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07-20-2010, 07:39 PM   #1
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Faulty Pentax Takumar Bayonet 28mm f2.8?

I got the lens off eBay and and took some test shots to make sure I wanted to keep it, as there's a 7 day return policy. I'm not happy with the results. My setup was my kitchen and an Oust can perched on the back of the stove. The overhead kitchen light was on, as was the overhead stove light. My K10D was set at ISO 400 and white balance was set at Tungsten. The K10D was in full manual mode, and I used stop down metering (the green button). I picked the Oust can because its colors had contrast.

I'm not happy because this lens wide open at f2.8 is DARKER than all the other settings (when it should be lighter), and I can't tell the difference between f8, f11, f16 and f22! They all look the same to me, and I know enough to know that should not be. The f5.6 setting was the best one - and lightest one. This holds true for other test shots I took, including some from outside. I thought I messed something up so I deleted all the first test shots and started from scratch. Same results.

Here are the shots. They are straight out of the camera. All I did was resize them for display on this forum. I think this lens is bad, so I'm sending it back and just paying the money to get the Pentax-M version. I read somewhere that the Vivitar 28mm's for Pentax were good, but I think I'm going to stick with OEM for my second-hand MF lenses from now on.

Anyhoo, any ideas as to what happened here? TIA.

#1 - f2.8


#2 - f5.6


#3 - f8.0


#4 - f11


#5 - f16


#6 - f22


07-20-2010, 08:01 PM   #2
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1: F2.8 should have the same brightness, not more than other apertures when using the green button. When you hit the green button it sets the appropriate shutter speed for proper exposure.

2: Manual lenses are known to have exposure errors. Edit: You just need to learn to compensate for these errors with a little compensation.

From my limited knowledge and experience with manual lenses it looks pretty normal to me.
07-20-2010, 08:04 PM   #3
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Well... the lens isn't really faulty, exactly. When you hit the green button, the camera sets the shutter speed based on its guess of the right exposure. It varies for different lenses and different apertures, but usually there is some degree of under exposure. That's all you have here. More the camera than the lens. Now if you kept the shutter speed the same for each shot, you should have progressively darker shots. If that didn't happen, then yes you would have an issue.
07-20-2010, 08:40 PM   #4
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QuoteQuote:
I think it really is a problem with all those 'old guys' out there actually doing shit instead of talking about it. They need some schoolin' from people who do a lot of talkin' bout shit but not doing. What would experience in the field be without being tempered by hours and hours of forum reading. Simply not right.
I just recently picked up a Rokinon 28mm f/2.8 that does the exact same thing. 2.8 is one stop underexposed, f/4 is right on, and all other stops overexpose by one stop.

The thing is, if I use stop down metering, all stops meter correctly.

It's not a huge deal for me, since I know that it does that, but it is curious.

07-21-2010, 05:20 AM   #5
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welcome to the wacky world of K10D metering

In my humble opinion there is nothing wring with the lens, but the issue is the K10D metering on Manual Aperture lenses.

this chart has been posted so many times before that I have a perminant link to it for quick reference.



the issue is the K10D focusing screen, which I can only suspect has some unusual reflective characteristics which send more light (I assume here from different areas other than what the sensor is pointed at i.e. "scatter") back to the sensor when a lens is wide open causing it to under expose (more light back = perceived brighter image) and when you stop down, the focusing screen sends less of what I think is "scatter" bacl to the sensor, causing the camera to compensate by slower shutter speeds at small apertures than the situation requires.
07-21-2010, 05:23 AM   #6
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QuoteOriginally posted by alohadave Quote
I just recently picked up a Rokinon 28mm f/2.8 that does the exact same thing. 2.8 is one stop underexposed, f/4 is right on, and all other stops overexpose by one stop.

The thing is, if I use stop down metering, all stops meter correctly.

It's not a huge deal for me, since I know that it does that, but it is curious.
I had this discussion with pentax when I reported the problem to them, and they claimed that stopped down metering was the "correct way" but I found on my K50mm F1.4 there was no difference.

I think they were inferring that the lens did not have time to respond due to a slow aperture, but that has been totally disproved. the fact remains the K10D and K20D have a real metering issue with manual lenses due to the focusing screen.

I demonstrated this by installing an *istD screen into my K10D and the K10D then "magically" duplicated exactly the *istD metering performance.

I posted a chart in the resopnse to the OP on this, and the chart includes my K10D with a Jinfinance split image as well, I also included the K7 performance, which is better but not perfect
07-21-2010, 08:26 PM   #7
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Thanks for the info, but I'm not so sure this is the issue I'm having. None of my other manual lenses - including another Takumar Bayonet - do this, and besides my 18-55 II, all my lenses are manual. I've already contacted the seller and told them I'm sending the lens back for a refund.

QuoteOriginally posted by Lowell Goudge Quote
In my humble opinion there is nothing wring with the lens, but the issue is the K10D metering on Manual Aperture lenses.

this chart has been posted so many times before that I have a perminant link to it for quick reference.



the issue is the K10D focusing screen, which I can only suspect has some unusual reflective characteristics which send more light (I assume here from different areas other than what the sensor is pointed at i.e. "scatter") back to the sensor when a lens is wide open causing it to under expose (more light back = perceived brighter image) and when you stop down, the focusing screen sends less of what I think is "scatter" bacl to the sensor, causing the camera to compensate by slower shutter speeds at small apertures than the situation requires.
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