Freitag, 12. Dezember 2014

Physicist's everyday

Inspired by my study of physics and enriched by a touch of self-irony the idea of this little comic strip crossed my mind :)

In English: Tidiness is for the dumb ones because the genius DOMINATES OVER THE CHAOS!!!

Dienstag, 2. Dezember 2014

RJ Lens Turbo Review

Probably most of you are familiar with the principle of Focal Reducers. If you are not, you might find this short explanation very inspiring.
Whilst the first product of this kind, the metabones speed booster, conmes up with a rather high price tag, there are already some lower-end alternatives and I have decided to go for one of those myself. The product I have picked in the end is the Speed Booster / Lens Turbo from RJ because you can find many positive reviews and it offers two special features: The
first is an aperture control ring for Nikon G lenses which is essential if you are going to use modern lenses. The second thing is the tripod mount shell which comes with the adapter and is important if heavy lenses will be used.

Those facts imply that the RJ should be a good pick, alright? So let us go a bit further and take a look at the actual performance and build quality of the adapter!

1. Build quality
The Adapter consists of a massive metal body that feels well made. There are some reports about other products revealing a misaligned flange distance as soon as you try to focus to infinity which is a major problem and renders the adapter unusable. 
But the RJ is different: As soon as a little screw, found at the side of the mount, is loosened, you are able to align the optical unit and adjust the flange distance.

There is one important thing to mention: The aperture is controlled mechanically by a little stick which is connected to the aperture Ring. The length of that stick is also adjustable with a little screw and you should alway check that length before you mount a new lens to the adapter. I did not when I mounted my Tamron 17-50mm lens and unfortunately the stick's protrusion was so large that it scratched the inner mount of my lens!

This inconspicuous pin turned out to be nasty!
Turn this to keep the heck in check!
Mounting the camera and lens on the adapter turned out to work very well: The RJ mounts very precisely with little play.

2. Image Quality
After we have seen that the adapter is well made, it is time to face the most important aspect: The image quality.
You can find many sample images from the adapter in the internet and examining them will most probably lead you to an impression that correlates with my findings: The adapter produces quite sharp images even at wide apertures. Yet, at higher aperture values there will appear a special kind of lens flare in the shape of a blue spot in the center of the image if there is a light source in front of the camera. This is not an issue in real life if you are not a landscape photographer who is dependent on small apertures, but should be mentioned. There is also a slight green cast introduced by the adapter which does not pose difficulties also, because it may be corrected in post very quickly and even on many cameras on set.

Yet, while the most Sample Images tend to look very promising, tack sharp from corner to corner, I have encountered serious edge softness with my copy on Micro Four Thirds cameras. Though the image center appears really sharp, there is a noticable decease of resolution in the corners, coming along with strong vignetting and distortion!

This issue is most present on short focal lengths with my Tamron 17-55mm, but still a bit noticeable with longer lenses.
After I had written to the RJ support regarding the Image quality, they recommended to try the flange distance adjustment. Unfortuntely I was unable to achieve an improvement by doing so and in consequence I decided to write an other email to ask about the fact that the most images in the web appear to be flawless and get an explanation if either I could have gotten a bad copy or there is a general problem with the quality of their adapters.
I have written to them two times and haven't gotten an answer in over one month. I guess that it is up to you to decide about the integrity of that company for yourself.
As you see on those samples, the adapter is unusable for serious use on mft cameras for which it is constructed and advertised.

So is it all bad in the end? Fortunately not because the adapter DOES produce a really decent image in the center and I found out that my Black Magic Pocket Camera, thanks to it's Super16mm Sensor, does only cover the sharper center of the Image so that the RJ Speed Booster turned out as a superb match for that camera! My Tamron gains one full stop of light and an usable amount of wide angle on the BMPCC. That is the reason why I am still satisfied with that product in a certain way.

But would I buy the RJ again or even recommend it to somebody?
No, I wouldn't because I feel cheated due to the lack of optical quality and the "unreliable" support. In the end this experience implies the conclusion that it might be worth spending some more money to get a reliable product from a more trustworthy company.

