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02-27-2009, 08:17 AM   #1
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K20D - Manual Focus Trapping - Lens selection HELP?

I am purchasing a K20D here soon, as my move from another system. The main reason for my switch is the value of the K20D with the SR built into the system.

I am trying to put together a kit, and at the same time keep the price down. Obviously if I had the money I would get FA 31, 43, 77 etc.. A wide angle Sigma 10-20, and a Sigma 70-200 or the Pentax *200.... but I can't afford all of that RIGHT NOW.

So, here is my question. If I choose to get some manual focus lenses.. so I can save some money.. how do they integrate with the camera when it comes to metering and f-stop? Are all of the lenses similar ( if manual focus ) in how they would work on the K20D?

I guess this is my question... if I use a DA lens.. lets say this DA lens is 2.8, but I want to shoot at F8.. the camera will keep the lens "OPEN" while I focus and meter.. and then when I press the shutter, it will stop it down to F8 and take the picture...

Am I right in assuming that ALL MANUAL FOCUS LENSES.. well not be able to do this? Say I want to shoot at F8.. will that always be dialed in prior to shooting at F8.. limited the light through the viewfiend.. and making critical focus more difficult?

Any help?

02-27-2009, 08:50 AM   #2
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Designinme, welcome to this forum!

No, you're not right with your assumption. I think in short there are 4 types of manual lenses (I hope I am complete and don't tell you stories here):

- preset, without auto: these act like you suppose: there is no way stopping these lenses down and keep the viewfinder as bright as it is open. So the way to work is here: focus with open aperture, choose your aperture and fire.
-preset, with auto: these are different from the first ones in having a button changing the apertur to a chosen value. Set your aperture, open it again, focus, push the button and shoot.
- manuals like the Pentax-M series: the aperture stays open all the time, so focussing wil be done with full lit viewfinder. For metering set your camera in M modus and push the green button.
- manuals with automatic aperture, like the Pentax-A series: set your lens in A and let the camera choose aperture. Focussing has to be done manual.
02-27-2009, 09:23 AM   #3
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welcome to the forum

to elaborate on the discussion by rense,

the first 2 types of lenses he is discussing are generally (but not always) M42 screw mount lenses that use an adaptor to connect to the K mount bodies. There are a couple, but only a couple of preset apature lenses in K mount, specifically the origonal 400mm F5.6 and the 500mm F4.5, but all other K mount lenses have open apature focusing.

with respect to lenses and metering, the lenses that will meter the best on the K20D are pentax SMC-A lenses. These lenses have apature contacts and the apature is controlled by the camera body, when the lens is in the A position.

These lenses will also allow you to use P-TTL flash, because this mode of flash calculates from a preflash at max apature, the flash duration you will need when the lens is stopped down, as opposed to TTL flash, which takes light reflected directly off the sensor. The K20D only supports P-TTL

With respect to "trap focusing" I am not sure on the K20 whether there is any difference from my K10 (someone correct me here if I am incorrect) but the K10 can't trap focus with most M42 screw moount lenses because the lens base is not wide enough to cover the contacts on the camera body mount. K mount lenses do permit trap focusing, but as noted earlier cannot be used for P-TTL flash.

If you are going to do a lot of manual focus, you should consider a split image viewfinder, there are several chineese ones and also KatzEye. Look in the forums for reviews and comments, there is usually an active thread at any time.


A word of caution if you elect to go with older, non A lenses. The K10D and K20D have some metering issues if they do not know the lens maximum apature, this has been linked to the viewing screen. see the link here
https://www.pentaxforums.com/forums/241716-post69.html

Also changing to a split image screen may help in this regard but introduce some errors in "A" lens metering. DO not be discouraged by this, simply check out each lens, and once you know how each lens you get performs, make adjustments for it.
02-27-2009, 10:27 AM   #4
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QuoteOriginally posted by Rense Quote
Designinme, welcome to this forum!
- manuals like the Pentax-M series: the aperture stays open all the time, so focussing wil be done with full lit viewfinder. For metering set your camera in M modus and push the green button.
- manuals with automatic aperture, like the Pentax-A series: set your lens in A and let the camera choose aperture. Focussing has to be done manual.
So am I right to assume then that any Pentax M series will function most closely to a automatic lens.. other than I will just focus manually?

