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12-01-2016, 10:21 PM - 2 Likes   #16
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Yikes! So much comment and so much confusion!

Simply put:
All lenses with the "A" contacts on the mount enjoy FULL support of all metering, exposure, and flash modes on Pentax dSLR cameras.
"A" contacts are found on Pentax-A, F, FA, DA, and D FA series lenses.

The only variance is with P-TTL flash using non-AF lenses. Without the distance information provided by those lenses, the body is unable to provide fine control to the pre-flash duration. As a result there is a tendency to overexpose when using P-TTL flash with high ISO and short distance.

In-camera metering for lenses without "A" contacts are limited to stop-down metering (center-weighted average or spot only). Stop-down metering on most dSLRs from all brands may be a little unreliable. That is why using a hand-held meter or the so-called Sunny-16 Rule are common backup methods.


Steve

12-01-2016, 10:29 PM   #17
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Not a Number has it right. Of course, if you have AF button activated for metering (which is the way most of us prefer for purposes of allowing re-composition in AF), then you also have metering activated via the AF button.

The confusion comes in regarding focus - as that is where the AF lenses have the added focus point features (beyond having AF itself, of course). "A" lenses have the single electrical contact, so you have no choice other than center spot for metering. This is still somewhat more dependable than the stop-down process for the M lenses. Electrical transmission of the aperture setting metering is somewhat more accurate.

---------- Post added 12-01-2016 at 11:38 PM ----------

QuoteOriginally posted by stevebrot Quote
Yikes! So much comment and so much confusion!

Simply put:
All lenses with the "A" contacts on the mount enjoy FULL support of all metering, exposure, and flash modes on Pentax dSLR cameras.
"A" contacts are found on Pentax-A, F, FA, DA, and D FA series lenses.

The only variance is with P-TTL flash using non-AF lenses. Without the distance information provided by those lenses, the body is unable to provide fine control to the pre-flash duration. As a result there is a tendency to overexpose when using P-TTL flash with high ISO and short distance.

In-camera metering for lenses without "A" contacts are limited to stop-down metering (center-weighted average or spot only). Stop-down metering on most dSLRs from all brands may be a little unreliable. That is why using a hand-held meter or the so-called Sunny-16 Rule are common backup methods.


Steve
Good analysis. I'd somewhat disagree with the pTTL providing any form of "fine control" related to the distance information provided in the calculation. If anything, it tends to just cause flash delay and confusion (especially so with the K-5 series bodies). Pentax pretty much admitted pTTL failure when it went to providing manual ratio flash with the K-3 (implemented very well, by the way). It might be old school, but it won't fail you nearly so often as pTTL.
12-01-2016, 10:39 PM   #18
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Also with the so-called flagship models such as the K-3 set the DOF preview to optical. Then you can use the EV bars to set the exposure in Manual mode with non-A lenses (M, K, M42, T-mount/Fixed Aperture e.g. telescopes and mirror lenses, and "A" lenses off the "A" mode).

In M mode with non-A lenses
- half press to turn the meter on
- operate the DOF preview - this stops down K-mount lenses and takes a light reading through the lens and displays the EV bars
- adjust the exposure from the EV bars by setting the shutter speed with the eDial and/or turning the aperture ring. Screwmount lenses with the A/M switch should be set to "M" so the iris will close down.

QuoteOriginally posted by ScooterMaxi Jim Quote
The confusion comes in regarding focus - as that is where the AF lenses have the added focus point features (beyond having AF itself, of course). "A" lenses have the single electrical contact, so you have no choice other than center spot for metering. This is still somewhat more dependable than the stop-down process for the M lenses. Electrical transmission of the aperture setting metering is somewhat more accurate.
With the AF set to AF-S Catch-in-Focus will work with Manual Focus lenses regardless of electrical pins or not. Caveat - with some bodies Catch-in-Focus may not work with non-conductive lens mounts such as T-adapters or some extension tubes. This is not definite and perhaps needs more exploration..

Last edited by Not a Number; 12-01-2016 at 10:50 PM.
12-02-2016, 03:17 AM   #19
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QuoteOriginally posted by stevebrot Quote
Simply put: All lenses with the "A" contacts on the mount enjoy FULL support of all metering, exposure, and flash modes on Pentax dSLR cameras.
That's right. If you set your camera on a tripod with a "A" lens on, "M" mode setted, point that to something with a bright/white center and dark corners, switch the metering from spot to whatever different setting, you'll notice different EV comp. in the viewfinder (this changes due to the metering method - with [(.)] meter. setting the exposimeter just suggests you to compensate - for the dark corners) . Already checked with my k5.

