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Thread: Exposure, Aperture, ISO & Shutter Speed

  1. #1

    Exposure, Aperture, ISO & Shutter Speed

    I have a 50D with EF-S 18-200mm 1:3.5-5.6 IS lens. I have couple of issues related to exposure, shutter speed , aperture and ISO.

    (1). Is there any rule of thumb/best practice of setting the shutter speed related to the focal length when you are not using a tripod ? Should the shutter speed be 1/focal length ?
    (2). If so when you are taking pictures in outside the best (probably easiest ) setting would be using Tv (unless you have an intention of controlling the DOF of the subject). So you can change the shutter speed based on the focal length you have. Correct or wrong ??
    (3). Is there any rule of thumb/best practice of keeping the minimum ISO level of your camera in all possible times ? For ex in a sunny day outside should you keep your ISO at 100 or 200.

    Actual Problem :

    When i m taking pictures outside i usually set the shutter speed to 1/200 (if i am not using a tripod) and try to keep the ISO at 100/200 (in day time and increase it to 800 at evening or night). So the only option left is to control the exposure is the aperture. Even thought i have set it to the maximum (this will vary based on the focal length but most of time its 5.6) , the image produced is underexposed.

    (4). What should i do in order to get correctly exposed photos with out shakes ?
    (5). What are the best combinations of aperture and shutter speeds to be used generally ?
    (6). Should i consider of using a lens with much higher aperture ?
    (7). What is the lowest shutter speed to be used generally when not using a tripod ?
    (8). How productive, suitable of considering the exposure meeter of the camera ? Because most of time i have heard professional are telling that they never believe the inbuilt exposure meeter. (I think some practical situations also avoid considering the inbuilt exposure meeter , such as using an external flash). So how can they know the correct exposure ? How do they know the best aperture, ISO and shutter speeds to give ???

  2. #2

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    Re: Exposure ,Aperture, ISO & Shutter Speed

    Hi sauron,

    Your problems require a lot of reading and practice. A complete answer would cover a good photo book you would need to read. Here a couple of hints from me:

    (1)
    Rule is known as "T = reciprocal of focal length" but should be ammended by crop factor of camera sensor and IS of lens.
    Thus when photographying from hand with your zoom on 200mm without IS maximum T should be cca. 1/320s. Canon says for you lens "... Optical Image Stabilizer for up to 4-stops of effective correction even at full zoom". Theoretical this would mean you could go with max. T = 1/20s for f=200mm or 1/2s for f=18mm (!!!). But:
    - You need la lot of experience to use shutter speed longer than 1/60s, even with IS. Check best practices for holding the camera, pressing smoothly the shutter release. Until you gain some experience with this consider your IS is effective only for 2 stops. Observe Canon says "... up to 4-stops"
    - For T > 1/30s camera shake can generate movements where any IS may not be effective. Do not believe "heros" who claim they can shoot handhold at T = 1s. Just make a test, same photo of static subject, from tripod and from hand (with IS). You will notice the difference when checking the photo at 100% magnification. Of course, exigence for an image to be CRISP may be different from person to person. So, result subjectively depends on what a certain person considers to be CRISP.
    - At 1/20s 50D has a vibration when raising the mirror. For T = 1/8 ... 1/30s crisp photos require Mirror Lock Up (MLU). MLU is thow best to use on tripod, from hand it is not very confortably.
    - Long T should be used only for static subjects or for creative reasons. Photographying even slowly moving people from tripod (no camera shake) with 1/20s may render unsharp photos. Just because subject is moving during the time the shutter is open. A creative example where you might still stick on long T would be the panning technique.

    (2)
    When shooting outside you should use Tv when you want to control speed. Freezing action may require 1/1000s or even shorter. For normal use go with ammended "reciprocal of focal length" rule. For static subjects you may use long T; or switch on Av to control DoF. For creative reasons you may may use any T dictated by your imagination.
    So, here the answer is not simple. Use the most appropiate program for the subject, observing other rules for getting a good photo. Such as rules for getting a crisp image, keeping ISO as low as possible, etc.

    I leave rest of answers for others. Have fun!

  3. #3

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    Re: Exposure ,Aperture, ISO & Shutter Speed

    Hi Sauron,

    Here begins a journey ...

    Quote Originally Posted by sauron View Post
    (1). Is there any rule of thumb/best practice of setting the shutter speed related to the focal length when you are not using a tripod ?
    Yes.

    Should the shutter speed be 1/focal length ?
    Yes, but you need to then multiply the result by 1.6 since you're using a crop-factor camera.

    (2). If so when you are taking pictures in outside the best (probably easiest ) setting would be using Tv (unless you have an intention of controlling the DOF of the subject). So you can change the shutter speed based on the focal length you have. Correct or wrong ??
    There are no "rights or wrong" ways - just consequences. In the vast majority of cases depth of field is important (either making sure you have enough, or making sure you don't have too much) - so I think you'll find that most people use Av mode outside.

    (3). Is there any rule of thumb/best practice of keeping the minimum ISO level of your camera in all possible times ? For ex in a sunny day outside should you keep your ISO at 100 or 200.
    Everything in Photography is a compromise. High ISO settings allow you to shoot at higher shutter speeds or smaller apertures, but at the expense of more image noise -- so it's really a case of which is the lesser of two evils. Personally, I believe that people worry far too much about noise; so long as the image was correctly exposed in the first place then you can shoot at just about any ISO you like with a modern DSLR and not have noise be an issue in the final print - although - having just said that it makes sense to keep the ISO as low as possible so long as doing so doesn't cause other image issues (like camera shake from having too low a shutter speed).

