OG BMPCC sensor "dust" visible only when lens stopped down?

Question: Does it make sense that stopping a lens down would make sensor contamination visible in your footage when otherwise it wouldn't be?

I've recently noticed a blemish in my footage that maybe is some dust or lint on the sensor of my OG BMPCC. However, it is only visible when the lens is stopped down significantly (e.g. f12 or higher). It does not appear when the lens is say below f10 or maybe f8. It is always in the same part of the frame, centered horizontally and a little below center vertically. If I shoot against a light colored wall, it is most visible: shaded slightly darker (but translucent) and is the shape of a fat "L", rotated slightly. The height of this blemish is about 140 pixels on a 1080 frame.

I've noticed this using a couple different lenses but just began to try to narrow it down with the Olympus 8mm Pro fisheye. I would post a picture but get the error "Sorry, the board attachment quota has been reached." (I post sparingly and rarely post pictures; not sure what this means.)

I'm not a heavy user of my BMPCC and lenses and keep them well-stored and take care when handling, changing lenses etc. I don't see any contamination on either my lenses or my sensor. I did use an air blower on both the lenses and sensor. Pretty sure it's not the lenses as I get the same blemish in the same spot with multiple lenses.

What confuses me further is I don't see this shape in the viewfinder when shooting, though so far I've not tried to see it with my external monitor attached, just using the built in BMPCC monitor.

EDIT: Pointing the BMPCC at my computer screen, I do see the blemish now on the BMPCC monitor, which makes this easier to trouble-shoot. The blemish clearly disappears below F8. Seems like it gets "washed out".

Thanks for any insights.
Steve

Make sure the picture you post is less than 1 MB in size. Try exporting a Jpeg from Photoshop and dial in the percentage to reduce the size.

Make sure the picture you post is less than 1 MB in size. Try exporting a Jpeg from Photoshop and dial in the percentage to reduce the size.

Donald, thanks. It's only 245KB and failed again. I'll try again later.

Steve, when you stop a lens down is when you see lens and sensor dirt, it becomes more defined as you stop down, same effect on back side of lens as increasing depth of field on the front side. This is expected behavior.
Cheers

hi, just put a pinhole on the camera and you might notice a lot of stuff.
Periodically I will put pinhole on and aim at an even, diffused light (sky would even do), then make note of the dust locations (a larger external monitor is helpful but not necessary), then take off pinhole and blow off dust with one of those blower bulbs, replace pinhole and check. Keep in mind that any dust reviled by the pin hole but not seen with a small aperture of a lens will still degrade the image quality. Best to keep that opening sealed with a lens or cap (hard to do when needing to change lenses.)

I rarely check my M43 cameras for dust as I never stop down to the degree that would show it. On my full frame cameras I'm obsessive about dust on the sensor as it will show up at apertures < than 5.6. This is one of the overlooked factors in the debate over sensor sizes....
BTW there is little practical benefit to stopping down a lens on a BMPCC to F12. If you need to control light use ND filters.

Thanks, guys. Nice to know what I'm seeing.

John, I take your point about not using anything too closed down. I just happened to finally try to troubleshoot what I was seeing when the Oly 8mm was attached and that lens does not take screw on filters. I can control the time of day I shoot and that should keep me below f8 if I use that lens.

Also interesting about contamination on larger sensors.

I've been unsuccessful even moving whatever is on the sensor with an air bulb so perhaps it's something going on under the cover. Besides an air bulb, are there any safe cleaning procedures for CMOS sensors? I don't want to make it worse and can live with it as is.

Still can't post a pic to the forum.
Steve

It's not dirt on the sensor but on the glass filter infront of it so you can't damage the sensor itself by cleaning. A magnifying glass and strong light should show it up and I find gel sticks sensor cleaning teh best way of tackling dust. Given that you are unlikley to get greasy fingers on the sensor I never thought wet cleaning was any use. I take your point about some lenses not being able to take filters so cleaning and stopping down is your only real solution.

John, thanks very much for the advice. Really appreciate it.
Steve

Question: Does it make sense that stopping a lens down would make sensor contamination visible in your footage when otherwise it wouldn't be?

Yes.

The way to test if your sensor is clean os to go to F22 or beyond, go point the camera at the sky (not the sun!) and defocus to minimum. You'll see all your sensor dirt....

Dirt on lenses doesn't tend to show up this way unless its on the rear element and REALLY bad....

JB

Thanks, JB.

Just to close the loop on this: I finally can attach a picture.

Got a magnifying glass and can see what looks like a piece of lint on the sensor. Unfortunately, a sensor swab won't budge it. I guess it's under the sensor glass and may have been there since it was built in 2013 for all I know. I never noticed it because until recently I never shot anything below F8.

In doing a little more searching, I see this issue has come up a few times on the forum so thanks again to those with the patience to answer the same question multiple times.
Steve

Thanks, JB.

Just to close the loop on this: I finally can attach a picture.

Got a magnifying glass and can see what looks like a piece of lint on the sensor. Unfortunately, a sensor swab won't budge it. I guess it's under the sensor glass and may have been there since it was built in 2013 for all I know. I never noticed it because until recently I never shot anything below F8.

In doing a little more searching, I see this issue has come up a few times on the forum so thanks again to those with the patience to answer the same question multiple times.
Steve

Yeah that's quite large.

Do you see it in the same place with every lens you use ?

JB

Thanks, JB.

Just to close the loop on this: I finally can attach a picture.

Got a magnifying glass and can see what looks like a piece of lint on the sensor. Unfortunately, a sensor swab won't budge it. I guess it's under the sensor glass and may have been there since it was built in 2013 for all I know. I never noticed it because until recently I never shot anything below F8.

In doing a little more searching, I see this issue has come up a few times on the forum so thanks again to those with the patience to answer the same question multiple times.
Steve

Yeah that's quite large.

Do you see it in the same place with every lens you use ?

JB

John, yes it's in the same exact place with multiple lenses. Like I said, who knows how long it's been like that without me noticing, given I was using ND's on most of my lenses outdoors.

Steve

Had something smaller than that on mine when it was new, you could see it on the glass when the lens was removed. I just cleaned the sensor protective glass with my glasses cleaner and it's been fine since. And that was about seven years ago! Just be very careful.

Steve

I have the same issue and also believe it is behind the sensor glass, always been there. I have to be very careful outside as I don't even need that deep of a stop, it will show at f8 against sky or light wall. I will often use cheap kit zooms that are f5.6 at the long end to avoid lens changes, so there is no room for stopping down. I use primes inside, generally f2-2.8, so usually not an issue there. I have seen it though when stopping down against a window, determining if I need nd, so it's definitely not just the cheap zoom.

I have the same issue and also believe it is behind the sensor glass, always been there. I have to be very careful outside as I don't even need that deep of a stop, it will show at f8 against sky or light wall. I will often use cheap kit zooms that are f5.6 at the long end to avoid lens changes, so there is no room for stopping down. I use primes inside, generally f2-2.8, so usually not an issue there. I have seen it though when stopping down against a window, determining if I need nd, so it's definitely not just the cheap zoom.

Curtis, sorry you have the same thing but glad I’m not the only one. Since I’m not a professional it’s just slightly inconvenient for me. But the first thing I’ll do with my next camera purchase is grab my magnifying glass in stand in good light for a sensor inspection.

Steve


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That looks more like fungus than dust. There’s some mystery crud that eats the IR filter in these cams. I couldn’t clean mine because it was between the laminated IR glass layers.

Good Luck