ISO settings, and other ramblings

phil from seattle wrote:

FingerPainter wrote:

phil from seattle wrote:

I tend not to use auto-ISO, preferring to keep ISO as low as possible.

But Auto-ISO does keep ISO a low as possible, for the lightness you told the camera you wanted.

Also, with auto-ISO, I found myself setting the max ISO frequently so why not just set the ISO directly - it's super quick to do on Olympus cameras.

Because, since you are not as smart as the camera, you will sometimes set it higher than it needs to be to get the target image lightness, and as a result your image will be noisier than it needs to be.

I believe you are confused.

Actually I believe you are confused about what exposure, EV and ISO are.

You need to study the exposure triangle to understand how this all works.

Perhaps I'd need to study the exposure triangle to learn your incorrect understanding. But nobody needs to study the exposure triangle if they want to understand how cameras actually work.

Exposure value (EV) is controlled by 3 factors: aperture, shutter speed and sensor sensitivity (aka ISO).

EV  indicates how much light the shutter and aperture settings will let in at one instant given the current scene lumiance. EV is controlled by precisely two parameters: Shutter speed and f-number, according to the formulas:

EV = log(base 2) of  ((N^2)/t)

or

2(^EV) = (N^2)/t

where N is f-number and t is exposure time (shutter speed).

Exposure, as distinct from Exposure Value, is how much light falls on the sensitive medium per unit area while the medium is exposed to light. Exposure is determined by three factors:

1. T-stop (approximated by f-stop)
2. Exposure time (shutter speed), and
3. Scene luminance (amount of ligh tin the scen.)

ISO is not the sensitivity of the sensor. Changing the ISO setting does not change the sensitivity of the sensor. The sensor will report the same voltage for a given exposure regardless of the ISO setting. What ISO does is map a given voltage reported by the sensor onto a specific digital number stored in the image file. The higher the ISO setting , the higher the digital number.

When you are using auto-ISO and set the camera to aperture or shutter priority, there are 2 variables that need to get fixed - ISO and aperture (if S-priority) or shutter speed (if A-priority). The camera will attempt to guess a good ISO to for the remaining variable.

Maybe on Panasonics. On cameras with a proper implementation of Auto-ISO, no guessing is involved.

The camera notes the meter reading as modified by the Exposure Compensation setting. This tells it the EV it needs to set at the camera's base ISO in order to get the lightness indicated by the EC setting. If it can get this EV with the user's selected aperture (in A mode) or Shutter (in S mode) , then the camera set s that EV, and takes the picture at base ISO. However if the shutter (in A mode) or the Aperture (in S mode) that it needs to set to get the indicated EV is beyond a limit, then the camera sets the shutter or aperture to the limit, and raises the ISO. The limit could be the widest aperture on the lens, the slowest shutter speed on the camera, or a minimum shutter speed the user set a a parameter to Auto-ISO.

In shutter priority where you set the shutter speed, I believe auto-ISO can do a reasonable job of guessing an ISO for a proper f/stop. Though, I mostly shoot in A-priority so haven't tested this theory.

There is no guess involved.

However, in aperture priority where you select the f/stop, autoISO doesn't know what you are taking a picture of and what the best shutter speed should be.

If you have chosen A mode, you have told the camera you don't care what the shutter speed. is.

A good ISO for playing kids is going to be different from one for a landscape or night scene in a city. AutoISO appears to bias towards action shots. I.e, picks an ISO value that will allow a fairly fast shutter speed to take a decent picture of the playing kids.

I think you are fantasizing. If you are shooting intelligently, it shouldn't matter what the ISO is.

And, as a result, my tripod mounted landscape shot gets a higher ISO than it should. All my night shots of London Christmas lights would have been at an unnecessarily high ISO because the fantastic IBIS in my cameras allows me to take long hand held exposures.

How did you tell the camera that you wanted the backgroudn black? This is just the camera doing what it is supposed to do. It doesn't try to guess what you want. It takes a meter reading and tries to produce a nice medium grey (or lighter if EC is positive, or darker if EC is negative.).

Maybe someday computational photography will get us there but it's not today.

I hear they are working on mind-reading.

So, you are wrong. I am smarter than my camera

Hmm, I thought the consensus of the previous posts was that our cameras were smarter than we are. You dissent from that consensus?

because I know what I'm shooting and it doesn't.

You are so smart that you forgot to tell a camera that you know doesn't  know what you are shooting how dark you want it. The EC setting is there for  a reason. If you don't want to use EC, don't use an Auto-exposure mode. BTW, you'd have the same problem in A or S mode without Auto-ISO except perhaps the camera would just refuse to take a shot at all.

ISO is not some mysterious voodoo.

What is mysterious is why people think it changes the sensitivity of the sensor.

I pretty much can look at a scene and know what the right ISO should be.

Uh-huh.

And if I'm wrong, I can quickly change it since it's super fast to set the ISO and slow to tromp through menus to set maxISO.

If  you are changing maximum ISO shot-by-shot, you are doing it wrong.