Part 3.1: Capture Format – What is it?

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Image courtesy of B&H.com

After choosing your optical process (Spherical or Anamorphic) and capture medium (Film or Digital), the next important decision to be made is your Capture Format.

Capture format refers to the physical size and shape (aspect ratio) of the film frame or digital sensor. Capture format is independent of capture medium. As the graphic above illustrates, some formats, have celluloid and digital versions.

Some common formats (a more detailed discussion in the following part) include Super35/APSC, Super16, Full Frame, & Micro Four Thirds.


What are the key factors when considering Capture Format?

What are some considerations when comparing different Capture Formats? Let’s discuss few.

Field of View (FOV)    The physical size of an imaging area has a direct relation to the field of view. Think of it this way, if your standing looking out a window, the larger the window, the more you would see.

Similarly, a larger imaging area results in a wider FOV, while a smaller imaging area results in a narrower FOV. Below are some relative comparisons between the FOV of various formats (all from same position and using same lens). 

Screen Shot 2016-08-05 at 10.51.17 AM.png
Super16 compared to Super35, both with a 18mm lens. (Image courtesy of AbelCine.)
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Super35 compared to Full Frame/Vista Vision, both with 18mm lens.                                     (Image courtesy of AbelCine.)

Lens Image Circle    Another factor to consider is Lens Image Circle. What’s the lens image circle? Simply put, it is the area of an image a lens can project (often expressed as the diameter of a circle in mm). With regards Capture Format, the lens image circle has to of sufficient size to cover the size of the capture format.

The image below is an illustration of the image circle of a Zeiss CP.2 lens which has an image circle of 43mm. With that large of an image circle, it can cover formats up to Full Frame 35mm (which has a min diagonal of approx. 43mm ). As such, it can also cover all formats smaller than Full Frame 35mm including Super 16mm, 3-Perf/Super35, and 4-Perf 35mm.

(Resource: http://www.hdvideopro.com/columns/help-desk/sensors-circles/)

Screen Shot 2016-08-05 at 2.22.59 PM

When a Lens Image Circle does not properly cover a format, the image will suffer from  “mechanical vignetting”, which is essentially a the lens blocking the view of the sensor. In the example below, you see the lens image circle is not sufficient to cover the sensor.

Screen Shot 2016-08-05 at 11.30.38 AM.png
A lens with an image circle of approx. 16mm on a M4/3 sensor (21.6mm min image circle).

Here is a database compiled by Matt Duclos of Duclos Lenses that lists popular lenses and their image circles and popular camera sensors with the minimum image circle required to cover them: cameramarket_image-circle-database.pdf.

Aspect Ratio    If you recall, the matter of aspect ratio was initially discussed in my post 0n Optical Process. As a recap, there are two aspect ratios to be concerned with. The first being the delivery ratio (what the audience will see) and second, the capture ratio. What are the considerations when looking at capture ratio?

Optical Format    The choice of optical process can impact one’s choice capture ratio. For example, to fully benefit from a 2.0x anamorphic lens, the ideal capture format should have a ratio of 1.33 /4:3 (4-Perf 35mm, ARRI Alexa 4:3, and Panasonic GH4 4:3 all have a native ratio of 1.33). 

[Update 01.19.17: Here is nice article discussing formats and anamorphic photography – http://www.hdvideopro.com/columns/help-desk/go-wide/]

Flexibility    There may be times when capturing on a different ratio than the one intended for delivery can have benefits. For example, capturing on a 1.33/4:3 format, on spherical lenses, when delivering a 1.78/16:9 has the added benefit of extra space above and below the frame (provided the 1.33 format is larger then the 1.78 format), which can be very beneficial when doing VFX where tracking markers are required but need to stay outside of the frame (tracking markers can later be cropped out).

Creative Choice    We have typically become accustomed to 1.78/16:9 and 2.35 ratio when watching tv and movies. However, these are not necessarily “walls that can never be broken”. Much like anamorphic format was initially designed to address a technical dilemma but is now more a creative decision, the matter of aspect ratio is as much creative as it is technical. (Resource: http://www.theasc.com/site/blog/thefilmbook/the-elastic-frame/) 

Resolution    Generally speaking, the larger the format, the higher the  resolution.  This can easier seen when comparing digital formats. For example, if you look across the ARRI line of digital cameras (same also true for RED Weapon sensor), you’ll note an increase in resolution equates to a larger format/imaging area. 

Screen Shot 2016-08-10 at 1.16.24 PM.png
ARRI ALEXA/AMIRA Format vs Resolution Chart.
phfx_REDDSMC2BRAINS_formatsAndFilm
RED Dragon Format vs Resolution Chart.