    When i m taking pictures outside i usually set the shutter speed to 1/200 (if i am not using a tripod) and try to keep the ISO at 100/200 (in day time and increase it to 800 at evening or night). So the only option left is to control the exposure is the aperture. Even thought i have set it to the maximum (this will vary based on the focal length but most of time its 5.6) , the image produced is underexposed.
    This is normal behaviour - you've told the camera that it can't go any slower than 1/200th - it can't be any more sensitive than 200 ISO - and by virture of the lens that you're using, it can't open up anymore than F5.6. So if you're shooting objects in full direct sun (or close to it) then you'll be OK, but if the object is in the shade or it's not a very bright day then under-exposure will occur. The simple solution is just use Av mode and increase the ISO if the shutterspeed drops too low.

    (4). What should i do in order to get correctly exposed photos with out shakes ?
    Use the 1 over focal length times 1.6 rule that we talked about earlier as your minimum shutter speed, and/or use an IS lens.

    (5). What are the best combinations of aperture and shutter speeds to be used generally ?
    Depends on what your shooting. Again, it's a compromise - if your main problem is camera shake then use higher shutter speeds.

    (6). Should i consider of using a lens with much higher aperture ?
    It would help - price is the other variable that enters into the equasion though.

    (7). What is the lowest shutter speed to be used generally when not using a tripod ?
    It depends on the focal length of the lens your using, and whether or not there is any subject motion.

    (8). How productive, suitable of considering the exposure meeter of the camera ? Because most of time i have heard professional are telling that they never believe the inbuilt exposure meeter. (I think some practical situations also avoid considering the inbuilt exposure meeter , such as using an external flash). So how can they know the correct exposure ? How do they know the best aperture, ISO and shutter speeds to give ???
    Build in camera meters are fine - but - as they only measure reflected light, you sometimes have to dial in up to two stops of positive or negative "EC" (Exposure Compensation). For general shooting - with the camera set to evaluative metering - they generally don't require any compensation though. External flashes come in two varieties; those that are ETTL compatable in which case the flash exposure is taken into account, and those that are strictly manual in which case the flash exposure isn't taken into account in the metering.

    Hope this helps

  4. #4

    Re: Exposure ,Aperture, ISO & Shutter Speed

    Thanks calexe and Colin for their fast replies .......

  5. #5
    Klickit's Avatar
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    Re: Exposure ,Aperture, ISO & Shutter Speed

    Quote Originally Posted by sauron View Post
    (8). How productive, suitable of considering the exposure meeter of the camera ? Because most of time i have heard professional are telling that they never believe the inbuilt exposure meeter. (I think some practical situations also avoid considering the inbuilt exposure meeter , such as using an external flash). So how can they know the correct exposure ? How do they know the best aperture, ISO and shutter speeds to give ???
    Another possibility, if your camera allows it, is to shoot your image as you think it should be, according to the information the in-built meter gives you, and then to bracket those settings with one slightly underexposed and one slightly over exposed. That gives you three chances of getting a correct exposure.

  6. #6
    Moderator Dave Humphries's Avatar
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    Re: Exposure ,Aperture, ISO & Shutter Speed

    Hi Sauron,

    Quote Originally Posted by Klickit View Post
    Another possibility, if your camera allows it, is to shoot your image as you think it should be, according to the information the in-built meter gives you, and then to bracket those settings with one slightly underexposed and one slightly over exposed. That gives you three chances of getting a correct exposure.
    True, but this will be better achieved in Av mode, because you're unlikely to 'run out' of shutter speed. I suspect you meant this Kit, but I thought it worth clarifying.

    Whereas, if still in Tv at 1/200 and 100iso; if they were underexposed anyway due to not being able to open up beyond f3.5 to f5.6, there won't be any benefit because you have 'run out' of apertures. If Auto iso could be used, it may work.

    FWIW, I agree with Colin about (far) too much emphasis being put on use of low ISO.
    Tv - If the subject is blurred, either due to subject movement, or camera shake, it is almost impossible to fix in PP.
    Av - If the DoF is too shallow or too great (depending upon subject), again this cannot be so easily fixed in PP.
    ISO - However, if the image is noisy, there are third party software plug-ins for this that can be quickly and easily applied in PP (if necessary)

    I generally also work in Av 90% of the time (and Manual 10%), set the aperture to suit the subject and composition, check the shutterspeed this gives and manually tweak ISO to achieve one to suit the subject (first) and focal length (second). I then shoot and immediately review on LCD for blinkies and RGB histogram and use EC for subsequent shots as necessary in half stop increments. Sometimes, experience allows some EC to be set before the first shot is even fired.

    The 1/(FL x CF) rule doesn't guarantee 100% success rate of sharp images, only say 50%, but that's where IS/VR helps improve the odds. On occasion, in low light, for static subjects, if without tripod, I'll push the rule and rely on IS/VR more.

    Cheers,

  7. #7

    Re: Exposure ,Aperture, ISO & Shutter Speed

    Thank you Klickit and Dave for their great replies ......

  8. #8

    Re: Exposure ,Aperture, ISO & Shutter Speed

    With this camera, ISO up to 8 should be fine. I'd rather be flexible in apperture settings than anything else but that's just me.

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