Getting Exposure Right?

Discussion in 'Beyond the Basics' started by Amnesia, Mar 16, 2006.

  1. Amnesia

    Amnesia TPF Noob!

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    Hi there,
    I'm trying to learn how to use Av mode, Tv mode etc and learn about exposure. But I can't get it quite right.

    I've been trying to get photos of my fish. The only light in the room is the light from the fishtank.

    First set of results:
    I've put the setting onto the Av - and I turned it to the lowest number (widest aperture).
    1st Pic
    ISO 800
    F 3.5
    And I got this.
    2nd Pic
    ISO 1600
    F 5.0
    And I got this.
    However, For some reason, MOST are coming out like the 3rd pic...
    ISO 800
    F3.5
    And I got this.

    (I have cropped and resized the first photo, and the last two have just been resizedfor quicker viewing :)).

    After I had a play around, and a read on the internet...
    Second set of results:
    (in Tv mode)
    I got a torch (all I could get my hands on) and I've shone i've got my brother shine it on the subject(s)....
    Pic 4. ISO 400 f/4.5 shutter speed 1/100
    Pic 5. ISO 1600 f/4.5 shutter 1/125
    pic 6. ISO 800 f/5.6 shutter 1/125

    I think I'm finally getting there - Is what I'm doing correct now... I've managed to keep a higher (kind of) ISO and increase the shutter speed up to 100+.....

    I've tried it with a lamp, but the bulb was a peachey/orangey - it definitely worked better with than the torch.... Then I tried using a flash - And that also worked, but they all came out as silver, with a glare on the tank:)

    Thanks
    Amnesia
     
  2. Rob

    Rob TPF Noob!

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    You are doing pretty well there, a bit more practise and fiddling and you'll have it perfect.

    A couple of fishtank tips - don't use flash as it'll either bounce or go wrong and it may upset the fish!

    Get close to the tank and make a hood out of a big black coat to cut out any reflections. Also keep the lens perpendicular to the glass. (Clean the glass if it isn't already near perfect!)

    Use a fairly narrow aperture to get the whole of the fish in focus.

    Illuminate the fish with the help of an assistant at 90 deg to you with an LED torch. This will give some depth to the scales and avoids casting light on the taking surface of the tank.

    Hope this helps - you're pretty much there already!

    Rob
     
  3. Amnesia

    Amnesia TPF Noob!

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    Thanks for the tips Rob!

    So I should keep the exposure between 800 and 1600? I notice in pic 4 it is only on 400 - if I had turned this up, I think the photo would have come out a bit better.

    And when you say a narrow Aperture - Do you mean the smallest number you can get with the zoom you are using (say if I'm at 55mm get it as small as possible, such as 2.6 etc?)
     
  4. Rob

    Rob TPF Noob!

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    Yes, you're probably going to need the ISO high.

    No, the opposite - a narrow aperture is a big f-number, keep it above 5.6 and it should do the whole fish.

    Rob
     
  5. Amnesia

    Amnesia TPF Noob!

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    Brilliant! Thank you.
     
  6. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Also, it looks as though the focus was off, in the third photo. The gravel & rocks are in good focus but the fish is not. Because you are shooting at F3.5, the depth of field will be smaller (than if you shot at a smaller aperture like F8). If the fish had been sharp, the background would have been out of focus...which would have made the fish stand out...probably a nice photo.

    It could also be that the fish was moving fairly fast at that moment, and the shutter speed was not fast enough to freeze the movement.
     
  7. Amnesia

    Amnesia TPF Noob!

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    Okay, I'm going to try getting a smaller aperture (bigger f/number right? lol ) this evening when I get home.
     
  8. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Yup, that's right...but remember that when you close the aperture, you have to slow down the shutter to keep the same exposure level...and if the shutter is too slow, a moving subject will be blurry.
     
  9. Amnesia

    Amnesia TPF Noob!

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    slow it down? I'm using about 1/100 at the moment...
    I thought I would need to increase it?
    If I was using ISO 800 f/2.8 1/100 and increased the settings to
    ISO 800 f/5.6 1/200
    Would that work?
    I thought, if I doubled the f/# I would need to do the same to the shutter speed?
    Maybe I've got that TOTALLY wrong?
     
  10. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    You are not TOTALLY wrong....just backwards. :lol:

    So you start with this.....ISO 800 F2.8 1/100

    Now if you change the aperture to F5.6, you are making the aperture smaller, and letting in less light. So you have to let in more light with the shutter.

    1/200 of a second is twice as fast (shorter time) than 1/100 of a second. So 1/200 is less light than 1/100.

    Also, F numbers are not on a lineal scale. F5.6 is not half the size of F2.8....it's actually 1/4 of the size. (two F stops). One F stop up from F2.8 is F4, then F5.6. Confused yet?

    Each "stop" is half or double the amount of light. So the aperture at F4 is half the size of the aperture at F2.8 and the at F5.6, it's half the size of F4. So, going from F2.8 to F5.6 is two stops, or 4 times as small (1/4).

    Now shutter speeds are easier. 1/30 is twice as long as 1/60...which is twice as long as 1/120 (although 1/125 is the value used). Each time you double or half the shutter speed, that is one "stop".

    So, if you reduce the aperture by two stops, to keep the same exposure, you have to slow the shutter by two stops to compensate.

    Got it?

    http://www.sizes.com/tools/photoaper.htm
     
  11. Amnesia

    Amnesia TPF Noob!

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    Right... I am a little confused, I'll have a long read of it later :)
     
  12. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Here's the common water in a bucket analogy:

    Pretend your film or sensor is a bucket, and light is water flowing from a faucet. The ISO you choose determines the size of the bucket: a ISO 100 bucket is twice as big as an ISO 200 bucket, which is twice as big as an ISO 400 bucket.

    Correct exposure is filling the bucket to the rim without overflowing: overflowing would be overexposure and under filling would be under exposure.

    How much you turn the faucet on is the aperture: f/2.8 is turned on full blast and f/22 is a trickle.

    How long you leave the water running is the shutter speed.

    Now you can turn the water on full blast for a little while (big aperture, short shutter speed), or turn it on at a trickle for a long while (small aperture and long shutter speed), or something in between as long as the bucket gets filled to the rim (not under, not over).
     

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