Functional Anatomy of the Hand

CAMERA ERGONOMICS

PART 4    Functional anatomy of the hand

By AndrewS

Ergonomics has been defined as the study of designing equipment and devices that fit the human body, it's movements and it's cognitive abilities. (Wikipedia, 2011)

Most cameras are hand held devices so it makes sense to begin a study of camera ergonomics with an exploration of the range of movements and capabilities of the hand and fingers.

I take it as self evident that a camera should be designed to fit the hand, not the other way around.

Lateral dominance   Before going further, I should raise the issue of dominance.  Most humans are right side dominant. This means they perform better  at writing and operating devices with the right hand.  Most also are right eye dominant.  These people have a left side dominant brain. About 10% of people are left handed, although about half of these also have left side brain dominance. Crossed and/ or mixed dominance is not uncommon.

Cameras are designed to be operated by right handed people. Those who are left handed are not catered for. In this case the user has to adapt to the camera which is ergonomically suboptimal. However, most manage to train their non dominant side to do the job. They have little choice, until someone comes up with a right/left inverted camera shape.

Photo 1 Dimensions

Size    The right hand you see in the photographs is the author's. In Photo 1, Dimensions you can see it measures 190 mm long by 96 mm wide.  Wikipedia gives 189 x 84 mm as average for an adult male and 172 x 74 for an adult female. Obviously hands vary in length, width and finger thickness. However reasonably healthy hands all function the same way.  Cameras need to be designed so that they readily accommodate most of the hand size variation found in the general population.

Photo2 Skeleton

Photo 2 shows the skeleton of a hand and wrist with 27 bones.   There are 8 carpal bones in the wrist giving it flexibility in all directions without which operating a camera would be almost impossible.  Next come the 5  metacarpal bones, then the phalanges, of which the thumb has 2, the other fingers 3 each.

Thumb Opposition    The thumb metacarpal bone can rotate to allow the tip of the thumb to touch the tips of the other fingers. This is called opposition and is essential to gripping and operating a camera. Look at Photo 3 Palm flat,  Photo 4, Clench no opposition and  Photo 5, Clench with opposition.  You can see that if the hand tries to grip onto anything without thumb opposition, the grip force is directed into empty space and is dissipated. But with opposition in play the grip force of the fingers is directed  into the tissues of the base of the thumb, lying over the rotated metacarpal bone. Thus the hand can grip and hold things securely.

Photo 3 Palm Flat

Photo 4 Clench No Opposition

Photo 5 Clench with Opposition

Sensory capability    To operate a camera the fingers must detect touch and position to a high level of sensitivity.  This is particularly the case with modern electronic cameras some of which have many controls. My reading of research on this subject would indicate that sensitivity increases towards the fingertips, being greatest just below the nail bed. It would also appear that the thumb is less sensitive to touch discrimination than the other fingers. These characteristics are important to the design of physical controls.

Photo 7 Relaxed Top View

Photo 6 Relaxed Rear View

Basic Hand Posture      See Photo 6, Half Closed Relaxed Rear View and Photo 7, Half Closed Relaxed Top View.  This is a relaxed natural position for the hand to adopt.  Muscle force is required to clench the fingers further,  straighten them, or move them into a different alignment.  A properly designed camera will be sized and shaped to fit easily into the relaxed hand, which can then grip the camera securely with minimal stress.

Movements    Each finger has a range of possible movements which are critically important to the design of the shape, layout and controls of a camera. I find it useful to categorise fingers as "grippers" or "controllers".

Photo 8 Index Pos 1

Photo 9 Index Pos 2

Photo 10 Index Pos 3

Photo 11 Index Pos 4

Index finger  This is a controller. It has good position and touch sense. It can curl and  straighten in the line of the finger. It can also angle from side to side at the metacarpophalangeal joint over a small but vitally useful range.  These movements can take place without the slightest change in the position of the hand or any other finger. You can demonstrate all this for yourself. These characteristics make the index finger the best choice for actions required to be made during the capture phase of photography, that is,  while one is actually in the process of making photos. Please see the series Photos 8, 9, 10, 11.    I elected to demonstrate these movements on a mockup camera to better illustrate their practical utility. The index finger can easily operate four items, as shown here, by feel, without having to look at the finger or the control items, and without having to disrupt the basic grip. On the mockup the four modules are Shutter Button, Main Control Dial, ISO Button and Exposure Compensation Button.  This enables all primary (Aperture, Shutter Speed, ISO) and secondary (Program Shift, Exposure Compensation) exposure parameters to be quickly and confidently adjusted, auto focus achieved, auto exposure achieved then the photo captured, all with just one finger.

