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Thread: ISO vs Aperture F stops

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    Default ISO vs Aperture F stops

    Hi All,

    I'm still learning the basics and trying to fully understand the balance between ISO, Aperture and shutter speed.

    My question is here, in Low light situation, people tend to refer to on stop in aperture as say F5.6 to F4, or F4 to F2.8 etc. Could someone tell me does this stop has the same effect dumping up the ISO to from say 400 to 800, or 800 to 1600 etc?

    I'm trying to understand if you don't have pro grade glass and went for cheaper alternatives which are considered soft wide open. In low light situations would you assume you get the same grade quality with the below. All else being equal, ie camera & shutter speed etc

    F2.8 at 400 ISO (lets say sigma)
    f4 at 800 ISO (lets say Nikon's pro grade)

  2. #2
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    Default Re: ISO vs Aperture F stops

    Some sigma lenses are sharper than Nikon pro grade. Example = Sigma 35/1.4 hsm.

    so your question is really hard to answer.

    ISO and Aperture value and shutter has to be balanced to return a certain exposure. When maintaining the same exposure (brightness), if you want close down your aperture by 1 stop, you need to up your ISO by one stop if you want to maintain the same shutter speed. What choice to make really depends on:

    1. what ISO you are already currently on
    2. what shutter speed are you trying to maintain
    3. If the aperture you want will achieve what you want (depth of field)
    4. If your subject is moving
    5. How stable are you? are you on a tripod? or other kinds of support? are you hands/stance stable?
    6. What is your personal tolerance on noise on your particular camera.

    No one can answer these questions for you. You have to consider all these yourself at the moment while you are in that situation, and you alone have to make a call on that.

  3. #3

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    Also note that aperture affects depth of field, and ISO affects noise...
    Alpha

  4. #4

    Default Re: ISO vs Aperture F stops

    it depends on what you think is important
    wise man tell me "Everything in life is a trade off" (ok, that was my engineering prof...)

    anyway.

    if sharp sharp important eg need thicker depth of field, front sharp behind sharp, then have to raise ISO at risk of noise.

    if shaky paws like me drink too much coffeeeeeeeeeeeeeee, and you don't have a tripod, and you must not have noisy image, then we sell off depth of field for more brightness through a wider aperture.

    you don't need "pro grade" glass to make a nice photo under low light. you could do with a "nifty fifty" 50 f.18 , relatively inexpensive (sub $200 brand new)
    you could use a tripod. or even a bean bag or sack of rice.

    kit lens is pretty good, too.
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  5. #5

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    Exposure depends the triple constraints: aperture, shutter speed and ISO. A correct exposure can come in several combinations of these 3 constraints.

    Eg. F2.8, 1/100 and ISO800 give you a correct exposure or 0EV.

    Reduce aperture by 1 stop, F2.8 to F4, with 1/100 and ISO 400 remains, it will give you -1EV. So if you want still get 0EV, then you can either adjust shutter speed 1/100 to 1/50 or ISO 800 to 1600.

    So F4, 1/50 and ISO 800 or F4, 1/100 and ISO 1600 will still give you the same 0EV.

    Basically you can varies all the 3 for the correct exposure. (Some pple like their pic to under or overexposure so you adjust accordingly)

    question now is what are the tolerances for all the 3 settings for that situation you are taking.

    Aperture: depth of field and focusing. How much you want to focus and where you want to focus on. f2.8 might be too thin for group shot but ok for a solo portrait.

    Shutter speed: your subject is moving fast or slow or stationary. To freeze your subject or create motion blur. 1/100 might be good for freezing but 1/50 can create a sense of motion.

    ISO: depends on your camera and you rather than the situation you are in. How much noise you are ok and how much details you willing to give up if noise reduction is applied.
    ISO 800 might be ok but ISO 1600 maybe too noisy for you. Most or all modern and new camera can handle ISO 1600 quite well.


    The rules applied to all lens be it pro or kit lens. So it voices down how you want your photo to turn out to be.
    Last edited by dracoid4; 13th October 2013 at 07:54 PM.

