Blackmagic Forum

I am using the BMPCC with the Panasonic 12-35mm lens and I am have an issue setting the exposure when shooting wide open at f/2.8. Everytime I press the Iris button it stops the lens down to f/8 and when I manually stop down to 2.8 everything is over exposed. What am I doing wrong? I've read other threads regarding exposure but have yet to find a solution. My camera is running at the latest firmware. Any help is appreciated.

I'm confused. You say, "I am have an issue setting the exposure when shooting wide open at f/2.8"- you've already set the exposure to 2.8. If pushing the iris button stops the lens down to f/8, then thats what your lens needs to be at in order to not overexpose. If you open it back up to 2.8 and everything is blown out, then yeah, you're overexposing and need to stop the lens down.

Briwil wrote:I'm confused. You say, "I am have an issue setting the exposure when shooting wide open at f/2.8"- you've already set the exposure to 2.8. If pushing the iris button stops the lens down to f/8, then thats what your lens needs to be at in order to not overexpose. If you open it back up to 2.8 and everything is blown out, then yeah, you're overexposing and need to stop the lens down.

that's my point, how would i be able to get a properly exposed picture if i plan to shoot wide open?

Lower ISO...ND filters...

A variable neutral density is worth the investment. I bought a large one, 82mm, and then bought a series of conversion rings to enable quite a few lenses, all the way down to 52mm filter size, to use the variable ND. Now that I am investing more heavily in small C-Mount wide angles, I am fated to buy a 138mm Variable Neutral Density filter for the Matte Box.

Variable ND allows you to tame daylight. Without it, the Pocket Cinema Camera is fairly inept under the sun. With it, you have much more creativity to pursue.

To reduce the exposure, use an ND filter and/or a smaller shutter angle. A lower ISO setting on the BMPCC doesn't increase your highlight range, at least not in ProRes film mode or raw mode.

balazer wrote:To reduce the exposure, use an ND filter and/or a smaller shutter angle. A lower ISO setting on the BMPCC doesn't increase your highlight range, at least not in ProRes film mode or raw mode.

Actually, never having touched a BMPCC, you'd be surprised by how little I know about the beast.

i actually tried to lower the iso and using a nd filter. i just don't understand why the camera stops down when the iris is pressed. logically a camera should adjust exposure based on the settings of the camera and lens correct?

The iris button will adjust the iris to make the image as bright as possible while having very little of it above 100% white. If you press the iris button and the iris stops down to f/8, it means there's a lot of bright parts to your image and you need a stronger ND and/or a smaller shutter angle if you want to have the lens be wide open and preserve nearly all of the brightest parts of the image without overexposing them. What were you shooting, and what shutter angle did you use?

But really, you should not rely on the iris button. You need to decide for yourself which parts of the image can be overexposed and which cannot, and set the exposure manually to make the image as bright as possible while not clipping any parts you want to preserve. Using zebras at 90-100% will help you here. As a very rough guide, any bright part of the image with saturated color should not be allowed to clip. (blue skies, matte objects) Unshaded lamps, the sun, and reflections of them should probably be allowed to clip.

geero wrote:logically a camera should adjust exposure based on the settings of the camera and lens correct?

Exactly. You already set the camera settings when you chose the shutter angle (equivalent to shutter speed on a still camera) in the menu settings, and then you stop your lens down to whatever you want. Pressing the iris button is no different than manually adjusting the f stop of your lens, it's just doing it automatically.

Iris button=f-stop adjustment

I think you are missing some very basic knowledge of exposure.

Cameras exposure is controlled by a few things.
1: Shutter speed (shutter angle) - in video this is pretty constant and has a limited range.
2: ISO - again a limited range
3: Aperture (or Iris)
4: ND filters - these are a "must have" for anyone shooting video.
5: Light in your scene

Many different combinations of the above will result in the same exposure.

If you wanna shoot wide open you'll need to lower the ISO, add ND, increase the shutter speed or reduce the light in the scene.

Pressing the Iris button only adjust the Iris (aperture) to correct the exposure. So that's why is stops down when there is to much light.

What is "correct" exposure? With BMD "cinema" cameras the answer is: there is no "correct" exposure while filming, only a "best" exposure, and that is the one where you get the lowest noise and no highlights clip. This approach is usually called "expose to the right" (ETTR), and that's what the iris button does (at least in film mode; video mode is different!): When you press iris, the camera sets the iris so that the brightest parts of the image just don't clip (if possible, of course - it can't open a f/4 lens up to f/2.8 ...). If you need f/2.8 - don't touch that button, but change the other parameters that influence exposure. Be carefull with shutter angle: you can lower the exposure by reducing the shutter angle, but then you'll loose motion blur, making your footage look jumpy and jerky; best to use 172° (as 180° seems to be broken) unless you need that "private ryan" look. Don't touch ISO either, let it sit at 800 - this camera doesn't do analog gain, all ISO changes are digital processing that will loose some dynamic range. Best way to do it is using ND filters. Period.