The Hidden Power of Contrast Sensitivity in the IOL world - bad night vision causes

Contrast sensitivity is the most underestimated, yet most critical side of vision quality after cataract surgery. and refractive lens exchange. It affects functional vision, as contrast is not just a quality of the picture, whether it has brighter colors or dimmer ones. It's about loss of details in the shadows, loss of ability to read emotions on face and even fine details recognition. It's all about more, that we expect, and no I will explain how wrong IOL model may affect your visual performance and despite your doctor says you have 20/20, and you really can read 10 lines in the doctor's office, your everyday life vision is nightmare due to poor vision, which your doctor can not confirm by tests (spoiler - by wrong tests...)

This topic became critical, when we are talking about presbyopia correcting IOLs, either EDOF or multifocal, trifocal and covering astigmatism correcting toric IOL. All segment.

Whenwe are talking about presbyopia-correcting lenses, we should focus on three key components: first — the defocus curve; second — positive dysphotopsia or night halo effects; third — possibly the most underestimated yet most critical for functional vision — the contrast sensitivity. In many real-life situations, it’s more important for human functioning than the visual acuity.

There have been publications on this since 1987 (which I’ll mention today), maybe earlier, and even newer publications from 2022–2023 in literature. Driving, facial recognition, signs, emotions — all rely on contrast sensitivity under low contrast conditions. There are two parts: the patient’s subjective contrast sensitivity and the Modulation Transfer Function (MTF) of the optics. A patient’s contrast sensitivity depends on many factors — corneal condition, tear film, retina, axial length, vitreous, optic nerve, the visual cortex etc.

But if we take two identical patients (theoretically), whether healthy or with retinal or glaucomatous conditions, under the same conditions, and implant two different lenses — one with lower MTF, one with higher MTF — the patient will have better contrast sensitivity with the higher MTF lens.

MTF describes how optics transfer fine detail and contrast. 

If you will look at the world around you, you will notice large objects, like chairs. They are large and have fewer elemenets per space that is low spatial frequency. And fine details like texture of fabric have more elements per space or high spatial frequency. And all these frequencies have different contrast. And we combine all this on one graph.

One lens or optical system can transmit, for example, 40% of visual information at certain contrast and frequency levels, and another can transmit only 20%. Transmitting 100% across all frequencies and contrasts is physically impossible. That’s a limitation.

So MTF essentially merges resolution and contrast into one metric.

As you see in this image: top left — crisp, detailed; bottom — less defined, blurry. When we think of contrast loss, we often imagine such type of images.

That’s wrong!

Contrast loss is actually the loss of image detail, or entire visual elements in shadows. And this is explained by the Spatial Vision Theory — a critical element in understanding of how do we see the world. It’s also the principle behind digital cameras — your phone camera works on this.

Briefly the idea is the following: visual information consist of elemental sinusoidal patterns with varying orientation, brightness, and spatial frequency. All the information we see is decomposed into those elemental patterns. Then, in cameras, mathematical Fourier transforms recompose the image from those pieces. In the human visual system, similar computations happen — not like a phone, but conceptually close.

These transformations rebuild the image from elements into a whole picture we see and perceive — like in this face example. First, we just see a person. And then magic happens our brain combines high contrast and large objects and low contrast fine details to a complete image. And we see a person and recognize emotions.

So, important point here is that contrast loss means the loss of details — or even entire components — especially in shadows!

This is a slide from an interesting 1987 study by Arthur Ginsburg — available online — which discusses all this thoroughly in an ophthalmology context. So, when we talk about losing details with a lens, we imagine a slightly lower-quality image as I said. And we think “It’s not good, but it's okay to live with it, nothing special”

But no — it’s about unrealistic images, where parts of the image or entire low contrast objects in the shadows may dissapper, like pedestian on a dark road. This is the problem.

And how do we avoid it? By using an IOLs which keeps as much of contrast as possible and that's why transmit more contrast-rich images to improve patient contrast sensitivity and needed details.

Contrast sensitivity isn’t just a detail — it’s the foundation of how we truly see the world. Losing it means losing safety, facial recognition, emotional cues, and more — even when your doctor says your vision is “20/20”.

In my new YouTube video, I explain it in more detail, show the foundation of the ground-breaking experiment of 1957 and explain how IOL choice may influence on your visual outcomes. Follow the link to see my video:

Ask your questions in the comments to the video!

Oleksii at IOL-adviser.com