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    Default Re: ISO vs Aperture F stops

    Quote Originally Posted by Ming1388 View Post
    F2.8 at 400 ISO (lets say sigma)
    f4 at 800 ISO (lets say Nikon's pro grade)
    The calculation itself is correct.
    EOS

  7. #7

    Default Re: ISO vs Aperture F stops

    Quote Originally Posted by Octarine View Post
    The calculation itself is correct.
    The calculation is correct but it does meant that only only F2.8 at 400 ISO on sigma and f4 at 800 ISO on pro grade. Both settings get your correct exposure and you can use it on any lens. The only issue is that whether your lens allows you to set it to F2.8 or F4?

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    Default Re: ISO vs Aperture F stops

    Quote Originally Posted by Octarine View Post
    The calculation itself is correct.
    Quote Originally Posted by dracoid4 View Post
    The calculation is correct but it does meant that only only F2.8 at 400 ISO on sigma and f4 at 800 ISO on pro grade. Both settings get your correct exposure and you can use it on any lens. The only issue is that whether your lens allows you to set it to F2.8 or F4?
    I think misunderstanding Octarine's post, the calculation itself is correct.

    the unsaid word is:
    but the image results of two lenses, he says: nobody can tell TS.
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    Default Re: ISO vs Aperture F stops


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    Default Re: ISO vs Aperture F stops


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    Moderator catchlights's Avatar
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    Default Re: ISO vs Aperture F stops

    Quote Originally Posted by Ming1388 View Post
    Hi All,

    I'm still learning the basics and trying to fully understand the balance between ISO, Aperture and shutter speed.

    My question is here, in Low light situation, people tend to refer to on stop in aperture as say F5.6 to F4, or F4 to F2.8 etc. Could someone tell me does this stop has the same effect dumping up the ISO to from say 400 to 800, or 800 to 1600 etc?
    Yes, the same amount of light will be enter into the lens.

    Quote Originally Posted by Ming1388 View Post
    I'm trying to understand if you don't have pro grade glass and went for cheaper alternatives which are considered soft wide open. In low light situations would you assume you get the same grade quality with the below. All else being equal, ie camera & shutter speed etc

    F2.8 at 400 ISO (lets say sigma)
    f4 at 800 ISO (lets say Nikon's pro grade)
    short answers;
    - both lenses might not produce the same the image quality, as both lens are design differently.
    - different aperture setting will produce different degree of depth of field.
    Shoot to Live, Live to Shoot
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  13. #13

    Default Re: ISO vs Aperture F stops

    Quote Originally Posted by catchlights View Post
    Yes, the same amount of light will be enter into the lens.
    Sorry boss, gotta disagree. Between f/4 and f/2.8, LESS LIGHT will enter at f/4 - it's just that the EXPOSURE and the shutter speed will be the same because the ISO is bumped up to compensate for the loss of light.
    Alpha

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    Default Re: ISO vs Aperture F stops

    Quote Originally Posted by Rashkae View Post
    Sorry boss, gotta disagree. Between f/4 and f/2.8, LESS LIGHT will enter at f/4 - it's just that the EXPOSURE and the shutter speed will be the same because the ISO is bumped up to compensate for the loss of light.
    you are absolutely correct, thanks for correcting me.
    when the shutter speed remain the same, will not have the same amount of light enter the lens if aperture size is different.
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  15. #15

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    Actually what's the question about ah?

    If it's about IQ from different lenses, then it's almost impossible to come to a conclusion because everyone quantifies IQ differently.

    Anyway the initial equations are lacking shutter speed, and that may affect IQ as well. No?

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    Default Re: ISO vs Aperture F stops

    Hi all,

    Thank you all very much for your responses and clarification. I was really just trying to undertstand whether I would get similar quality using a full frame camera with a 2.8 lens vs f4 lens.

    I have a D600 and looking for a telephoto lens. I just wanted to understand that a camera with decent low light performance, without going to extreme ISO. Would you generally get the same or similar quality picture if you used F2.8 with ISO 400 vs F4 with ISO 800. I guess I need to have a test and try myself to really understand how it all affects the imagine quality.

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    Default Re: ISO vs Aperture F stops

    Quote Originally Posted by Ming1388 View Post
    Hi all,

    Thank you all very much for your responses and clarification. I was really just trying to undertstand whether I would get similar quality using a full frame camera with a 2.8 lens vs f4 lens.

    I have a D600 and looking for a telephoto lens. I just wanted to understand that a camera with decent low light performance, without going to extreme ISO. Would you generally get the same or similar quality picture if you used F2.8 with ISO 400 vs F4 with ISO 800. I guess I need to have a test and try myself to really understand how it all affects the imagine quality.
    Shooting at Night is challenging, you need to get the right ISO, Aperture and shutter speed to achieve the image you want.
    Remember to bring a tripod along for night shots.

