The Simple Math of Correct Exposure

so what is wrong with this photograph well if you know a little bit about photography you might say this is under exposed underexposed and this is a term that has to do with and it's related directly to today's topic which is exposure so it seems like it might go together exposed and exposure but the question is what does this mean what is underexposed mean or what is exposure exposure is a way of measuring how well you have done getting the right amount of light onto your film so imagine over here we've got a piece of film or a digital sensor just light-sensitive something I always seem to use the little brown squigglies for that so you've got your camera you've generally your film or your sensor it's hidden away from all the light and when you open up your camera by taking a photograph it pops open the shutter light rushes in and it's exposed so it touches your film or your sensor and then it creates a sort of either a chemical or a electric Rhea interaction where the light interacts with that surface and it creates then an image as a result and that image is measured as its measured partially for its exposure that exposure is just a measure of how much light it picks up while the shutter is open and Wildish while the sensor or the film is exposed to light so this is a really important thing because exposure has to do with the quality of an image how it looks you want to have good exposures so that your photographs are crisp and clean and look nice well now what does it look like when you do the opposite looks like something like this this is overexposed overexposed just means too much light fell on the sensor or on the film and all the colors are washed out all the darks are gone you can see the light at the darkest thing you have on here is sort of still a pretty light color so that is overexposed now this is somewhere closer to correct exposure so still not perfect but bit closer and you can see here that now I've got tones that range all the way from high white so very bright white down to a pretty dark true black almost so you know we're here especially the true blacks and lots of things in between now tones are not actually colors tones are sort of like a range of I guess I'm not the amount of light that an object gives off so you can see here that this is a white tone this is a gray tone and this is sort of a black tone and everything in between is a part of the tonal range of an image and what your what your camera is trying to do when it's looking for exposure is it's trying to create sort of a contrast range that has a little bit of dark a little bit of light and lots of in-between so when your camera looks at this scene it wants to make 18% gray sees everything in black and white and it's like that we want to make it 18% gray so that everything averages out so if you were to take all the colors or take all the tones so you're taking a black and white version of the image and throw it in a paint mixer the resultant color would be 18% gray if your meter had it its way but sometimes that doesn't always match reality in the in reality this scene had a lot of darker tones in it and a lot of a lot of lighter tones but ironically when it sort of works out it comes out to about 35% gray so here you can see the problem that metering and exposure create for photographers you've got your camera wanting to make 18% gray your photo isn't actually full of anything that's 18% gray it's actually if you look at it's about 35% gray so you want to try to find a way of tricking your camera into exposing correctly for the 35% that it actually is not the 18% that it wants to make let's try and now to make really quickly sort of a diagram of what is actually happening here let's imagine we've got a stool here and it is a three-legged stool so you've got one two three legs on your stool and they're kind of funny legs sorry about that now one of your legs is going to be your ISO this is going to determine how fast the photograph can be taken because you've got a certain sensitivity on your film you're also going to have your shutter speed and we're just going to right shutter is going to be how fast the photograph is taken so how long the camera is open to the light and your last thing is going to be your aperture and this is sort of like the size of the window that's allowing the light into your camera now you can imagine this as a three-legged stool where if one of the legs is too short the stool falls over so the correct exposure is sort of like sitting is sort of the flat stool that you imagine that you can sit on or put an egg on and it won't roll off now if any one of these things is incorrect or not matching then you're going to have a problem your stool is not going to be flat or usable really and sometimes this will be called the exposure triangle so these three things are all related they're variables of good exposure and if you change one well then you have to change others so just like if you were shortened one of these legs you would then have to shorten all of the legs or you'd have to sort of adjust them so that they would all create then a flat stool a lot of times what a lot of photographers when they're first starting out they will then first lock in one so for example I always four it was really easy back when I started ISO was always set by your film so you had a piece of film and then your only two variables were your shutter speed and your aperture and then you just could mess around with those two and always try to make sure that they were bringing everything into equilibrium so what you're trying to do when you're aiming for good exposure is you're trying to fill the film with a certain amount of light that it needs in order to expose the photograph correctly and you can kind of imagine then the film or your sensor to be sort of like a bucket so we're going to write film our sensor here and your bucket you can change the size of your bucket so your bucket can be big it can be small and in this case you could think of a small bucket as being a film with high high ISO so it doesn't take very much water to fill a very small bucket it doesn't take very much light to fill a very sensitive piece of film or a sensors like so something at ISO 800 is like a very small bucket so you're trying to imagine that you need you know maybe this much water in your bucket no matter which bucket you have if it's a big bucket or a small bucket you need to have it you know maybe reach this 70% full level or something like that say and you're doing this with a pipe so the pipe is running from here to here you've got your pipe right here and you're running water through it and you can change then with this in this setup you can change how long the water runs you can change how big the pipe is and