The purpose of this article — to learn from what material it is made and how much of plastic there are, and how reliable is it.
The lens is inexpensive, but it doesn’t change an essence. On feelings it is shakier, than my Canon 100/2.8L.

sigma 28-105

I foresee a lot of criticism concerning irrevocable dismantling of a lens (I am not going to bring it together, though it is possible, I accurately mounted it and I lost nothing). On the other hand this lens fell one time, at something is broken inside it (we will clarify in the course of disassembling) and the electrician doesn’t work.
There was the strong desire to repair a lens, but I overcame it and I am proud of it :) Let there will be a full analysis of a lens with comments on quality of accessories and check of lens material and qualities of an enlightenment.
I will express the opinion on a lens, and you, dear readers, offer your options what it to be necessary to look.
Especially I was pleased by existence of a nonspherical element. The sigma “is famous” for use of hybrid nonspherical elements.


Though all firms of lenses use hybrid nonspherical elements, at Sigma they meet more often.
Sigma uses other type of an enlightenment, than Canon, Nikon and Zeiss, the high layer of an enlightenment reflects a violet range.

Sigma is ashamed of this lens (though, it is unclear why) and cleaned information on Sigma 28-105/2.8-4 Aspherical IF DG from the official site. There is no optical circuit.

optical_sigma28-105This diagram is necessary to us, where to look for a nonspherical element which requires special attention. It is clarified that it is very easily to get it, it is from the lens hood at the beginning (and we will begin disassembling exactly from there).
Besides, it is quite good to get the list of spare parts for this lens. I didn’t begin to download the list before disassembling of a lens since wasn’t going to bring it together but if you want to collect such lens, the list is very useful to you.
I pay attention that there is any special preparation on disassembling of a lens. But I had no doubts that I will sort it. The only thing that can stop a technically competent person (who doesn’t use big efforts while disassembling, doesn’t pull for loops and other details) — is need to be accurate and sequential if you want to collect a lens. It is necessary to fix on paper (to write) and on a photo all steps of disassembling. Otherwise then it is easy to confuse sequence. It is also possible to make a video.

Process of lens disassembling

The lens Sigma 28-105/2.8-4 Aspherical IF DG is with Canon bayonet joint which also should be removed first of all.
Before I had an experience of disassembling only of manual lenses to which it was possible to reach from a front lens. But here I didn’t find dimples for a key and it was difficult to unscrew the plastic slip. So I decided to do it in the other way.


Here there are two sets of small screws. The first set on an outer edge, fixes a bayonet joint. The second set on a black plastic ring inside, fixes the plastic stub protecting a lens interior from hit there of dust and dirt.
We turn off a plastic stub and we take it out.


It is visible that one goes from a bayonet joint to the green printed circuit board. It is soldered to “dandelion”. And the green board has other loop fixed on itself which is soldered by one end to a board, and another to mechanics of a focusing motor. It is sad, it won’t be possible to take out loops from connectors. For reliability of a lens it in plus, but we won’t be able to shoot and put a bayonet joint aside so simply.
We remove a bayonet joint aside, but we don’t pull for a loop. Bayonet joint weight will withstand a loop, but not our pulling efforts.

sigma len

Now pay attention to yellow ringlets. These are justified rings which are inserted in order that the lens precisely corresponded to a working segment of the camera.
We remove ringlets. They are floppy and get through a bayonet joint outside.

From pluses — continuous rings, but not from slices. It is simpler to put back if it is required.
Under rings there is a black plastic ring with the “autofocus / manual focus” which opens. We turn off small screws and the ring is removed.

sigma28-105 mm

Further it will be necessary to turn off screws which retain a board. Pay attention, there where is a white ring and a small thong there is a small engine under a small white ringlet. It is the autofocus engine.

sigma 20105

Because of the broken pinches one part of a lens (after that rings with the switch) was disconnected from part with lenses and sagged.
Also we turn off the screws retaining gears of the focusing mechanism and a motor. What screws are need to be twisted it is visible in a slot under a board. We need to twist that screws are twisted in plastic pinches.
On the left side, there is a loop. It is necessary to turn off a small screw which keeps a loop in the put status. To take out black and red wires from the holder.
Now we lift a board and the focusing mechanism up, together with the engine (it is taken out with shaft).
When I began to pull out a board, there was a breakaway of the loop going to a board. Probably, it is also the reason why the autofocus doesn’t work.
Tracks, however, can be connected back.
But it not so important. We will look how it fastens.

sigma lens

A “spider” fastens to the chromeplated tube and we should remove at first a plastic tube with a front lens.
There are small screws on perimeter in plastic cones. Everything is very simply turned off and taken out. Finally, I made couple of photo “double-dealing fellow”. I put Sigma optics a front lens to the Canon 100/2.8. It turned out very impressive macro. If to attach it and make a video, there is a quite good construction for shooting of small insects.

sigma 28_105

At the left, on a leg there is a visible plastic flask in which the we can twist a screw.
In the same way, turning off screws on perimeter, we get the block with a diaphragm.

That is all, the lens is completely sorted.
Tests and my impressions about quality of materials and their types will be farther.
It will be in the second part of article, in some days.

Preliminary results

Two tubes holding optics (chromeplated and black, slightly bigger size) are metal. All the rest is plastic, including a big tube with a frontal lens and a big tube with a focusing ring and a zumm.
Excess of plastic in a design also brought this lens when falling.
Except a “double-dealing fellow”, the optics of Sigma can work and it is simple as a qualitative magnifying glass. In comparison with usual magnifying glasses it is day and night. If to sort a lens on separate lenses, some increase very decently, but at the edges we have a strong aberrations and it is not really comfortable to look through them, unlike a collected optical design.
The enlightenment of internal lenses\ has no protection. I didn’t touch them by fingers, but the slightest contacts of other subjects left on them traces which you see on a photo.