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To fog or not to fog, that is the question.

With regard to ophthalmology and optometry, fogging refers to the technique of adding plus sphere power during refraction and/or retinoscopy in an attempt to control accommodation. Accommodation refers to the ability of the natural lens in the eye to change shape, and thus to focus light on the retina for sharper vision. During retinoscopy or refraction, the technician does not want the eye to accommodate, because accommodation introduces an uncontrolled variable into the measurement process.

The goal is to move the focal point in front of the retina. The eye accommodates in order to see more clearly.  If an eye is optically fogged,  and the eye accommodates, the vision will get blurrier,  not clearer. Thus,  accommodation is discouraged.  Care must be taken to not fog the eye too much.  If the vision is blurred too much, accommodation may actually be stimulated in a effort to see better.

How fogging works

The eye in the diagram to the right has a glasses correction that has left it under-plused. When accommodation is relaxed,  light focuses behind the retina and the vision is slightly blurry.

The patient accommodates (the natural lens gets "fatter") to see better.  Accommodation moves the focal point onto the retina.

We fog the eye by adding enough plus power to move the focal point in front of the retina.

If the patient accommodates now,  his vision will get blurrier instead of clearer.  

Accommodation now causes the focal point to move furthur forward. Thus, accommodation is discouraged.

Fogging must not be used when using the cross-cylinder to check the cylinder power and axis. Cross-cylinder techniques work best when the eye is able to accommodate to see better.

In some situations, the question is not "to fog or not to fog", but rather how much to fog. The question has come up a lot recently with regard to the COT "skill" exam that the COA must take after passing the COT written examination. Some test takers are not passing the computerized retinoscopy skill exam at least partially because they failed to fog the follow eye before performing the retinoscopy exam.

I have never fogged the fellow eye during retinoscopy and my accuracy has been just fine, thank you. If you talk to an optometrist, without fail you will get the recommendation to fog the fellow eye. The problem with this practice is that it can often be "under-effective" and it can be "over-effective". Suppose you are performing retinoscopy on a +3.00 D hyperope OU for the first time, and he does not have an old Rx that would give you a clue. Using even +1.50 or the "R" lens would not get you close to fogging the fellow eye. Conversely, if the eyes where -3.00 D OU, the "R" lens over the fellow eye would be serious overkill.

Never the less, JCAHPO expects you to fog the fellow eye as prep for the retinoscopy skill evaluation. JCAHPO is not saying how much they expect you to fog (despite repeated attempts to get an answer). The consensus seems to be to fog with +1.50, or the "R" lens.

What does research have to say? I have done an extensive literature search in an effort to discover the “correct” amount to fog.  What I found was that recommendations range from .5 D to 6 D.  I did find a research study reference that concluded that the amount of fogging made very little difference in the accuracy of retinoscopy (link below).
http://www.ncbi.nlm.nih.gov/pubmed/9170797

By the way, if you don't fog during retinoscopy, accommodation can be discouraged sufficiently by having the patient view a non-accommodative target with the fellow eye. Examples are the big "E", or just a circle or square of light on the screen. Children are great accommodators, so a good practice is to perform retinoscopy on them after they are dilated with a good cycloplegic drop such as cyclopentolate.