Technical

Andy Hutchinson is a no-nonsense photography YouTuber from Australia. A few days ago he talked about some standalone tools that do a better job than tools built into post-processing programs like Photoshop and Lightroom.

One of the tools he described is DxO Pure RAW, a demosaicing and noise reduction tool.

What is Demosaicing

Digital camera sensors don’t know what colour light is. To the sensor, it is simply more light or less light. So sensors have a colour filter array that sites over the sensor’s pixels. The array is a mosaic of red, green, and blue colour filters.

When you want to process a RAW image on your computer, the program has to ‘read’ the raw data in the picture, using a demosaicing engine.

Programs like Photoshop and Lightroom have demosaicing engines built into them.

Some other programs use the demosaicing engine built into the operating system of your computer.

As Andy Hutchinson describes it, DxO’s approach to decoding RAW digital sensor information is to train their machine learning model to recognise real-world noise patterns and to differentiate between genuine image features and unwanted artifacts.

At the same time, the software runs DxO’s denoising algorithm on the image data.

Because they do the denoising at the same time as demosaicing, you get a purer and cleaner image than you would if you ran the image through a denoising engine after demosaicing.

There is an added advantage to using DxO Pure RAW if you use Fujifilm cameras.

Fujifilm took their own route to the construction of the colour filter array over the sensor. The result is that programs like Photoshop and Lightroom have more difficulty demosaicing the RAW images than with cameras that use a BAYER colour filter array (almost all other camera brands).

The benefit in using Pure Raw is that its end product is a DNG RAW file rather than a RAF file. So if you then want to use Photoshop or Lightroom on the DNG file, all the difficult bit has already been done by Pure Raw.

Plus, Pure RAW also includes a built in lens softness compensation feature and also corrects for lens vignetting, chromatic aberration and distortion.

Quite a mouthful. 

Does it work?

I downloaded a trial version of DxO Pure Raw and processed a Fuji RAW file with it. I processed the same image with Photoshop and then compared the two images. 

What you are looking at is a photo processed with DxO Pure Raw with part of the same image processed in Photoshop overlaid on it.

Compare the two – click twice on the photo below to blow it up to see the detail.

Let’s start with mechanical focus systems. They have a focus ring that is directly coupled to and moves the lens elements.

Focus by wire uses electronic signals to control focus. The photographer turns the focus ring but that doesn’t change the focus. Instead, turning the focus ring controls the motor(s) built into the lens. The motor(s) take their instruction from the movement of the focus ring, and the motor changes the focus.

Focus by wire gets its name from fly-by-wire systems used in aircraft. Except on small aircraft the pilot doesn’t move the control surfaces on the wings directly because it would be impossibly hard. Instead, the pilot presses a pedal or turns a dial and electronic motors move the control surfaces..

And because the aircraft is so big, any small errors are not relevant.

In cameras it is different because small movements can be seen and felt. This is true in still photography and in video.

And that has been the source of the criticism of focus by wire – that the systems are laggy and prone to overshoot.

The photographer turns the focus ring quickly, and the system plays catch-up.

Photographers report that they feel divorced from the focusing, which is the exact opposite of what one should feel when trying to take a photo that needs critical focusing. The pressure might be off in a studio or with landscapes, both of which are situations where the photographer has time to focus. But on the street or with any fast paced action, the photographer needs to feel that the response is immediate and consistent.

The situation is improving and some focus by wire systems have smooth focus changes.

It’s helpful to know how the focusing system is on a lens feels before you lay down money for it.

In London today the sun set at 16:36. Between then and dusk at 17:15 there is still enough natural light for the human eye to see features in the scene and do most activities.

Compared to the human eye, however, cameras have a much more compressed range of being able to detect all the gradations from dark to light.

I shot this at 16:59, so fifteen minutes before dusk.

You have two choices if you want to photograph in this low light – increase the ISO or keep the camera at base ISO and put it on a tripod.

I shot this at ISO 6400 and f3.2 and 1/200th of a second. If I had put the camera on a tripod I would have had to use a slow shutter speed. At base ISO I would have had to shoot at 1/4th of a second and the person coming out of the station would have been a blur.

So what are the downsides of ISO 6400 in poor light?

