The roles of a lens and sensor in compositing

Here is a 10K photo I took a week ago on my 5d mark 3 and Isco anamorphic lens.

the_port_photo

Here is 370% crop of that photo.

final_comp0001

What is wrong with this crop? What about the next crop below?

start_comp0001

Did the stop sign grab your attention? Sure it did. The element doesn’t look like a part of the photo. Actually, it is a vector element which I placed on top of the photo. So what is an actual difference between the first image crop and the second one? What does it take to properly integrate an element into an image?

It is not a secret that the main ingredients of  good CGI integration are lighting and fine-details. In the world of VFX physically accurate lights, materials and HDRIs are becoming a guaranty of the realism. However a good 3d model, lighting and textures make only 70% of the realistic integration. Then what are the rest 30%?

I intentionally chose the stop sign. It is a screaming and attention-getting element. Moreover, it has no lighting, no textures and it doesn’t even have a 3d shape. It is just a flat, vector image. However throwing in the last 30% of magic would integrate even such screaming element as the Stop sign.

So what are the final steps in making the integration realistic? The answer is simple: It is everything which has to do with a lens and camera which were used for image acquisition.

No matter how realistic the CGI element is, it wouldn’t  feel real if it wasn’t projected through a lens on a camera sensor.  The main differences between the crop images above is that the very top crop image was processed as if the stop sign was photographed and not drawn in a vector app.

Every lens and camera sensor has their own characteristics and imperfections. By studying these details and applying them on the CGI elements we would further increase the level of integration with the photo.

So in order to integrate the stop sign, I studied my equipment which I used for that photo. I took a picture of a text book to observe the lens optical artifacts such as chromatic aberration, fringing and focus rendition.

text_crop

Sorry. The jpeg compression killed all the CA details.

Due to the fact that my lens is a retro lens it has a lot of optical artifacts. I needed to study them and simulate these imperfections on the stop sign image as if it was photographed through the same lens. I didn’t have a resolution chart so I used a Renderman book which was a nice alternative. By looking at the text on the page I could observe the lens CA and fringing. The left page was slightly out of focus so it helped me to understand how the lens rendered the out of focus regions.

The second thing I’ve done, I took a photo with the lens cap on. I got a pitch black image. I deliberately set high ISO to capture the sensor noise. I pushed the noise signal five stops over to simulate the development process I used on the seaport photo.

noise_sample

I like the noise coming from the 5d. It doesn’t have a strong digital pattern. With some post processing it may arguably look like film grain.

I had the noise pattern which I could overlay on the stop sign image as if it was captured with 5d mark III FF sensor.

Now when we combine all of the elements and apply them on the stop sign we will get this.

sign progress

The next step is color match the images. Never completely  trust your eyes! The recent event with the blue or gold dress is a good example of how our vision can play a joke on us. Throughout years I spent in post production and dark rooms I got into the good habit of using all kinds of scopes which measure video signals. In order to precisely match the images we need a waveform and vectorscope monitors. Gladly the compositing packages like Fusion and Nuke provide these tool out of the box.

value_reading

I took the lifebuoy as a color reference. Using the waveform I matched the luminance values by pushing and pulling the gain and gamma of the image. Then when it came to the color adjustments I monitored a vectorscope to match the saturation level and the hue of reds and whites.

The blue dress examined with the vectorscope.

blue-dress

Judging by the vectorscope the blue dress has blue and yellow colors. The colors are equally saturated and perfectly complementary pointing at exactly the opposite directions of the hue wheel.

Ok, back to the seaport photo. The adjustments I’ve made were subtle. But this is where all the magic takes place. I could work on the stop sign and add dirt, rust and erosion, but my goal was to study the effects of the lens and camera sensor on the composit. Even though lens manufacturers try to reduce the optical artifacts of their products, these tiny imperfections add life and character to images. Use scopes, make pictures, be happy!

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