Diving Into Depth of Field
About two years ago I had a cataract surgery patient who changed how I look at my refractive goals. My patient was a busy and social person who wanted to see pretty well at distance but also wanted to be able to do everyday tasks around a room without glasses. She was willing to wear readers if she had to.
I aimed for -0.70 Diopters OU to achieve a distance dominant middle vision path. There was nothing unusual about any of this.
She became remarkable on the first day after her second eye cataract surgery. I entered the room and saw that she was beaming. “Thanks” she said. “This is amazing. This really blows me away. I can do everything. You didn’t tell me I would be able to do this!"
No, that’s not really possible I said. You can do most things, but I doubt you are going to feel comfortable reading fine print or driving a car. You and I talked about this before surgery. These are monofocal lenses.
She persisted. “No”, she said. “I can do it all.”
She pulled out her cell phone and began reading the news. (Well, I thought, looks like I made you a little myopic. I hope you are ok with that!). Next I put the 20/20 line up on the distance chart and said; can you do this?
"It is just a bit fuzzy", she said, "but I can read it no problem."
"E V O T Z"
“And, she continued “my focus is perfect everywhere between as well. I tell you, I can do it all”. You have worked a miracle!”
I suppose I like being a miracle worker, but I also like to know why, so that I might be able to do it again some time in the future.
I did a careful refraction. This is what I got:
OD -0.37 – 0.50 x 175 -0.62
OS -1.00 – 0.50 x 010 -1.25
That explained a lot. I had given her about 0.60 Diopters of micro monovision. But was this the whole story?
I asked myself if the astigmatism had played a role. I considered that she really had four refractions, not two, because the astigmatism gave her different refractions in different dimensions. My dimensional refraction then became:
OD -0.37 x 085
-0.87 x 175
OS -1.00 x 100
-1.50 x 010
The secret to her depth of field was now revealed to me; this patient was experiencing a synergistic combination of small amounts of anisometropia and astigmatism. There was a 1.12 Diopter difference between the two bolded dimensional refractions!
Monofocal lenses have some depth of field, but we don’t consider this to be a primary function. This patient taught me that we can greatly expand the lens’s intrinsic depth of field by combining small amounts of anisometropia and astigmatism, and that we can make the patient very happy by doing so. For a theoretical discussion of expanding depth of field using monofocal lenses, click here.