Panasonic GH4 User Review Part 3.1 Focus Technologies

GH4 with 100-300mm lens

 

From 1989 to 2009  my main cameras were Canon SLRs then DSLRs.  In many respects these were decent cameras with good image quality and performance. But every one of them suffered from inconsistent autofocus accuracy.

Then in 2009 I bought a Panasonic G1, the first mirrorless interchangeable lens camera (MILC) with EVF.  Compared to DSLRs of the time, this camera had second best image quality, performance and ergonomics. But it focussed accurately. First time, every time, all the time. This was a revelation. In single shot use the G1 hardly ever missed focus.  The G1 and all subsequent Panasonic MILCs used a different type of focus technology which clearly delivered a big advantage in accuracy. 

For several years camera reviewers kept asking the wrong question: "how fast does it focus" ? when they should have been asking "how accurately does it focus" ?

In the event Panasonic brought it's MILC AF speed up to match and now surpass that of DSLRs, while maintaining a clear advantage in accuracy and consistency.

 

DSLR focus  systems  Refer to the DSLR schematic.

A standard flipping mirror DSLR has three separate focus systems.   

For manual focus with eye level view the image coming through the lens reflects off the main mirror and is focussed onto  the focussing screen which lies beneath the pentaprism.  The user turns the focus ring on the lens back and forth until the image appears at it's sharpest.  In  manual SLRs of yesteryear this was the only way (apart from setting a distance by scale on the lens) to focus.  On my 50 years old Pentax Spotmatic this actually works quite well with a fast prime lens feeding plenty of light through the optical pathway.  Fast forward to the autofocus era with zoom lenses of smaller aperture and manual focus becomes very difficult at the wide end of a kit zoom. In the digital era sensor size reduced to the 28mm diagonal and pentaprisms gave way to pentamirror design. Manual focus became even more difficult.

For autofocus using the optical viewfinder (OVF)  some light from the lens passes through the main mirror, reflects off a submirror and proceeds to an autofocus module at the bottom of the mirror box. This module uses mirrors, lenses and sensors to tell the main lens which way it should turn to bring a specific part of the image into focus. There are multiple potential sources of inaccuracy in this system. It uses phase detect AF which is inherently both imprecise and inaccurate. It also depends on all the optical and mechanical components in the focussing chain being precisely aligned in the correct position. This is frequently not the case leading to further inaccuracies.

For autofocus in live (monitor) view a completely different focus system is used.  In this case, focus is assessed directly on the imaging sensor. Some cameras use a phase detect array built right into the sensor, many use a contrast detect system like most MILCs.  This has the potential to be more precise and accurate than the PDAF module in the bottom of the mirror box.  Whether it actually is or not depends on the implementation of the technology in each case.

 

Panasonic MILC focus system  Refer to the MILC schematic.

Focus arrangements in Panasonic MILCs are conceptually much more simple. No doubt the actual technology involved is very sophisticated and complex but in principle the ideas behind it are straight forward. 

There is just one focus system. It works by contrast detection (CD) directly on the imaging sensor. This works the same for eye level or monitor view. It is also the same for manual focus except that the contrast detect function is carried out by the operator's eye and the lens is focussed by hand.

Panasonic is one of the few makers to rely entirely on contrast detect focus technology. In the past this meant good single shot AF performance but sub standard follow focus capability using AF continuous on a moving subject. However as I will describe in a subsequent post, Panasonic has  been able to achieve excellent follow focus capability with the GH4 while staying with CDAF.  

In  the next three posts I will describe how the GH4 works and performs with manual focus, single shot autofocus and continuous autofocus on moving subjects. As you will read I have found it to perform exceptionally well in all three focus modalities. In fact the GH4 has the best focus performance of any camera used by me, ever. It is a big step up from the GH3  particularly in AF Continuous on moving subjects.