Sunpak's E-TTL flashes are designed to work with Canon digital single lens reflex cameras.
Companies such as Sunpak build multi-dedicated flashes, which are designed to be used with various camera brands, rather than a single brand. Sunpak's external flashes are mostly designed to work with Canon, Nikon and sometimes Sony cameras.
Sunpak E-TTL Flashes
Sunpak produces a number of E-TTL external flash units, including the RD2000, PZ42X, PF30X and PZ40X. These flashes are designed to work with the E-TTL flash control mode, which is used on Canon digital single lens reflex (DSLR) cameras.
Pentax K20D Specifications
According to Digital Photography Review, the flash control mode for the internal flash on the Pentax K20D DSLR is P-TTL. The external flash hot shoe on this camera model is designed to work with Pentax-dedicated flashes, which means those external flash units that are manufactured by Pentax, and some made by third-party manufacturers.specifically to work with Pentax.
Compatibility
None of the E-TTL Sunpak external flash units on the product section of the company's website are explicitly described as working with the Pentax P-TTL flash control mode. Though these flashes could be used with a Pentax K20D in manual mode, they would not be capable of utilizing through-the-lens metering, so an external light meter would be required.
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