Sweet spot exposure settings for BM URSA 4.6K?

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litote

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Sweet spot exposure settings for BM URSA 4.6K?

PostTue Jan 15, 2019 3:13 pm

I am just getting started with the BlackMagic URSA mini 4.6K model and could benefit from someone's shooting experience.

My research shows that the optimum ISO for this camera is 800, and for a frame rate of 24fps a shutter angle of 180 gives optimal results for smooth, natural motion of objects (neither streaked or stuttered).

Thus, in order to set the correct exposure, if I want to have more control of depth of field, I am limited to adjusting ISO and shutter angle, rather than aperture. How far from these optimal settings (ISO: 800, SA: 180) can I stray before noise, dynamic range and unnatural motion become apparent? What sort of ranges seem to work? Presumably ISO should be adjusted rather than shutter angle?

Also, in setting exposure, what is a good workflow? Should I set the aperture first to get the desired DOF, then use the zebra bars to guide the adjustment of the shutter angle/ISO until I reach an acceptable reduction of zebras on highlights?
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Uli Plank

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Re: Sweet spot exposure settings for BM URSA 4.6K?

PostTue Jan 15, 2019 4:16 pm

In cinematography, you solve that issue with ND filters.
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Re: Sweet spot exposure settings for BM URSA 4.6K?

PostWed Jan 16, 2019 4:08 am

On that camera for most situations, you just set ISO 800 and shutter angle 180 degrees as you mentioned.

Optionally you can use ISO 400, but I don’t recommend using ISO 1600 if you can avoid it as the increased noise may be unattractive.

Optionally you can go to larger or smaller shutter angles such as 270 degrees or 120 degrees but most folks stay with 180 degrees. In situations where motion is modest, I’ve liked shooting 360 degrees.

As for aperture, it really depends upon the depth of field and sharpness you want in a shot within reason for the lens you are using. Most very fast lenses are soft wide open unless the lens cost a boatload of money. That’s usually true of most lenses less than about T2. The sweet spot for focus is usually about a stop less than wide open and then very good for a few more stops. But you may not want to stop the lens all the way down as diffraction sets in as the iris closes down. Let’s say ideally you want to shoot around T2.8-T5.6 and you control that by using neutral density filters to reduce the amount of light so you can shoot as wide as you want (since a fairly shallow depth of field is usually desired).

But what is a proper exposure? How do you know when you have it? On most of the BMD cameras, you have two important exposure aids: zebras and false colour.

Zebras indicate when you might be clipping portions of your image but seeing some zebras starting to appear may be more of a warning; conversely even when you don’t see zebras you may be clipping in one colour channel. That’s bad. You can adjust the sensitivity of zebras to help avoid that. The default is 100%, but I feel that’s risky and set it to 90%. Many people use 95%. So that’s going to give you an early warning that you might be clipping and be careful. You may rely a lot on zebras if you are shooting raw and exposing to the right (ETTR) to feed the sensor the most light. If you are shooting ProRes, you want to expose for the scene.

False colour is like manna from heaven. It just makes choosing exposure much easier in most situations. You can read about it in the camera manual, but essentially you want to sound seeing red or purple in your frame. Yellow can be okay and blue is likely fine, but most of the important content in your frame will be dark grey or green or medium grey or pink or light grey. Skin tones will often be somewhere from green to pink. You’ll get the feeling for what works for you.

This is a very simple view of exposure as there are many other techniques that will also work all the way up to an exposure meter calibrated for your camera. But I love false colour and rely on it heavily.
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rick.lang

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Re: Sweet spot exposure settings for BM URSA 4.6K?

PostWed Jan 16, 2019 4:10 am

Let us know if you need any guidance regarding ND filters. All filters are not alike so you want to be well-informed.


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soroksorok

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Re: Sweet spot exposure settings for BM URSA 4.6K?

PostWed Jan 16, 2019 7:52 am

You should also be really careful with apertures smaller than f4 on most of the lenses. In some cases (like shooting skies/water/walls) you might get ugly magenta/greenish blobs of color in corners. I learned that the hard way while shooting nature and trying to control exposure with aperture instead of ND filters.
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litote

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Re: Sweet spot exposure settings for BM URSA 4.6K?

PostWed Jan 16, 2019 11:22 am

@Rick Lang

Thanks for your reply Rick.

Could you explain a little about what you mean by: “If you are shooting ProRes, you want to expose for the scene.”? Up until now I have only been using zebras to guide exposure, but I will start researching false color exposure.

As for ND filters, I have a Firecrest 82mm ND 0.9 filter. My research indicated this was a good quality filter. I believe the ND 0.9 provides three extra stops to allow me to widen aperture. I have yet to test it, but was advised that a three-stop filter would be a good general purpose ND to start with for shooting outdoors.

BTW, when photographers talk about stopping “up” or “down”, regarding apertures, I am confused. Does “up” one stop refer to increasing the aperture, or decreasing the aperture diameter by one f-stop?
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Re: Sweet spot exposure settings for BM URSA 4.6K?

