Learn and understand how autofocus works while photographing underwater

Underwater , the focus is almost always done automatically , which doesn’t mean that the camera will do the entire job by itself !

On the contrary , many factors are to be taken into account in order to get an accurate and quick focus .

In order to avoid your pictures from coming out terribly unsharp, because of the inaccurate « AF-Auto Focus », have a look at the explainations below.

The three different types of Autofocus ( AF )

When it comes to Autofocus settings, you DSLR has three options :

La Mise au Point en photo sous-marine-Mode_AF.jpg

  • One Shot / AF-S

It is done by pushing the shutter button half-way down , once this is done , you will have to re-focus after each shot. It is the basic setting that suits steady or slow subjects very well (Nudibranch, moray , etc….)

 

La Mise au Point en photo sous-marine-AF_mode_Nikon.png

  • AI servo / AF-C

 The camera detects whether or not the subject is moving and autofocuses according to the subject’s movements. It’s the ideal setting for moving subjects such as Sharks or Anthias.

 

  • AI focus / AF-A

It’s a mix of the above modes : One Shot and Al servo . You focus on the subject and the camera detects it’s movements.

 

  • Le AF-On button

I’d like to provide some precisions about the AF-ON button of your camera.
the focus for underwater photography technics -AF-ON

This button remembers where the focus was done .

In other words, it won’t focus when you push the shutter button. Whereas it will make an exposure measurement half-way throught.

« This trick is very usefull if you are waiting for your subject to have a specific behavior. »

The subject has to stay in the part of the frame that’s in focus during this lap of time.

 

The collimators

the focus for underwater photography technics -Collimators

 If you have got a point and shoot camera , your choices are  very restricted . The autofocus will be made at the square’s level , and therefore will be rather slow .

“This is the big drawback of these point and shoot cameras.”

On the contrary , you will find a whole bunch of options for your DSLRs.

 

First of all , there are 4 types of collimators :

  • Vertical collimators : It focuses on vertical plans
  • Horizontal collimators : It focuses on horizontal plans
  • Cross-type collimators : It focuses both on horizontal and vertical plans
  • Star-type collimator : It focuses on horizontal, vertical and diagonal plans !

the focus for underwater photography technics -collimator focusing

The more plans of focus your collimator has, the quicker and more efficient it will be in low light.

 

Which is a big advantage for underwater photography. By default, i always use the central collimator.

 

 

How can you choose the best collimator according to the type of shot you are going after?

In macro photography, you are to use only one collimator.

Indeed, the spot you focus on has to be precise. According to the wanted composition, you will often find yourself selecting another collimator than the middle one. 

 

In environmental photography, you have a wider range of choices.

You can either keep using a single collimator or use a group of them .

I personally use a single collimator which as move across my viewfinder according to the composition i use.

” Most of the time , we haven’t got enough time to select the perfect collimator. Therefore, we make focus on the desired spot and modify the frame later “.

For photos of cetaceans , or photos taken during free diving , i use all collimators.

Indeed, in those moments, the focus has to be really quick. It is necessary to optimise speed and autofocus.

Selecting all collimators can be quite dangerous, because we don’t know which focusing point the camera is going to choose.

Therefore, chances are the camera will choose a focusing point on a reflection on the surface (as there is more contrast), than on the animal itself. This is a compromise to be made.

 

Where should the focus be done?

The depth of field notion is intimately linked to the focus .

It’s even more true in macro photography as the plan on which you make the focus will determine the realisation of your photograph.

For portraits , most of the time , the focus is made on the subject’s eye . Indeed , if there is a single field that is to be sharp , it’s the eyes  !

” Emotion is transmitted by the gaze  “

 

The notion of hyperfocal

To be know when we want to maximise the depth of field.

According to where you focus, the are in focus will be dispatched  with the following proportions :

  • 1/3 fore
  • 2/3 rear

Without going too scientific on this, hyperfocal is the closest sharp point when the focus is set to infinite. Once you found this spot, it’s the place where you are to make your focus.

Doing so will extend the plane of focus to 1/3 , in front of the focusing point.

 

To conclude on the focusing !

You understood so, focus defines a bunch of factors which will allow you to have the wanted composition.

In order to master it , you have to know these basic rules, which will soon become automatic.

It will allow you to think more about your creations, even before making them, and especially to obtain a result closer to your expectations!