Autofocus Systems for Underwater Photography

You will notice that the featured image is actually a bird in flight. When we think about fast autofocus birds in flights is what is really going to test performance.

This image was taken by my wife with a Nikon D7100 and a Sigma 70-200mm lens in the Galapagos Islands.

I also shoot birds with my Panasonic G9 and have a direct experience of focus systems for moving subjects and I can comfortably say that today AI has become more important than anything else for those kind of shots. Artificial intelligence predicts movement and ensures that once the camera has reached focus the first time it reacts automatically to movement without the need to refocus.

Let’s start from the basics first.

Types of Autofocus

There are two types of systems for auto focus in digital cameras:

  • Phase Detection
  • Contrast Detection

Both systems need contrast to focus despite the naming convention, so phase detection works on contrast too.

In situation of low light low contrast EVERY camera switches to contrast detection without exceptions.

Contrast Detection AF

This is the simplest and cheapest way to obtain focus and is what is typically implemented in compact cameras. Contrast detection moves the focus back and forth to find the maximum contrast and then locks on subject. This is sometimes perceived as hunting by the user when the camera fails to find focus.

Contrast detection is the most accurate method of autofocus as it looks for perfection without prioritising time. With exception of Panasonic no other major brands use contrast detection AF on high end or semipro models.

Phase Detection AF

With this technique the image goes through a prism and it is split then when the two parts match the subject is in focus and the focus locked.

Phase detection is less accurate than contrast detection in particular there are instances in which focus is achieved in front or behind the subject. This is the system implemented by Nikon, Canon, Olympus and Canon.

Hybrid AF

This system combines both methods, it starts with phase detect to determine the focus start and then uses contrast detect to make sure the focus is accurate. Sony is the main driver of this technology.

Low Light Focus

All autofocus methods need light to function without exception, when the scene is really dark cameras have some methods to achieve focus, this includes:

  • Using the lens widest aperture to focus
  • Bump the ISO and then adjust later
  • Auto focus illuminator and modelling lights

Generally low light is less than 1.25 Lux or candela per square meter representing a really dark scene.

Pro and Cons of Each System

If we look at the three systems each one has positive and negatives and depending on the subject this are more or less important.

SystemSpeedAccuracy
Phase DetectFasterLess accurate
Contrast detectSlowerMore accurate
HybridSlowestMore Accurate
AF comparison Table

Performance Requirement for Underwater Photography

Many underwater photographer think that they need a system that focus fast, can track moving objects and work well in the dark, this system of course does not exist.

In particular considering the availability of focus lights the performance in low light is definitely not a show stopper. More important are speed and accuracy. For the purpose of a comparison I have included here some models from Sony, Panasonic, Olympus, Nikon and Canon with a variety of formats representing some popular choices among underwater photographers.

I have included 3 performance metrics for comparison:

  • AF time
  • Low Light Low Contrast Ev
  • Low Light High Contrast Ev

The first measure tells you how quick the camera focuses in normal conditions, this is in my opinion the most important parameter as generally underwater photography is not below 1 Ev.

The second measure is the number of Ev of low light the camera can still focus with a low contrast subject, and finally the third is still a low light scenario with a high contrast subject. Let’s look at the results that are build using test data from imaging resource.

AF comparison table

I have used conditional formatting so green is good amber is average and red is bad for each category.

AF Time

First observation is that hybrid AF is very slow, second contrast AF as implemented by Panasonic is faster than most of DSLR peers in this table. If we consider 0.2 seconds as acceptable the full frame mirrorless Sony A7RIII has unacceptable performance. While the Nikon D850 AF is in another league both MFT Olympus and Panasonic are faster than other APSC and even the canon full frame.

Low Light Low Contrast AF

Mirrorless cameras dominate this category, the Panasonic GH5 can reach focus at -4.5 Ev that is practically dark on a low contrast subject, second is the Sony A7 RIII and third the Olympus OMD-EM1MKII.

In a low light scenario phase detection fails sooner so some of those cameras switch to contrast detection to achieve focus.

Low Light High Contrast AF

All cameras are able to work at least at -3 Ev so this is not a distinctive category, it is worth nothing that some phase detect system that failed in the low contrast target scenario perform well in this category but generally performance is pretty decent.

Why are your shot blurred?

Some people that have the camera in the table still struggle to get shots, why is that? I have found that for most users do not read instruction manuals and to make it worse modern camera have far too many AF settings. My GH5 for example has 6 options of AF area, 4 options for AF Mode, 3 parameters for tuning the AI (artificial intelligence) engine, plus additional custom modes to select the 225 focus points in any random shape you like. The average person will skip all of this and select one option and then fail the shots.

Conclusion

Surprisingly for some if we look overall at the camera that has green in all categories we find two mirrorless micro four thirds. Even more surprisingly both those cameras are faster to focus than APSC DSLR from Canon and Nikon although it is not really a great distance.

Typically when it comes to comparison between camera there is someone that says but camera X gets the shots blurred so speed does not matter. I talk by direct experience with outdoor and birds not just fish and I can tell you that each system will miss shots in burst mode but more importantly underwater photography is nowhere near requirements for birds in flight.

I have performed tests with a light meter at less than 1 candela per square meter with my GH5 with a 60mm macro lens and with my surprise it focuses just fine without the AF illuminator. I have to admit I do not really trust auto-focus so in most situation I use back button and peaking however based on my recent findings I need to trust autofocus a bit more it seems!

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