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Thread: ISO, Aperture and Depth of Field.

  1. #1
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    ISO, Aperture and Depth of Field.

    Hi all,

    Apologies if these questions have been asked but I have had a look through the forum and found some posts about what I am asking but none that really answer my questions fully. So, I have a few and here they are:

    1. Am I correct in assuming that is diffraction that means that a wide aperture (say f/2.8) results in a shallow depth of field? Its just that if you think about it without the science, surely a big hole letting in more of the image would mean the image had more in focus (greater depth of field).
    2. I think I am finally understanding iso but itís still a bit cloudy as I donít see why a DSLR needs this optionÖ. So I guess my statement is, even if you leave the shutter open for 30 minutes, it cannot capture enough light to provide the sensor and therefore you have to fool the sensor into thinking that whatís being presented to it is greater than what it actually is. therefore, iso is needed as shutter speed and aperture are unable to fulfill the role of iso in fooling the cameras sensor.

    With regards to the iso statement, these two (virtually identical clips Ė not sure who stole whose presentation) YouTube clips helped me a great deal in finally getting my head around what the sensor is doing in changing the iso.

    http://www.youtube.com/watch?v=WEApLA-YNko
    http://www.youtube.com/watch?v=BSMDdpPtOqo

    anyway, sorry if i have not understood things but this is all a bit difficult to get my head round. separately they are easy to comprehend... put them all together and their function boundaries cross each others function boundaries if you see what i mean.

    thanks
    Aaron

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    Re: ISO, Aperture and Depth of Field.

    Aaron,

    Welcome to the forum.

    There are excellent tutorials on this site, under the tutorials/ learn photography concepts page http://www.cambridgeincolour.com/lea...y-concepts.htm, that provide a through explanation of these issues. Please start by going through them. Then if you post here with any remaining questions, folks will chime in with answers. This is the most helpful photo forum I know, but it doesn't make sense for us to try to re-create the wheel when someone has already written such excellent explanations.

    Dan

  3. #3

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    Re: ISO, Aperture and Depth of Field.

    Aaron welcome to CIC, now to somewhat answer you questions, when the aperture is set to a small number say f/1.4 or your f/2.8 then the hole is big letting in lots of light. However it creates a very shallow area (front to back) that is in focus, now a larger number such as f/29 or f/32 creates a very, very small hole for light to pass through resulting in some diffraction, however it creates a very large area (front to back) that will be in focus. As for diffraction in the real world do not worry about it, it only shows up if you were to view the image at a large magnification as it will not be as sharp, when was the last time you looked at something that was 100ft away and it was tack sharp, never. Now here is a funny thing, when you print a image you apply sharpening to it, now a small image you apply more as you look at it up close, however if you print a very large image you apply less as it is viewed at a greater distance so it does not need to be sharpened as much. Case in point if you look at a billboard along side the road it looks clear and sharp, now look at it up close, it is fuzzy, out of focus as it is been make to be viewed at a great distance and so our eyes and brain work together to give up the correct view. So I do not worry about diffraction because when I print a larger image it is not as sharp as if I was looking at it on the viewer. So if you want shallow depth of field that is with either/both the foreground or background soft than small number = large hole, large depth or field than big number = small hole.
    Now ISO, this comes from the old world of film, we still use it, all it means is how sensitive the sensor is to light, the higher ISO the more sensitive (less exposure time needed), lower the less sensitive (more exposure time needed). Now the camera's onboard computer know what ISO you selected, you can adjust the other two setting aperture and shutter speed (manual) or you select to adjust aperture and the camera will adjust shutter speed (aperture priority) or reverse it and you have shutter priority. Now these three items are the Holy Trinity of Photography, from film to today's digital, learn them without them you will be lost.
    Now what I have written is very simple, it is somewhat more complex however when you break it down to the basic concept that is what you have. Being new as your questions show keep it simple for now, the 30 minute shutter speed if shot during daylight would involve the use of at least 6 stops of netural density filters to fool the sensor into thinking that the light is not as strong. At this point in time stay away from this information until you get the Holy Trinity down, then start to push it as this is what happening with doing long exposures the Holy Trinity still applies here the same as any other shot.
    To better understand them you need to practice, practice, practice and practice again, learning from what you did before. Also do not be afraid to post an image and let us know what you want, we a pretty nice group here.

