A brief over-simplified explanation of ISO for Newbies...
Oct 14, 2011 20:21:44 #
...as I understand it. I want to double the emphasis right up front that this is an over-simplified explanation for those who can use it to help them understand what's going on. There is no need for anyone to turn this into a highly technical discussion and get carried away here and become argumentative. If you know more about this than I do, I'm happy for you but please don't confuse inexperienced people any more than they already are by tossing a bunch of useless jargon, acronyms, abbreviations, and other techno speak at them.
Further, for those of you who benefit from this, there is no reason to come forward and reveal who you are unless you are just in a self-effacing mood. I don't need for you to openly express your appreciation. I understand your situation. If you can use it, good, if not, good. It didn't hurt anyone, especially me. I feel your pain. I've been where you are. You can't get a straight answer even with Google.
ISO, or in film, ASA, is a rating of the speed by which a digital sensor or film receives light that creates an image when the shutter is clicked. Like many, if not most, things with digital cameras, the speed with which an image is made is based on the way it worked in film cameras and is done that way so those who understand film can easily understand digital, with certain modifications. This is not about which one is better. It's about how the terms in digital came about from the way it was with film cameras.
Film for color prints was rated from ASA 100 up to 3200, maybe 6400 in the end - it isn't important. These numbers represent the "speed" of the film. You could get ASA 200, 400, 800, and 1600 in addition to 3200 and whatever. Most people used 100 ASA film because that usually gave them the best pictures. They set their cameras for 100 ASA and you left alone for the duration of the number of exposures the film was capable of taking, 24 or 36 usually. There are exceptions to that but they are not important for this discussion. The film, a roll of special paper, had flattened crystals of a silver compound. It was the silver compound that was sensitive to light and upon which an image was created. ASA 100 was the slowest film of the bunch and it was called slow because it had more crystals glued to the backing and hence took longer to expose. That meant that your shutter had to say open longer to let in enough light to properly expose the film in creating a photo. It was referred to as a "daylight" film because it didn't do very well in low light without a flash. You could enlarge photos bigger with 100 ASA than with the other film speeds without it causing the picture to become grainy. The grain was caused by enlarging the negative so big that you started to see the film backing between the silver crystals and the bigger the picture was enlarged, the grainer it became. This aspect of film is what we now refer to as resolution in our digital images.
The faster the film, the fewer crystals were glued to the backing and the less light you had to let in to expose the crystals so they would capture the image. The faster film would allow us to use a faster shutter speed to equal out the exposure time. Unfortunately, the faster the film and the fewer the crystals, the quicker you would see the grain when you enlarged a negative but it let us shoot pictures in lower light or where fast shutter speed was needed, like action shots. ASA 800 would usually only get you an 8x10 enlargement before it started to become grainier at any larger print size, but it was good for sports and other action shots because you could set your shutter speed up high enough to stop the action. ASA 3200 would allow you to get pictures even while it was nearing dark outside but they were really grainy. I mean really, really grainy. Why? Because they didn't have much silver on the backing and it let you see the film backing without very much enlarging at all.
Let's explore how that comes into play with digital. Digital sensors are made to emulate film and for a simplified understanding of how, imagine that there is a light sensitive plate where the film was and that the plate is made up of a bunch of light sensitive dots on it. The more the megapixels, the more dots there are. Your digital camera will get the clearest, highest resolution, image when the ISO is set on 100, just like the film and when it is set that way, all the dots on the plate are ready to accept light to make up the dots that will become the image. You will need a good bit of light to take a picture so your lens is going to have to be more open and/or your shutter speed will have to be set at a relatively slow speed. A good sunny daytime setting for your camera when it is on 100 ISO would be an aperture of f8 and shutter speed of 125. If you want to have a faster shutter speed to get a good clear shot of a fairly slow moving object at that same aperture of f8 for fairly good depth of field, you can move your ISO up to 200 and move your shutter speed up one notch, whatever that is on your camera. Your highest resolution, read that best, pictures will be taken at 100 ISO.
