LUX vs Diopter Difference aka. How good you see on the brightest of bright days?

So I’ve been thinking about the difference in LUX and the ability for my eye to see.
I know that @BiancaK has written a bit about this before. Sorry Bianca, I couldn’t find it in your thread.

Anyway I find my eyes have a huge difference depending on lighting and weather conditions.

My other threads around this conversation are here and here

I was wondering. Is there a simple calculation between LUX and diopter difference?

Like when I look at the average LUX of enviromental conditions from Wikipedia. There is an extremely large difference between the brightest of sunlight and a very overcast day.


But there is obviously not a direct correlation between LUX and diopter difference in your eyes.

Has anyone thought about this or has an interesting take on it?


My take on it is that your eye (and visual cortex) relies on blur for focus cues, and in bright light it’s much easier for it to figure out that it’s not focused precisely.

For the past several days I’ve been screwing around with red light therapy using a $6 LED flashlight and it’s already making a difference. You might experiment with that. I’ve been very surprised with the results so far. In People Over 40 Years of Age (maybe you’re not there yet), and Lab Rats (I guess you’re not one unless you choose to be) it’s been shown to make a big improvement in night-vision, color discrimination, and ability to detect low-contrasts.

Besides your own conscious perception of blur quality, there are probably some internal “auto-focus” triggers that don’t get fired as easily at night. You might find a few minutes a day with a red flashlight improves your night vision by several lines on the eye-chart.

Also, this: (I have not read it yet)


Thats was good.

It also links to this post which talks about the right amount of LUX being 1,000.
I don’t necessarily agree with that but its a good post.

It also relates to my previous query about can there be too much light for active focus.


Light levels cause your pupil to constrict or relax. When your pupil opens up, the natural spherical aberration of the eye causes “focus shift”, meaning for the same shape of your eye and lens, the optimal focus point moves closer to you (in other words you get slightly more myopic in dim light). Correct prescription with a fully dilated pupil can be 0.75 diopters stronger than with a daylight constricted pupil, though at dusk it’s usually more like 0.25 diopters.

Also the eye focuses blue light more than red, so in evening light the ambient light is bluer, meaning that you are more myopic in dusk light due to the bluer light in the environment. This effect is why blue lights can appear out of focus when red lights aren’t, at the edge of blur.

Finally, the wider your pupil is, the narrower the “depth of field” that you see in focus. With a small pupil (such as daylight), you have a large area in front of and behind the focus plane that looks also in focus. This is called “depth of field”. When your pupil is wide open, that depth shrinks. This is important because if you can’t focus squarely on something, but come up short, it can look sharp still with a constricted pupil (daylight), but fuzzy with a wide open pupil (dusk).

All of this conspires to make your eyesight “worse” in low light situations (compared to bright daylight).

This is also why optometrists test in a dark room, they want to be sure you see sharp when driving at dusk (liability issues). This results in over correction by sometimes a half diopter during daylight, which of course, helps the myopia progression.

So it’s not light levels or lux that matters, but the optics of your eye and its pupil that matters.


A good article explaining this for astronomy can be found at:

1 Like

I don’t think so. It will be down to your eye’s sensibility.

I have a handheld LUX meter. A sunny day indoors, even near the window will not be more than 1000 LUX (indirect light). A totally gloomy grey rainy day outdoors starts around 1000 LUX.
My subjective perception was different and thought the sunny day next to the window had more lights than a gloomy day outdoors.
Something to consider for @gemilymez , too.

But the credit goes to @Humble . It was his journal that started me on measuring LUX. He always recorded the LUX, too. This may give a N=1 answer to your question.
Jenik Humble - journal from - 3.5

The other thread was this. It has examples for different LUX for indoors and outdoors.
What is the definition of poorer lighting conditions

And this was @Merlin93 's input on the international ophthalmic standards


Thanks Bianka,

From this thread and a bit more reading I think you’re right.
All I could find from Jake’s resources was a simple rule of 0.5 diopters.
Meaning, that generally between indoor and outdoor, low LUX and high LUX, is about 0.5 diopters. Which sounds about right to me…


Fascinating! I loved reading this. That is all.