  18. #18

    Default Re: ISO vs Aperture F stops

    Quote Originally Posted by Ming1388 View Post
    Hi all,

    Thank you all very much for your responses and clarification. I was really just trying to undertstand whether I would get similar quality using a full frame camera with a 2.8 lens vs f4 lens.

    I have a D600 and looking for a telephoto lens. I just wanted to understand that a camera with decent low light performance, without going to extreme ISO. Would you generally get the same or similar quality picture if you used F2.8 with ISO 400 vs F4 with ISO 800. I guess I need to have a test and try myself to really understand how it all affects the imagine quality.
    You might get an excellent quality f/4 lens and a lousy f/2.8 lens. Or vice versa. You cannot compare IQ based on the aperture.

    Using f/2.8 with ISO400 vs. f/4 with ISO 800 will not change the QUALITY of the picture overall. Simply put: At f/2.8, your lens may be a bit soft, but sharper at f/4. Then again, at ISO800 you would have a bit higher ISO noise than at ISO 400, thus you might also lose some details. Also, depth of field will be different.
    Alpha

  19. #19

    Default Re: ISO vs Aperture F stops

    Quote Originally Posted by Ming1388 View Post
    Hi all,

    Thank you all very much for your responses and clarification. I was really just trying to undertstand whether I would get similar quality using a full frame camera with a 2.8 lens vs f4 lens.

    I have a D600 and looking for a telephoto lens. I just wanted to understand that a camera with decent low light performance, without going to extreme ISO. Would you generally get the same or similar quality picture if you used F2.8 with ISO 400 vs F4 with ISO 800. I guess I need to have a test and try myself to really understand how it all affects the imagine quality.
    this is quite an invalid comparison to start with, cause' there are a lot of intertwined factors within the scenario.

    1) f2.8 vs f4.0 - there is a difference of 1 stop of depth of field and this will affect how much stuff will be rendered in focus
    i.e shooting portrait. both eyes are in focus when shot with f4.0 lens but only the left eye is in focus when shot with f2.8 lens and the right eye is slightly blur. does it mean the f2.8 lens is lousy?

    2) ISO 400 vs ISO 800 - 1 stop difference in ISO which can translates to better/worse dynamic range + noise performance
    pretty much self explanatory. the photo which is shot with the f4.0 lens will be noisier due to the increase in ISO. does it mean the f4.0 lens is lousy? cause ISO performance focus more on the sensor, rather than lens performance.

    3) print size
    if you are just exporting to web size, nothing is gonna matter much here.

    if you want a general conclusion, especially with point 3) in consideration. i would say yes, IQ is generally similar but definitely not the same.
    Last edited by tecnica; 14th October 2013 at 04:05 PM.

  20. #20

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    Quote Originally Posted by tecnica View Post

    this is quite an invalid comparison to start with, cause' there are a lot of intertwined factors within the scenario.

    1) f2.8 vs f4.0 - there is a difference of 1 stop of depth of field and this will affect how much stuff will be rendered in focus
    i.e shooting portrait. both eyes are in focus when shot with f4.0 lens but only the left eye is in focus when shot with f2.8 lens and the right eye is slightly blur. does it mean the f2.8 lens is lousy?

    2) ISO 400 vs ISO 800 - 1 stop difference in ISO which can translates to better/worse dynamic range + noise performance
    pretty much self explanatory. the photo which is shot with the f4.0 lens will be noisier due to the increase in ISO. does it mean the f4.0 lens is lousy? cause ISO performance focus more on the sensor, rather than lens performance.

    3) print size
    if you are just exporting to web size, nothing is gonna matter much here.

    if you want a general conclusion, especially with point 3) in consideration. i would say yes, IQ is generally similar but definitely not the same.
    Some things you just have to try it out to understand. I suppose the TS is inexperienced and is trying to decide which lens to purchase - f/2.8 or a cheaper f/4 lens (I assume they are zoom lenses)

    When I started out, I didn't understand why the f/2.8 zoom lenses are so good, especially when that have such a short range (e.g. 17-35mm). I was way happier with my Tamron 28-200mm. As I shoot more, I figured out why.
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