The relationship between ISO, f-stop, and shutter speed is that if you have a perfect meter reading to take a picture in a certain light, with a certain shutter speed and a certain ISO, if you want to change one of the three to account for a changing condition, you must change one of the other two. If the light dims and you want your camera to react just the same to the change in light, you can raise your ISO. If you don't want to change your ISO but you need a faster shutter speed then you have to lower your f-stop to f7.1, the next stop down or perhaps f5.6, depending on which you must use to get your meter to say you're going to get a good exposure.
What happens when you raise the ISO from 100 to 200 is that some of the dots are turned off on that plate, i.e., fewer silver crystals, so you can get an exposure with less light. Going on up from 100 ISO to 400 ISO, like putting a roll of 400 ASA film in your camera, gives you the ability to raise your shutter speed up 2 notches to catch faster action OR take your aperture down 2 notches which will lower your depth of field but open your lens more so you can let more light in. What you have also done is turn off some more of your dots on the sensor which creates noise, which is the equivalent of graininess in film. Noise usually is more evident in dark parts of your image, or shadows. If you try to make a big print from an image, the noise will show up like the grain did in film and for the same reason - parts of the sensor are turned off and hence not accepting any light to make up the image which causes gaps between the dots as did the absence of silver crystals did with film. The big plus is that we now have software that will smooth out noise whereas there was no way to fix grain in film. And that's the relationship between ISO, f-stops, and shutter speed. When you change one you have to change one of the other two. That's a major part of what your camera does for you on auto and that's the basics of what you need to know and be ale to do in order to shoot manual.
Shooting in auto is not a bad thing. The problem with it is that light and dark can fool your camera because it cannot see individual things in the scene. It sets the meter for an average of darks and lights in a scene and tries to average it out to a shade of grey referred to as 18% grey, whatever that is. If you aim your camera on auto at a big part of your scene that is very light or white, the meter thinks that's what's in the scene and sets the f-stop and shutter speed accordingly and thus makes everything that's real dark, go even darker and lose detail. In an outdoor scene, if you aim at the sky the ground goes dark. If you aim at the ground the sky gets washed out. You can make the camera compromise by splitting the center of your view through the camera so you get half lights and half darks, what you do is compromise both the sky and the ground and you don't get a good picture of either.
There are ways to fool the camera but in order to do that, you have to know what the camera is doing in any given situations and unless you know the relationship between iso, f-stop, and shutter speed, it will make it more difficult to outsmart your camera in auto and get good pictures when you want to and not be at the mercy of an out of control little beast that'll fit in your pocket only to come out just to frustrate you. If you know what the camera is doing, and can outsmart it, there's really no reason to not shoot in auto, especially when you are beginning. Often, if you don't give the camera a situation that confuses it, you can get better pictures on auto than any other way. For those of you who are just beginning and are stuck on auto as you're trying to learn, try to take pictures during the first hour of light in the morning or the last hour before dark. Try to have the part of your subject that's facing you aimed toward the light source which should be at your back, and respect the light/dark example I gave you above. Read all you can without falling asleep, especially those pesky "tips and tricks" manuals. They contain some good information.
Further, for those of you who benefit from this, there is no reason to come forward and reveal who you are unless you are just in a self-effacing mood. I don't need for you to openly express your appreciation. I understand your situation. If you can use it, good, if not, good. It didn't hurt anyone, especially me. I feel your pain. I've been where you are. You can't get a straight answer even with Google.
ISO, or in film, ASA, is a rating of the speed by which a digital sensor or film receives light that creates an image when the shutter is clicked. Like many, if not most, things with digital cameras, the speed with which an image is made is based on the way it worked in film cameras and is done that way so those who understand film can easily understand digital, with certain modifications. This is not about which one is better. It's about how the terms in digital came about from the way it was with film cameras.
