My experience with getting to grips with using Aperture Priority to create shallow depth of field in my images has led me to learn more about exposure – how the camera actually works.
Basically, I’ve learned that light enters through the aperture, which moves from wide open (creating shallow depth of field) to narrow (allowing full depth of field) allowing more (open) to less (closed) light to reach the sensor.
The shutter speed determines the length of time the shutter stays open, from fast to slow, allowing less light (fast speed) to more light (slow speed) to reach the sensor.
And the camera sensor, which is a different size depending on the type of camera (bigger is generally better), can also be set to be more sensitive (high) to less sensitive (low) to the available light – this is the ISO. Unfortunately greater sensor sensitivity goes hand in hand with greater ‘noise’ (poorer quality) when it comes to image quality.
So spot the common denominator here – letting more or less light in to the sensor, where the image is created. I’m learning about the rule of equivalent exposure, where if a certain level of light is needed to give a particular image the correct exposure, then it is possible to achieve this level in many different ways.
Opening up the aperture (letting in more light) requires compensating for this by letting in less light via the shutter (using a faster shutter speed) and/or reducing the sensitivity of the sensor (lowering the ISO), maintaining the correct balance of light for achieving the correct exposure.
Or alternatively closing down the aperture (reducing the available light) therefore requires a slower shutter speed to compensate (allowing more light in) and/ or increasing the ISO (making the sensor more sensitive to light).
And these three values, aperture, shutter speed, and ISO, are all measured in stops of light, with one stop relating to either a doubling or halving of the available light. It seems that photography, therefore, is all about manipulating the amount of light allowed through the lens onto the sensor in order to create an optimum exposure within the camera…
Today’s decidedly ‘how-not-to’ picture is actually supposed to be a cute pic of my baby granddaughter bouncing up and down in her baby bouncer with a huge grin on her face – unfortunately in the very low indoor winter light, even with the aperture wide open the relatively slow shutter speed has given me some phenomenally impressive motion blur! 🙂