More info for poor focus problem

Sounds like the autofocus motor is having issues, I'd suggest contacting Sigma.

The Nikon D40 does not have an autofocus motor, relying on the lens having one. This Sigma lens does not have such a motor, relying on the camera having one. Nikon designates lenses with an autofocus motor AF-S, Sigma calls theirs HSM.

The Sigma 70-300mm f/4-5.6 DG will not autofocus with the D40 since the lens does not feature an internal focusing motor (HSM). The D40 does not have an AF motor in the body and hence it can autofocus only with lenses featuring an integrated motor (eg. Nikon AF-S lenses, Sigma HSM lenses, Tokina DX II series etc). Unfortunately you can use the said lens only in MF mode.

Hi there,

Well you can please follow the link below to get your manual -

http://www.dphotojournal.com/sigma-105mm-f28-ex-dg-macro-reviews-sample-photos/

(click on the manual tab and then click on - Download Sigma 105mm f/2.8 EX DG Macro User Manual (PDF – 67kb), thats it)

Good Luck!!

Thanks for using FixYa!!

Hi is the manual focus working from inf to close focus and not stoping then it needs a new hsm motor

Your Sigma 28-300mm doesn't have a HSM-motor as far as i know. When you switch it from MF to AF, you also have to disengage the manual focusing ring by pulling it towards the camera, or pushing it away from the camera. Using the AF without unlocking the manual focusing ring may damage the AF motor. 
I hope this helps, 
/Chris

1. with this combination of camera/lens you will not have AF when a x2 converter is applied! that is because in all canon cameras (except 1D series) there is no AF when the Aperture is smaller then F5.6.
2. if you don't mind using MF, u need the sigma 2x EX DG Tele Converter.

usandy,
You can set the focal point by toggling the menu switch (you can see it moving around into different locations as it "wraps" around the viewfinder, now depending on your f-stop the area that is not in focus will be soft, you can also experiment with the autofocus lock on you subject and evn try amnually focusing on your subjects.
randy320sgi

Unfortunately, this lens is not supported by the D40 due to Nikon's unfortunate decision to leave out the autofocus motor from this camera.  This is why I tell folks not to waste their money on the D40 - access to the many, many, many lenses that Nikon has made over the years, many of which are available used for a substantial savings, is lost on this camera.  The only Nikon lenses that work are AF-S lenses, and some third party lenses that have their own internal motors.  I got a list of Sigma lenses that are compatible with the D40 for you:
The list of compatible Sigma lenses for the Nikon D40 as of the current date is as follows:

• 10-20mm F4-5.6 EX DC
• 12-24mm F4.5-5.6
• 17-35mm F2.8-4 DG
• 50-500mm F4-6.3 DG
• APO 50-150mm F2.8 EX DC HSM
• 70-200mm F2.8 DG MACRO
• 80-400mm F4.5-5.6
• 100-300mm F4 DG
• 120-300mm F2.8 DG
• 300-800mm F5.6 DG
• 14mm F2.8 EX
• 30mm F1.4 EX DC HSM
• 150mm F2.8 EX DG
• 180mm F3.5 EX DG
• 300mm EX DG
• 500mm EX DG
• 800mm f5.6 EX DG

from here:  http://www.imaging-resource.com/NEWS/1177682545.html
Sorry for the bad news.  The cheaper thing at this point may be to sell the D40 and get a D80.  I personally have a D70S, but that is no longer made.