Film for color prints was rated from ASA 100 up to 3200, maybe 6400 in the end - it isn't important. These numbers represent the "speed" of the film. You could get ASA 200, 400, 800, and 1600 in addition to 3200 and whatever. Most people used 100 ASA film because that usually gave them the best pictures. They set their cameras for 100 ASA and you left alone for the duration of the number of exposures the film was capable of taking, 24 or 36 usually. There are exceptions to that but they are not important for this discussion. The film, a roll of special paper, had flattened crystals of a silver compound. It was the silver compound that was sensitive to light and upon which an image was created. ASA 100 was the slowest film of the bunch and it was called slow because it had more crystals glued to the backing and hence took longer to expose. That meant that your shutter had to say open longer to let in enough light to properly expose the film in creating a photo. It was referred to as a "daylight" film because it didn't do very well in low light without a flash. You could enlarge photos bigger with 100 ASA than with the other film speeds without it causing the picture to become grainy. The grain was caused by enlarging the negative so big that you started to see the film backing between the silver crystals and the bigger the picture was enlarged, the grainer it became. This aspect of film is what we now refer to as resolution in our digital images.
The faster the film, the fewer crystals were glued to the backing and the less light you had to let in to expose the crystals so they would capture the image. The faster film would allow us to use a faster shutter speed to equal out the exposure time. Unfortunately, the faster the film and the fewer the crystals, the quicker you would see the grain when you enlarged a negative but it let us shoot pictures in lower light or where fast shutter speed was needed, like action shots. ASA 800 would usually only get you an 8x10 enlargement before it started to become grainier at any larger print size, but it was good for sports and other action shots because you could set your shutter speed up high enough to stop the action. ASA 3200 would allow you to get pictures even while it was nearing dark outside but they were really grainy. I mean really, really grainy. Why? Because they didn't have much silver on the backing and it let you see the film backing without very much enlarging at all.
Let's explore how that comes into play with digital. Digital sensors are made to emulate film and for a simplified understanding of how, imagine that there is a light sensitive plate where the film was and that the plate is made up of a bunch of light sensitive dots on it. The more the megapixels, the more dots there are. Your digital camera will get the clearest, highest resolution, image when the ISO is set on 100, just like the film and when it is set that way, all the dots on the plate are ready to accept light to make up the dots that will become the image. You will need a good bit of light to take a picture so your lens is going to have to be more open and/or your shutter speed will have to be set at a relatively slow speed. A good sunny daytime setting for your camera when it is on 100 ISO would be an aperture of f8 and shutter speed of 125. If you want to have a faster shutter speed to get a good clear shot of a fairly slow moving object at that same aperture of f8 for fairly good depth of field, you can move your ISO up to 200 and move your shutter speed up one notch, whatever that is on your camera. Your highest resolution, read that best, pictures will be taken at 100 ISO.
The relationship between ISO, f-stop, and shutter speed is that if you have a perfect meter reading to take a picture in a certain light, with a certain shutter speed and a certain ISO, if you want to change one of the three to account for a changing condition, you must change one of the other two. If the light dims and you want your camera to react just the same to the change in light, you can raise your ISO. If you don't want to change your ISO but you need a faster shutter speed then you have to lower your f-stop to f7.1, the next stop down or perhaps f5.6, depending on which you must use to get your meter to say you're going to get a good exposure.
What happens when you raise the ISO from 100 to 200 is that some of the dots are turned off on that plate, i.e., fewer silver crystals, so you can get an exposure with less light. Going on up from 100 ISO to 400 ISO, like putting a roll of 400 ASA film in your camera, gives you the ability to raise your shutter speed up 2 notches to catch faster action OR take your aperture down 2 notches which will lower your depth of field but open your lens more so you can let more light in. What you have also done is turn off some more of your dots on the sensor which creates noise, which is the equivalent of graininess in film. Noise usually is more evident in dark parts of your image, or shadows. If you try to make a big print from an image, the noise will show up like the grain did in film and for the same reason - parts of the sensor are turned off and hence not accepting any light to make up the image which causes gaps between the dots as did the absence of silver crystals did with film. The big plus is that we now have software that will smooth out noise whereas there was no way to fix grain in film. And that's the relationship between ISO, f-stops, and shutter speed. When you change one you have to change one of the other two. That's a major part of what your camera does for you on auto and that's the basics of what you need to know and be ale to do in order to shoot manual.
