What is ISO, aperture and exposure?

Discussion in 'Photography' started by aaronnorth, 1 Aug 2008.

  1. aaronnorth

    aaronnorth Member

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    can somebody give a description of what these 3 are? or a link to a useful guide,

    thanks, Aaron
     
  2. tko187

    tko187 Member

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    Was going to post the same question, hopefully someone can help. Im new to the world of DSLR, or even if someone can point us in the direction of a good forum.Cheers.
     
  3. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

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    ISO is a term used originally in film and refers to the films sensitivity to light. It is an acronym for International Standardization Organization. The higher the number the higher the sensitivity. In bright light therefore one uses a lower ISO value to avoid overexposure. In dim light one uses a high ISO in order to capture the image. ISO values typically double in value - from 25, 50, 100, 200, 400 and so on (although there are intermediate values). Again in the film days film stock was sold in these values. ISO 100 film is twice as fas, or twice as sensitive to light as ISO 50. In digital cameras setting an ISO 100 means that the sensors will capture and process twice as much light as if the setting was 50.

    You can see the ISO film rating on these two cartridges. The Kodak color film on the left is rated at ISO 100 while the Agfa B&W on the right is ISO 400. That means the Agfa can be used under lighting conditions 4X more dim than the Kodak. Another use for fast film or fast ISO is taking sports pictures or pictures of moving objects, where fast shutter speeds are used.
    [​IMG]



    Aperture refers to the opening at the back side of a lens. Light must travel from the subject through the lens and then to the film plane or photosensors. The aperture is the gate which controls the amount of light that reaches the film plane. Apertures are also typically referred to as f-stop. f-stop is a ratio of the aperture diameter and the focal length of the lens. So that a 50mm lens having a maximum aperture opening as wide as 50mm will have this f-stop of 1. Apertures vary to admit different amounts of light depending on the brightness of the scene. Again on a bright day there is too much light so you would use a smaller aperture in order to block most of that light. Since this ratio is focal length/diameter you can see that the f-stop number increases as the diameter of the aperture decreases. Using this 50mm lens, if we restricted the diameter of the aperture from 50mm to 6.25mm this would be an f-stop of f8. When we restrict the aperture control of a camera this is referred to as "Stopping Down". There are other advantages in using a smaller aperture. Smaller aperture give the lens a deeper area in which things are in focus so we can vary the aperture size depending on whether we want more things in focus or if we want to blur the background/foreground.

    Aperture size f22. Notice that the aperture is constructed by weaving several thin metal leaves. In this lens 7 "blades" were used to enable the aperture to vary it's size.
    [​IMG]

    Aperture size f5.6. Here you can see more clearly the seven sided polygon created by the movement of the blades. These polygons reveal themselves in the out-of-focus highlights of a picture which betrays the blade count of the lens. More expensive lenses use nine blades to approximate a circle.
    [​IMG]

    Exposure was the effect that light had on photographic film or paper. Chemicals in the film or paper responded to light by turning dark. The more exposure to light caused a darker reaction within the film or paper. There are two ways to control the exposure. The first is to keep the aperture fixed at a certain diameter and to vary the amount of time that light is allowed to penetrate to the film/paper (this is called the Shutter Speed). The second way is to keep the amount of time the aperture is open at a fixed value and to vary the opening of the aperture to admit more or less light. In digital cameras there is a third way - and that is to vary the sensitivity of the photosensors by keeping both shutter speed and f-stop constant and to vary the ISO.

    Photography is therefore all about light and controlling it's quantity and quality. A properly exposed photo may have darker areas and lighter areas but the goal is to keep the darker areas from being too dark and the brighter areas from being too bright (if you are trying to show information in those areas). The photographe uses various combinations of f-stop and shutter speeds to create the effect he is after.

    Hope this helps. :D

    Cheers,
     
  4. Mark Evans

    Mark Evans Expert

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    nicely written clive,

    you'll also find that a "sweet spot on alot of lenses is at about F11 where everything is pin sharp. seems to be on my kit. certainly was on my 135mm f2 Lseries :D

    the smaller the F stop eg f2 the more "out of focus" look. increase this number and youll lose shutter speed which in turn fuels the need for a tripod (camera shake blur) OR bump up the iso and gain shutter speed.
    if you bump up the iso, this introduces "noise" little grains, dots whatever you want to call it. the more expensive kit you'll not notice it. ( i can shoot at iso1600 with no noise! honestly the 5d is that good) when i first started photography it took a while to get that in my head but when you understand the relationship between all of these factors you'll glide your way through an slr. the NEXT thing to learn is composition!

    mark
     
  5. aaronnorth

    aaronnorth Member

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    thanks guys, you know whats coming next Mark... whats composition?I a bit confused on exposure but i am sure it will come in time ;)
     
  6. Mark Evans

    Mark Evans Expert

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    composition?...how you frame a picture.a bit like an aquascape.we all see it differently. again using the rule of thirds. slightly different to the golden ratio? i dont think so. :D

    mark
     
  7. Steve Smith

    Steve Smith Member

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    Great writeup Clive :) I've had an SLR for a while (300D) and I usually shoot on manual, but I tend to guess settings more than anything (though I sometimes get a good picture).

