Talk to me about Aperture and ISO

Discussion in 'Photography Beginners' Forum' started by ricepudding, Oct 24, 2007.

  1. ricepudding

    ricepudding TPF Noob!

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    I'm just a beginner and trying to read up on this stuff. Tell me if I have this right:

    Aperture is the "amount of light" allowed in by the lens. I can set this to high f numbers (less light) or low f numbers (more light). This contributes to the exposure of the picture and also the depth of field, high f numbers giving a deeper depth of field, lower f numbers giving a shallower depth of field.

    ISO I'm a bit more confused on, but I believe when ISO is set higher the camera grabs more light from whereever it can to help expose the picture, risking more "noise" in the photo...graininess.

    So, tell me more about these two and how you use them. Many of my pictures are indoors and I want to get a natural feel to them, not whited out skin due to flash. So I would like to forgo using my flash. Even with a small aperture f number I don't have enough to expose (if I'm doing things right). I haven't messed with ISO yet but is that what I need to do is bump up the ISO if I don't want to use a flash indoors?

    Is there any other way to get a more natural looking photo indoors?

    Thanks!
     
  2. Helen B

    Helen B TPF Noob!

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    You have got the basics, pretty much. The higher the ISO, the less light the sensor needs to make an image. Turning the ISO up does not actually affect the sensitivity of the sensor, it just amplifies the signal coming off it more. Not only does it amplify the image signal, it also amplifies the electronic noise coming from the sensor, so the higher the ISO, the higher the noise - ie the grainy look - especially in the darker parts of the image.

    As you have the camera you could try some exercises with different ISO settings. You can use noise reduction software like Noise Ninja and Neat Image to improve the smoothness of high ISO images.

    You might see references to EI instead of ISO. Strictly speaking an ISO speed has been obtained by following the procedure laid down in the relevant international standard, and any other speed should be referred to as an exposure index, or EI. It is numerically equal to ISO. Almost everyone, including the camera manufacturers, ignores this and uses ISO instead of EI - for example ISO 1600 should be EI 1600, to show that it is not the 'native' speed of the camera.

    If you are going to use flash, one way of avoiding the rapid fall-off in the light is to bounce the light off the ceiling - ie point the flash towards the ceiling. If the ceiling isn't white you will have problems with colour casts. There are a variety of other tricks to get more even light.

    Best,
    Helen
     
  3. ricepudding

    ricepudding TPF Noob!

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    Helen,

    Thanks for the answer. What are the other tricks? Right now I have an on board flash so I can't aim it. I just tried turning up my ISO to get a pic of hubby watching tv with no flash. We just have florescent bulbs on in the room (fairly bright). It took using the maximum ISO to get an image that was close to the "actual light" in the room. Using flash makes hubby look like a ghost. :-( I don't want all my images to be of poor quality with the iso turned high...!?
     
  4. davidfromoz

    davidfromoz TPF Noob!

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    I'm am extremely new at this but let me have a crack at it.

    The first factor is lighting. That is how much light is coming from the subject. You can alter this by choosing a time when there is lots of natural light or using artificial lighting (room lights or a flash). Flashes can be made to look more natural by muting them, using more of them or by bouncing their light off something before it hits the subject.

    The other factor in the equation is shutter speed. Shutter speed and aperture determine how much light gets into the camera. Different lenses have different aperture capabilities. "Fast Lenses" can have bigger apertures and hence get more light.

    You can get more light into the camera by using a larger aperture (sacrificing depth of field) or by using a slower shutter speed (frequently sacrificing clarity). With a slower shutter speed you will need to reduce movement, so it works best with static subjects. And you can reduce movement of the camera by using a tripod or some other technique to steady the camera. Vibration reduction (a feature of a camera or lens) might also help with camera movement related lack of clarity. With a very still subject and a well stabilized camera you can open the lens for much longer to get more light.

    Once you have the light inside the camera, if there is not enough to
    get what you need you can increase the gain of the signal coming from the sensor. This is basically what changing the ISO setting does. But you will increase noise when you do this. (sometimes this can be used artistically). Remember when watching a night scene in a movie when you turned up the brightness of the TV. You might start seeing a bit more but you also get a lot of noise. The same thing happens with ISO setting as you increase it.

    As far as I know, thats about all you have to play with.

    cheers,
    david
     
  5. Helen B

    Helen B TPF Noob!

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    What aperture and shutter speed are you using? With your 50 mm you should be able to go as low as f/1.8 and 1/30 hand-held - maybe try holding the camera steady at 1/15. At ISO 800 that should be OK in bright room light.

    Good luck,
    Helen
     
  6. EOS_JD

    EOS_JD TPF Noob!

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    First off, buy a book called Understanding Exposure by Bryan Petersen which describes everything you'll need to know about exposure. ISO, Aperture and shutter speed info is there to hel you get a correct exposure.

    The aperture is the opening within the lens itself. At small fnumbers the opening is large and at high fnumbers the opening is small therefore letting in less light.

    When less light is allowed through the lens, the shutter speeds will need to increase to compensate.