12-02-2016, 03:32 AM   #20
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QuoteQuote:
You don't need to use the Green Button for M or K lenses either if you know what you're doing and are shooting in M mode on the camera.
Trouble is still working out what I'm doing

---------- Post added 02-12-16 at 03:42 AM ----------

Thanks everyone for your input. Not even got involved with flash yet so that still a bit confusing.

My understanding is i can use a Pentax A lens just the same as my da 18-135 with no problems in av mode.
That's great. Hoping to get out this weekend.
12-02-2016, 03:49 AM   #21
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QuoteOriginally posted by photolady95 Quote
When I shoot I always use M mode so for M lenses I just select the aperture I want to use. For me it works and I don't have to use the green button. With the camera in M mode, it's like using a film camera like I learned on my K1000. This doesn't work if you use AV mode though.

I have never used a K lens to my knowledge on my dslr's so can't confirm that what I do works on them as well.

But if you want samples I can provide those as well.
As far as I know you need to use the green button on M series lenses for the camera to set the shutter speed for the selected aperture setting. Using an M 50 f1.7 on my K-3 I do the following
1. Camera is in M mode and use of aperture ring is enabled.
2. Set desired aperture using the aperture ring.
3. look through viewfinder on desired subject and press green button for camera to set shutter speed
4. focus and shoot.
Are you saying that I can skip step 3?
I have used Av with the M 50 f1.7 on my K-3 but only with the aperture ring wide open at f1.7 and that works.
12-02-2016, 05:33 AM   #22
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QuoteOriginally posted by jddwoods Quote
Are you saying that I can skip step 3?
You only really need to take green button meter readings when there is significant change in the overall EV of the scene/subject. The shutter speed will remain at the same setting until you press the green button again. Say you are taking macro shots of a flower. You would only have to use the green button again if the lighting changes or you recompose and say have more or less of a dark background in the new shot(s).
12-02-2016, 05:49 AM - 1 Like   #23
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QuoteOriginally posted by Kyrsten Stone Quote
My understanding is i can use a Pentax A lens just the same as my da 18-135 with no problems in av mode.
Yes, that's true. There are only two things Pentax A lenses won't do that the 18-135 does (apart from being weather resistant).

1) They won't focus themselves, of course. Never mind; you still get a focus lock indication based on centre-spot focus, so that's good.

2) A series zoom lenses will AFAIK not report their focal length to the camera. Because many of these change their maximum aperture depending on their zoom setting, this may occasionally throw exposure off or cause the wrong aperture to be written into the EXIF. It also means the shake reduction isn't optimised. The best thing to do there is probably to set the SR focal length (when the camera asks for it) to the maximum, unless (for example) you have a very large focal range (e.g. 35-135) and are shooting either zoomed all out or zoomed all-in a lot at low shutter speeds where SR is doing a lot of work.

12-02-2016, 06:52 AM   #24
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QuoteOriginally posted by pathdoc Quote
A series zoom lenses will AFAIK not report their focal length to the camera.
This is true. The data contact that provides focal length info was added on the KAF mount and no A-series lens (KA mount) reports focal length to the body.

QuoteOriginally posted by pathdoc Quote
Because many of these change their maximum aperture depending on their zoom setting, this may occasionally throw exposure off or cause the wrong aperture to be written into the EXIF.
I don't believe this is true, though I don't have a lens to test with to confirm. The base contacts define 1/3 stop increments the same as the camera's aperture increments. It has been my understanding that the variable aperture zooms are wired to step the reported maximum aperture up/down as the lens is zoomed even on the A-series lenses.* The EXIF will report whatever the body "thinks" it is setting, which is always an offset (in 1/3 stop increments) from the reported maximum aperture. Exposure accuracy follows and always has a 1/3 stop granularity for aperture regardless of exposure mode.


Steve

* Again, I have no way to test this and have my doubts that this would be true for 3rd-party KA mount zooms.

