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12-31-2014, 06:37 PM   #1
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Pentax 67 lens on 645Z metering issue

I am new to Pentax and got my 645Z one week ago, but I do have many Pentax 67 lenses, so I got the PT76-PT645 adapter for using the 67 lenses on my 645Z. Today, I found one issue on metering that I am not sure is my operation problem, or something I am not aware of. I were trying two lenses of Pentax 67 on the 645Z, the 500 f5.6 and the 105 f2.4, both have the same metering issue. I was using the average metering, when shooting wide open, the resulted exposure seems slightly over, but still look OK, however, when I step down the aperture on the lens by one step, I found the resulted photo will be over explosed by about one step, so will need to provide a "-1" exposure or more ( I was using "A" (Aperture Priority) mode) on the camera, when step down the aperture by 2 steps, then it might need to do "-3" to provide correct exposure. However, the LV on screen were always correct no matter your do "-1" or "-3" ( speed will be slower of course). Then I thought perhaps the aperture was re-open in actual shooting, so I watched the iris while shooting was done, but I saw no change on the iris, it is always closed as the aperture control doing it. It also seems the PT67-PT645 adapter will not do anything between the lens and the camera. So I really wonder why when using the 67 lenses, the exposure will be over according to their aperture step-down. While the LV on the 645Z do able to get correct exposure. When using the Pentax 645 lens ( I have the 35FA and 55DA lenses) on the 645Z there is no such issue. Anybody have any idea what am I doing wrong?

12-31-2014, 07:06 PM   #2
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Do you have the AUTO/MAN lever on the lens set to MAN?
12-31-2014, 07:09 PM   #3
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The 645Z supports open aperture metering, so it's normal for the aperture to stay open as you turn the aperture ring.

It's also normal for viewfinder metering to be slightly inaccurate with legacy lenses (this applies to k mount DSLRs too). Since it sounds like a -1EV compensation is what's needed with that particular lens, I'd recommend either using that or switching to live view, which will always meter consistently regardless of the lens type.

Finally, I doubt that the metering will be any more accurate in manual diaphragm mode, but if your lens supports it I guess it wouldn't hurt to try.
12-31-2014, 07:12 PM   #4
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QuoteOriginally posted by tuco Quote
Do you have the AUTO/MAN lever on the lens set to MAN?
Yes, the AUTO/MAN lever on the lens are at MAN.

12-31-2014, 07:22 PM   #5
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Do you have the genuine Pentax adapter? The genuine adapter does have the required mechanical links for Av exposure to work.
12-31-2014, 07:35 PM   #6
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QuoteOriginally posted by Adam Quote
The 645Z supports open aperture metering, so it's normal for the aperture to stay open as you turn the aperture ring.

It's also normal for viewfinder metering to be slightly inaccurate with legacy lenses (this applies to k mount DSLRs too). Since it sounds like a -1EV compensation is what's needed with that particular lens, I'd recommend either using that or switching to live view, which will always meter consistently regardless of the lens type.

Finally, I doubt that the metering will be any more accurate in manual diaphragm mode, but if your lens supports it I guess it wouldn't hurt to try.
The 645Z does not control the 67 lens aperture thru the adapter, the aperture on the lens is closed as controlled, I watch in slow speed shooting ( over seconds) and the iris did not wide open at any time. The strange thing is that in LV, the metering is correct and speed are correctly changed according to the step down of the lens aperture, however, at actual shooting the metering is over, and the way its over is according to the step-down, i.e. step down more, than over expose more. I further tested the speed ( using tripod and facing same objects) and found following data on the 105 f2.4 lens:

At f4 speed is 1/250
f5.6 1/80 ( correct sped should be 1/125)
f8 1/20 (correct speed should be 1/60)

So obviously, the metering on the 645Z is getting less light than the CMOS when using the Pentax 67 lens with PT67-645 adapter while step down the aperture, therefore, the speed is incorrect when step down the aperture. While LV on the 645Z is not the actual view, so will not notice the exposure has been over. Anybody have any idea why?

---------- Post added 01-01-15 at 10:42 AM ----------

QuoteOriginally posted by thomaskplee Quote
The 645Z does not control the 67 lens aperture thru the adapter, the aperture on the lens is closed as controlled, I watch in slow speed shooting ( over seconds) and the iris did not wide open at any time. The strange thing is that in LV, the metering is correct and speed are correctly changed according to the step down of the lens aperture, however, at actual shooting the metering is over, and the way its over is according to the step-down, i.e. step down more, than over expose more. I further tested the speed ( using tripod and facing same objects) and found following data on the 105 f2.4 lens:

At f4 speed is 1/250
f5.6 1/80 ( correct sped should be 1/125)
f8 1/20 (correct speed should be 1/60)

So obviously, the metering on the 645Z is getting less light than the CMOS when using the Pentax 67 lens with PT67-645 adapter while step down the aperture, therefore, the speed is incorrect when step down the aperture. While LV on the 645Z is not the actual view, so will not notice the exposure has been over. Anybody have any idea why?
Above have some statements confusing, the exposure in LV on 645Z is also not correct, just user will not notice it, as it automatically make adjustment on the display.
12-31-2014, 11:54 PM   #7
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Just found the solution of the problem. First of the the PT67-PT645 adapter is genuine ( I have three, one is from Japan of $250, two is made in China - Widepan which is only $ 110, they all operate the same), I should leave the lens AUTO/MAN switch in AUTO, so thru the adapter, the lens aperture is controlled by the camera, although the aperture is manual switched on the lens, and the camera does not show the aperture information ( its show F 00), however, during the shooting process, the camera somehow controls the lens aperture to do open and close operations to provide accurate metering information to the camera. Now after using the AUTO mode on the lens, then the exposure is correct when stepping down!

---------- Post added 01-01-15 at 03:00 PM ----------

Furthermore, I have been using Sony's mirrorless cameras since day one, including now using A7r / A6000, and have used many old manual lenses of various brands onto them which are manually set aperture on the lens, however, Sony's camera does not care for such aperture control on their metering, and it can still get accurate metering result. So the Pentax 645Z metering design is quite different compare to Sony's although it is using the Sony CMOS.

---------- Post added 01-01-15 at 03:23 PM ----------

More: I am not all correct above. I then tried to use my Hasselblad V lenses on the 645Z, with HD-PT645 adapter that does not have any aperture lever, the 645Z are getting correct exposure no matter which aperture I am switching on the lens. So my latest conclusion is that, the 645Z when using with the PT67-PT645 adapter which able to let it know it can control the aperture, then the metering operations are different, it will try to open the aperture on the lens to do metering first, before shooting. In my very first test, I switch the 67 lens switch to MAN, the camera does not know it cannot open the aperture, so gather the step down lighting result and thought it is the wide open lighting, so it calculates to do a longer exposure, end up provide more over exposed result when step down more. When using non Pentax adapters, with no aperture lever, it will not do same thing, it will treat it as wide open all the time, and do not do the step down calculation.
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