Using low contrast filters for video

In a previous post, I discussed using a Tiffen Low Contrast filter when filming with an AVCHD enabled camera. I didn’t illustrate the point with any of my test footage.

Here it is.

Low contrast filters and video encoders

To recap, we use low contrast filters in AVCHD DSLR video because AVCHD compresses footage using a perceptual filter: what your eye’s can’t perceive gets the chop in the quest for smaller file size. Our eyes cannot see into shadow, so AVCHD ignores (filters out and discards) most of the shadow data. ACVHD knows we can’t see the difference between small variations in color, so it removes such slight differences and replaces them with a single color.

That’s fine if you will not be editing the footage (because your eye will never see the difference), but if you do any post processing involving exposing up the footage, the missing information shows up through macro blocking or color banding. To fix this, we can do one of three things:

  1. Use a low contrast filter. This works by taking ambient light and adding it to shadows, thus lifting the shadows up towards mid-tones and tricking AVCHD into leaving them alone. The low contrast filter thus gives us more information in shadow not because it adds more information in the shadows itself, but because it forces the AVCHD encoder to leave information that the encoder would otherwise discard.
  2. Use Apical Iridix. This goes under different names (e.g. Dynamic Range Optimisation or DRO for Sony and i.Dynamic for Panasonic), but is available on most DSLRs and advanced compacts. It is a digital version of a low contrast filter (its actually a real time tone mapping algorithm). It works by lightening blacks and preserving highlights. Although it again doesn’t add any new information of itself, Iridix is before the AVCHD encoder, so again can force the encoder to leave shadow detail intact.
  3. Use both a low contrast filter and Iridix together.

The video uses the third option.

Deconstructing the video

The low contrast filter allows us to see detail.. even though it is in full shadow

The video consists of three short scenes. They were taken with a Panasonic Lumix LX7, 28Mbit/s AVCHD at 1080p and 50fps (exported from Premiere as 25fps with frame blending), manual-exposure, custom photo style (-1, -1, -1, 0). It was shot hand held and stabilized in post (Adobe Premiere warp stabilizer). The important settings were

  • ISO set to  ISO80 and
  • i.Dynamic set to HIGH.

I chose the lowest ISO so that I could set i.Dynamic high without it causing noise when it it lifts the shadows.

The cameras had a 37mm filter thread attached and a 37-52mm step up ring, on which were attached a Tiffen low contrast filter and Variable Neutral density filter. The reason I did not use two 37mm filters rather than two 52mm filters (i.e. bigger than the lens diameter) is that stacking filters can cause vignette unless you step up as I have done.

Here’s the three scenes. Left side is as-shot, right side is after post processing. Click on the images to see larger versions.

Scene 1
Scene 1

Notice in this scene that the low contrast filter is keeping the blacks lifted. This prevents macro-blocking. Also note that the highlights have a film-like roll off. Again, this is caused by the low contrast filter. The Variable ND filter is also working hard in this scene: the little white disk in the top right is the sun, and it and the sky around it were too bright to look at!

Scene 2
Scene 2

Scene 2 is shot directly at the sun, and you would typically end up with a white sky and properly exposed rocks/tree, or a properly exposed sky and black rocks/tree. The low contrast filter and Iridix (i.Dynamic) give us enough properly exposed sky and shadow to enable us to fix it in post. Nevertheless, we are at the limits of what 28Mbit/s AVCHD can give us. The sky is beginning to macro block, and the branches are showing moiré. I shot it all this way to give us an extreme example, but would more normally shoot with the sun oblique to the camera rather than straight at it.

Scene 3
Scene 3

Scene 3 is a more typical shot. We are in full sun and there is a lot of shadow. The low contrast filter allows us to see detail in the far rock (top right) even though it is in full shadow. It also stops our blacks from clipping, which is important because near-black holds a lot of hidden color. For example, the large shift from straw to lush grass was not done by increasing green saturation in the mid-tones, but in the shadows. If you want to make large color changes, make them in the shadows, because making the same changes in the mid-tones looks far less natural (too vivid). Of course, if we didn’t use a low contrast filter to protect our blacks (and therefore the color they hold) from clipping, we would not have the option to raise shadow colors!

Conclusion

Shooting flat is something you should do if you will be post editing your video footage.  Many cameras do not allow you to shoot flat enough, and to get around this, you can use either a Tiffen Low contrast filter, or the camera’s inbuilt Apical Iridix feature. To maximise the effect, you can use both, as illustrated in this example.

The main advantages of using a low contrast filter are:

  • Protects blacks from clipping, thus preventing shadows from macro-blocking and preserving dark color. The latter is important if you are going to make substantial color correction in post because raising shadow color usually results in much more natural edits.
  • Better highlight roll-off. The effect looks more like film than digital (digital sensors have a hard cut-off rather than a roll-off).
  • Lower contrast that looks like film. Although many people add lots of contrast (i.e. dark, blue  blacks) to their footage, true film actually has very few true blacks. The low contrast film gives this more realistic look.
  • Removes digital sharpness and ‘baked-in’ color’. Many cameras cannot shoot as flat as we would like, and produce footage that is obviously digital because of its sharpness (especially true of the Panasonic GHx cameras). Adding a low contrast filter is useful to mitigate against these issues.

