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Comment by bumbledraven | Hacker News Reader

Why aren't the stars more visible in the background? Does it have to do with the camera's exposure?

Note that the Moon is fully lit by the Sun (it's a "full Moon" from the perspective of the satellite). And it's gray on the picture, as the Earth is significantly brighter.

Now, you can check it yourself, take the best camera you can have, go one night when the Moon is full somewhere outside of the city where you see the stars and then try to make a photo where on the photo both the stars and the Moon surface are visible at the same time. It simply can't work, the light of the stars is very faint, as soon as you see any details of the surface on the photo you take the stars will surely be invisible. Our eyes are still much better than any camera we can produce.

Isn't this just because of the processing? I imagine if you took additional logarithm of the values returned by the sensor, you could have both starts and the bright moon visible at the same time.

No, our bodies can simply do more, see:

http://photo.stackexchange.com/questions/21579/how-does-the-...

We could use additional "tricks" with cameras too (e.g. physically obscuring the moon and earth for one frame to shoot the stars then mixing it with the next photo) and with the additional post-processing (which would certainly include reducing the dynamic range to be able to show the photo on the screen) get something that some skeptics would think is "normal" but it's not what was being done on the photos presented, as nobody designs that much for features that actually don't bring anything: It would be easier to shoot just the sky in the same direction (the stars would look effectively the same, they are that far -- the satellite is just one hundredth of one AU away from the Earth and Alpha Centauri is some 271 thousands AU away) and photoshop it.

That's basically the idea behind HDR (High Dynamic Range) imagery in photography/photo processing. (And my understanding is that it's hard to figure out exactly what the best scaling function is in order to mimic typical human perception: it's not just a simple log.)

Earth is a sunlit object in this picture, the luminosity is the same level as during the day on earth - would you see stars in your photos if you took a correctly exposed photo of something during daylight on earth? I don't think this would happen :) The dynamic range difference between day light objects on earth and the stars on the sky is huge.

The satellite is 1 million miles away from Earth at the L1 Lagrange point. What is it about optics that allows us to compare a photo of the stars taken at such a distance with one taken from the surface of the earth? [Edit: "compare", in the sense of understanding whether or not the stars will or won't show up in each case. I get that the laws of optics are the same throughout the universe.]

> The dynamic range difference between day light objects on earth and the stars on the sky is huge.

I agree, but would a person sitting in the satellite see stars if they looked in that direction? If so, do our eyes just have better dynamic range than the camera used by the satellite?

Yes, our eyes just do have a fantastic dynamic range.

Having said that, if you were sitting on the satellite, you would see even more stars if you blinkered out the brightness of the sun, moon, and earth.

Don't cameras have much better dynamic range, but we just limit it in software by reducing everything into 8-12bit/color images? After all, the hardware is just a bunch of sensors counting photons.

Go here and see for yourself, for example: http://www.dxomark.com/Cameras/Nikon/D810---Measurements

ISO50, 14ev, ISO 12800 8ev. Then read here:

http://photo.stackexchange.com/questions/21579/how-does-the-...

We as humans can probably obtain 24 stops when our body uses all the tricks it has, not only optics of our "hardware".

Thanks for links here and under another comment, I learned something new today :).

>The satellite is 1 million miles away from Earth at the L1 Lagrange point. What is it about optics that allows us to compare a photo of the stars taken at such a distance with one taken from the surface of the earth?

The fact that the earth in the picture is lit by the sun and has similar luminocity levels to looking at the sky from the surface during the day.

The background of the picture (the stars) on the other hand, have the same meagre light they always have.

Eyes have about 24 stops dynamic range, there are sensors that have over 20 stops dynamic range with a single exposure, with multiple exposures (HDR) you can of course get way more than that.

The problem is that our monitors have very limited dynamic range - so even if the original had very high dynamic range, it would not show up properly on a current monitor.

There are HDR panels coming to the market this year which have more dynamic range.

And of course it's possible to tonemap HDR images for viewing on normal screens - there are various algorithms for that - https://en.wikipedia.org/wiki/Tone_mapping

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