Why have a "man wearing sunglasses standing in front of window"? (ISO)

[saurabh.d21]

Hi
I have a Kodak digicam and learning about the technicalities of photography through DPS, but being an amateur photographer, I just could not understand as to how the sensitivity of the image sensor/film impacts the exposure as explained under ISO or for that matter what does image sensor sensitivity actually mean?

To further clearify my question, I would like to ask that when Shutter Speed controls the duration of light & aperture the quantity of light how does ISO really matter? I mean why have a "man wearing sunglasses standing in front of window" when we can control the exposure with the other 2 elements of exposure Triangle. The fact that it acts like how IRIS acts in a video camera or a night vision is well understood but why to affect the sensitivity of Image sensor with it when we have Shutter Speed & Aperture?

If I am still unclear please tell me.

[OsmosisStudios]

I have no idea what youre getting at with your man with sunglasses in front of a window. The analogy is either poorly explained or not explained and Im being thick.

ISO gives you some latitude when it comes to using both shutter speed and aperture. Aperture is the opening of the iris of the lens, shutter speed is how long that iris is open for. ISO allows you to play with the two while maintaining a proper exposure.

ISO is actually an older concept pertaining to film, but the concept is unchanged. Different sensitivities means you can control the other two factors independent of eachother.

Say you need to freeze action at a sports event, but it's night time and the stadium lights simply arent cutting it. For freezing action you generally want at least 1/200 shutter speed: let's say youre using a long lens and want 1/400. Using even maximum aperture only gets you to 1/300: you need "more light". You cant open the lens up any more and you cant lengthen your shutter speed for fear of blur: stadium lights are out of the question. Well, if you can increase the sensitivity of the sensor, then you can make up that "lack of light".

In the opposite direction, let's say youre doing some long-exposure work of a waterfall to get that streaky look, but dont have a Neutral Density filter to play with. Even stopped way down, youre not getting enough blur, or you're blowing out the water. If you can lower the sensitivity of the sensor in your camera, you can get that extra bit of time or get the blown highlights back into range.

[Doug Pardee]

The ISO setting determines what overall exposure is needed for a particular photo.

The lower the ISO, the higher the required exposure (slow shutter speed, small f-number), and the cleaner the photo will be.

In low-light situations, at low ISO the shutter speed may be unacceptably slow even with the lens wide open. By raising the ISO you can increase the shutter speed and/or stop down, although at the cost of introducing some "noise" into the captured photo.

To recap, you say "we can control the exposure with the other 2 elements of exposure Triangle" but there are many cases where that is not sufficient. You can only open your aperture so far, and slow shutter speeds usually result in blurry photos.

[dcclark]

Think of ISO as giving you more options. You have more ways to adjust your exposure. Each one has a trade-off:

Aperture: More open - less depth of field.
Shutter speed: Longer - motion blur.
ISO: Higher - more noise.

In each case, you get more light (or more sensitivity to light), but you have a downside as well. In each photographic situation, you may have different requirements. For example, in a landscape, you may value depth of field above all else. In a portrait, you may want no noise to speak of. There are always options, and only you as the photographer can properly choose among them.

[jrgarcia619]

I like to think of iso as film speed. If you buy a film that has a higer iso number it is more sensitive to light. Lower numbers are less sensitive to light. Basically it allows you to be more flexible with your choice of dof and shutterspeed depending on the situation. It all works together....

[saurabh.d21]

Thanks all for replying to my query so fast. ur feedback is of great value to me. though I just so perfectly understood the waterfall example by OS (with all ur pro knowledge u cant be thick), I guess in the example we need to increase the ISO only when light is very bright. Through "man with sunglasses" I was just referring to another article - Learning about Exposure - The Exposure Triangle.

Therefore to make it understandable to me, am I right if I say: increasing or decreasing ISO manipulates the exposure level in such a way to give you the desired exposure under different lighting conditions or movements by increasing or decreasing the sensitivity of the image sensor i.e. it makes image sensor more or less sensitive to same amount of light?

Wheras an ND filter manipulates the sensitivity or sharpness of light itself and not the sensitivity of image sensor?

Also Mr. dcclark can we really equate both & say exactly that: more light = more sensitivity to light
or do u mean something different.

Also just yesterday I was visiting a night fair and clicked some photos of my friend standing in foreground against a dimly lit background; now I see this burning red effect in his eyes in photos whereas the background was underexposed and my friend somewhat overexposed. I was using a Nikon digicam in auto mode & did not change the ISO. The flash was working fine.
should I attach the photo for more accurate analysis?

please all give ur feedback.

[teaking]

ISO is the sensitivity value applied to the camera sensor. Idealy the lowest ISO is used which gives less noise. In circumstances where by changing shutter anf f stops does not get an exposed image changing the ISO and how sensitive to light the sensor is can achieve the desired exposure but in return you gain more noise.

In bright light you would always use a lower ISO its just another tool for the photographer.

[dcclark]

Originally Posted by saurabh.d21

Thanks all for replying to my query so fast. ur feedback is of great value to me. though I just so perfectly understood the waterfall example by OS (with all ur pro knowledge u cant be thick), I guess in the example we need to increase the ISO only when light is very bright.

Backwards -- you only need to increase the ISO when light is dim. Increasing ISO in bright light just allows even MORE sensitivity to light, which you don't need, while adding noise (which you really don't need).

Originally Posted by saurabh.d21

Also Mr. dcclark can we really equate both & say exactly that: more light = more sensitivity to light
or do u mean something different.

No. Sensitivity to light is a property of the camera's sensor. With the same amount of light, "more sensitivity" means that the sensor will see more of the light which is already there. "Less sensitivity" means that it will see less of the light. Higher ISO = more sensitivity = apparently brighter images.

Also, not meaning to be mean, but would you mind spelling out all words and punctuating? I'm having a difficult time reading some of your posts. Thanks!

[saurabh.d21]

Thanks Mr. dcclark for making it simple for me to understand through your example that "sensor will see more/less of the same amount of light". Good words to explain things for a non technical amateur like me. And sorry for my misspelled English.

Thanks Teaking.

[belfyta]

Originally Posted by saurabh.d21

Also just yesterday I was visiting a night fair and clicked some photos of my friend standing in foreground against a dimly lit background; now I see this burning red effect in his eyes in photos whereas the background was underexposed and my friend somewhat overexposed. I was using a Nikon digicam in auto mode & did not change the ISO. The flash was working fine.
please all give ur feedback.

I'm going to try to answer this one. First of all I want to apologize for posible mistakes, I'm spanish and practicing english too... so sorry...

In my experience, the flash incorporated in the camera used in auto mode gives you this problem: overexpose the main subjet if you are near it, and underexposed the background. Exactly what happened to you.

Using manual mode instead, you have other posibilities: (I don't know if your camera has this options)

1.- Maybe your flash has its own configuration, so if you need to stand near the subjet you can decrease your flash power.
2.- Use Slow mode: this option decreases the shutter speed of your flash, so the camera can include background light.

I hope that will help....