Shooting in auto is not a bad thing. The problem with it is that light and dark can fool your camera because it cannot see individual things in the scene. It sets the meter for an average of darks and lights in a scene and tries to average it out to a shade of grey referred to as 18% grey, whatever that is. If you aim your camera on auto at a big part of your scene that is very light or white, the meter thinks that's what's in the scene and sets the f-stop and shutter speed accordingly and thus makes everything that's real dark, go even darker and lose detail. In an outdoor scene, if you aim at the sky the ground goes dark. If you aim at the ground the sky gets washed out. You can make the camera compromise by splitting the center of your view through the camera so you get half lights and half darks, what you do is compromise both the sky and the ground and you don't get a good picture of either.
There are ways to fool the camera but in order to do that, you have to know what the camera is doing in any given situations and unless you know the relationship between iso, f-stop, and shutter speed, it will make it more difficult to outsmart your camera in auto and get good pictures when you want to and not be at the mercy of an out of control little beast that'll fit in your pocket only to come out just to frustrate you. If you know what the camera is doing, and can outsmart it, there's really no reason to not shoot in auto, especially when you are beginning. Often, if you don't give the camera a situation that confuses it, you can get better pictures on auto than any other way. For those of you who are just beginning and are stuck on auto as you're trying to learn, try to take pictures during the first hour of light in the morning or the last hour before dark. Try to have the part of your subject that's facing you aimed toward the light source which should be at your back, and respect the light/dark example I gave you above. Read all you can without falling asleep, especially those pesky "tips and tricks" manuals. They contain some good information.
Oct 14, 2011 22:06:36 #
CORRECTION: This is the third paragraph from the bottom in the original post. The corrections are in all caps below. Please excuse me. I was getting a little hungry and wandered off the point.
What happens when you raise the ISO from 100 to 200 is that some of the dots are turned off on that plate, i.e., fewer silver crystals, so you can get an exposure with less light. Going on up from 100 ISO to 400 ISO, like putting a roll of 400 ASA film in your camera, gives you the ability to raise your shutter speed up 2 notches to catch faster action OR take your aperture UP 2 notches which will RAISE your depth of field close your lens more so you can let less light in. OR YOU CAN RAISE BOTH APERTURE AND SHUTTER SPEED UP ONE NOTCH EACH. What you have also done is turn off some more of your dots on the sensor which creates noise, which is the equivalent of graininess in film. Noise usually is more evident in dark parts of your image, or shadows. If you try to make a big print from an image, the noise will show up like the grain did in film and for the same reason - parts of the sensor are turned off and hence not accepting any light to make up the image which causes gaps between the dots as did the absence of silver crystals did with film. The big plus is that we now have software that will smooth out noise whereas there was no way to fix grain in film. And that's the relationship between ISO, f-stops, and shutter speed. When you change one you have to change one of the other two or you can change both. That's a major part of what your camera does for you on auto and that's the basics of what you need to know and be able to do in order to shoot manual.
What happens when you raise the ISO from 100 to 200 is that some of the dots are turned off on that plate, i.e., fewer silver crystals, so you can get an exposure with less light. Going on up from 100 ISO to 400 ISO, like putting a roll of 400 ASA film in your camera, gives you the ability to raise your shutter speed up 2 notches to catch faster action OR take your aperture UP 2 notches which will RAISE your depth of field close your lens more so you can let less light in. OR YOU CAN RAISE BOTH APERTURE AND SHUTTER SPEED UP ONE NOTCH EACH. What you have also done is turn off some more of your dots on the sensor which creates noise, which is the equivalent of graininess in film. Noise usually is more evident in dark parts of your image, or shadows. If you try to make a big print from an image, the noise will show up like the grain did in film and for the same reason - parts of the sensor are turned off and hence not accepting any light to make up the image which causes gaps between the dots as did the absence of silver crystals did with film. The big plus is that we now have software that will smooth out noise whereas there was no way to fix grain in film. And that's the relationship between ISO, f-stops, and shutter speed. When you change one you have to change one of the other two or you can change both. That's a major part of what your camera does for you on auto and that's the basics of what you need to know and be able to do in order to shoot manual.