    Depth of field and controlling it is my next nemesis :lol:
     
  8. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

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    Aaron, it helps to think about light as if it were a fluid like water. The exposure is a bucket. The idea is to allow enough water to enter the bucket so that you fill the bucket exactly to the brim. If too much water is let in it spills over and makes a mess. If not enough water is let through the bucket doesn't get filled. if you have a garden hose with a spray handle you can vary the size of the outlet by squeezing the handle a little or a lot. If you squeeze the handle just enough (small aperture) the water just dribbles out and you have to wait a long time to fill the bucket. The amount of time is the shutter speed i.e, how long the shutter is open. You can also fill the bucket by squeezing hard (wide open aperture) and you only have to wait a short while for the bucket to fill. So there are lots of different combinations of aperture + shutter speed that will fill the bucket exactly. If I get the combination wrong though I'll have problems. If I open wide for a longer time than I should I overfill. This is called overexposure and the image looks too white and washed out. If I restrict the opening too small and don't wait long enough the bucket doesn't fill. This is called underexposure and the image will look dark and muddy.

    All modern cameras have an automatic exposure control. They sense out how much light is streaming through the lens and they can figure out the combination of aperture and shutter speed fills the bucket exactly. The camera also allows you to pick say, an aperture and then figures out what shutter speed to use to fill the bucket exactly. Lets say you choose f11. You point the camera at the subject and it measures the amount of light in the scene. if the scene is a bit dim the camera might figure that you need to have the shutter open for say, 1/30th of a second, which is a long time.

    You take the shot. For our 50mm lens this means that the circular aperture with diameter 4.5mm is opened for 1/30th of a second and lets in a certain amount of light. But because this is such a long time your hand was shaking as you held the camera so the image looks blurry. In order to stop the action of the hand movement the aperture needs to be opened for less time. If I'm going to reduce the length of time the shutter is opened then I better have a bigger hole right? If I halve the time - 1/60th of a second then I have to double the aperture's cross sectional area. That means a 6.25mm diameter opening which is f8. I can double the aperture again f5.6 (8.93mm dia) but then I have to now halve the time again 1/125th of a second. So these three combination of time and area give exactly the same exposure: f11 at 1/30th second= f8 at 1/60th second = f5.6 at 1/125th second.

    It's better to forget about actual diameters and just remember the typical progression of f-stops larger to smaller=> f2.8, f4, f5.6, f8, f11, f16, f22. Each subsequent f-stop is half the size of the preceding one. At the same time you'll see typical shutter speeds like 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, and so on. Lenses that open up to 2.8 and larger, like l.8 and 1.4 are expensive - and that's what you are paying for, the ability to have a really large opening so that you can use higher shutter speeds (shorter opening times) and freeze the action or to capture the image in really dim light.

    Hope this helps.

    Cheers,
     
  9. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

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    Hiya Steve!,
    I use manual mode when shooting scenes with "high dynamic range". That means there is a big difference between the brightness of the brightest part of the scene and the darkest part. This happens all the time when shooting a tank with tank lighting only. But this is what's so great about digital cameras you can just guess at the exposure and make adjustments from there because you have instant feedback on the screen. :D For normal scenes I just use aperture priority and let the camera figure out the shutter speed.

    Don't know why depth of field is problematic. On the lower right hand side (facing the camera's front) there is a depth of field preview (DOP) button. Just press this button while looking through the viewfinder and the aperture stops down to the its shooting value. You can see what will be in focus before you take the shot. You can hold the DOP button down while focusing manually and get it right the first time. Sure, the image goes darker but that's no big deal. The higher the f-stop (smaller opening) the greater the depth of field. As saintly mentioned though, you want to try to keep the aperture no smaller than about f8/f11. At smaller aperture there is a phenomenon called diffraction which is light bending around small edges, that then lowers the overall sharpness. This is especially evident on macro lenses or otherwise shooting close up.