By the way:
As you can read in the right column of my blog, if you would like to say "thank you" for the hopefully useful posts, you can easily do this by making your next Amazon purchase using the links provided in the "Note" box. It won't cost you anything more, but helps me keeping this blog running! Thanks! :)

Dienstag, 12. August 2014

Better video stabilization in Blender


Blender has got a great tracking tool, which can also be used to compensate unwanted camera movements. A common use case is represented by the intention to make a shaky handheld video look stable.

A very good introduction to the basics of Blender's stabilizing features is found here.

Though if you try to deshake a video you will encounter the Problem that the algorithm compensates all camera movement. This is fine if you are going to stabilize a steady shot like it is shown in the tutorial above, but in most cases there is some intended movement like a camera pan that should be preserved while annoying jittering needs to be removed.

To make this kind of selective video stabilization possible I have written a small addon which applies a rudimentary highpass filter to a selected tracking marker. After the filter is applied, only the fast, jittering moves are represented by the marker so that you can use it to get much better stabilization results!

You can download it on BlenderArtists and I have a quick example comparing the standard and highpass stabilization for you:

Please notice that the wobbliness is caused by the rolling shutter of my camera und thus not introduced by Blender ;-)

Using the addon is very simple:
Find a good feature and track it, just like in the tutorial.
When you have done this, select the marker and use the addon whose UI is located on the bottom of the Clip Editor's tool panel.


The numeric value represents the range of frames over which the motion is smoothed. A higher value makes slower motions being stabilized, too. Usually a value between 5 and 15 gives good results.
The Apply operator filters the selected marker in the scene's frame range (the start and end frame that are found in the timeline window's header).
Please make sure that the marker is tracked/keyframed for every frame in that range; otherwise the addon will fail.

Now a second trackpoint is created. Simply use it for 2d stabilization instead of the original track.
You can also repeat the procedure for a second track point and use it for rotation stabilization!

That's it. Have fun!

Montag, 23. Juni 2014

Skin Softening

In the last Blender tip I stated that I would write something about frequency extraction. Here is one useful technique which depends on that.

Although I am not a fan of skin retouching in general because I believe a human's natural and individual appearance is most appealing and candid, I am sure that some of you might like to soften an actress' or actor's skin sometimes. The main difficulty doing this is that you want to obtain a smooth looking skin without making it look like wax and to get an idea how achieve this, it is useful to understand what we are really trying to do here:

Do we really want to Soften the whole skin, as the term softening refers to removing every detail that is smaller than the user-specified blur radius?
No, we don't. We want the skin to look gentle and clean, not smooth! So it is not our Intention to remove every feature, but only the ones that are annoying, while keeping the detail which make the skin look natural.
Here we can benefit from the fact that the bad features like little flecks are bigger than the skinpores that should be kept - which leads us to frequency extraction.

We want to create a little node setup that blurs the image while preserving the edges, but keeps the small scaled details. This is surprisingly simple:

Let us go through this step by step:
As described in an earlier post, we use the two Gamma nodes to work in a gamma corrected color space. After the Gamma node, the tree is split into two branches: The lower one blurs the the image but keeps the edges. Using the Color Sig value we can control which details will be blurred and the Space Sig controls the blur amount. This branch alone creates this:

To recover the little details, the upper branch compares the raw image with a version that is less blurred than the one from the lower branch by subtraction. The output is an Image containing only the features that were blurred and the Space and Color Sig values enable us to control which details will be there. In the end, the two branches are added up and we get the final result.
I have exaggerated the rather subtle effect a bit to show it more clearly.

 Be careful: Because we have removed a whole layer of details, if the image gets rescaled to a size where the recovered details are lost, there is only the blurred surface left. So this technique is mainly suitable where the result will not be scaled in an unregulated way!

Now, of course it makes sense to apply this nodeset selectively to skintones using color keying. Have fun!