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Thread: The Basics - Sensitivity/Shutter Speed/F stop

  1. #11
    Barracuda
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    Next up... F stops.

  2. #12
    Grouper
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    thanks for posting that. can't wait for the next one.

  3. #13
    Barracuda Founding Member
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    Can you explain exactly what ISO is and how to select what is "practical"?
    The water's more exciting.. with CHUM in it!

  4. #14
    Grouper Founding Member
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    Quote Originally Posted by ReefHound View Post
    Can you explain exactly what ISO is and how to select what is "practical"?
    I would have to agree with RH here, what conditions would you use 100 and what conditions would you bump it up?
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  5. #15
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    When one is using film in the 'old' days before digital camera, ISO is the measurement of the film's sensitivity to light.
    Generaly, most film speed people used then were/are ISO 100, for daytime photography. If you use ISO 50-200 at night or in a room, the flash would go off (if there's auto flash). I used to use ISO 200 film because it allows me to take photo at lower light without firing off the flash as often. There are also ISO400 film, which can be used at lower light without flash.
    So if one has a high ISO, long shutter speed (Puffer will cover this), and if you have steady hands (pr tripod), and steady subject, it is possible to take a nightshot without using the flash.
    However, one down side of high ISO is that the photo can get very grainy. The higher the ISO the grainier it gets, and it's especially obvious if it's enlarged.
    In the digital camera age, the same thing applies in term of grain, though I can testify that the better camera, as in DSLR, can take higher ISO with less grain effects. When I used my G5, I never shoot past ISO 200, because anything beyond that gets very grainy. However, with my Nikon DSLR, my ISO is normally set at 400, and I have pushed it up to 1600 with acceptable (to me) grains. (low light, indoor condition with no flash or night shot with no flash)

    Edit: Each digital camera is different, the sensor size, resolution, manufacturer's measurement and quality means that each camera reacts different to different ISO. Canon point and shoot starts at ISO 50, and other manufacturer's starts at ISO 100.
    But they may end up with similar sensitivty to light. It would pay to play around with the different ISO settings on your camera, take some photos (like what Puffer did), and zoom in on when you look at it on the computer and see at which ISO setting is the photo unacceptable to you. Generally, higher ISO is used to take 'action' photo without flash or in lower light condition without flash.
    Last edited by Silverlode; 11-22-2007 at 15:54.

  6. #16
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    Quote Originally Posted by ReefHound View Post
    Can you explain exactly what ISO is and how to select what is "practical"?

    Here is a very complete definition:

    Film speed - Wikipedia, the free encyclopedia

    But keep this in mind... It is the combination of ISO/F-stop/shutter speed, along with the light present, that defines how much light gets to the sensor (what used to be film).

    When should you use it? Whenever possible.. we will get more detailed as I go thru the other settings, but keep this in mind - If you could.. you would use the best of each setting, but that is almost never possible, so you are going to have to compromise...the trick is to know which to compromise first (will get in much more detail shortly)

  7. #17
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    Ok, next up, F-stops. The camera I have has a range of 2.8 to 8. The range varies a lot, depending on the camera. The number actually means the size of the lense(opening) versus the focal length. Each number, in the old days, used to be halving or doubling the amount of light that gets to the film... modern ones will have lots of partial stops, by the way.

    For those that want to know:

    f-number - Wikipedia, the free encyclopedia

    The important things to know are:

    1. The smaller the number, the more light it lets in.

    2. The bigger the number, the more will be in focus.

    So my camera has four standard stops - 2.8 - 4 - 5/6 - 8 These are steps like the units of ISO 800 - 400 - 200 - 100, and shutter speeds (in portions of a second) of 1/60 - 1/120 - 1/240 - 1/500 (really 1/480, but there is a lot of rounding).. So.... 1/120/F 5.6/ISO 200 is the exact same exposure as 1/240/F 2.8/ISO 100, but they will not look the same. Mostly because of the change in F value.

    This test takes a very accurate setup, so I used a tripod, set the paper 10 inches away, focused on the grey line in the middle and took images. I used the widest angle, which make this look the best, but it is the one that is mostly used underwater.

    The image looked like this:

    Note: This image is compresses, made smaller and I am showing it just so you can see the size of the original.




    Here is the bottom of that image, taken with F 2.8...



    Here it is with F 8....



    So, so far, one should use the lowest ISO number and the biggest F number... but there are still two issues left.... the shutter speed and the amount of light left to begin with.

  8. #18
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    Due to the really small actual focal length of the lense, the F8 image above is truly amazing... no normal SLR or large sensor DSLR can do that, as their focal lengths are typically much bigger, and this effect is both focal length and f-stop, with the focal length being the most important (small is better here)

  9. #19
    Grouper
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    Quote Originally Posted by frankc420 View Post
    Quote Originally Posted by ReefHound View Post
    Can you explain exactly what ISO is and how to select what is "practical"?
    I would have to agree with RH here, what conditions would you use 100 and what conditions would you bump it up?

    What ISO to use depends on the ambient light, and whether you have a strobe. If you are using a strobe, use a low ISO (like 100); as the lower ISO speeds have less noise.

    If you have limited ambient light and no strobe - that is when you use higher ISO's. As Puffer Fish showed, you add noise as ISO increases. You really need to test your camera to see what ISO levels are acceptable to you - most non-DSLR cameras start showing noticable noise increase above 400 ISO.

    You use a higher ISO to give you sensitivity in low light, so you can use an acceptable shutter speed (anything lower than 1/60 sec is likely to blur unless you have a really short focal length lens). Another place higher ISO's can be used are places that prohibit flash (museums for example). Shooting a higher ISO here can allow you a little more flexibility in shutter speed.
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  10. #20
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    So, what about shutter speed?

    Well, shutter speed effects the exposure, the same as F-stops, with faster letting in less light, and slower letting in more.

    I took a large picture of a map, and then just cut out the center portion, so that you could see what it is doing.

    All images were hand held, at wide angle, using macro.

    At anything faster than 1/125th of a second, it is easy to get a sharp image... if you jerk the camera, you can still mess it up, but in general, it works really well. This is a 1/256 shot:



    Remember, this is a very small section of a picture.

    Hold the camera steady.. and you can go lower... this is a 1/106 image:




    Going slower:

    A 1/64th Picture:



    Here is a 1/30th:




    Look carefully and it is starting to blur.

    Here is a 1/26th of a second:



    The camera I am using has anti-shake.. and it attempts to adjust for that movement, and I can, but only about one stop...

    A 1/39th and a 1/26th with the shake control on:





    So, there are the first three big ones.... if you could, you would use the lowest ISO... the smallest lense opening (bigger number) and a shutter speed of at least 125th or maybe 1/60 (if you have anti-shake).

    How likely is this to happen? If it is a nice sunny day, and you are in clear water, 30 ft or less... all the time. If you have the correct strobe (more on that later), all the time. In 70ft of water, with 50 ft of vis..never.

    So, what should you adjust?

    First should be f-stop (particularly in wide angle), most camera's actually have slightly better resolution in the middle of the range... so go to F4 or f5.6. Then, adjust the ISO, but beware of going over 400.

    So, if 1/125 shutter, F5.6 and ISO 400 does not work... then consider what you are taking a snap shot.. in don't worry about it.
    Last edited by Puffer Fish; 11-24-2007 at 16:52.

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