A little while back I took the opportunity to run some tests during the CML camera assessments at UWE Bristol with the Sony PMW-F55 and both the 4K and 2K Optical Low Pass Filters. The results were largely as expected, but I didn’t have the time until now to share those results, so here they are.
I shot two different series of test shots. The first series are of a resolution test chart, the second set of the model that was on set, in order to asses the impact on skin tones etc. It should be noted that the resolution chart did not have patterns that can show 4K resolution, really what I was looking for was aliasing and other image undesirable artefacts.
What’s the 2K OLPF filter for?
When the PMW-F5 and PMW-F55 are shooting at high speeds above 70fps the camera sensor is read as a 2K sensor instead of the normal 4K. There are two ways that this can occur. 2K Full Sensor Scan and Center Crop 2K.
Full Scan 2K uses the entire sensor but now the sensor is read as a 2K sensor instead of a 4K sensor. The camera is fitted with a 4K optical low pass filter (OLPF) at the factory. However when shooting using 2K full scan the 4K filter is ineffective and needs to be replaced with a 2K OLPF. Fortunately Sony made the OLPF on the F5/F55 interchangeable and an optional 2K filter can be purchased from Sony. It takes just a couple of minutes to swap the filters.
The 2K OLPF is also required when shooting in 2K raw when the Sensor Scan in the cameras “Base Settings” is set to 2K Full Frame (2KFF).
Another possible application for the 2K OLPF is to soften the pictures a bit when shooting in 4K. If you find the 4K images with the 4K OLPF too sharp you can use the 2K OLPF to provide a softer, creamier look. Unlike a soft effect filter in front of the camera lens a filter behind the lens is not effected by changes in aperture of focal length, so the results are highly consistent.
On to the tests.
Click on any of the images to enlarge them and see the full size 4K images. NOTE THAT YOU MAY SEE ALIASING THAT ISN”T IN THE TRUE IMAGE WHEN VIEWING A SCALED IMAGE ON YOUR COMPUTER SCREEN, SO PLEASE VIEW WITH 1:1 PIXELS.
The first test was to see what happens when you shoot using 2K Full Frame with the standard 4K filter. Really there is no surprise here. Because the factory fitted 4K OLPF is ineffective at 2K you will get a lot of aliasing and moiré as can be seen by the rainbow patterns on the test chart.
So what happens if you swap the 4K OLPF for the 2K OLPF while shooting in 2K Full Frame? Well all the unwanted aliasing simply goes away and you have a nice artefact free image.
So it’s easy enough to see that if you want to shoot 2K Full Frame, whether for slow motion or for 2K raw you really do need the 2K OLPF option.
But what about when shooting 4K Full Frame, how does the OLPF effect the sharpness of the image. Below are both the full frame, frame grabs plus a couple of crops of the girl so you can see how skin tones are effected.
Notice how the shot done with the 2K OLPF is noticeably softer, the texture in her hair is softer, her skin looks smoother, yet the difference between the two images is not really that great. The 2K OLPF does not excessively blur the image, but it does give it a pleasing softness. This could be beneficial for cosmetics or fashion shoots, dream sequences etc. However it is also possible to soften footage in post production to produce a similar effect. Below are the full frame originals if you want to see the entire shot.
One thing that you definitely don’t want to do is to accidentally use the 2K OLPF when you are shooting in any of the center scan modes. Remember that in center scan mode the sensor is still in effect a 4K sensor, just now you are only reading out a smaller 2K section from the center of the 4K sensor. As a result the 4K OLPF is still optimum. Below are some further crops from the whole image just shown the center of the test chart. Starting with 4K full frame scan using the 4K OLPF, which is beautifully sharp and clear.
Next is with the 4K OLPF in 2K crop mode. The lens was changed to give a similar field of view.
It’s not really that surprising that the image is a little softer, the first image is part of a 4K image while this is from a 2K image, so it is lower resolution. There is some coloured moiré in this image, probably a result of changing the the lens, perhaps this lens is slightly sharper than the original lens so has greater resolving power. It is almost impossible to entirely eliminate coloured moiré with a bayer sensor and a chart like this will show it up, it’s a very tough test for a sensor. But compare this to a similar section of the shot done with the 4K OLPF with the camera shooting in 2K full frame scan mode and you can see that actually the moiré isn’t actually all that bad. In most real world applications you are not going to see the aliasing above unless you have a very very fine, in focus, repeating pattern similar to the one on the chart:
But what about using the 2K OLPF in 2K center scan mode, well take a look at how soft the image is. It just looks completely blurred and soft.
You really don’t want to shoot like this your pictures will look very soft indeed.
So there you have it. The 2K OLPF really is needed if you want to avoid heavy aliasing and moiré when shooting in any of the 2K Full Scan modes. You can also use it if you want to soften your 4K images a little for a smoother creamier look. But you definately don’t want to use the 2K OLPF in combination with any of the sensor center scan modes.