That being said, as a caution, format is not always associated with resolution. Similar formats can have very different resolutions. For example, the newly released RED Weapon HELIUM Sensor is a Super35 format with a resolution of 8k while the “standard” Weapon sensor is also 8k but is a larger Vista Vision/Full Frame format. (Resource: http://thecinelens.com/2012/05/23/sensor-size-vs-sensor-resolution/)

[Update 8/16/16: I just realized example above was a little confusing. Point being made is that format and increase in resolution is not absolute. Perhaps a clearer example is comparing the resolution of the SONY F3 and that of the SONY FS7. Both are Super35 sensors and yet boast different resolutions, F3/HD and the FS7/4k.]


Depth of Field and Crop Factor

I choose to separate these two factors separately. Why? I feel they are the most often misunderstood factors when discussing formats.

Let’s discuss Depth of Field (DOF) first. Some associate DOF with format. However, technically speaking, “Capture format has no impact on DOF. DOF is a function of optics, not capture device.” In other words, a lens will have the same DOF no matter what format it is paired with. 

So why then is DOF often part of the discussion when choosing format? Looking again at the examples above, what is the first thing you notice? The difference in framing or field of view!

Screen Shot 2016-08-05 at 10.51.17 AM.png

Now let’s say you wanted to keep the same framing across the two formats. What would you need to do? You can either 1) change focal length or 2) move position of camera. Both of these actions can impact DOF as wells as other factors (perspective for one if moving camera). How?

The three factors that effect DOF are 1) lens aperture, 2) lens focal length, and 3) Focus distance. Assuming the you DO NOT change aperture, what happens if you move the camera?

If you were to move the camera closer, you change the focus distance resulting in less DOF, conversely, moving the camera away would increase DOF. What happens if you change focal length?

To match the FOV of a smaller format, you need to use a longer focal length, which results in shallower DOF. To match the FOV of a wider format, you would need to use a wider lens, which results in a deeper DOF.

[Note: I fully intended to conduct camera tests to illustrate points above but ran a lil’ short on time. As soon as I can, I will update this post with camera tests. So, please stay tuned!]

To be fair, making those changes are predicated by the difference in Format. Yet, as you can see, what ultimately affects DOF is changes in optics not format.

“DOF is a function of optics, not capture device.”

Now let’s discuss Crop Factor. Firstly, I hate that term. No, not the technical meaning  – ratio of the dimensions of a camera’s imaging area compared to a reference format.  “That’s actually useful!”  

What I hate is the emotional attachment some place on Crop Factor. For example you may often hear, “I don’t like that because it has a crop sensor. This is better cause it has no crop!”. When I hear that, I ask:

“What is your reference?”    If you love movies, then what your accustomed to seeing is  3-perf/Super35/APSC and not Full Frame.

“Better how?”    Better Color, Dynamic Range, better Skin Tones? Or is it just ‘bigger’?

“Crop Factor is simply a mathematical expression to show the difference between two formats and is NOT a qualifier for quality!”

So, how is Crop Factor useful? Knowing the Crop Factor between formats is helpful when filming simultaneously on different formats. Looking at the image below, you’ll see that a M4/3 format has a crop factor of 2x when compared to a Vista Vision/Full Frame format. To match the FOV of the Vista Vision/Full  Frame format, you would need to use a lens that is twice as wide on the M4/3 format and conversely, to match the M4/3, you would need to use a lens that is double to focal length on a Full Frame format.

Screen Shot 2016-08-05 at 2.28.54 PM.png

Here are some additional resources on the topics discussed above:

Practical Optics – Testing Different Sensor Sizes

Practical Optics – Notes on Lenses & Elastic Formats

http://petapixel.com/2016/05/18/depth-field-explained/


Take Away Points:

Here are our take away points…

  • Capture format refers to the physical size and shape (Aspect Ratio) of a film frame or digital sensor.
  • A larger format will have a wider Field of View and will require a larger lens image circle.
  • Choice of Capture Format aspect ratio can be affected by choice of technical process (Optical Process and Cropping) and creative choice.
  • A larger format typically results in more resolution but may not be true in every case. 
  • While depth of field technically has no direct relation to format, larger formats often exhibit shallower DOF because longer lenses are needed to match the FOV of smaller formats. 
  • Crop factor is a simply a mathematical ratio used to compare two formats and is not a quantifier for quality. 

In the next part, “Part 3.2: Capture Format – Choosing a Format”, we’ll discuss the most commonly available formats and considerations when choosing a format.

Post No.13

 

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