Third party generic DSLR camera lenses

A third party camera lens, is one that has been developed by a different company than those who built your camera's body. For example, if you own a Canon or Nikon DSLR camera, you might choose to purchase a generic Tamron or Sigma lens. Tamron and Sigma lenses are often much cheaper than the Canon or Nikon equivalents. Therefore photographers generally purchase these lenses for cost saving purposes.

Advantages of third party lenses

• Towards the upper end of the market, they can be much cheaper. Especially when it comes to faster or telephoto lenses, for example 400mm and longer.
  
  
• Tamron specifically are known for their excellent image quality at a much less price tag than Canon or Nikon.
  
  
• Sometimes you can buy a generic brand lens that isn't available in Canon or Nikon models. For example, Sigma sells an 8mm fisheye lens that Canon doesn't make.

Disadvantages of third party lenses

• It's important to keep in mind that third party generic lenses are optimized for price. Therefore the optical quality isn't always up to those found on Canon and Nikon brands.
  
  
• If you're intending on upgrading and reselling your lens in the future, generic brands don't hold their value as good as Canon or Nikon.
  
  
• There is no guarantee that the generic lens you buy will be compatible with any Nikon or Canon EOS camera. It's always best to really look into this fully before purchasing a third party lens. Don't simply presume because the salesperson (who is paid a commission) says its compatible, that it will fully work with your particular camera. Or that there won't be any problems. I usually browse message forums to see what photographers themselves say about the lens.
  
  
• Many third party lenses don't have an option of turning on fully manual modes. They also don't offer the nice extra's such as Canon's USM for fast focusing.

Should you buy a generic third party or a brand lens for your digital SLR camera?

If money is no option, you should buy the real deal. After all, Canon lenses are developed specifically for Canon camera bodies. While Nikon lenses are developed especially for Nikon camera bodies. No one knows their camera bodies better than Canon and Nikon.

Even if the price difference is only a few hundred dollars, then wait until you can afford the brand lens. Personally, I've been caught out a couple of times going cheap and always had to buy again, because I wasn't fully happy with the third party lens.

However, I do understand that the low price of a generic lens is sometimes a photographers only choice. Especially while they are first learning. As I recommended earlier, check out the compatibility fully and ask around on message forums for the opinions of other photographers who have purchased these lenses.