Overexposing and underexposing

[sarahkate]

Hi all. I'm shooting with Kodak portra 400 in my Canon AE-1 and I'm unclear about overexposing and underexposing.

To underexpose or overexpose do you change the ISO or the aperture?

For instance. If I'm shooting Kodak Portra 400 and I change the ISO to 320 (or 500)...would that be considered underexposing or overexposing? Or another example of keeping the ISO at box speed and changing the aperture based of the cameras built in metering system.

What is the difference/benefit?

In the past I've been changing my ISO based on lighting conditions and shooting at the recommended f-stop. Is this correct?

Thanks, Sarah.

 

[ann]

You can over/under expose by changing ISO, which will change the fstop of course or the shutter speed depending on your chice.

Fstop controls depth of field (along with focal length and shooting distance)

What your doing is correct. It is very common to over expose negative film and underexpose slide film.

With negatives one then changes the development times to match.

 

[compur]

If I'm shooting Kodak Portra 400 and I change the ISO to 320 (or 500)...would that be considered underexposing or overexposing?

Theoretically, if you set the film speed on your camera to a value lower than the film's rated speed you are over-exposing and vice versa. This assumes normal lighting conditions.

Most print films, however, can tolerate quite a bit of over or under exposure though image quality may not be optimum. Many color print films often even improve a bit by an over-exposure of about 1 stop.

 

[Big Mike]

Exposure is an equation with several variables; the shutter speed, the aperture, the ISO (sensitivity of the film in this case) and the amount of light.

On the camera, you have control over the shutter speed, aperture and on a film camera, you can choose the ISO of the film you use plus you can override the ISO on the camera.

Changing either of those three things will change your exposure (provided you aren't countering that change with another variable).
So you can over or under expose by changing either of those settings. Although, overriding the ISO setting on the camera doesn't change ISO on it's own...it just affects the camera's metering, which in turn may change your exposure choice.

But know you need to ask, over or underexposing from what? Where is your starting point? If you use the auto exposure mode, the camera will give you exposure settings based on what is in front of the lens...but that system has inherent flaws. Sometimes it will give you underexposed images, and sometimes overexposed. Usually, it's not too far off and it's corrected by the lab when developing and/or printing. But in order to get it 'just right', it helps to know what the camera is trying to do, and how to counteract the camera's metering system.

One method is to use a grey card.
How to use a Grey Card ~ Mike Hodson Photography