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07-10-2018, 07:44 PM   #1
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Pentax K1 + Asahi SMC Pentax 24mm 2.8 Manual : Overexposes

Hi All,

I recently purchased Pentax 24mm 2.8 and 50mm 1.4 manual lenses to go with my pentax K1 and had taken few test shots. I have enabled the tvShift from the e-dial programming and also enabled the aperture ring (Setting 26). This is what i follow to take a picture.


Set aperture on lens + Manual focus + Press green button to enable camera to do metering and calculate shutter speed and then take the exposure.

This works if am having aperture at 2.8. But when I try to keep the aperture at f22, the camera constantly overexposes even if it calculates the shutter through metering when I press the green button. Seems like it does affect only the 24mm lens and the 50mm does not seem to have this issue. I have to use exposure compensation to underexpose up to 5 stops. This again affects if am changing the aperture to 2.8. Any tips will be really helpful to understand where am going wrong with this.

07-10-2018, 08:01 PM   #2
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Several people have reported issues with Green Button over the years across multiple cameras. Try using the DOF preview method since the K-1 I assume can do this. I have seen past threads that clearly indicated this method is of higher accuracy than the green button method.

How to use/meter Manual & M42 Lenses on all Pentax DSLRs (K-1, K-3, K-5, K-30, etc) - PentaxForums.com
07-10-2018, 08:08 PM   #3
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I usually set my front dial to exposure compensate in Manual setting. That's because I shoot a lot with manual lenses and even those with A setting can suffer from exposure issues. After I take a test shot (or remember how the lens behaves) I am usually good to go.
07-10-2018, 09:26 PM   #4
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Tried with DOF preview button and I can see the metering which changes when I manually change the shutter. But with 2.8 the exposure is good but with f22 its over exposes but the metering shows it as correct exposure

07-10-2018, 09:44 PM   #5
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QuoteOriginally posted by matroxication Quote
Hi All,

I recently purchased Pentax 24mm 2.8 and 50mm 1.4 manual lenses to go with my pentax K1 and had taken few test shots. I have enabled the tvShift from the e-dial programming and also enabled the aperture ring (Setting 26). This is what i follow to take a picture.


Set aperture on lens + Manual focus + Press green button to enable camera to do metering and calculate shutter speed and then take the exposure.

This works if am having aperture at 2.8. But when I try to keep the aperture at f22, the camera constantly overexposes even if it calculates the shutter through metering when I press the green button. Seems like it does affect only the 24mm lens and the 50mm does not seem to have this issue. I have to use exposure compensation to underexpose up to 5 stops. This again affects if am changing the aperture to 2.8. Any tips will be really helpful to understand where am going wrong with this.
Try using live view. Through the VF, matrix metering is unavailable, so results may vary and exposure compensation may often be needed.

Could also be a problem with the lens's aperture diaphragm.

Adam
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07-10-2018, 09:50 PM   #6
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Thanks Adam. How to identify any issues with len's aperture diaphragm?
07-10-2018, 09:54 PM   #7
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Could be sticky blades. Put aperture to 2.8 and manually move the lever on the lens. Then close the aperture to 22 and do the same. If the blades are sticky, the time the blades need to open again should be noticeable longer with 22.
07-10-2018, 11:53 PM - 1 Like   #8
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QuoteOriginally posted by matroxication Quote
Any tips will be really helpful to understand where am going wrong with this.
You are not doing anything wrong. This problem has been with Pentax owners since at least the K10D and is not limited to Pentax. The reasons are complex and poorly understood by the user community. What we do know is that the focusing screen brightness (what is read for both open-aperture and stop-down metering) does not decrease linearly as the lens is stopped down. This is not a problem for open-aperture reading when the lens controls the aperture, but it is for stop-down metering. Here are the talking points:
  • While stop-down metering used to be the gold standard, the implementation on Pentax dSLRs often results in inconsistent metering
  • The inconsistency varies by physical aperture and by lens mounted
  • Both over and underexposure may be demonstrated for a particular lens depending on the stopped-down aperture and may be as much as 2-3 stops over or underexposed
  • Incremental improvements have been made starting with the K-7 model, but only in M mode. Believe it or not, metering on the K10D and K20D was MUCH worse.
  • The root cause is the stock focus screen and brightening treatments applied to compensate for the half-silvered mirror required by the PDAF system. The brightness of the screen does not vary linearly as the lens aperture is stopped down.
So much for the problem, here is how to get good results:
  • The most consistent meter on the camera the one used in live view (the image sensor itself). It does not suffer from this problem and will meter consistently and accurately both in Av mode with manual aperture lenses or using the green button in M mode with K-mount lenses.
  • The second most consistent metering is to use M mode with either the green button or the optical DOF preview*. The camera is able to detect the case when a non-controllable lens may be mounted (no-A contact detected). In that case the meter output is treated differently than for lenses that the camera can control. The results are much better than on past cameras. This is true for both K-mount and M42 lenses. Yes the green button will still calculate an exposure even if its action does not actually stop down the lens. The meter does not care. This generally works very well except for wide aperture lenses.
  • Least dependable is Av mode with manual aperture lenses
  • For tricky lighting, a hand-held meter is a good tool to have in the bag
In actual usage, the flow goes something like this...
Meter once and shoot freely until either the light or subject changes.
Long-time users of stop-down or non-metered cameras are very familiar with this concept and it works incredibly well. Rare is the need to meter and massage exposure in real time. There is one more thing that all users of stop-down metering should be aware of and that historic users likely remember well. That is the so-called meter coupling range. The meter sensitivity in the camera's specification sheet is with the aperture wide open. While the K-1's meter may be linear to -3 EV(100), that level of screen brightness comes up much more quickly as the lens is stopped down. In a dim room at f/11, the meter may well be at or below its linear range and will report more light than is actually there with resulting severe underexposure. This happens regardless of the set ISO. The solution is to meter wide open and manually calculate the shutter speed for the desired aperture -or- use one of the more sensitive hand-held meters. In true available darkness, neither technique will work.

