Statistics below shows that Pentax have the least amount of lens failure rates compared to other brands. cannon lenses have the most defects. You can read more at

LensPlay - Lens defects
Canon lenses - 12012 with 907 defects

The probability of getting a good lens is 92 %

The probability of getting 5 Canon lenses in a row is 68 %

Sigma lenses - 2676 with 587 defects

The probability of getting a good lens is 78 %

The probability of getting 5 good Sigma lenses in a row is 29 %

Tamron lenses - 1323 with 203 defects

The probability of getting a good lens is 85 %

The probability of getting 5 good Tamron lenses in a row is 43 %

Tokina lenses - 436 with 70 defects

The probability of getting a good lens is 84 %

The probability of getting 5 good Tokina lenses in a row is 42 %

Nikon lenses - 2198 with 173 defects

The probability of getting a good lens is 92 %

The probability of getting 5 good Nikon lenses in a row is 66 %

Pentax lenses - 1239 with 86 defects

The probability of getting a good lens is 93 %

The probability of getting 5 good Pentax lenses in a row is 70 %

Minolta/Konica/Sony lenses - 591 with 60 defects

The probability of getting a good lens is 90 %

The probability of getting 5 good Minolta/Sony lenses in a row is 59 %

Other lenses - 865 with 92 defects

The probability of getting a good lens is 89 %

The probability of getting 5 good Other lenses in a row is 57 %