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08-20-2014, 11:24 PM   #1
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Help - Spotty bokeh using Pentax tilt/shift

Hi, I've been shooting with my Pentax MX for about 2 years now... great camera, great lenses, great results.

I recently followed an online tutorial on how to make a tilt/shift lens on the cheap.

I tore apart an old 50mm f1.7 prime that was in perfect condition, but just to be sure I gave it a good clean while it was dismantled.

So, now I have a good as new, f1.7 Pentax tilt lens... BUT...

I've gone through a couple of rolls of c41 b&w film only to find that while the areas in focus are clean and clear, the areas where there is bokeh (ie. out of focus areas) have a distracting, random spotting. This effect gets worse the more out of focus the area, and in some cases looks like I've shot through a car window while it's pouring with rain.

I've never seen anything like this and I can't seem to find any information about it online. I'm aware that by making my own lens there is always a risk of something weird/wonderful happening, I'd just like to know what is going on so I can minimise it, or indeed use it to my advantage... Knowledge is power after all.

Thanks, Luke

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08-21-2014, 01:44 AM   #2
Ole
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I'm speculating here, but I'd think that a tilt lens would have to have a much larger image circle than the one the camera otherwise requires. By using a 24x36 lens as a tilt lens on a 24x36 camera you get all sorts of optical aberrations. It would be interesting to see how a medium format lens would perform.
08-21-2014, 05:29 AM   #3
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Somehow there's quasi-focus on some dusty element in/on the lens?
08-21-2014, 05:48 AM   #4
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QuoteOriginally posted by Ole Quote
I'm speculating here, but I'd think that a tilt lens would have to have a much larger image circle than the one the camera otherwise requires. By using a 24x36 lens as a tilt lens on a 24x36 camera you get all sorts of optical aberrations. It would be interesting to see how a medium format lens would perform.
I agree that both for tilt and shift, the lens circle must be larger, in particular to accommodate the shifting of the lens and thus the lens circle. Also, as I understand it the lens is ground and designed a bit better to extend the center area out to the lens' edges again to assist in the shifting and in this case the tilting also. Using this lens on a cropped sensor would help in this regard, since the sensor being smaller would make the lens' circle appear to be larger thus simulating a real tilt/shift design.

QuoteOriginally posted by Nesster Quote
Somehow there's quasi-focus on some dusty element in/on the lens?
I am thinking that there is some tilting applied to this image and thus with the focal plane moved from its normal position, other items are now in focus - or at least in play within the lens itself.



08-21-2014, 10:03 AM   #5
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...or the sensor is seriously filthy.


Steve
08-21-2014, 02:58 PM   #6
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QuoteOriginally posted by stevebrot Quote
...or the sensor is seriously filthy.
Can't be the sensor since the MX is a film camera.
Did you process the film yourself or take it to a lab? I'm wondering if this is a result of poor processing or handling. I think an easy test of the lens in various positions should help to figure out what's going on.
08-21-2014, 03:26 PM   #7
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QuoteOriginally posted by FrankC Quote
Can't be the sensor since the MX is a film camera.
My bad


Steve

---------- Post added 08-21-14 at 03:27 PM ----------

Looks like an enlarged finger print.


Steve
08-22-2014, 06:12 AM   #8
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QuoteOriginally posted by Lukemx Quote
I recently followed an online tutorial on how to make a tilt/shift lens on the cheap
Could you please point me to this article ? I would be really interested in making one myself, providing I have the tools to do it ?

As for your problem.... well, I have absolutely no idea what could have caused it

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