Conversion factors
Article Index
Start | refractive indices
Accomodating
Reading addition
Lens distance compensation

Accommodating

Underwater everything seems bigger (even outside US territory) and appears to be closer than it does out of the water. As mentioned before this is a result of the differences between the refractive indices of water, glass and air. As a result an object at 100cm seems about 25% closer. This means that underwater you should focus your camera at 75cm to get a sharp image.
The same is true for your eyes. They will need to accommodate more to focus on closer objects. This can be a problem at very short distances. As we get older our power of accommodation decreases. Noticeable by your arms getting to short to hold a book at a comfortable reading distance. So you need help.
Enter reading glasses, your close (distance) friend. They help you read even the fine prints again, as long as you keep it in focus range.
This range however is quite small which means that if you use reading glasses to read a book at say 40cm you probably have difficulty reading at 30cm (without getting a headache). You'll need glasses with a higher refractive power to comfortably read at shorter distances. As you would underwater!
As a matter of fact, it could well be that even when you don't need reading glasses yet you actually do need them to see comfortably underwater!
(how's that for a selling argument)
.

An object at distance X is projected by the corrective lens or eye) at distance Y

 

When the object gets closer the projection distance Y will increase also and gets out of focus.

 

To get the projected object back into focus again the eye(lens) needs to accommodate more (if possible) or an increased refractive power is needed (as pictured)