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09-16-2012, 04:58 AM   #1
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Incorrect exposure with Miranda 35-70mm K-mount


This is my first post on this forum ! Can anyone help me with a problem I am observing on a Miranda 35-70mm 1:3.5-4.5 MC Macro (PK95109349) that I got off ebay.

The lens has clean optics and the aperture ring stops down from f/3.5 - f/22 perfectly. The aperture lever works as expected i.e. I can move it from wide open to the selected f-stop. I am able to use the lens in manual mode as per the usual steps on Robert Donovan's website. The lens works as expected indoors i.e without a natural lighting source. However when I step outdoors e.g. a bright sunny day i observe that shots taken from f/5.6 -f/22 are progressively over exposed. At f/22 I have to underexpose by around -4.0 to get the correct exposure.

I am wondering if this can be classified as a problem with the camera (Pentax K5) not being able to control the aperture lever. I do see that the aperture change is snappy when controlled by the lever (no delay whatsoever). Please let me know what you think and shout if you need any more information. The lens takes decent pictures and hence I would like to see it work completely.

Thanks a lot!

09-16-2012, 05:11 AM   #2
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Does it have an A mode? Take a photo with it stopped down and look into the lens, see if the aperture blades move as they should. Also, do you do green button metering?
09-16-2012, 05:27 AM   #3
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Nopes! It does not have a 'A' mode. I should have mentioned but I did look into the lens while taking a photo (used the default 12 second delay) and could see the aperture blades stop down. I am using green button metering. I also use the Depth of Field preview function to double check that the exposure is correct. Strangely the final image is overexposed.

09-16-2012, 06:35 AM   #4

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But does it stop down enough? At f/22 the hole between the aperture blades should be around 3mm (not round of course).

09-16-2012, 08:12 AM   #5
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yep! I just took a few shots with a shutter speed of 4 sec. I can see that the camera does stop down to the correct aperture across the entire range.
09-16-2012, 09:21 AM   #6
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Interesting. What happens if you use the "sunny 16 rule" and set the aperture to f/16 and the shutter speed to 1/ISO for a cloudness sunny daylight scene shot, e.g. ISO 100 shutter set to 1/100sec ? Is the exposure close (+/- 1-1.5 stops). How does the calculated (1/ISO) shutter speed compare to what the green button sets?

Confirm Optical Preview and Use Aperture Ring is enabled/allowed and Green button for Tav & M mode is set to Tv shift.

Last edited by Not a Number; 09-16-2012 at 09:27 AM.
09-16-2012, 12:32 PM   #7
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Okie! sounds interesting. I will give this a try and get back later in the week.
11-18-2012, 06:45 AM   #8
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Finally a day came when the sun was out in the UK and I could take out my camera I gave the sunny 16 rule a try and I get the correct exposure using the lens i.e. at f/16 and ISO 1000, a shutter speed of 1000 gives the correct exposure under sunny conditions. The exposure calculation using the green button seems cuckoo as it gives a shutter speed of 200 in the same situation. AFAIK, when the green button is pressed an exposure calculation is done by closing the diaphragm to the set aperture. I can see this happening snappily enough. So why is the camera making an incorrect calculation.

Any help is appreciated.

thanks a lot.


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