Issue with ND filters on Pentax 12-24mm with Pentax K3

Dear Pentaxians,

I recently bought a used Pentax 12-24mm f4 lens to start into wideangle photography, so far very good results with this lens mounted on my K3.

As I wanted to start taking long exposure shots during daylight, I bought two ( expensive...) brand new 77mm ND filters, one ND8 and one ND1000 from the Hoya PRO ND series.

In order to reach 30 seconds of exposure during daylight without need to stop down up to f22, I mounted ND8 and ND1000 together on the lens, but then the results are quite surprising at 12mm as you can see in the attached image:

-There is a physical vignetting from the second ND filter ( which remove all the interest from the wideangle lens...)
-Worse there is a kind of chromatic aberration or more likely interference/Moiré that occurs on the lower end of the picture !

Important to note that both ND Filters works fine when used separately.

Do you have any solution or explanation ?

I did a quick seach, but I did not found such issue reported before...so any help will be very appreciated!

Cheers from Toulouse, France

If the vignetting is more noticeable at 12mm than at 24mm then tis likely that the second filter is actually intruding into the field of view, ie. it is being 'seen' by the lens? Alternatively, if you are also using a lenshood on top of two filters, then that could intrude into your field of view...

Roger

Yes what I meant by physical vignetting, was indeed that the second filter is intruding the field of view. No solution there but it is a bit too bad...
No problem with the lenshood, but I noticed that it is shaped quite tight, at 12 mm if the lenshood is not properly set in place ( i.e slighty rotated ) it intrudes immediately.

But still what concerns me the most is the strange "Moiré" in the bottom..

The light interference in the bottom part of the frame might be the problem I sometimes get - forgetting to block the viewfinder with the wee cover or some black tape, so light bleeds in through there. I've had some good images ruined from this .

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Tim

Some of my vaguely better stuff

If you're aiming at long exposures in bright sunlight at wide angle, an ultra thin 16 stopper is what you may need...stacking causes vignetting at this focal length.

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'Photography...it remembers little things, long after you have forgotten....' (Aaron Siskind)

The lens hood on these needs a confident and firm click into place, or it will be visible at the corners. I was surprised as to how precise the shielding is, but that is a good thing.

Using two filters will also, as already mentioned, certainly result in vignetting.

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Best regards, John

Would part of the problem be that larger filters are needed for such a wide angle lens? The vignetting is probably the filter mount encroaching into the shot. Try a shot with the mount without filters to see if that is the problem. 77mm is fine for standard lenses, maybe 100 mm filters are needed. On the FA 15-30 mm lens for instance only 150 mm filters will fit.

I would say that the line at the bottom of the image is the lower edge of a square ie it has not been far enough down.

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Pentax K-1, K-3,K-5,

I don't see the result as being surprising at all. If your going to use screw in filters on a ultra wide angle lens best to use specific slimline ones. And if you intend to stack them then you might still get vignetting, so best to get step up rings and go for bigger sizes. You're blocking the field of view. Just the same as if you'd had a finger in front of the lens

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Alan


PPG
Flickr

Hadn't read your post properly as usual, I take it you are using round screw-on filters. My comment still applies, I guess you are seeing the filter frames so you would be better with a larger square format filter. You will be able to use the 77mm filters on other lenses, with a step-up adaptor if necessary to suit smaller diameter lenses.

Hello guys,

First of all thanks for all your answers.

-Indeed I am using 77mm round filters, and when stack together they block the field of view, so I am good to buy a 16 stops ND filter (more difficult to find, any recommendation ?) That's explain the "vignetting".
- Meanwhile regarding the aberration or interference I retain the explanation given by Northgrain... I will do some tests with some tape..I keep you posted!

That's why I always wear a hat Tim To put on top of the camera

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Alan


PPG
Flickr

What can I say. I just get overexcited sometimes and forget the proper workflow . Hats a great idea though

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Tim

Some of my vaguely better stuff

Hello guys,

A short message to comfirm you, that the interferences where due to some light leaking from the viewfinder! So next step: buy a viewfinder cover.

There should have been one in the box the camera came in.