inches from the wall and you'll see a circle of light with an image of the window - move the lens back and forth so the window is sharp and note the size of the image circle on the wall. Now hold your other hand in front of the lens, say a couple of feet in front and move the lens forward very slightly so the hand is sharp, and note that the size of the image circle hardly changes. Now hold your hand much closer to the front of the lens and you'll need to move the lens quite a bit further forward to get it sharp, and at the same time the image circle will become noticeably larger.

What's happening in terms of exposure is that as the image circle becomes larger, so the light is spread over a wider area and it becomes diluted and less bright. The physical lens aperture has not changed, but the effective f/number goes up.
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Thanks for the info Richard, very in depth and makes total sense even though i've not tried it yet. I had looked on something regarding the inverse square law online with regards to portraiture and how ISL affects light fall off. With that in mind, your description makes sense. Thanks again, :-)
 
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