Checking your SI2K for hidden SENSOR DUST

[Justin Lovell]

From some rigourous tests, I've found that I will not shoot with my SI2k above 5.6 when pointed at bright objects (eg. SKY).

When you have any dust on your IR filter or on the sensor itself, they become noticeable in certain situations shooting stopped down from 5.6.

The brighter the source of the object, the more the dark spots appear on the image. This is because the dust is either covering sensor pixels, or creating a shadow on the sensor - I'm not sure.

Does anyone have any practical solutions for cleaning the sensor/IR chip?

I'm currently using either lens tissue, and a blower bulb/canned air (cautiously). I have a lens cleaning pen which I use for the sensor when it gets dusted.

BTW, the dust is VERY VERY hard to see, even the tiniest piece can cause an issue.

Not sure if this issue is also apparent with other CMOS cameras ie RED.


Justin
FrameDiscreet.com transfers // cinematography

[Bob Hart]

A quick and dirty check whilst cleaning is to point your sensor at a reasonable and sensible light source, like a window if you are in a house or office, not at the sun or an overbright point source.

Make a 1/8" or about 4mm slot of your fingers with hand and fingers flat (a Mr. Spock slot if you are agile enough). Slowly slide the narrow gap between your fingers across the front of the IMS mount. You may have to do it in different directions. You should see any dust spots on the sensor if you close your fingers enough to make the image a little dark.

Clean the IR glass then try the finger trick again. It is quicker and just as effective as dismounting and refitting a stopped-down lens.

For cleaning I have been using a clean lens brush, a washed and dried fine art brush and either canned air or blowing as I brush. Don't breath on it from close or it will mist up.

I have not been game enough to take the IR glass out and will not do so until it is unavoidably necessary.

In lieu of the fingers, I tried making a cardboard slot and it did not work, something to do with the rounded thickness of the fingers I guess. Strange things are optics.

[Alex Raskin]

Justin, same here.

Here however, i only start seeing dust at f8 and up.

It's always the same couple of spots that become more and more apparent in the video as you keep closing the lens down. I was unsuccessful cleaning them up with either a blower or a lens pen. Am pretty sure the cam came like that from the factory.

ND filters in matte box do resolve the issue, as you no longer need to stop the lens itself down that much.

But what if you need to actually close aperture to increase DOF?

Hmmm

[Kaspar Kallas]

I have very bad experience with PS in this regard,
I had two minis shipped back to PS for matching for 3d when they come back one had dust on sensor and both sides of low pass filter - in the end of the day the blamed me for making the cam dirty!?
I clean the mess up with compressed air (for lens cleaning) and fiberless cotton(not really but dont know the word in english) swabs.
Never again will I deal with this company!

-Kaspar

[Alex Raskin]

What's PS?

[Alex Raskin]

BTW, with PL lenses, my dirt spots become virtually invisible even with small apertures. Go figure.

The dirt is most visible with C-mount adapter and lenses closed down.

[Bob Hart]

My understanding is that dust on sensors is normally an operator's check and maintenance task for all detachable lens digital cameras, stilll-image or movie.

The SI2K system is not alone in this.

Maybe this responsibility should be more carefully defined in the user-manual if it is not already.

It is unfortunate that Kaspar experienced this problem in what might seem to be a sealed area but may not be.

P+S are usually very careful with their work. They and Silicon Imaging have been very helpful with past technical enquiries I have made.

PS is a shortform for P+S Technik GmbH in Germany, who build the SI2K camera system, accessories and recorder unit in conjunction with Silicon Imaging in the USA.

[Steve Nordhauser]

The current Mini design treats the area behind the IR cut filter as a sealed area. Some earlier designs were not sealed as well as the new design. Even so, it is possible that a speck that is left from anodizing and threading could come loose in shipping. Dust behind the filter is a warranty issue to be handled by your distributor.

The IR cut filter exterior can be cleaned using the same methods you would use on a coated lens - blow air first (preferably not a can as this can get propellant on the filter), lens brush next and a lintless swab with a bit of lens cleaner last.

We don't suggest removing the IR cut filter to clean behind it unless you have a good dust-free environment like a laminar flow table. Many rental houses can do this. Removing the 4 screws surrounding the filter will make it possible to remove the filter assembly and clean the back of the filter and the sensor face. Please keep the orientation the same as it was.

Bob, I hadn't heard of that test for crud before, thanks. Dust is hard to see - individual pixels are 5 microns in size.

[Bob Hart]

Steve.


I forgot to mention that for my test, the camera is operating and you view the monitor or a big screen when you do the finger slot test.

[Steve Nordhauser]

Bob,
No problem - I got that. You don't go hunting 5 micron dust without the camera on or a microscope.
Thanks,
Steve

[Chris Lakes]

Steve,

I have been removing the ir filter for years on both cameras I have. What is this about filter orientation? Thanks.

[Bob Hart]

Chris.


I'm not sure until Steve comes back with an answer. I think the IR filter and anti-aliasing optical filter are integrated. The anti-aliasing filter layer, if that is the way it works, should probably be the closest to the sensor surface as it has a slight controlled softening effect.

On the lens side it might behave differently and degrade the image - my imaginings only. I am not about to take it off until it is unavoidable.

[Steve Nordhauser]

Yes, sorry for the delay. As Bob says, the IR cut coating is on one surface of the OLPF so they are integrated together.
The optical assembly of this is done at P+S and I haven't received the definitive answer from them. I would expect that they are consistent, both for the reason Bob gave and because so many of our cameras are being used for 3D, you want them to be as matched as possible.
It certainly isn't catastrophic if it is reversed.

The only time it should be removed is if there is known crud between the filter and sensor and then only in a clean atmosphere.
Steve

[Steve Nordhauser]

Within the next day or so, we should be posting two new application notes in the support section of our website. One is on collimation and the other is on sensor cleaning. Both are from P+S and pertain directly to the SI-2K/Mini.
-Steve