The main disadvantages of using a low contrast filter/Apical Iridix are:

  • The filter loses you about 1/3 stop of light.
  • You usually have to use the low contrast filter along with a variable ND filter (which you need to control exposure). The two filters have associated optical defects apart from their intended function (may cause vignette because you are stacking filters, loss of sharpness). However, remember that you are shooting at a much lower resolution for video, so the sharpness effects will be much smaller than for stills. You can eliminate vignette by using larger filters and a step-up ring.
  • Apical Iridix will increase shadow noise. Use it at maximum only with very low (typically base) ISO.

Notes

To answer a reader query about whether using a loc contrast filter is a viable alternative to using log footage, I have added the following section below.

Comparison of Low contrast filter vs log footage.

The three pictures are frames from three movies shot with (top to bottom) rec709, rec709 plus low contrast 3 filter, and log.

rec709 vs LC3 filter vs log
rec709 vs LC3 filter vs log

All were shot on a camera capable of shooting rec709 and log (a BMPCC), using a high bitrate (Prores HQ which is about 200Mbit/s) so that there are no codec effects to confuse the issue. The lens was a SLR Magic 12mm, set wide open (T1.6, which is about f1.4), no ND.

The rec709 frame is what you would see on all  DSLRs that do not support log.

Once you add the low contrast filter, you see the highlights spread out across the frame so that the darks (lows) and mids (gamma) are lifted, and because the highlights are spread out, they also dim slightly. The effect is however, low, and depends on the light – if there are no bright highlights in your shot (i.e. sky or a bright ambient), the low contrast filter will not work.

Looking at the log frame, you see that all color is subdued and this will always (it does not depend on you having highlights in the shot). There is hardly any green (or in fact, any color) at all. The image also loses a lot more contrast, and although you cannot see it, there is also no sharpening at all.

We can also have a look at this more objectively by taking a look at the YC waveforms;

rec709 vs LC3 filter vs log - YC waveforms
rec709 vs LC3 filter vs log – YC waveforms

The YC waveform shows you the brightness of the image left to right (why video enabled cameras don’t give it as an option over the photographer’s histogram I don’t know; the YC is much more useful because it shows you not only that your image is clipping, but where it is happening!).

You can see here that the filter has a slight effect in reducing contrast (the blacks are lifted slightly and the lights are dropped slightly), but the log footage has a much more pronounced effect. Height of the YC waveform represents contrast, and you can see immedatly that log footage has much less contrast. Although this results in very subdued looking footage, it actually enables you to push your footage harder in post without it breaking up, and also gives you a much more neutral starting point for your post-work, preventing your footage looking ‘similar to everyone else with the same camera’.

So, although a low contrast filter does reduce contrast by lifting the blacks and dropping the lights, it does not have as large an effect as log footage.

Log footage also gives you other things that make your footage more gradable in post, such as;

  • Reduces color saturation considerably, allowing you to boost color farther in post without it breaking up (banding).
  • Removes all color styling, allowing you more creative options to create your own styles to match your production.
  • Gives you access to better and more flexible third party look up tables (LUTs)
  • Removes all in-camera sharpening and other post production (and if you are shooting raw it also removes the effects of color temperature; color temperature is only saved as metadata and not as something that permanently affects your footage data). The lack of sharpening allows you to make better selections, and add superior sharpening in post (a desktop generally has better sharpening algorithms than your camera).

And finally,

  • allows you to learn proper grading, because you are no longer relying on a ‘nearly there’ rec709 output, but instead start with neutral log footage.

I quickly edited the log footage towards a final look. Here’s where I got to after a couple of minutes with Première and Colorista II;

The edited log image
The edited log image

Here’s what I did;

  • Selected the background and reduced its clarity and made it colder (this is to visually separate it from the plant in the foreground and make the background less busy). Having unsharpened footage enabled me to quickly make the selection to do this (bear in mind that this is a selection on a moving image; sharpening gives you halos and all sorts of trouble as sharpened edges move!).
  • Boosted the greens and yellows of the plant, overall exposure, and highlights.
  • Sharpened the plant by increasing its clarity slightly.

Note that using log footage does increase your effective dynamic range but doesn’t stop you clipping; I did not use a variable ND filter in this test, and that caused clipping in all the footage! However, you can see that the dynamic range of the final edited image looks better than the rec709 footage you would get out of camera, and rec709 footage probably could not be pushed as far as the image above without causing banding on the deep greens and/or a nasty transition at the boosted leaf highlights.

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