Disambiguation: FoV & DoF on 4k & 2k
Lens Characteristics and shooting formats.
* UPDATE: (06/22/2009) I have updated this posting with more “scientific” images that do not have so many variables at work. While they are not as pretty, they do show what is going on better then the rose pictures. I have also tried to more clearly convey the over all point. Enjoy!
It has come to my attention after hearing some things said on various sets, as well as reading posts online that there is some confusion out there when it comes to lens characteristics and shooting formats especially when it comes to shooting 4k and 2k on the Red One. For some reason there seems to be the perception that when you switch from shooting in 4k to 2k using the same lens, you automatically get the same results as sooting on a lens that is twice as long. To help provide some clarification on the matter I decided to shoot a series of tests to provide a visual example of what is actually going on when you choose various shooting formats and how that impacts the final image.
**If you get nothing else from this article get this: If you keep the lens the same, and only change shooting formats the only thing that changes is the Field of View, everything else about the lens stays the same.**
But before I can dive into the test, I need to provide some background on some of the characteristics of lenses and what makes them different from one another. While there are many nuanced differences between lenses, for the purpose of this article I’m going to limit them to these three: Field of View (FoV), Depth of Field (DoF), & Perspective Compression (PC).
Definitions: (These are non-technical definitions.)
- FoV = The total area viewed by the lens, or what the lens can see.
- DoF = The total area of the image that is perceived as being in relative focus.
- PC = The size of the individual picture elements and how close together or far apart they seem to be within the given picture. PC is most noticeable when going from a wide angle to a very telephoto lens (18mm VS 80mm). For the purposes of this article 20mm & 40mm focal lengths were used so some of PC is less noticeable.
When it comes to the characteristics of wide angle lenses they exhibit the following: FoV = Wide, DoF = Large, PC = Minimal. A wide angle lens will yield a picture that has a wide view, more of the image will be in relative focus, and the elements in the foreground, mid-ground, and background will appear to be further away from each other and each individual element will be relatively small. (These are just generalities.)
For a telephoto lens the characteristics they exhibit are as follows: FoV = Narrow, DoF = Small, PC = Great. A telephoto lens will yield a picture that has a narrow view, less of the image will be in relative focus, and the elements in the foreground, mid-ground, and background will appear to be closer together to each other and each individual element will be relatively big. (Again, just speaking in generalities.)
And now on to the test* ...
(The images posted here are rather small, to view them in a larger format, you can view the HD 1080p video on vimeo by going here: www.vimeo.com/5236056)
* A quick disclaimer: at the time of this test it was rather windy (as seen in the video) so the rose bush was moving around a lot. I did my best to pull samples from where it was the most still. However, the wind did effect the position of the contents of the frame.
The common misconception that I have heard, and read when it comes to changing in-between 4k and 2k on the Red One while using the same lens is that when going to 2k you just double the lens focal length and that is the lens you are using. For example if I were shooting on a 20mm lens in 4k when I change over to 2k I now have a 40mm lens. This is not entirely true, and here is why ...
First off lets examine our 4k image at 20mm:
(The lens used throughout these tests is the Red 18mm - 50 mm T3 Lens which covers a super 35mm image area.)
The three characteristics of the lens:
- FoV: Covers many flowers and a large portion of the sky.
- DoF: The focus was set on the yellow rose center frame. The dead rose above it appears to be in relative focus. The leaf in front appears to be just slightly out of focus. The yellow roses in the background are out of focus but the details are clearly discernible.
- PC: Each element is spaced apart. There is clear separation between the foreground leaf, the mid-ground yellow rose, and the background yellow roses.
The “Scientific Image”
The three characteristics of the lens:
- FoV: Covers from ~6” to ~3’ 3.5”.
- DoF: The focus was set on the yellow tape measure. The text on the bottle is discernible, but blurry- as are the details on the bunny in the background.
- PC: Each element is spaced apart. There is clear separation between the sheep in the foreground and the background, Each duck is clearly separated as well.
Now keeping everything the same with the lens, Im going to switch over to 2k. What happens when I make this change is the camera is taking a center crop of the 4k sensor as detailed by the following image:
Here is the resulting 2k image:
Upon examining this new image here is what is going on:
- FoV: Covers just three of the yellow roses, and a small portion of the sky.
- DoF: The focus was set on the yellow rose center frame. The dead rose above it appears to be in relative focus. The leaf in front appears to be just slightly out of focus. The yellow roses in the background are out of focus but the details are clearly discernible.
- PC: Each element is spaced apart. There is clear separation between the foreground leaf, the mid-ground yellow rose, and the background yellow roses.
The “Scientific Image”
The resulting 2k image:
The three characteristics of the lens:
- FoV: Covers from ~1’ 2.5” to ~2’ 6.5”.
- DoF: The focus was set on the yellow tape measure. The text on the bottle is discernible, but blurry- as are the details of the bunny in the background.
- PC: Each element is spaced apart. There is clear separation between the sheep in the foreground and the background. Each duck is clearly separated as well.
So what was achieved by going to 2k? The only difference in the image was that in 2k the FoV was decreased. To further verify that this was the only change in the image, here is the full 4k image shown at 100% and then the center was cropped out, and the 2k image:
The 4k Cropped image:
The 2k image (again):
The “Scientific Image”
The 2k image (again):
Now, going back to 4k, the lens will be changed to 40mm:
Here is what is going on in this image:
- FoV: Covers just the three yellow roses and a small section of the sky.
- DoF: The focus was set on the yellow rose center frame. The dead rose above it has now gone soft. The leaf in front appears to be more out of focus. The yellow roses in the background are more out of focus & the details are less discernible.
- PC: Each element is closer together. The separation between the foreground leaf, the mid-ground yellow rose, and the background yellow roses has been flattened / diminished.
The “Scientific Image”
The three characteristics of the lens:
- FoV: Covers from ~1’ 2.25” to ~2’ 7”.
- DoF: The focus was set on the yellow tape measure. The text on the bottle is NOT discernible, nor are the details on the bunny in the background.
- PC: Each element is closer together. The space has been compressed making it feel as if one element is on top of the next.
Here are the two images for direct comparison:
Just for kicks - the same image (40mm) shot at 2k:
The “Scientific Image”
To recap what is going on when you change from 4k to 2k using the same lens: only FoV is being reduced, all other characteristics of the lens remain the same. So if using a 20mm lens in 4k, in 2k the FoV is now roughly 40mm, however, the DoF and the PC still remain the same as the 20mm lens. Unfortunately, switching over to 2k does not give you a “new” set of longer lenses, as you can achieve the same results by recording in 4k and then just taking the center crop of the image.
And now a new question comes to mind “What if I want to shoot in 2k and have all the characteristics of the 40mm lens? (FoV, DoF, & PC)”. If you want the characteristics of a 40mm lens and you want to shoot in 2k then you need to be using a lens designed to cover that format (16mm film lenses).
So are you confused yet? Hopefully not - I hope this exercise has helped clear the air on what is going on when switching between 4k and 2k when using the same lens on the Red One.
Until next time, get out there and shoot!
Cheers,
Ryan E. Walters, Cinematographer