The photo is very noisy. Look at the close-up of the face of the man in the shadows.

So then you might think noise is a terrible thing. But at a normal viewing distance the photo will look OK. It is only when we get close that we see the noise.

Of course if I were to print it and stare at it at the same distance as I am from the computer screen then I would see the noise. But if the print was in a frame and hung on a wall, the a normal viewing distance might be more than two metres (a bit over two yards) and you would hardly see the noise.

The amount of light entering a lens at the maximum aperture is the same no matter the size of the sensor. It’s the hole in the lens that lets the light through that counts. So, it is the same, for example, on an APS-C lens with a maximum aperture of f4 and a full frame lens with a maximum aperture of f4. When you think about it, that must be true because f-stops are defined as the amount of light entering a lens.

What does change is the depth of field, and to look at that we have to look at equivalent apertures.

Equivalent Apertures

Equivalent aperture is the aperture value on one sensor format that shows the same depth of field (DoF) and background bluras the aperture on a different sensor.

For example, on a Canon APS-C sensor with a crop factor of 1.6, an f4 aperture would be equivalent to the depth of field on an aperture of f6.4 on a full-frame sensor. To get the answer of f6.4, multiply the aperture by the crop factor. In this example it is 1.6 x f4, which give an equivalent aperture of f6.4

To put it in a more general way, a wide aperture on a full-frame sensor will have a more shallow depth of field than the same aperture on a crop sensor.

Q: How do you get a shallow depth of field?

Use a lens with a very wide aperture. And remember that a bigger sensor format will achieve shallower depth of field, whereas the same settings in a smaller format will not have as shallow depth of field.

Q: What is the magic and why would I want shallow depth of field?

Shallow depth of field makes the foreground subject stand out from the background. it works best when the foreground is further from rather than near to the background. And it works best when the subject is near to the camera. If the subject is far from the background but both the subject and the background are far away from the camera then they will merge together and the foreground will not stand out from the background. So shallow depth of field works best when the subject is near the camera.

Q: Help me choose?

Well, f1.8 is considered to be a standard wide aperture. Anything wider than that is where the magic starts. So f1.4 is gong to give good separation between the foreground and the background in an image. A wide aperture means the hole in the lens is big compared to other f stops where the hole is smaller. And the bigger the hole the more shallow the depth of field possible. That is why bigger formats such as full frame or medium format give more shallow depth of field than smaller formats such as APS-C or Micro Four Thirds.

Q: Any downsides?

Yes, you might miss focus completely when the depth of field is very shallow. It is easy to find you focused on a nose or a cheek and not on the eye, for example. Also, wider aperture lenses are more expensive and heavier. They are more expensive to make because the lens element at the front of the lens has to be big enough to open up wide.

And glass is heavy, so wide lenses weigh more.

It’s not necessary to chase super wide apertures on long focal length lenses because they are going to be used at greater distances and longer focal lengths compress the distance between foreground and background. So there is no advantage is chasing something that isn’t going to show.

A long lens with a widest aperture of f4 is as good as anyone needs. Actually, if you are shooting in poor light – maybe wildlife in the early morning or in a wood – then a bigger aperture of say f2.8 is better. But that is for light gathering rather than for separating foreground from background.

Q: Some numbers?

OK. Let’s clear one thing up and get it out of the way. A 50mm lens on a full frame camera and a 35mm lens on APS-C will have the same field of view. So to compare like with like means comparing these two focal lengths.

A camera with an APS-C sensor and a 35mm f1.4 lens and a subject 1.5m away has a depth of field of 10 cm. A full frame camera with a 50mm f1.4 lens and a subject 1.5m away has a depth of field of 7 cm.

A camera with an APS-C sensor with a 35mm f1.4 lens and a subject 2.5m away has a depth of field of 29 cm. A full frame camera with a 50mm f1.4 lens and a subject 2.5m away has a depth of field of 21 cm.

A camera with an APS-C sensor with a 35mm f1.4 lens and a subject 3.5m away has a depth of field of 56 cm. A full frame camera with a 50mm f1.4 lens and a subject 3.5m away has a depth of field of 41 cm.

From these numbers we see that depth of field increases the further away the subject is from the camera and we also see that the difference between full frame and APS-C narrows the further way the subject is.