More than you ever wanted to know, right?


Steve

* The optical DOF preview displays an EV display in the viewfinder that allows for easy exposure compensation and old school center-needle operation. Very cool.


Last edited by stevebrot; 07-10-2018 at 11:58 PM.
07-11-2018, 03:18 AM   #9
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Live view does not help if not set in Av mode. In Av mode histogram updates and metering sort of works. Manual mode is as far out as can be, green button included.
07-11-2018, 06:56 AM   #10
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Another approach some users take is to set the exposure wide open, then manually adjust for the number of stops of aperture changed. So if you wanted f/8 and metered at f/2.8 you would apply 3 stops of correction to the shutter speed or iso to compensate for the 3 stops you stopped the lens down by.
07-11-2018, 07:56 AM - 1 Like   #11
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This is not a joke but I do carry external spotmeter. I used it a lot with my previous Leitaxed 28mm Zeiss. Primarily I have it for large format purposes but it solved all inaccuracy issues at once for manual K-mount lens.
07-11-2018, 09:55 PM   #12
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QuoteOriginally posted by MJKoski Quote
Live view does not help if not set in Av mode. In Av mode histogram updates and metering sort of works. Manual mode is as far out as can be, green button included.
Not my experience. That being said, things may be different in K-1 land. Are you saying that you get different shutter speeds from a manual aperture in Av mode than with green button in M mode with the same lens and settings. I get the same numbers on my K-3 in both modes and those numbers agree well with my handheld meters. I too always have one in the bag, but seldom have to consult it if metering in live view.

As for the liveview histogram. There is another recent thread where its deficiencies are plumbed in detail. The short story is that it represents the rear display characteristics and may not reflect an actual exposure or even the review mode histogram. One should expect it to be worthless with a manual aperture lens.


Steve

(...shoots a healthy mix of non-A K-mount and M42 lenses...)

Last edited by stevebrot; 07-11-2018 at 10:03 PM.
07-11-2018, 09:59 PM   #13
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QuoteOriginally posted by UncleVanya Quote
Another approach some users take is to set the exposure wide open, then manually adjust for the number of stops of aperture changed. So if you wanted f/8 and metered at f/2.8 you would apply 3 stops of correction to the shutter speed or iso to compensate for the 3 stops you stopped the lens down by.
That works when needing to avoid the low end of meter sensitivity, but does not address the general problems with stop-down meter consistency when using the optical viewfinder. Many lenses are at their worst behavior wide open.


Steve
07-12-2018, 12:12 AM   #14
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QuoteOriginally posted by stevebrot Quote
Are you saying that you get different shutter speeds from a manual aperture in Av mode than with green button in M mode with the same lens and settings.
Yes. Only aperture which worked good with Leitaxed Zeiss 28/2 was wide open. And that was when shutter speeds were "fast" under 1 second. Even wide open started to fail when light levels dropped ~10 seconds being some kind of limit. However in Av mode histogram updates as aperture is selected and it works in good light.
07-12-2018, 07:03 AM   #15
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QuoteOriginally posted by stevebrot Quote
That works when needing to avoid the low end of meter sensitivity, but does not address the general problems with stop-down meter consistency when using the optical viewfinder. Many lenses are at their worst behavior wide open.
I have not encountered this but I do little with manual exposure due to limited tolerance for manual focus these days (poor vision)
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