    Cheers,
     
  10. PM

    PM Member

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    My melon aches now :lol:
     
  11. Mark Evans

    Mark Evans Expert

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    :lol:

    clives right though, these are all things you need to know. especialy with an slr.TBH the depth of fiel preview button is a waist of time IME but it's there. over time you'll just kinda know roughly what exposure settings you'll need for certain situations.

    except for canons 50mm 1.4 prime for about £200 a stunning lens and as quick as you like. :D
    all other lenses in this bracket are big bucks.

    dont forget that even in manual mode the metering system will change things alot too. eg center weighted, spot metering etc etc ;) ( actualy this is very important. for eg

    your in manual mode shooting a landscape for inastance, you then leave the camera in spot metering mode (without knowing) shoot the scene, but the scene keeps over exposing!...why? cant figure it out. it in manual it should be fine shouldnt it?....
    little did you know it was in spot metering mode and youve been focusing on an area thats darker than everything else in the scene a tree, a dark blue car, now the camerea exposes for THAT item rendering everything else lighter than that over exposed.especialy sky! put it in Evaluative metering. should help things


    what i did when learning was learn the camera and every function it has! its very important.
    im not as good as clive at writing stuff like this, so i hope you understand what i mean

    mark
     
  12. George Farmer

    George Farmer Founder Staff Member

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  13. LondonDragon

    LondonDragon Administrator Staff Member

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    Links I have used in the past:

    These are great tutorials from the same site, just saves you browsing it:
    http://www.shortcourses.com/guide/
    http://www.shortcourses.com/use/
    http://www.shortcourses.com/workflow/
    http://www.shortcourses.com/sensors/
    http://www.shortcourses.com/tabletop/

    Good sites for camera reviews:
    http://www.steves-digicams.com/
    http://dpreview.com/

    Good site to post your images and look for photos from particular camera models and lenses:
    http://www.photosig.com/
    http://www.pbase.com/cameras

    Possibly the cheapest place to buy cameras:
    http://www.pixmania.co.uk/
    http://www.7dayshop.com/ (Canon stuff and acessories)

    Hope that helps :)
     
  14. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

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    This is true only if you are shooting subjects that are far away, like landscapes. If this is the case then most of the image is in focus. If you are shooting close up and macro such as plants in the tank where DOF makes or breaks the composition then the preview button is enormously useful. :idea:

    Cheers,
     
  15. aaronnorth

    aaronnorth Member

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    Thanks Clive, the bucket explanation helped a lot. I will have to read those pages when i can be bothered lol.

    From what i am getting:

    >Higher ISO is needed in darker lit places,
    >ISO 400 can get 4x more light than ISO 100,
    >A lower ISO is used for fast moving objects

    >Lower aperture (smaller f-stop = smaller width of lens)
    >Smaller aperture is usefull for finer detail in close ups - (or is it for everything?)

    >You reduce shutter speed by altering the aperture (lower f-stop, quicker shutter speed)
    >High exposure - more light (whiter image)

    Thanks for everyones help :D

    Aaron.
     
  16. tko187

    tko187 Member

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    I think this will take me a while, its kind of scary, but at least i have my feet wet, cant wait to get some results. Thanks for all the links and help.
     
  17. aaronnorth

    aaronnorth Member

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    think about when starting fishkeeping, plants, and other things in life... you get used to it :D
     
  18. George Farmer

    George Farmer Founder Staff Member

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    Hi Aaron

    A high ISO can be useful for darker places, as you can use a faster shutter speed, but the compromise is image quality. Higher ISOs in all but top-end cameras give grainy images. Most are fine up to 400.

    Lower ISO isn't necessarily used for fast moving - normally you'd use higher ISOs, if anything, to enable a faster shutter speed to capture movement.

    Lower f/stop = larger aperature size, so f/2.8 is much larger apeture than f/32.

    Larger aperature let in more light, so useful for capturing movement and/or shallow depth of field where you want to blur backgrounds i.e. macro (close-up) photography.

    Smaller aperature give a larger depth of field, so more detail is in focus. Useful for landscape photography etc. where you want as much detail fore-background as possible. The compromise here is slower shutter speeds, so a tripod is considered essential.

    Slower shutter speeds are also useful for creating the illusion of movement i.e. in waterfalls where the rushing water is blurred.

    The great thing about DSLR and some better compacts is that the shutter speed, aperature (depth of field) and ISO are all manually adjustable. There are other adjustments such as exposure metering modes and white balance that are also useful.

    I like to shoot mostly in 'aperature priority' (Av), where you adjust the aperature (depth of field) and the camera automatically compensates for shutter speed. ISO is normally manually selected, using the lowest ISO possible (for max. quality) to still maintain prefered shutter speed to capture movement and prevent camera shake if shooting handheld.

    Shooting in (shutter priority) Tv mode allows you to manually adjust the shutter speed and the camera will choose the aperture for you.

    I hope that makes sense.
     
  19. aaronnorth

    aaronnorth Member

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    Thanks George,

    would faster shutter speeds for photographing fished used as they are fast?
     
  20. PM

    PM Member

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    Anyone got a Nikon D40x? I have one but I'm not great with it yet...
     

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