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To prove to yourself that this is true, stand in front of a mirror.  Now make a movement with your right hand, or wink your right eye.  Looking at yourself in the mirror, from your viewing perspective, was the image of the hand (or the eye) on the right or the left that moved?

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The ND filter is a filter that, placed before the lens reduces the amount of light reaching the camera's sensor. It acts like a pair of sunglasses to your lens.

1 stop, 2 stops, 3 stops, etc. Sometimes these are referred to as EVs, for exposure value. These are probably the most convenient measurement because they tell you how many stops they'll adjust your exposure by.

Video shooting with wide aperture lenses outside is another application if you want to shoot in a 2.8 or even wider. you are doing the day when it's very bright. way too much light coming into the camera kind of locked into our shutter speed. You cannot turn the ISO down further and that is a problem you can't solve without an ND filter. many high-end video cameras have ND filters built into them.

2x, 4x, 8x, etc. Sometimes these are referred to as ND2, ND4, ND8, and so on. These refer to the amount by which the light is diminished. An ND2 filter halves the light, while an ND8 filter reduces it to one-eighth.

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The forefinger of my right hand is on the camera button that takes the photograph.  Notice that it is on your right.  Now if I was really facing you, i.e. not reflected from a mirror,  the forefinger on the button would be on your left side.  You might not like taking that on faith.

The neutral density (ND) filter is one of the basic gears that should be included in every landscape photographer’s camera backpack. Many beginners may don’t know how, where, and when to use them. In this article, we will introduce some basic knowledge about it.

One might wonder why is it that when you see writing through a mirror that the writing is backwards, but not upside down.  Here we try to understand why it is we see objects in reflected from mirrors as we do.

ND filters come in different strengths or darkness levels. For the photographer, the easiest way is to have ND filters that tell you how many stops of light they will darken your exposure. Most brands of ND filters label the ND filters with either a factor number or optical density number. But for the photographer, neither of them is referred to the number of stops by which the light is reduced.

For example, the long exposure shots like if you’re shooting a river or a stream or waterfall and you want to get that misty look. An ND filter is a spectacular way to go. However, it might be confusing in terms of the grade of ND filter you have. You have to know how dark it will get and the problem is these nomenclatures are used interchangeably so let’s just dive into the chart.

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So, ND500 sounds like a lot, but it'd be the same as stacking an ND16 and an ND32 (16 x 32 = 512; manufacturers round it to 500).

So if you have an ND filter with strength or intensity of 2 all you need to do is turn it into a fraction by putting a 1 above it. So ND filter of 2 is going to let in half the amount of light. An ND filter of 4 is going to let in 1/4 amount of light and so on. So if it’s a 3 stops reduction, it will let in 1/8. if it’s a four-stop reduction so it’s an ND 16 it’s going to let in 1/16. if it’s an ND filter then reduce it by 5 stops that would be an ND 32 . and that’s going to let in a fraction of 1/32. and so on and so forth.

Since I am not standing on my head, my head is up and it appears so in the reflection from the mirror. Return to Physics of the Visual Stimulus

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When one looks into a mirror it appears as if you are looking through it and seeing the object on the other side.  It also seems as if the object is flipped left to right. In reality the right half of the real object is on the same side as the right half of the virtual object.

When you go to purchase an ND filter often it is going to have an ND number that will say something like nd2, nd4, nd8, nd16.

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Again if you’re trying to go with a very very wide aperture on a sunny day well cameras have sync speed limits which means you cannot really use super-fast shutter speed and so one of the best ways to manage that is to lower the amount of light coming in-camera through an ND filter.

Up down is not a problem because the reflected image is up just as you would expect it to be if the object was really opposite you.

If you are still doubtful as this explanation lets look at the following photo which I took of myself in front of a mirror.

By the way, have you ever noticed that Ambulances use this illusion by writing the word "Ambulance" backwards on the front of their vehicle. They do this because when you see the vehicle through your rearview mirror the word Ambulance appears normal and then you are absolutely certain about what is behind you.

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One question you probably have is why and in what situations would you want to reduce the amount of light coming into your camera sensor.

A stop is an arbitrary amount of light. If you have a shutter speed that’s twice as long as one before. You’re letting in twice the amount of light that is one stop. ND filters are rated in the amount of light that they reduce coming into the lens. so if we say this is an ND filter with the one-stop intensity that means it would let in half the amount of light as it did before and this is how ND filters are rated. So they have different intensities from 1-9 or 10,000?

So, take a look at the writing across my sweatshirt.  Obviously it is backward.  It is not actually backwards on my sweatshirt.  But what seen reflected from the mirror it appears backwards.  The word "The" is on the right side of the sign, under my right hand.  If I was, in fact,  facing you, i.e., not reflected from a mirror,  the word "The" would be on your left side and you would be able to correctly read the words "The Joy of Visual Perception.

Let's define some terms.  By real object (image) I mean the object that is being reflected.  By virtual I mean the image or the reflection of the real object.   When you hold words in front of a mirror by standing behind the text (as I am, below) and look at its reflection, the right side of the reflected word will  be on your right side and will appear backwards. If  I was actually standing with the sign in front of you, the words, from my perspective, would read from right to left because I am behind the sign. They would read from left to right for you, as does this text on the video monitor.