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01-28-2009, 07:18 PM   #1
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DG lens on K20d

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?

01-28-2009, 07:26 PM   #2
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If the lens is a K mount lens, it will mount and focus properly on the k20d. If the aperture ring has an A setting, all the exposure modes should work as well. If, however the lens is for a Canon or Nikon or Sony or Leica or ... it will not work on the k20d.

Search the forum on "using manual lenses" to come up with the good and bad of non - A lenses. Sometimes they are worth it. I have two.
01-28-2009, 07:51 PM   #3
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The lens is from my dead *ist body and so is the ring flash but I can only get it to work in the manual mode. I would like to use a fixed aperature of 22-32 and get the flash and lens to compensate but I have not figured it out.
01-28-2009, 07:55 PM   #4
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QuoteOriginally posted by vomer6 Quote
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?
Those are Pentax's abbreviations. Sigma has different ones. DG for Sigma is like DA for Pentax, so it should work fine.

And FWIW, you don't *need* F (or FA) or DA lenses even from Pentax - older lenses work too. But they don't have autofocus, and many work may work in "M" mode.

QuoteQuote:
Is this why when I tried to use it I could only get a deceit image by manually settings for the ttl ring flash?
No idea.

01-28-2009, 07:56 PM   #5
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I have the non-DG version of this lens on my K100DS and it works great. Make sure the aperture ring is set to the "A" setting if you want to use any of the program line modes (Tv, Av etc) or in Manual if you want to control the aperture via the camera. If not set your aperture via the ring and shoot fully manual. I have no experience with your ring flash so I cannot not help you there.
01-28-2009, 07:59 PM   #6
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Your lens will work fine. Your ttl flash, while it works okay on the ist series of cameras, will not work fine. You will have to use it in manual mode as the latest batch of Pentax dslrs don't do ttl.
01-29-2009, 05:55 AM   #7
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QuoteOriginally posted by Myoptimism Quote
Your lens will work fine. Your ttl flash, while it works okay on the ist series of cameras, will not work fine. You will have to use it in manual mode as the latest batch of Pentax dslrs don't do ttl.
So I'm correct in saying that with the K20d I MUST only use manual settings for all my macro with flash?
There is not another flash that I could buy that would correct the exposure automatically?
01-29-2009, 06:06 AM   #8
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QuoteOriginally posted by Marc Sabatella Quote
Those are Pentax's abbreviations. Sigma has different ones. DG for Sigma is like DA for Pentax, so it should work fine.
Actually, it's more like D-FA. DG means that it's Digitally Optimised, but Full Frame. DC means it's APS-C only, IE: DA.

Incidentally, the closest equivalent of this lens on from Pentax is their D-FA 100mm F2.8 Macro.

Still shouldn't make a difference. leave the Aperture ring in "A" and let the camera make the setting. Eva2000 has one, and he loves it

QuoteOriginally posted by vomer6 Quote
So I'm correct in saying that with the K20d I MUST only use manual settings for all my macro with flash?
Yes

Cameras after the *ist DS (i think) are P-TTL only.

QuoteOriginally posted by vomer6 Quote
There is not another flash that I could buy that would correct the exposure automatically?
there is

Pentax's AF160FC Macro Ring Flash, when it's out

01-29-2009, 06:27 AM   #9
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QuoteOriginally posted by vomer6 Quote
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
01-29-2009, 08:06 AM   #10
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One more suggestion if you would rather use the aperture ring on the lens (instead of setting the lens to A and controlling the aperture from the camera body):

You need to dig in to the menus and set the camera to allow use of aperture rings. The default (for some unknown, useless reason) is to disallow use of aperture rings.
01-29-2009, 12:26 PM   #11
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QuoteOriginally posted by vomer6 Quote
So I'm correct in saying that with the K20d I MUST only use manual settings for all my macro with flash?
There is not another flash that I could buy that would correct the exposure automatically?
For macro, the Pentax ring flash would work in pTTL as will the other Pentax flashes and some models from others. If you do a forum search, you should come up with the list of compatible flashes. They are in the Accessories forum.
01-31-2009, 06:20 PM   #12
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Thanks for the help- lens details

the actual lens is Sigma 105mm dg macro f stops from 2.8-32 and has A setting on the f stop.
With the ring light I can take a nice picture at 1:1 at F-32 and shutter 1/180.

I will most likely buy the new Pentax ring flash that they just came out with
01-31-2009, 06:50 PM   #13
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Sigma has a macro ring flash that works with Pentax P-TTL exposure system.
02-02-2009, 09:04 AM   #14
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QuoteOriginally posted by Lowell Goudge Quote
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.

*snip*

All will work on the K20, but the SMC-Pentax and SMC-A will not work with the P-TTL flash
Lowell, don't you mean that SMC-M lenses will not work with P-TTL flash but SMC-A lenses will work?

I'm also trying to understand the limitations of P-TTL with older lenses before I dive into a Pentax DSLR system.
02-02-2009, 09:25 AM   #15
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QuoteOriginally posted by indytax Quote
Lowell, don't you mean that SMC-M lenses will not work with P-TTL flash but SMC-A lenses will work?

I'm also trying to understand the limitations of P-TTL with older lenses before I dive into a Pentax DSLR system.
you're right, my error SMC-M have no A setting, also.

as for older lenses, it is really that you need A or newer to use P-TTL.

if you want to do flash photography with M42, K or M lenses you might be better with an *istD which has TTL flash and works just fine. Also, and I am not sure of this, check someone who uses a lot of M42 lenses on a K10, P-TTL might work since you are stopped down perminantly, (in theory it could) but I am not sure.

Also, the AF540FGZ has an "AUTO" mode with its own sensor, to give you auto flash with old lenses, just like pre TTL in the early 1980's
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