Once and for all! Explain ISO? - Orchid Board

Izzie · #1 06-30-2010, 01:42 PM

I've been trucking a camera around for years now. Never taken classes, won a few awards, but starting to finally get my act together to really learn about the trade.

So far I've figured out most of what I need to.

Except ISO.

Had a couple people try and explain it, but always in cryptic jargon that was hard to understand and remember.

So- once and for all! Can someone explain exactly what ISO is (as I understand, it has something to do with light sensitivity?), and how fiddling with it on my camera can help me improve my photography. (for example, I always have my ISO on AUTO, but I have 100-900 or something available)
I have a dinky little (but very trusty!) Canon Powershot SD1100. It dreams at night of being a Canon Rebel.

These are some better examples of my photography, taken with that camera, just...for reference. As always, I'd love to get tips on how I could have done these better!

I've just recently started learning how to adjust curves/levels, which has really helped me get back to the 'real' color of what I photograph.

Izzie · #2 06-30-2010, 01:42 PM

:X Clicked submit before I added pictures- ACK! Give me a moment!

RosieC · #3 06-30-2010, 03:05 PM

I'm not a camera expert and only vaguely understand this myself but this is my understanding.

On old film cameras it related to how sensitive the the film was to light and used to relate to the chemical make up of the film being used. A very sensitive film will only need to be exposed for a very short time to give the same brightness of picture as a less sensitive film exposed for longer.

So in dark conditions you either need to open the shutter for a long time to get lots of light or you need to use a sensitive film which can absorb a lot of light in a very short time. (It also relates to aperture, but that has other 'depth of feild effects' so I will stick with assuming a fixed aperture, also will help keep the explanaition simpler).

Opening the shutter for a long time can lead to blurring due to movement so a sensitive film may be preferable. There is a BUT. Sensitive film tends to give a grainy looking picture and you get far better clarity on a film with low sensitivity... but then if the camera moves in the time it takes to get enough light it will blur.

On an automatic camera it will measure light levels and automatically optimise the shutter speed for the type of film you have loaded... but on a real film camera the user has to choose which type of film (which ISO) to load.

Roll forward to modern digital cameras. The ISO setting effectively mimics the old film types by changing the sensitivity of the light receiver (a light sensitive silicon chip). The BIG difference here is that a digital camera can have an automatic choice of ISO. The camera then measures the light levels and tries to optimise the combination of ISO and and shutter speed (and aperture) for the level of light, to give you the best picture.

Remember low ISO and short shutter speed give the best picture as low sensitivity (ISO) will have the sharpest clarity, and fast shutter speed will reduce the chance of movement/bluring... BUT in low light you have to compromise with higher sensitivity and or slower shutter.

Of course there is a range of options between automatic and manual depending on your camera. I have a top end Cannon compact, which allows full manual control even without an SLR (especially when I use the hack software which opens up extra options on the control chip) but also has really good auto options.

Hope I've got all that right and that it makes some sense.

Call_Me_Bob · #4 06-30-2010, 03:11 PM

i dont know the exact reasons or explanaitions, but the higher the ISO the brighter the picture, and the granier it gets. the lower the ISO the darker the picture is, and less grainy. if you are taking pics, and have enough lighting without having to make your iso high, then keep it low. but if its darker and you cant get your pics bright enough while having the shutter speed be fast enough for clear pictures, then make the ISO higher. When making the ISO higher, you are sacrificing by making the picture granier

RosieC · #5 06-30-2010, 03:18 PM

I'm not an expert at all on knowing which to pick, I think some experts carry light meters to help then decide.

RosieC · #6 06-30-2010, 03:20 PM

Good concise explanaition Help. Mine ended up a bit wordy

RosieC · #7 06-30-2010, 03:22 PM

One more thing to add.

If you have a tripod and are taking pics of still images (such as plants) you can have longer shutter speeds and therefore lower ISO even with less than optimum light. But if you are holding the camera in your hand and/or taking moving objects then short shutter speed is far more important and so you have to use a higher ISO (but the sacrifice is the grainy nature).

Call_Me_Bob · #8 06-30-2010, 03:44 PM

Quote:

Originally Posted by RosieC

One more thing to add.

If you have a tripod and are taking pics of still images (such as plants) you can have longer shutter speeds and therefore lower ISO even with less than optimum light. But if you are holding the camera in your hand and/or taking moving objects then short shutter speed is far more important and so you have to use a higher ISO (but the sacrifice is the grainy nature).

yes! a general rule of thumb is, the shutter speed should be at least the length of your lense. so if you have a 300mm the minimum shutter speed should be 300. if you have a tripod, and are taking pics of something still, such as ORCHIDS!!!!, put the ISO as low as i can go, and then cahnge your F-stop and shutter speed accordingly, for sharp images.

ps. rosieC- thanks for the props!

Tindomul · #9 06-30-2010, 05:31 PM

Ok, thats cool. So change the F-stop to what. How do you know what to change it to, and I forgot what f-stop means. Sorry, thanks

ronaldhanko · #10 06-30-2010, 06:19 PM

If you think of ISO in terms of what it meant in the film age,is relatively easy - the higher the ISO the more sensitive the film is to light (more or less the same as the sensor in your digital camera). The more sensitive the film (or sensor) the less light needed to get the exposure you want. Thus, with a high ISO you can use a faster shutter speed (less light) or a smaller aperture (less light). you may want the faster shutter speed to stop motion or the smaller aperture to get greater depth of field (more of the subject in focus) and thus a higher ISO.