you can change the size of the bucket so we're going to make sure it makes inside now the length of time that the water is running into the end of the bucket is sort of like your shutter speed so just like you can open the shutter of your camera for a really long time you could open the faucet and let water run for a really long time so that it fills up the bucket even if you have a tiny pipe your pipe could be really mini it could be like a f-22 pipe so really tiny pipe but and it could stay there for a long time for the water to go through and fill up to fill up your bucket but at the same time you could also maybe spring for a bigger pipe so you can maybe make the pipe like this big and then you've got tons of water coming through which therefore means you don't need as much time to let the water run through or in this case you don't need as long of a shutter speed so you can let more water in more quickly that's really important so remember that your aperture is like the size of the pipe now the length of the pipe but the width of the pipe how much bandwidth it has how much water or light in the case of a camera is able to pour through and into your bucket and your shutter speed is the amount of time you have that you're running that water through and your sensor is the size of the bucket itself and all of these things are adjustable so you can run the water longer you can make the pipe bigger or smaller and you can change the size of the bucket and either way you've kind of come up with a piece of piece of math that again is a triangle so you have this three so you have these three variables and you have your your aperture your shutter speed and your ISO each image has an interesting and unique combination of these things so right here we've got our ISO to say ISO shutter let's put shutter with SH and we'll have our f-stop right here now this image to the left was taken on a bright evening at ISO 200 its shutter speed is about a 500th of a second and it was taken at around F 5.6 so it's pretty standard for an evening bright day that's about pretty average shot really this shot was the Irish cottage it was a cloudy rainy day it was ISO 200 still though I took the shot though at a hundredth of a second and it is also at F 5.6 so you can see here one you can see here that we've got our st. we've got the same bucket so it's the same two hundred sized bucket but in this case I let the water run on the Irish cottage it ran a lot longer five hundredths of a second and a hundredth of a second so that's a lot fast that's a lot faster this image here of the abandoned airport then of the Irish cottage because I didn't have as much light so I just didn't have as much light to expose the film and so I needed a little bit longer to do that now like take a look over here at the Berlin situation so here I'm standing on top of a house looking out over downtown Berlin you can see the TV tower is very far away and that is actually taken at a 400 ISO shutter speed was 115 and the f-stop was five points sorry 3.5 Wynette concentrate here 3.5 all right so what's going on here now we've got a bigger bucket first of all so the bucket is art sorry a smaller bucket I need to remember my own metaphors here so you've got a smaller bucket so it takes less time to fill that that bucket and we've got now more the water's running for even longer so we've got a fifteenth of a second which is a lot more than a hundredth of a second and even more than a five hundred thirty seconds so a lot more and in this case the pipe is actually a lot bigger because we've got a smaller number so a smaller number means a bigger pipe and in this situation we've got a 3.5 f-stop which means that it's almost one entire stop of light difference between here and this is a very very important concept you will all have to memorize went from the shutter speed lesson and from the aperture lesson that you've got sort of these variables for so for F stops for example you have you have maybe f4 and then you have F five point six and there one stop away from each other really important to remember so that's one stop you also have for example maybe one hundred and twenty-fifth of a second or one hundred and 250th of a second and you've got one hundred and twenty fifth of a second now that is also one stop of light so shutter speeds can be one stop of light so shutter speeds can be one stop of light and F stops can be one stop of light away from each other and that is a really important thing to remember because another important thing is that your ISO speeds are also one stop apart so if this was 100 for example right here instead of 200 you would then have to change this shutter speed and cut it in order you'd have to cut it in half or you'd have to double it I guess so it kind of hard to explain with these numbers but you would have to move if you were to move this down to 100 you have to move this shutter speed up to two hundred and fiftieth because you have in this situation you have doubled the amount of time that you need to expose that image and therefore you need to either double the amount of time or if you don't do that so say you were not to double the amount of time that you need you would have to add more light so you'd have to move this to f/4 so you've moved one stop here so we've got minus one stop so now we need to add a stop somewhere else source we've added our stop here and the same thing if you were to do that you know maybe to this 400 here say you were to say it take this and make this one hundred speed film well that means that's two stops so we've got - two stops here and that means you need to make up for it somewhere else well you can't really make it up for it here because maybe your lens isn't faster so you'd have to just you have to just do it with your shutter speed so that means you'd move down to an eighth of a second and then down to a quarter of a second so you would already be at a very slow photograph right there so here you've got your plus two right here so it's always a matter of remembering these distances and that's why it's important to go back and take a look at what the difference between F 4 is and F 5 point 6 and F 8 and remember the traditional stuff so traditional full stops so that you know for each photograph that you're taking how many stops you have to move each of these variables when you're shooting manual photographs alright so that was your lesson about exposure and check out more lessons like this at all varsity or

Channel: Allversity

Views: 1,440,181

Keywords: exposure, Photography (Invention), Light (Art Subject), Camera (Invention), Art (Collection Category), aperture, shutter, iso, digital, camera, help, photos, photographs, tutorial

Id: EtQoLZN6K6Q

Length: 14min 58sec (898 seconds)

Published: Tue Jul 03 2012