Sigma 105mm macro f1.2


derek897

Link Posted 07/04/2017 - 17:50
Does anyone know how this is possible



Lens is a sigma 105mm macro f2.8
Ext tubes attached.
I know what i like, If not always why.

sebas77

Link Posted 07/04/2017 - 17:59
don't quote me here as I am not looking up to see if I am saying something totally foolish, but if I remember correctly, as the multiplier increases the tele and reduce the luminosity, there is the opposite that does the opposite. something like speed booster, never really interested in researching it further.
Last Edited by sebas77 on 07/04/2017 - 17:59

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emleyman

Link Posted 07/04/2017 - 18:17
It looks as if the lens electrical contacts have somehow all been shorted. If you look here: http://www.bdimitrov.de/kmp/technology/K-mount/Ka.html there is a table that shows the combination of contacts for each aperture range. If all of them are shorted then the camera will think it has a f1.2-f22 lens fitted.

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derek897

Link Posted 07/04/2017 - 20:18
Is this a problem I should be worried about or enjoying the fact that I have an f1.2 macro.
I know what i like, If not always why.

derek897

Link Posted 07/04/2017 - 22:25
A couple of shots at f1.2 taken with a sigma 105mm macro f2.8 and one at f2.8


IMGP1580edsm by dr.shutter, on Flickr


IMGP1564edsm by dr.shutter, on Flickr


IMGP1577edsm by dr.shutter, on Flickr
I know what i like, If not always why.

davidwozhere

Link Posted 08/04/2017 - 01:18
It does a nice job
Both the *istDS and the K5 are incurably addicted to old glass

My page on Photocrowd - link

emleyman

Link Posted 08/04/2017 - 08:30
It shouldn't be anything to worry about. Does tha camera recognise the lens properly without the teleconverter? If so I guess its something to do with the contacts between the lens and the converter.
I think the only issue might be a problem with exposure, dependng on what mode you use. Manual and green button should be ok, but I think that some other modes might get it wrong if they are calculating for a stopped down lens.
I'm no expert though, so I could possibly be talking complete rubbish.
Maybe everything will work but your exif data will show a very expensive, rare lens.
Your photos certainly look ok.

derek897

Link Posted 08/04/2017 - 08:45
Cheers. The lens works normally without the tubes.
Cant see much use for it other than the odd dreamy shot. But it might come in handy sometime.
I know what i like, If not always why.

redbusa99

Link Posted 08/04/2017 - 10:49
surely the mechanical action of the lens cannot open beyond 2.8 , that is as far as it can go, so i would think that is what you are actually getting and the 1.2 is the electronics misreading it.
odd lens or 2

Flickr

derek897

Link Posted 08/04/2017 - 12:19
redbusa99 wrote:
surely the mechanical action of the lens cannot open beyond 2.8 , that is as far as it can go, so i would think that is what you are actually getting and the 1.2 is the electronics misreading it.

If you look at the latest 2 shots of the blossom.
Top one is f1.2 as recorded in exif and 2nd one is at f2.8.
Even without the exif data the dof difference is huge.
So I dont think it's just a case of it recording it as 1.2 and shooting 2.8. There is a huge difference in exposure shooting manually too.
I know what i like, If not always why.

emleyman

Link Posted 08/04/2017 - 16:00
The way Iunderstand it is that the lens transmits the info on largest aperture and number of stops to smallest. In this case, incorrectly f1.2. When you set aperture to f2.8 on the camera you are actually telling the lens to decrease aperture by 4 stops, so although the exif says f2.8 (f1.2 + 4 stops) you are actually shooting at f11 (f2.8 + 4 stops).

derek897

Link Posted 08/04/2017 - 17:10
emleyman wrote:
The way Iunderstand it is that the lens transmits the info on largest aperture and number of stops to smallest. In this case, incorrectly f1.2. When you set aperture to f2.8 on the camera you are actually telling the lens to decrease aperture by 4 stops, so although the exif says f2.8 (f1.2 + 4 stops) you are actually shooting at f11 (f2.8 + 4 stops).


Dont understand that at all
I know what i like, If not always why.

sebas77

Link Posted 08/04/2017 - 18:21
haha I actually just realised you were asking about your lens

derek897

Link Posted 08/04/2017 - 20:17
sebas77 wrote:
haha I actually just realised you were asking about your lens

Ah yes. It's not just a random question.

Emelyman, what you're saying makes no sense to me at all.
It sounds like regardless of the f1.2 question, anyone shooting with tubes can't shoot wide open because at f2.8 it's really f11, at least thats what im getting from what you're saying.
The lens will stop at the following extra stops.

F1.2 f1.3 f1.4 f1.5 f1.6 f1.7 f1.8 f1.9 f2 f2.1 f2.2 f2.3 f2.4 f2.5 f2.6 f2.7

I do shoot macro wide open at times
(Previous thread )
So I know what my dof should look like.
And it's nothing as shallow as the f1.2 shots at the least magnification, furthest focusing distance.
I know what i like, If not always why.

emleyman

Link Posted 09/04/2017 - 18:10
I'll try again. Your lens is F2.8 wide open, but your camera thinks it's f1.2 due to some issue with the contacts on the teleconverter.
When you alter the aperture via the camera, it doesn't actually tell the lens the aperture it wants. What it tells it is how many stops from the maximum to reduce by. Therefore, if your camera thinks it's F1.2 wide open, and you tell it you want f2.8, it is actually telling the lens how many steps to reduce by.
I realise I assumed f1.2-2.8 was 4 steps (1.2-1.4-1.7-2-2.8 ), and you may have more available options than that, but the principal is the same. You are telling the lens to reduce x steps from the maximum aperture, not to go to a specific aperture value.
Therefore, if your camera says f1.2, and the lens is actually f2.8, then when you reduce to f2.8 on the camera, the lens will have reduced the same number of steps to a higher f value. (I think I was wrong about f11, and it is actually closer to f6.7). That explains the different dof between the f1.2 and f2.8 exif shots. They are actually at f2.8 and something closer to f6.7.
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