02-27-2009, 10:55 AM   #5
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QuoteOriginally posted by designinme_1976 Quote
So am I right to assume then that any Pentax M series will function most closely to a automatic lens.. other than I will just focus manually?
No, the A series does! Sorry if I am somewhat vague. If you set an A-lens at "A", the lens will stay wide open, until you fire. At that moment, the shutter is set to the chosen value. ONLY focussing has to be done manually, and different from the setting with an M-lens you don't have to push the green button. So, an A-lens behaves like an A-lens, on a K20d not different from when it's on, lets say, a P30 camera.

Regards, R

Last edited by Rense; 02-27-2009 at 11:42 AM.
02-27-2009, 01:53 PM   #6
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QuoteOriginally posted by designinme_1976 Quote
So am I right to assume then that any Pentax M series will function most closely to a automatic lens.. other than I will just focus manually?
As others have said, no, it is "A" lenses that are most similar to DA lenses. With an "M" lens (or any other K-mount lens with no "A" position on the aperture ring), you can't use any mode but "M", and you have to use the Green button to get it to stop down momentarily and meter before it sets a shutter speed. With "A" lens, you can use all metering modes. In either case the lens stays wide open while focusing and only stops down when you take the picture (or do a DOF preview, or in the case of "M" lenses, momentarily when you hit the Green button).

The "M"-type lenses are by far more common than "A". There's lots of M42 lenses out there too - but I don't know that world.
02-27-2009, 04:07 PM   #7
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QuoteOriginally posted by Marc Sabatella Quote
The "M"-type lenses are by far more common than "A". There's lots of M42 lenses out there too - but I don't know that world.

Marc, this sounds like a parallel universe.... and it's a great one, IMO!

R
02-27-2009, 05:08 PM   #8
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QuoteOriginally posted by Rense Quote
Marc, this sounds like a parallel universe.... and it's a great one, IMO!
Technically, I think M42 is classified as a "nebula", not a "universe" :-)

[ For those wondering what the heck I'm talking about, M42 is the official name for the "Orion nebula", a big gaseous cloud that's one of the first things you learn to find if you get bit by the astronomy bug the way we're all bit by the photography bug. It's the astronomical equivalent of a fast 50 :-). And yes, the word "nerd" doesn't begin to describe those of us familiar with both connotations of the term "M42" ]

02-27-2009, 11:57 PM   #9
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QuoteOriginally posted by designinme_1976 Quote
So am I right to assume then that any Pentax M series will function most closely to a automatic lens.. other than I will just focus manually?
NO. To elaborate:

The oldest bayonet lenses are the "K" or "M" series lenses. These have an aperture ring WITHOUT an "A" position, and must be manually focused and manually metered before shooting. The metering is actually pretty easy to do - just select the aperture on the ring and then press a button to set the shutter speed before shooting.

The next newer lenses are called the "A" series lenses - these have an "A" position on the aperture ring. If you set the aperture ring to the "A" position, then they act just you indicated in the quote above - they will meter automatically and all you have to manually is to focus before shooting.

All of the newer lenses, "FA", "FAJ", "DA", "DFA", "DA*", etc. etc. will meter AND focus automatically.
02-28-2009, 02:28 AM   #10
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QuoteOriginally posted by Marc Sabatella Quote
Technically, I think M42 is classified as a "nebula", not a "universe" :-)

[ For those wondering what the heck I'm talking about, M42 is the official name for the "Orion nebula", a big gaseous cloud that's one of the first things you learn to find if you get bit by the astronomy bug the way we're all bit by the photography bug. It's the astronomical equivalent of a fast 50 :-). And yes, the word "nerd" doesn't begin to describe those of us familiar with both connotations of the term "M42" ]
Great, thanks!
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