Originally posted by vomer6 Ok I have a sigma 105 macro dg lens that I wanted to use with my new k20d but the manual says I can't use it. It seems I need a "F" or "da" lens instead to use it. Can anyone explain why?
Is this why when I tried to use it I could only get a deceit image by manually settings for the ttl ring flash?
you touch a lot of issues here, but I'll start with the easiest - flash
the K20 uses a system called P-TTL which sends a pre-flash before the lens stops down and the mirror flips up, in order to measure the flash. It does this because it uses the camera's light meter for flash control, not a dedicated sensor off the focusing plane. TTL flash does not work on the K20 (only *istD and *istDS)
Also, because P-TTL flash meters with the lens wide open, and then calculates the flash needed when it stops it down, it can only work with lenses that have an "A" on the apature ring,
AND are in the "A" mode.
Now for th enext part, lens designations.
There are several pentax lens designations for the pentax bayonet mount(i'll leave screw mount out of this for the moment)
SMC-Pentax, and SMC-M use what is referred to as K mount. these lenses do NOT have an "A" on the apature ring
SMC-A have an "A" on the apature ring and use the KA mount
SMC-F, and FA lenses use the pentax KAF mount which has an "A" and is also auto focus. Note also that some FA lenses also have what was called power zoom, and use a KAF2 mount.
SMC-FA-J, and DA lenses use what is called by many the "crippled" K mount, because the mechanical apature coupling that tells the camera what apature the lens is manually set to is missing. Note some newer DA lenses have HSM focusing motors built in, and use a varient of the KAF2 mount, but no apature ring coupling to the body
All will work on the K20, but the SMC-Pentax and SMC-A will not work with the P-TTL flash
Now for sigma.
Sigma uses the pentax mount, but it is difficult to tell in total from the lens description which version it has. BUT, in your case it is easy. the latest sigma lenses have 2 different designations DC and DG. D stands for digitally optimized coatings (usually multi coated back element to prevent flair and reflections off the sensor and rear element) C is for ASP-C sensors, which mean the lenses don't project an image large enough to cover a 35mm frame, only the 24x16mm frame of many DSLRs. G stands for full frame (ie covers a 36x24mm image)
Your lens is a DG lens which means it is new enough that it is also probably autofocus and automatic apature, and if you have a p_ttl flash flash fucntions will work