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11-01-2012, 11:08 AM   #1
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[HELP] PROBLEMS 645 lens with K-5 !!!

Hello everyone. I just bought a Pentax 645 and 3 lens 75/2.8, 55/2.8 and 150/2.8 and I am please with the performance of them.

But the problem is when I shot 645 lens with my Pentax K-5, use third-party adapter I bought from Ebay (45$), the problem happens that the image is highly sensitive with bright. With the largest aperture 2.8, there is flare if there is strong light in the background, but in normal it works so so even I have hood.

Real problems is when the aperture is set from next f stop, start from f4 and the image is unusable, the following picture will describe it more clearly:

55/2.8 at f2.8


55/2.8 at f4


------------------

55/2.8 at f2.8


55/2.8 at f4


------------------

I do many test at home and it show clearly that when the aperture is not 2.8, the largest, the picture will always get that center bright, even in my room with a diffused light from windows.

I also understand that if I use a long hood (I use my hand to extend the length of the hood) the bright at the center disappears. I do not know what happens there and do you have this problem as I am having ? Even in f2.8, the picture usually lose many contrast if shot outside. Or the problem is I am using a cheap adapter ??? HELP ME PLEASE !!!

UPDATED: Today I go out with Pentax K-5 and 645-75/2.8. I shot always at 2.8 and the problem still there.

The center bright appear clearly even in f2.8 and with hood around 4cm. We all see contrast is terrible here.


In the next picture, I tried to use hand to extend hood with same f2.8. The contrast definitely better and there is no center bright at all !


Some pictures are very good, no problem with the center bright and some pictures have problem, I really cannot understand. I hope someone have experience use 645 lens in D-SLR can help me...


Last edited by Omnis; 11-01-2012 at 11:54 AM.
11-01-2012, 11:27 AM   #2
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Looks to me as if stray light is reflected back from the sensor or the adapter, then hits some bright metallic parts and then heads back onto the sensor.

Since there is no reflection at F2.8 the culprit could be a reflection from the aperture blades?

A longer hood may shield the stray light and eliminate the problem.
11-01-2012, 11:31 AM   #3
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QuoteOriginally posted by Ole Quote
Looks to me as if stray light is reflected back from the sensor hitting some bright metallic parts and then heads back onto the sensor.

Since there is no reflection at F2.8 the culprit could be a reflection from the aperture blades?
Your idea of the reflection from aperture blades looks quite a reasonable, but....why there is fine if I use my hand to extend the hood ? (Like over 10cm hood from the lens)
11-01-2012, 11:32 AM   #4
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QuoteOriginally posted by Omnis Quote
use third-party adapter I bought from Ebay (45$)
The image that the 645 lens is throwing is much bigger than image area of a APS-C sensor... did this adapter come with enough light baffles and flat paint inside as to hold off the light that bounces around... especially the mechanism that operates the aperture coupling. It could be that at a wide open position the reflecting surface is not in the light path, but when the aperture is adjusted is when it comes in contact with the unused edge of the image circle... the larger image projected by the lens.

11-01-2012, 02:08 PM   #5
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I've had this when using a highly reflective filter (baader astrofoil), but you're not using one though I think? One thing maybe to bear in mind is that the hood on there is a hood specifically for the 645 image circle. 645 image circle is a big image circle. The point being that maybe it's just not 'severe' enough for your K5 - it's for a 645, which needs a 'bigger' image circle than APSC. That bigger image circle maybe bouncing around and letting in too much light ie making the haze for APSC. This is why using FF lenses on APSC I now always have a hood that actually vignettes if used on 35mm but hoods the image circle down to APSC size for me.
11-01-2012, 03:53 PM   #6
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I have an adapter for using 6x7 lenses on a 35mm camera and it was causing the same type of internal reflection. Even tho the adapter surface was black, it still reflected too much. I had to coat the surface with a thin glue and used powdered charcoal as an anti reflection coating. This solved it.
11-01-2012, 04:08 PM   #7
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Adapting pentax -M lenses to M43 I had similar problems to Omnis, but not nearly so visible.
This paint is non reflective and adheres solidly to Aluminum
Krylon: Products: Camouflage Paint with Fusion Technology
Use the "Ultra Flat Black"
Hey, keep the camera and lenses in another room when you are spraying the adaptors.
11-02-2012, 05:24 AM   #8
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Today I tried to extend the hood and tried some test and the problem is solved. But because I use 55mm and 75mm lens and if in the future I have 35mm lens, the hood can make vignette, so.....re-spray the adapter looks completely the perfect solution ? So I need to spray the inside of the adapter, right ?



11-02-2012, 08:05 AM   #9
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Have you considered a mask at the rear of the converter? Some strategically placed gaffers tape or a carefully cut black paper mask secured to the rear?

You know... I'll try that with my K20D and the 6x7 to K adapter and see... I've not had the chance to try a 6X7 105mm / F2.4 outdoors... usually indoors. I'll make a mask and test it out.

I'll be back.
11-02-2012, 11:47 AM   #10
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While I admire both the desire to solve a problem and utilizing existing gear, I can't help but wonder why bother with so much selection in 35mm lenses that are more trouble-free and convenient to use.

Last edited by tuco; 11-02-2012 at 12:22 PM.
11-02-2012, 08:22 PM   #11
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QuoteOriginally posted by tuco Quote
While I admire both the desire to solve a problem and utilizing existing gear, I can't help but wonder why bother with so much selection in 35mm lenses that are more trouble-free and convenient to use.
What he said. There may be an engineering solution to every problem, but that doesn't it mean it is the best solution. In this case, it is likely that most M42 and K-mount lenses of similar focal length will likely outperform your 645 optics and will probably be cheap to boot.


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11-03-2012, 06:07 AM   #12
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As much as it is easier to buy a lens to fill the need, it is also convenient to press into service a lens at hand that will accomplish the task and you need not go a buy a lens or locate a place to rent... as if there are many Pentax lens rental places... I sometime twist on a 6X7 lens to my K20D to do an indoor shot. That larger image circle means that I'm using a much sharper center while the lens is wide open and it delivered a few prized shots for publishing. I'm sure that if we have all the lenses for the formats that we shoot then this question would not come up.
11-04-2012, 06:24 AM   #13
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I just bought a Pentax-645 300mm/5.6 and i'm waiting my adapter which should be here early this week. If that lens is as good as my Pentax-67 M*400mm/4 it will be a great addition to my kit. Medium format lenses are very good (at least the Pentax ones), the Takumar (6X7) 75mm/4.5 is another very good lens.
11-04-2012, 10:16 AM   #14
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Shot it and tell us if it work normally ! :-D
11-04-2012, 12:26 PM   #15
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Here is a data point. It was a simple test at f8-f16. I shot my 6x7 M* 300mm ED IF and 35mm SMC A* 300mm f4 on a 1.5 crop sensor camera then pixel peeped. My conclusion. No difference in image IQ worth talking about but the A* 300mm was easy to carry around, no adapter needed, didn't need stop-down metering and had aperture priority mode. And if I recall correctly, pretty much the same conclusion with my 67 generation 55mm and SMC A 50mm f1.2.

Last edited by tuco; 11-04-2012 at 01:24 PM.
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