ISO speed, exposure time..???

[DJVege]

Hi guys,

I'm still trying to wrap my head around these topics.

I have a crappy digital camera (Canon IXUS 400). If I'm out in broad daylight, an ISO setting of 400 (max) seems to make the images too bright (over exposed???). So I set the ISO setting lower, and the picture looks good. In dark situations, I have to set the ISO setting to the max, 400, because otherwise I get a blurry image. This is, I think, due to the fact I can't stay still long enough for the exposure time required...yes?

I guess what I'm asking is, in what situations should the ISO setting be high, and in what situations should it be low? Same goes for the exposure setting. (Which you can only change a fraction in my camera )

Thanks.

[[1ponders] (Paul)]

The lower the light conditions the higher you set your ISO (generally avoid over ISO800). ISO has nothing to do with the blurriness of your image, that has to do with your focus ( durh ) and your exposure setting. The lower the light conditions the slower your exposure the more steady you need to hold the camera. My suggestion would be that anything longer 1/25 sec you would benefit from using a tripod.

Oh and blurriness can have to do with your f ratio. Fast f ratios have short depth of field, ie close object focused on will mean object further away will be blurry. Slow f ratios increase your depth of field, ie objects at greater distance from the camera stay in focus as well as the object being focused on (If that makes sense)

[Terry B]

ISO (or what use to be called ASA) is the "speed" of the detector. With film the lower the ISO, the slower the film but the higher the resolution with less grain(noise). The trade of for faster and hence more light sensitive film is less resolution and more noise in the final image.
The same thing applies with digital cameras. With your camera in bright sun it probably doesn't have the ability to do a fast enough exposure when you have the ISO set at 400. The end result is an over exposed image. You are better off setting the ISO at a lower number. This will result is less overexposure, less noise and a better picture.
Film/CCD need a certain amount of light to hit them to achieve what we would perceive as a well exposed picture. This is determined by the shutter speed, aperture of the lens and the speed or ISO of the film/CCD. Doubling the ISO has the same effect as doubling the exposure time or opening the aperture up 1 stop.

With astro photography things get a bit more complicated. We usually work with under exposed images due to the dim objects being photographed. This takes long exposures-usually many minutes to hours. ISO change for a CCD is essentially performed just by amplifying the signal that has been received and not by actually increasing the amount of light detected. This will increase noise as well as the signal. Often it is better to use a lower ISO setting that allows you to have less noise in your final image and use other techniques to improve the exposure.

[DJVege]

Oooooohhh.....ok. I think I get it. Thanks guys.

Durh yourself, 1ponders!

[Dennis]

In the good old days of 35mm film photography, a "slow speed" film was 25 ASA, a "medium speed" film was 100 ASA and a "fast film" was 400 ASA. So, when there was lots of light available, you would normally select a slow to medium speed film and in dimmer conditions, select a faster film, right up to the very grainy Konica 3200 ASA film. Examples of film I used to use were:

• Kodachrome 25 - a 25 ASA slide film.
• Kodacolor 100 - a 100 ASA print film.
• Fujicolor 400 – a 400 ASA print film.
• Ilford FP4 - a 125 ASA B&W film.
• Ilford HP4 - a 400 ASA B&W film.

The grain (film structure) was very fine at 25 ASA and much coarser at 400 ASA, giving a 400 ASA photo a gritty look, much like noise in short exposure CCD image. You could enlarge a 100ASA film to produce a 16x12 inches print before the grain became noticeable, whereas grain would show at 10x8 inches from a 400ASA film negative.

Films improved over the years and a 1990’s era 400 ASA film had grain as fine as a 1980’s 100 ASA film. Generally, CCD’s with small pixels are “slow” whereas CCD’s with large pixels are “fast”; that is, they can record in dim light more efficiently.

Cheers

Dennis

[leon]

Isn't the IXUS 400 auto, set it and forget it and it works it out it's self, I have both the IXUS 500 and IXUS 50, they both are just point and shoot, so i figured the 400 would be the same.

Leon

[DJVege]

Yeah, I can set it to Auto, 50, 100, 200 or 400. Sometimes setting it manually seems to produce a better picture. Unless my eyes are just weird.

[Solanum]

Originally Posted by DJVege

Yeah, I can set it to Auto, 50, 100, 200 or 400. Sometimes setting it manually seems to produce a better picture. Unless my eyes are just weird.

In general you want to use the lowest ISO you can. With film the grain size was bigger in the high ISO film so low ISO was better. Similarly, with digital cameras, in general, there is an increase in noise with the higher ISO settings.

[DJVege]

Ok. Got it! Thanks again!

I'll just set the camera to auto from now on. For night pics, I'll try to steady the hands.