camera Blackmagic Cinema Camera 2.5K

sammy81z

Member
Hi all, I have 3 questions:

1. Regarding the Blackmagic Cinema Camera 2.5K, I think the crop factor on the sensor is about 2.3x. I read somewhere that because this camera’s sensor is small, it is very hard to get a cinematic shallow depth of field. This confuses me because I read that long lenses and therefore long focal lengths produce shallow depth of field and the longer the lens, the shallower the depth of field. So if a 35mm lens is placed on the BMCC, it will therefore be an 80mm lens. Therefore, would this not produce a shallow cinematic depth of field? Also, a 50mm lens will therefore become a 115mm lens, making the depth of field even shallower! Then, why do people still say that this particular camera is very difficult to produce a shallow cinematic depth of field? Have you used this camera for yourself to be the judge?

2. Between the Blackmagic Cinema Camera 2.5K and the Blackmagic Production Camera 4K, apart from the difference in resolution, price, shutter type and media storage etc. which camera would you choose? Do you think the Blackmagic Cinema Camera 2.5K is great for making feature films that can be played at actual cinemas and look as if they were shot on an Arri Alexa?

3. I have heard of 1080p HD films that were blown up to 2K and 4K etc. without loss of quality. Can you comment on this? Do you think the Blackmagic Cinema Camera 2.5K can blow up a 2K master successfully to 4K without losing quality? I am asking this in the event one wants to meet the Netflix 4K requirements for movie submission.

Thanks!
 

IvonV

Member
On point # 1... a 35mm lens is a 35mm lens no matter the sensor size of the camera it's being used on. Crop factor is a specific instance of field of view that assumes the 135mm still camera format is somehow the standard frame size. So, crop factor is a ratio of the field of view seen by one sensor as compared to a 135mm sensor (often referred to as "full frame"). The physics of a 35mm lens don't change because of the sensor under it. That said, sensor size IS one of the inputs to Depth of Field (DOF). You'll find that for a given lens focal length, distance to subject and aperture, the smaller the sensor the deeper the DOF. So, all other things being equal, a Blackmagic sensor that is 15.8mm x 8.8mm will have a deeper depth of field than a sensor that is 23.76mm x 17.82mm.
 

jax_rox

Moderator
Staff member
Moderator
sensor size IS one of the inputs to Depth of Field (DOF)
Sort of. Sensor size only really affects depth of field due to the need to use different lenses. The larger a sensor is, the longer lens you need to use to match field of view. Consider a 25mm lens on a standard S35 camera. To get the same FOV on S16, you'd need a lens ~12.5mm. On 65mm film, or an Alexa LF, you'd need something around ~50mm. So suddenly, to get a close-up that would match what a 100mm lens nets you on S35, you need a ~150mm lens at the same distance.

An IMAX camera would need ~300mm lens at the same distance.
 

AidenJames

Business Member
indieBIZ
I've been waiting almost an entire month to reply to this post. Both of the above are good answers, regarding your first point. Here's a full on article I found regarding the issue:

Effectively the way to think about it is the lens is projecting the exact same image regardless of the sensor size back on to the sensor plane. It's just that since the sensor is smaller it is only capturing the center portion of that projected image. Thus you are effectively getting a field of view that matches a 115mm (as in your example) with a 50mm, but nearly every other aspect of the lens' qualities will still be that of the 50mm.

Regarding point three, when you say "blow up" people may be referring to film. Due to the development process of film and the creation of film prints, the effect resolution of a film print that is shown in theaters is usually little better than 720p. You'll be hard pressed to actually maintain detail and quality if you're increasing the resolution using some extrapolation algorithm. There's other ways to do it but it's really of little value, as all you're effectively doing is dividing your existing pixels into four smaller pixels of the same color. From what I know Netflix's 4k requirements are kind of like Discovery's requirements, that only certify certain cameras, not just resolution, codec or format.

BTW, 1080p is effectively 2k.... 2k is typically 2048 x 1080 (sometimes 1998 x 1080), 1080p is 1920 x 1080.


Having said all the above, have you considered the Pocket Cinema 4k instead?
 
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jax_rox

Moderator
Staff member
Moderator
As for your other questions:
Between the Blackmagic Cinema Camera 2.5K and the Blackmagic Production Camera 4K, apart from the difference in resolution, price, shutter type and media storage etc. which camera would you choose?
All of the things you’ve mentioned are what separates the two cameras. The only thing left, really, is sensor size. You’re asking ‘which sensor size do you prefer’ and the answer, as it often is, is ‘it depends’.
Why are you asking the question? What do you want to make and what budget do you have?

Do you think the Blackmagic Cinema Camera 2.5K is great for making feature films that can be played at actual cinemas and look as if they were shot on an Arri Alexa?
Any camera is ‘great’ for making feature films. The only way to ensure something looks like it was shot on an Alexa is to actually shoot it on an Alexa.

I have heard of 1080p HD films that were blown up to 2K and 4K etc. without loss of quality. Can you comment on this? Do you think the Blackmagic Cinema Camera 2.5K can blow up a 2K master successfully to 4K without losing quality? I am asking this in the event one wants to meet the Netflix 4K requirements for movie submission
Depends what you mean by ‘loss of quality’. Most cinemas still project at 2K, though there are plenty these days that do project at 4K.
Ultimately, when it comes to cinema, viewing distance is also a big factor as to how something will look given an under-sized master.

As for Netflix, their 4K requirements are for their originals productions, and they have select cameras that they qualify for use on their productions. If you’re not shooting a Netflix original commissioned by Netflix, the resolution requirement doesn’t matter. Plenty of series and films that Netflix have bought (not commissioned) have not been 4K.
 

IvonV

Member
Sort of. Sensor size only really affects depth of field due to the need to use different lenses. The larger a sensor is, the longer lens you need to use to match field of view. Consider a 25mm lens on a standard S35 camera. To get the same FOV on S16, you'd need a lens ~12.5mm. On 65mm film, or an Alexa LF, you'd need something around ~50mm. So suddenly, to get a close-up that would match what a 100mm lens nets you on S35, you need a ~150mm lens at the same distance.

An IMAX camera would need ~300mm lens at the same distance.
I wasn't talking about changing the lens to match field of view. Same lens, same focus distance, same aperture, two different sized sensors will yield two different depths of field. Try it on your favorite DOF calculator. From PCAM:

camerasensor sizefocal lengthfocus distanceaperture
DOF​
Sony NEX-FS70023.62mm x 12.767mm35mm10'f/2
1' 8.4"​
Blackmagic Pocket Cinema 4K25.34mm x 14.25mm35mm10'f/2
1' 7.8"​
 
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onebaldman

Member
Wouldn't you want less DOF for film? Everything I see that looks cinematic only blurs the background out a little, while focusing on the character in the foreground.

The deeper bokeh, the more it looks like a DSLR in my opinion.

Is there a good resource on what makes a cinematic scene look good, or how much focus is too much?
 

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