Obviously it was a thing for film cameras, but now that everything is digital, something like “just make the picture darker” seems extremely easy to do with software
quick edit, I know what ND filters are for and how to use them, no need to explain. it just seems to me that it could be engineered in a way that doesn’t require them, which is what I’m asking about
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Digital cameras work by using special material (CMOS semiconductors) that produces an electric signal when hit by light, sort of but not exactly like a bunch of small solar panels. This signal is then amplified (similar to volume on a headphone) and converted into a whole number so it can be stored on computers. The more light that hits the camera in a given spot the higher the number, which creates a brighter region in the image.
But there is a limit, at some point you max out the signal, so even if more light hits it the image won’t get any brighter. There is also a minimum amount of light too. This is similar to how if you turn a speaker all the way up music gets distorted, the speaker is simply not capable of being louder so it cuts off sounds in the music.
You can edit digital images, but you can’t get back anything that isn’t recorded to the image in the first place. If the signal gets maxed out, you will lose all detail in that part of the image. This cannot simply be corrected with software, because the detail was never in the digital image to begin with.
There are a few main ways to adjust your image to get the right amount of light, but they can also have other side effects:
1. Exposure Time and Shutter Speed. The exposure time is how long the camera collects light that hits it. Each pixel basically “adds up” all the light over this time. Most cameras (though not all) also regulate this by having a small screen called the shutter that will block out the sensor and scan over the image, and the timing of this can also be adjusted. This allows for very short exposure times.
You can turn down the exposure time if the image is too bright, but this has 2 possible unwanted effects:
The less time the camera has to gather light, the less likely it is to detect it. This is because the camera only has some probability to actually detect light hitting it (due to physics). Low exposures can create random noise on the image if some pixels detect more light by chance (called shot noise), but with longer exposures the probability evens out across the image and you get less noise.
long exposures create motion blur, which can be stylistic. For example, a long exposure of a river is likely to smooth out the appearance of the river and make it more artistic than photorealistic. Short exposures lose this.
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2. ISO/gain. This is the electronic amplification that is given to the image, basically making its pixel values more spread out so when you digitize it into whole numbers you retain the most information. The issue using this though is that there is a minimum gain – 0 amplification – which in bright images may not be low enough.
Also, if the image has very bright but also very dark spots, low gain will create poor dynamic range (how spread out the pixel values are in the final digital image), so the darker areas may be under saturated and lose detail.
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3. F number/numerical aperture. This is a more complicated setting, but basically it is a measurement of what directions your camera captures light from. Low F number captures light from a wide direction, and high F number captures light from a more narrow direction – meaning less light overall and a darker image.
However, F number also affects the focus of the camera. Low F numbers have a narrow range of focus, so the background will be blurred. High F numbers have a large range of focus, so both the foreground and background can be seen.
In addition, this number influences the resolution of the camera, meaning changing the F number can affect the size of details you can clearly see. Exactly how it does this depends heavily on the lens, but know that changing F number may blur or sharpen certain details even when in focus.
Also, some lenses may not have variable F number, or a limited range. So you are not always free to change this.
So, most photographers will choose F number based on artistic style, the lens they have, and other technical requirements for the desired image. Changing this to alter light levels is usually not preferred.
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ND filters on the other hand just uniformly reduce the brightness of light coming in. You can still keep a longer exposure, use enough gain to get a good dynamic range, and pick an F number based on image requirements while not over exposing the image. This maintains the artistic capabilities of the camera. Of course, in some situations ISO or exposure time is enough and is preferred to an ND filter. It all depends on what the image calls for and what tools you have on hand.
Another factor, especially with very bright images like solar photography, is that the light can actually damage the camera due to the high intensity of light. So an ND filter is absolutely required in this situation.
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