Can Someone Explain Iso and Exposure?

nikks24 · Dec 3, 2008

I know that the Iso controls the sensitivity of the "film", but does it have any thing to do with Exposure? What exactly is exposure? And how does adding exposure or reducing exposure controls affect the photograph?

Thanks for the help

iflynething · Dec 3, 2008

Hmm......those are pretty broad questions in a specific forum.

I'll have a go. ISO and effect on exposure. To keep it simple, it will effect exposure. Say you're shooting on manual, 1/60 sec and ISO 100. You bump the ISO up to 800. That same shot at 1/60 will be very much brighter, might even be completely blown out.

Now as for your question about "what exactly is exposure,?" I'm going to have to leave that for someone else. I would sum exposure up as what the picture looks like and the histogram.

Adding/reducing exposure: I assume you are talking about exposure compensation? I don't know since everything else is broad answers. They mainly effect the ambient light.

I'm going to stop now. I'm getting repetative I think. Where's Big Mike

~Michael~

stsinner · Dec 3, 2008

Very simply, to me exposure means how light or dark a photo is.. If it's too dark and lacking details, it's underexposed (ISO too low), but if it's too bright with blown-out highlights, it's overexposed (ISO too high). Now let the pros respond.. I speak in laymans' terms.

TWoods450 · Dec 3, 2008

i've been on this forum for less than a month and I have already seen this question asked quite a few times. I hate to be a dick being a new guy around here but, the Search feature can help find answers to this stuff faster than waiting for a response to a newly asked question.

iflynething · Dec 3, 2008

I do need to correct and say, just because it's overexposed doesn't mean the ISO is too high. That's not actually MAKING the exposure if you get what i'm saying. If you're shooting during the day at 1/60 of a second (with no flash) at ISO 3,200, then yes, the ISO would be too high, but ISO doens't only contribute to the over/underexposure

~Michael~

prodigy2k7 · Dec 3, 2008

ISO increases the sensitivity of the sensor too... by 100% for a double of the ISO...
Look up a "stop" of light...
ISO100 to ISO200 is a full stop of light, therefore your image will be over exposed, to compensate, you speed up your shutter or close your aperture more...etc...

stsinner · Dec 3, 2008

iflynething said:
I ISO doens't only contribute to the over/underexposure

~Michael~

Wrong.

Of course it depends on shutter speed, as well, but ISO definitely contributes.

iflynething · Dec 3, 2008

stsinner said:
Wrong.

Of course it depends on shutter speed, as well, but ISO definitely contributes.

Well, that's what I mean. It's more shutter speed. Lets not get technical. I mean of course it's having to do witih aperature, shutter speed and all that.

I should have reworded that and said that first would come the shutter speed and aperature and everything inbetween, then the ISO. Ever seen many people adjust ISO to 800 then adjust shutter speed?

~Michael~

yogibear · Dec 3, 2008

With that argument you could blame it on aperture, ISO, and shutter speed. All three contribute to under or overexposure.

To put it simple: Exposure is the name of the image or process of creating an image by "exposing" the film or sensor to light.

ISO, aperture, and shutter speed are the basic functions of an exposure that are all connected. ISO for me was best understood as the rate at which film or the sensor picks up/absorbs light. So the more ISO the faster you gather light. Search for threads that talk about the relationship of ISO, aperture, and shutter speed to better understand how a correct exposure is made.

mrodgers · Dec 3, 2008

Here's how I look at it....

Shutter - set to stop or show motion.
Aperture - set for the depth of field, or how much of the photo is in focus.
ISO - the sensitivity of the sensor/film. Also, the higher the ISO, the more noise in the image.

All three are set to get the exposure wanted and the settings of each are set for what you want to accomplish.

Example, shooting some type of sports, your dog running in the yard, your kid riding past on their bicycle... You want to freeze the motion of your subject. You will want to have a specific shutter speed to freeze that motion. You set your ISO as low as possible for less noise, set your shutter speed to what you want, then set the aperture to what you want, sometimes you have a specific number and sometimes you can have a range that will work. If you can not get the shutter speed or aperture where you want them, you increase the ISO higher to achieve the S and A that you want.

With taking a portrait shot, flower shot, or landscape shot, usually you have a determined depth of field that you want and will set the aperture to a specific number. The shutter speed can be in a bit of a range that you can select, but you may not be able to achieve your image because the wind is blowing the flower, or your portrait subject may move. So, if you can't get into the range of shutter speed you want, you increase the ISO until you can.

Basically, you want the ISO as low as possible because the higher it is, the more noise you get. How much noise depends on the camera.

That's my take on it.

Tom3 · Dec 3, 2008

Look at ISO as the sensitivity of the film / sensor; as others have mentioned, 200 is 2x more sensitive than 100, etc.

the ISO is an absolute value; a film has a specific ISO, a sensor has a ISO range, but it is set, at a given time, to a single value.

Exposure is just a measure of the light has been recorded; and this is a combination of the sensor sensitivity, the shutter speed, your lens and the scene (and probably a few other things).

to take a poor analogy: pinch someone: first they feel nothing, then a whole gradient of pain and then it's too painful and they don't feel a difference anymore; their pain sensitivity is like the ISO.
how much you pinched at the end in similar to the exposure, it's the total pain he recorded.

(and no, I'm not a sadist, but it's the first analogy that came to mind

)

saycheese76 · Dec 5, 2008

Exposure is the amount of light that needs to hit the film/sensor in order to create an image. Lets say you have a bucket. If you use a small hose, you need to leave the water running for a little while to fill the bucket. If you use a big hose, the water needs to be on for less time. Too much or not enough water as a result of leaving the tap on too long or too short, or using the wrong size hose will over or under fill the bucket.
Hose=aperture
time water runs=shutter speed
size of the bucket=ISO
Hope this helps!

Dionysus · Dec 5, 2008

really? this question again? There really should be a sticky or something either explaining the basics, or explaining the search function, or even just giving an amazon link to Understanding Exposure. I've suggested understanding exposure like 390848005 times this month.

In this day and age, and with the power of the internet, i do not understand how someone can ever be uninformed long. it's as simple as going to wikipedia and searching "exposure" or up clicking search on this forum and searching "exposure". Maybe it's just me, and the way I think, but before I ask a question I like to be as informed as possible, and make sure that I have at least TRIED looking for information before i bother anyone else w/ my question.

This isn't really an attack on the original poster..just some frustration for the lack of effort from the newer people in general to try to find answers for themselves before just blindly asking a question.

For god's sake, most noobie questions can be found asked within the 1st three pages of this forum.

/rant.