Oct 15, 2011 03:20:42 #
This is an absolutely great "Primer" for the uninitiated. I assume you took the time to do this so in the future, new arrivals can be directed here when they ask the same question others have asked. (The other reason is that you're a nice guy and I know from watching that you do love to teach).
Great job.... and for all the new folks who dont know the gift that this post is to them...Thank you Gessman!
Great job.... and for all the new folks who dont know the gift that this post is to them...Thank you Gessman!
Oct 15, 2011 08:25:05 #
nikondaddy
Loc: Mayfield,Kentucky
Iso and Asa are the same dog with two different names and in film low numbers 64-100 meant low grain and in digital mean less noise .. Exposure is determined by these numbers but most d igital cameras alllow you to see exposure by using the programs and will allow you to venture out into manuel on your own like f/16 at 125th of sec as iso or asa 100 on sunny day.
Oct 15, 2011 14:45:39 #
rocco_7155 wrote:
This is an absolutely great "Primer" for the uninitiated. I assume you took the time to do this so in the future, new arrivals can be directed here when they ask the same question others have asked. (The other reason is that you're a nice guy and I know from watching that you do love to teach).
Great job.... and for all the new folks who dont know the gift that this post is to them...Thank you Gessman!
Great job.... and for all the new folks who dont know the gift that this post is to them...Thank you Gessman!
Thanks rocco. It's not so much that I like to teach but more that I like to see people learn and see them light up when the "bulb" comes on. I suppose that would be the other side of "loving to teach" so, I suppose, in a sense, your point is a good one. Some of us don't speak "techno" and if that is understood and we can break a complicated concept down and clearly communicate it to those who don't, most anyone can "get it." I am building a little stack of reusable explanations that can be cut and pasted to good advantage. Rather than telling someone simply to go do this or that who is new to cameras, photography, and they're not the same, computers, and the Internet all at the same time would almost make an ordinary person want to give up. Talking to them in terms that only give them more stuff to look up is not an answer if the question is, "how can I help this person." I'm not trying to build a monument to myself here, merely help cut down on some of the repetitive confusion.
Oct 15, 2011 14:53:09 #
nikondaddy wrote:
Iso and Asa are the same dog with two different names and in film low numbers 64-100 meant low grain and in digital mean less noise .. Exposure is determined by these numbers but most d igital cameras alllow you to see exposure by using the programs and will allow you to venture out into manuel on your own like f/16 at 125th of sec as iso or asa 100 on sunny day.
Thanks. Well said.
Oct 21, 2011 22:14:00 #
Oct 21, 2011 22:30:17 #
I think you did an excellent job! Wish I could have that broken down for me like that years ago!
Oct 21, 2011 22:31:53 #
Janice wrote:
Yes - I agree with rocco - this is a great gift - thank you gessman!
Thanks. :-)
Oct 21, 2011 22:35:34 #
sgbrown wrote:
I think you did an excellent job! Wish I could have that broken down for me like that years ago!
Thanks. Some things about photography do not lend themselves to over-simplification. That does, fortunately. Glad you feel you can benefit from it.
Something I'd really like to see is for some of the folks who are getting the wonderful outstanding pictures to share some of their routine techniques with the rest of us - you know, those little tricks that give them that crisp sharpness and clarity they get in their pictures. Maybe later we can get them to do that.
If you want to reply, then register here. Registration is free and your account is created instantly, so you can post right away.