Middle, ring and little fingers.  These are grippers, usually operating together as essentially one gripping unit. They will function  best if the camera is designed so all three can get a proper hold on the camera's handle.

Thumb  The role of the thumb in holding and operating a camera is complex. In most cases it has to perform both gripping and controlling functions. As we will discover it is very difficult to achieve both at once.  The thumb has three main movements: Opposition, Flexion / Extension and side to side movement. Opposition as we have already discussed is by rotation of the metacarpal bone at the carpometacarpal joint. Flexion / extension can occur at the metacarpophalangeal joint and the interphalangeal joint. Side to side movement occurs at the carpometacarpal joint. Note this is quite different from the index finger which uses the metacarpophalangeal joint for side to side movement.

So what ?? do I hear you asking ?? Well, so quite a lot as it happens.

You need to have the thumb metacarpal in opposition to be able to hold onto the camera with the right hand. Try this for yourself. Hold a camera as usual, in your right hand.  Then open up the ball of your right thumb (the part over the metacarpal bone) derotating the metacarpal bone and releasing the opposition posture. The camera will immediately fall to the floor unless you catch it with the other hand.

Photo 12

Photo 13

Now look at photos 12, Press Thumb IP Joint Extended and  13, Press Thumb IP Joint Flexed. There are lots of buttons and dials on the back of modern cameras. Some can be pressed / operated with the thumb held straight, as in Photo 12. With the thumb in this position opposition at the metacarpal bone is present and the user's grip on the camera can be maintained.  But other cameras have controls which require you to flex the thumb in order to operate them. Sometimes the controls are inaccessible to a  straight thumb, sometimes a semi submerged dial can only be operated with the tip of the thumb.  In either case, flexing the thumb forces the metacarpal to derotate and opposition is lost. In this case you cannot hold the camera with the right hand  and operate the thumb controls at the same time. You must support the camera with  the left hand while the right thumb is operating the controls. During this process the index finger is sitting idle, when it could have been operating controls with no disruption to the right hand grip at all.

Photo 14,  Thumb Lateral Movt Metacarpal  illustrates side to side movement of the thumb, shown in place on a mockup camera. This movement is articulated at the bottom of the photo, not where the side to side arrows are shown. As a result, this movement disrupts the placement of the ball of the thumb and to some extent destabilises the grip. In practice a small amount of side to side movement by the thumb is compatible with maintaining grip. In the case of the mockup shown here the AF start button can be activated by rolling the thumb 3mm to the right and the AF position jog controller is easily reached and operated by moving the thumb 10 mm to the left, maintaining opposition.

Why does all this matter ??  There are four phases of camera operation; Setup, Prepare, Capture and Review. In the Setup, Prepare and Review phases, it is perfectly satisfactory to fully support the camera and lens with the left hand while operating buttons, dials or even touch screen controls with the right thumb. But in the Capture phase that is not acceptable. While actually in the process of taking photos the operator needs to be able to adjust all primary and secondary exposure and focussing parameters while continuously viewing the subject  with both  hands firmly supporting the camera.

Many modern cameras require derotation of the thumb metacarpal or in some cases complete removal of the base of the thumb from the camera in order to access the controls required during Capture phase operation.  Many also require the user to look at the controls in order to hit the right one. This destabilises the right hand grip, jiggles the camera, takes the user's eye off  the subject and disrupts the flow of taking photos.  Many of these camera work just fine in one of the fully automatic, snapshot settings, where the camera makes all the key exposure and focussing decisions. But they are most frustrating to operate in one of the user control modes, for instance one of the P,A,S,M shooting settings.