PostWed Jan 16, 2019 11:56 am

Litote, I use Firecrests and love them on my URSA Mini 4.6K. They also work with ARRI and RED sensors.
I have up to ND1.2 4x4 Firecrest (4-stops) and up to ND1.5 Circular 77mm Firecrest (5-stops) for my camera. Most of the time when using a matte box with 1.2 I also use a Polarizing filter for an extra 2-stops and to increase contrast and get more detail out of the sky and clouds. This means I cut about 6-stops, or equivalent to ND1.8. It all depends on your aim. That’s why it’s best to buy a pack of ND filters with each increasing by one level.


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Re: Sweet spot exposure settings for BM URSA 4.6K?

PostWed Jan 16, 2019 1:24 pm

I use an Ursa Mini Pro 4.6K and have the luxury of using the internal ND filters which are a great feature. Coming from the still film world (a long time ago), I am most comfortable with the histogram from working with Photoshop "levels" for many years. Why? It shows you exposure, clipping (if any) AND the distribution of range. It is the origin of ETTR and ETTL approaches to exposure, and with experience it can not only tell you if you are screwing up, but if your images are hi key (to the right without clipping) or low key. It can be vital when shooting day for night while avoiding excessive noise, and if you are lighting a difficult scene you actually watch the "mountains" move to the right as you add fill. Other than a good calibrated monitor, I think the histogram is the most valuable tool on the finder landscape.

Each tool has it's fans, and to a degree, false color and histogram show the same info in a different format. I like the histogram because I don't have to flip back and forth. It just sits in the corner doing it's job while I can still compose and even during the shot I can watch it with recording on.
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rick.lang

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Re: Sweet spot exposure settings for BM URSA 4.6K?

PostWed Jan 16, 2019 11:57 pm

litote wrote:... Could you explain a little about what you mean by: “If you are shooting ProRes, you want to expose for the scene.”?


When you use a faster aperture (or minimal ND filtration) you let in more light. On a histogram, you’ll typically see the scene shift to the right. The zebras are a hint you may have let in too much light and are clipping. If you use the histogram you can judge clipping by seeing a white vertical bar at the right. You usually don’t want that so you may add a stop of ND or close down the aperture a stop, but most of your scene is on the right of the scale. Often raw is shot ETTR and you pull down the exposure in post as appropriate.

But ProRes is not quite as flexible as raw and so you often expose ProRes so that there’s a good balance in the histogram, not too far left or right. The histogram doesn’t tell you where your skin tones are though and that’s where false colour is so helpful. You can have the histogram indicate your scene is mostly to to the left, but if the false colour shows you have skin tones around green to pink, you know you may have a safe exposure for ProRes. So you’re not shooting ETTR or ETTL, but shooting for the scene where the skin tones fall in the colour you want them to be in that particular scene. Scene 1 you may want skin green and scene 2 you may want skin pink or even light grey. You’ll quickly get used to judging what you want when you expose for the scene.

BTW, when photographers talk about stopping “up” or “down”, regarding apertures, I am confused. Does “up” one stop refer to increasing the aperture, or decreasing the aperture diameter by one f-stop?


Well I hope my explanation Is correct: technically to be more precise, aperture refers to the ratio of the focal length and the opening of the iris or diaphragm so that a 50mm lens with a 25mm maximum opening for the path of light is an f/2 lens. That’s the simple approach used in photography lenses. Cinema lenses Mark their f-stops by measuring the amount of light that is transmitted and hence are more accurate when determining and exposure. That f/2 lens becomes a T2.1 lens allowing for the reflection and absorption of light in the lens, not the simple math.

You open up the iris to use a larger opening for the path of light and a fast aperture like T1.5 and you close down a stop or two to use a smaller opening and a slower aperture like T2.8 or T4. T4 is a one stop smaller aperture than T2.8.


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Uli Plank

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Re: Sweet spot exposure settings for BM URSA 4.6K?

PostThu Jan 17, 2019 2:31 am

If you'd like to see what slower or faster shutter can do aesthetically, have a(nother) look at "Collateral", where most of the night exterior scenes are shot with 1/24th of a second and show some 'smearing' of fast movements. Like the pedestrian bridge scene, where the taxi driver throws the suitcase to the highway.
(That was where I started wondering in cinema: what's going on here?)

Then look at the Omaha Beach scene in "Saving Private Ryan", where very short exposure times are corresponding with the machine gun fire and underline the horror of the scene. Every bit of explosions frozen in motion.

It's all about aesthetical decisions. Well, for Michael Mann it was both, the cameras at the time were not yet sensitive enough and he wanted to shoot available light.
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litote

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Re: Sweet spot exposure settings for BM URSA 4.6K?

PostThu Jan 17, 2019 8:38 am

@ Rick Lang

Thanks Rick, that explains it very well. From what I have since learnt about false colour exposure, it seems to be particularly useful for optimizing skin tones, as you mention, which are of course of the utmost importance to get right in a scene as they will be most noticeable.

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