Last edited by stevebrot; 12-02-2016 at 06:58 AM.
12-02-2016, 08:35 AM - 1 Like   #25
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QuoteOriginally posted by stevebrot Quote
I have no way to test this and have my doubts that this would be true for 3rd-party KA mount zooms.
Funnily enough, my experience in this regard IS with a 3rd-party KA-mount zoom, a Sigma 70-210 f/3.5-5.6
12-02-2016, 10:57 AM   #26
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Everyone knows there is a non-destructive method of modifying your Pentax dSLR body and pre-KA lenses to act like a KA lens is mounted? I've done it for a couple pre-KA lens and K-r, K-30 and K-3 bodies. The only tools needed are a screwdriver and scissors, and the supplies are a small bit of aluminum foil and magictape.

See this post:
https://www.pentaxforums.com/forums/22-pentax-camera-field-accessories/113756...ml#post1965512
and my followup a couple posts below this one.

As with many of these DIY projects, the explanation is a lot more complex than the actual doing. The first time around will likely take some time, but once you have done it, you could repeat the whole thing for body and lens in 15-20 minutes. And the beauty is, you only have to repeat it if you undo it and there is no need to undo any of it.
12-02-2016, 12:06 PM   #27
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wow ok
not sure i am ready for diy stuff yet.
my lens does always ask for the focal length when first attached,
but then will beep for a focus lock (which is nice)

happier i know what im doing is ok now, thanks everyone
12-02-2016, 02:38 PM   #28
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QuoteOriginally posted by JimJohnson Quote
Everyone knows there is a non-destructive method of modifying your Pentax dSLR body and pre-KA lenses to act like a KA lens is mounted? I've done it for a couple pre-KA lens and K-r, K-30 and K-3 bodies.
The only problem is that playing loose with the mount does not also provide the calibrated aperture mechanism common to all "A" contact lenses except the new KAF4. A number of slick tricks have been proposed since I first joined this site in 2007 and I don't know of any that actually worked easily to provide open-aperture metering with accurate exposure being one of the results. The challenges are:
  • Fooling the body into treating the lens as being A-series or newer
  • Getting an aperture opening appropriate to the metered exposure
Satisfying the first point is fairly easy. The second is not so easy since K and M series lenses typically have an aperture actuator whose travel is not proportional to the lens opening (not calibrated to the travel of the body's coupler). The actuator on some lenses is hair trigger (full stop-down with little motion) while others may require full travel to realize the full aperture range of the lens. Only in very a few cases (some late-model M series) is the aperture mechanism matched to the coupler motion. The only solution that I remember that actually worked went like this:
  • Mount contacts spoofed as per your link
  • Av, M, X, or TAv mode with aperture ring set to desired aperture and aperture on body set to the same
  • Lens being one with little motion required to stop-down to the desired opening
Unfortunately, there is no magic solution. Your extension tube project worked because the aperture coupling was a pass-through to a real A-series lens. If you got consistent results with actual M or K series lenses, congratulations for excellent good fortune.


Steve

(...went around this bush several times before finally deciding the return was not worth additional effort...)

Last edited by stevebrot; 12-02-2016 at 03:06 PM.
12-02-2016, 02:54 PM   #29
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I should have linked this a long time ago in this thread. It confirms the Matrix data from earlier plus adds a heap of additional data about mode/metering/etc by body type and lens type.

Summary of the K-Mount Evolution, Names, and Features
12-02-2016, 05:38 PM   #30
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QuoteOriginally posted by stevebrot Quote
Unfortunately, there is no magic solution. Your extension tube project worked because the aperture coupling was a pass-through to a real A-series lens. If you got consistent results with actual M or K series lenses, congratulations for excellent good fortune.
Really all you are doing is two things, turning on the various forms of open aperture metering (shorting out the 'A' contact on the body) and telling the body what the lens' aperture range is (magic tape on the lens mount). I do agree this is typically not worth the effort unless the photographer is going to regularly use a specific older lens or encounters a unique photographic opportunity.

I'll also admit, this started with my desire to pass 'A' functionality through my macro converter; and it grew from there. Maybe it is only due to good fortune that it continues to work for me. If so, I'll take it. The worst I have seen so far is only enough variation from 'accurate' exposure that was well within my ability to adjust within post processing. That said, when there is a variance, it was also consistent enough that after a couple shots I could simply use exposure compensation at the camera.

And in my opinion only two exposure modes can be used effectively - aperture priority (Av) and manual (M) because the photographer has to adjust the aperture ring to match what the body is also reporting. The aperture values change too much with other exposure modes to be constantly fiddling with the lens.

Oh and by the way, you also get to use P-TTL flash with this technique.
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