    Cheers:

    Allan
    Last edited by Polar01; 29th August 2013 at 03:10 PM.

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    Re: ISO, Aperture and Depth of Field.

    hi Allan and Dan. many thanks for the info. as always its created more questions but in a good way. and sorry dan, i trawled through the forum but completely missed the tutorials section. that's my homework for this evening :-)

    thanks again guys

  5. #5
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    Re: ISO, Aperture and Depth of Field.

    Quote Originally Posted by Polar01 View Post
    Aaron welcome to CIC, now to somewhat answer you questions, when the aperture is set to a small number say f/1.4 or your f/2.8 then the hole is big letting in lots of light. However it creates a very shallow area (front to back) that is in focus, now a larger number such as f/29 or f/32 creates a very, very small hole for light to pass through resulting in some diffraction, however it creates a very large area (front to back) that will be in focus. As for diffraction in the real world do not worry about it, it only shows up if you were to view the image at a large magnification as it will not be as sharp, when was the last time you looked at something that was 100ft away and it was tack sharp, never. Now here is a funny thing, when you print a image you apply sharpening to it, now a small image you apply more as you look at it up close, however if you print a very large image you apply less as it is viewed at a greater distance so it does not need to be sharpened as much. Case in point if you look at a billboard along side the road it looks clear and sharp, now look at it up close, it is fuzzy, out of focus as it is been make to be viewed at a great distance and so our eyes and brain work together to give up the correct view. So I do not worry about diffraction because when I print a larger image it is not as sharp as if I was looking at it on the viewer. So if you want shallow depth of field that is with either/both the foreground or background soft than small number = large hole, large depth or field than big number = small hole.
    Now ISO, this comes from the old world of film, we still use it, all it means is how sensitive the sensor is to light, the higher ISO the more sensitive (less exposure time needed), lower the less sensitive (more exposure time needed). Now the camera's onboard computer know what ISO you selected, you can adjust the other two setting aperture and shutter speed (manual) or you select to adjust aperture and the camera will adjust shutter speed (aperture priority) or reverse it and you have shutter priority. Now these three items are the Holy Trinity of Photography, from film to today's digital, learn them without them you will be lost.
    Now what I have written is very simple, it is somewhat more complex however when you break it down to the basic concept that is what you have. Being new as your questions show keep it simple for now, the 30 minute shutter speed if shot during daylight would involve the use of at least 6 stops of netural density filters to fool the sensor into thinking that the light is not as strong. At this point in time stay away from this information until you get the Holy Trinity down, then start to push it as this is what happening with doing long exposures the Holy Trinity still applies here the same as any other shot.
    To better understand they you need to practice, practice, practice and practice again, learning from what you did before. Also do not be afraid to post an image and let us know what you want, we a pretty nice group here.

    Cheers:

    Allan
    The technical term for the combination of aperture, shutter, and ISO is the exposure triangle. As Allan related you need to learn these, and how they work together.

    Bruce

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    Re: ISO, Aperture and Depth of Field.

    Aaron,

    I hope I did not come across as snippy. I was just suggesting that it is by far more efficient to start with the detailed and polished tutorials. But please post your questions once you have had a chance to go through them.

    Dan

  7. #7
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    Re: ISO, Aperture and Depth of Field.

    >> I think I am finally understanding iso but it’s still a bit cloudy as I don’t see why a DSLR needs this option….

    ISO is a leftover from film days. A digital sensor has only one sensitivity to light regardless of which ISO number you set. The difference in the image brightness is taken care of by the electronics.

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    Re: ISO, Aperture and Depth of Field.

    Quote Originally Posted by benm View Post
    >> I think I am finally understanding iso but it’s still a bit cloudy as I don’t see why a DSLR needs this option….

    ISO is a leftover from film days. A digital sensor has only one sensitivity to light regardless of which ISO number you set. The difference in the image brightness is taken care of by the electronics.
    To elaborate a bit: it is possible to amplify the image in post-production as well. But, that means you use the digitised values from the base ISO image. For several reasons, it's better to do the amplification
    on the analogue signal directly from the sensor, and avoid excessive brightening in post-production (the latter results in much higher/ much more visible noise)

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