HOW MUCH ISO SPEED AFFECTS FILE SIZES?

I noticed a strange phenomenon with my G3: file sizes are

dramatically affected by ISO speed! For example in JPG Large

Superfine mode (AE, exactly the same frame):

 

50 ISO - 1217 KB;

100 ISO - 1380 KB;

200 ISO - 1631 KB;

400 ISO - 1975 KB;

Auto - 1260 KB

 

It also affects RAW mode

 

50 ISO - 3392 KB;

100 ISO - 3558 KB;

200 ISO - 3755 KB;

400 ISO - 3989 KB;

Auto - 3425 KB

 

In RAW mode differences aren't so big but in JPG modes they are

so dramatically big that I am concerned about the quality. Is it a

normal phenomenon or my G3 is mulfunctioning?.In the whole big

manual Canon says nothing about this and states estimated

numbers which fit best ISO 400 mode. The funny thing is that the

counter doesn't care about the ISO settings and always shows

numbers based on biggest file sizes. But taking into consideration

that people generally don't use speeds higher than 100 (poor

quality) I wonder if it is possible to calibrate the counter and

enjoy proper values.

The higher the ISO speed, the higher the noise that is introduced. The noise is saved along with the image as a larger file

First of all, don't worry, your G3 is not malfunctioning.

<p>When you set the ISO speed of the camera higher, the signal from the sensor is amplified more. That allows you to take pictures when there is less light. Unfortunately, noise is also amplified. If you take the same picture at ISO 100 and ISO 400 you will notice that there's a lot more noise in the latter picture.

<p>I won't go into the details of the compression algorithm that is used for JPEG images, but a picture can be compressed more if there are more smooth areas in the picture. The noise at higher ISO settings makes the picture less smooth, so the picture can't be compressed as much as with ISO 100.

Thanks for your explanations! I also received the letter from Canon support which says:

 

"Dear Krzysztof,

Thank you for contacting Canon product support.

According to my resources, this is a normal phenomenon. As far as I can tell, your PowerShot G3 is not malfunctioning. Unfortunately, there is no way to calibrate the shots remaining counter. To my knowledge, the camera calculates the approximate shots remaining by taking the remaining space available on the storage medium and dividing it by a fixed value determined by the resolution and compression settings (not the ISO speed, since this will tend to change from image to image). I apologize if this is an inconvenience.

Thank you for choosing Canon.

Sincerely,

John

Product Support Representative"

 

Anyway I still think that the counter should take into consideration the ISO setting. I understand it can't predict how I am going to change ISO during whole session but it could show the calculated number of remaining frames in relation with presently chosen ISO.

Krztsyof,

 

The remaining shots display is a rough guideline. There are many factors at play that determine how much compression can be applied to an image, what the resulting file size will be, and thus the number of images there is remaining space for on your flash card. You just have to develop a feel for this knowing what settings are currently in effect in your camera and the nature of the pictures you are taking. i.e. how much information is in a shot. The more detail in a shot (including noise, which is random and thus results in less compression) the less the shot can be compressed. No matter what any manufacturer does to try to make the 'remaining shots count' display accurate, it never can be as the future cannot be predicted. It will always be a rough approximation and it will always be up to you to gauge this information accordingly. Even if Canon adjusted for ISO the display would still remain a very rough approximation.

Krzysztof, it's almost impossible to gauge accurately picture file sizes. Even on the same ISO there may be huge variations between shots of, say, a plain wall or fine intricate patterns with lots of colors.

 

For my 6.3MP pics, the best quality file sizes range from 1.5Mb to 3.3Mb, depending on the picture I'm taking. With other factors included, like sharpening etc. ... ISO isn't really a major concern, nor should it be.

Erb,

 

What you just said is not correct. ISO setting is a huge factor in determining JPEG file size. Higher ISO = higher noise = less compression possible. I did some shots recently where I set ISO to 200 and then to 1600 for the same shots (same content). The ISO 1600 files were twice as a large as those shot at ISO 200.