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Periodically I get a communications error when using my Sigma 70-300mm lens with my Canon 70D. If I change lens all works. Half an hour later if I change the lens back to the Sigma it works - so the problem is intermittent. Any ideas what is going on?

  • Does it do it at particular aperture setting(s) or focal length(s)? – Michael C Dec 23 '18 at 21:51
  • What specific Sigma 70-300mm lens are you using? There have been many in the Canon EF mount over the years. – Michael C Dec 23 '18 at 22:12
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The most common communication error seen with any lens is due to poor contact between the camera and lens at the interface between the two.

  • Check and clean the electrical contact points between the camera and lens. A soft microfiber cloth should be able to clean any dirt or other residue off the electrical contacts. Be careful not to allow particles to fall into the camera body or the back of the lens.
  • Check to see that the spring-loaded contact points on the camera side (Canon, Sony, Olympis, Panasonic, Pentax) or lens side (Nikon) of the mount are not stuck in a recessed position.

Apart from dirty/misaligned contacts between camera and lens, the most common communication errors for zoom lenses are caused by flex cables inside the lens failing.

  • If the issue presents each time the lens is zoomed beyond a certain point, it's likely a flex cable losing contact. This could be due to a loose connection at either end, or due to cracks in the flex that only lose continuity when the cable is moved to certain positions as the lens moves when zoomed or focused.
  • If the issue occurs when any aperture setting other than wide open is selected, then the issue could be a data connection issue inside the lens between one of the PC boards and the aperture assembly. Or it could be a firmware issue (see below).

With third party lenses, sometimes a camera body newer than the lens' firmware can cause issues. Since third party lenses are almost always reverse-engineered to work with existing cameras that are already around before the lens is introduced, newer camera models (or firmware updates to existing cameras) may throw the lens a "curveball" it doesn't know how to deal with. Sometimes the issue can be fixed by updating the lens' firmware.

Lens manufacturers, including third party makers, will vary with how much they stand behind and support their lenses. The two or three largest names in the third party lens market tend to support them very well. In the past, firmware updates would often be made available but required a trip to a service center to be applied to the lens. Often these were offered at no or very little cost for a specified time period after issues were discovered and corrected through a firmware update.

Recently, though, Sigma and now Tamron have developed hardware docks and made them available to end users so that the firmware running in lenses can be updated by the owner without having to send the lens off to the manufacturer's service facility. It's as simple as downloading the updated firmware to your computer and using a USB dock to load it onto your lens.

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