    Fstops are generally quoted as f2.8 - f4 - f5.6 - f8 - f11 - f16 - f22 - f32. Each position from f2.8 down means a narrowing of the aperture. f4 lets in twice as much light as f2.8. F5.6 lets in twice as much light as f4 etc etc. So at f2.8 your shutter speed is 8 times faster than at f5.6 and 32 times faster than at f22. This is one of the reasons why faster lenses are more expensive and why landscape photographers use tripods (see below about depth of field).

    Your camera may also show other fstops that are probably 1/2 or 1/3 stops like f3.5, f4.5, f13 and many other combinations to allow you to tweak your exposure more accutrately.

    Depth of Field controls how much of the image is in acceptable focus. Depth of field is controlled by focal length (magnification), distance to subject and aperture.

    Large apertures allow narrow depth of field (like a portrait with ablurry background) and narrow apertures are often used by landscape photographers to get the maximum amount of sharp detail righth through from foreground to background).

    Lens choice is also critical to determine dof. Wider lenses allow greater dof. Distance is critical in determining dof. The closer your subject is to the lens, the shallower the dof will be.

    It's using all this information together that will allow you to create a "correct" exposure. There may be many exposures that will result in a well exposed image but only one that is correct for what you want to interpret in the image.

    As noted above the sensor's sensitivity is controlled by the amplification of the signal. As you increase the gain (sensitivity) more noise is generated. Try to keep your ISO down and use faster lenses to control the aperture and shutter speeds (obviously taking into account the dof you want)

    When shooting indoors, what you see as a bright room is probably dark to a camera with that small hole letting in the light. Without flash, you need a fast lens (f2.8 or faster) - faster means wider aperture like f1.8 or even f1.2.

    With the faster lens you get faster shutter speeds allowed. Lets say you are using that 50mm f1.8 and you can only get a shutter speed of say 1/30th. this is a slow shutter speed for the focal length you are using . You should try to aim for 1/focal length (1/50th) or faster.

    In order to get a faster shutter speed you need to increase the ISO. By doubling the ISO, you will double your shutter speed however you will increase the noise in your image. This might be ok though up to ISO400 or even ISO800 so i would not worry too much about this. Most dSLRs are capable of producing relatively noise free images up to ISO1600!!

    So when you double your ISO your exposure should now read f1.8 and 1/60th shutter speed.

    But be careful as f1.8 means a very narrow depth of field particularly at close range.

    You need to manipulate ISO, Aperture and Shutter speed to get the exposure you want.

    Flash is a whole different area that i'm not able to explain!
     
  7. Helen B

    Helen B TPF Noob!

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    Though this is a rule-of-thumb that is often quoted, if you are going to be doing a lot of shooting in low light you should experiment and practice with much slower shutter speeds. The aim is to get to the point where subject movement is the limiting factor, not camera movement - then to choose the brief moments of stasis rather than the moments of action if you can. It's easier with people you know.

    You may already have worked this out, but rest your elbows on whatever you can. Find the best way of holding the camera that enables you to release the shutter with the minimum of movement - I use the second joint of my right index finger, not the tip. It's a tiny movement.

    If you have to use flash, then I recommend getting a flash rather than using the on-camera flash. You can then blend a flash exposure with the ambient light, both in terms of colour and amount. There are many ways of doing this, depending on the situation and how you want the results to look.

    Best,
    Helen

    Fluorescent lamps, very poor colour ones at that:

    [​IMG]

    Low wattage incandescent lamps:

    [​IMG]

    Mixture of direct flash (to get under the brim of BG's hat) with an orange gel to balance with the unfiltered stage lights - so the filters give their true colours. The person whose shoulder you see in bottom left was closer to the camera than Boy George:

    [​IMG]
     
  8. EOS_JD

    EOS_JD TPF Noob!

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    No doubting you can shoot at slower shutter speeds - particularly using flash (as in many cases the speed of the very quick flash burst will stop any motion - assuming the subject isn't moving too quickly) however as you shoot slower, you need to take more time over your shots - even more so when handheld (as you intimate).

    I can shoot at 200mm 1/15th sec with my 70-200 f2.8L IShowever it's a bit hit and miss.

    1/focal length is indeed just a guide but it's a good one - particularly for a learner. There's another thread around here where a guy is asking why his shots are blurry. He was shooting at about 280mm with a shutter speed of 1/125th. With lots of practice this can be done however if he'd shot at 1/250 he's have had more chance of getting a sharp image.

    Not saying you are wrong here.... onthe contrary you are correct but to shoot consistently without camera shake, 1/focal length is the rule I'd always aim for even if it means a higher ISO. I'd rather guarantee I got the shot. When shooting for money you can't take the chance.
     
  9. ricepudding

    ricepudding TPF Noob!

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    Thanks. I adjusted the shutter speed to more like 1/10 and adjusted the white balance to indoor lighting, I kept ISO at 100 and the pics seem pretty acurate to room lighting now. And I don't have shadows or super white people from the flash. YAY. Thanks for walking me through!
     

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