question about aperture priority

Hi Mike,
I took your Starved Rock class back in August. I actually went back there with my husband and took more pictures! The reason I'm writing is that your son Scott said that he recommends only using the aperture priority setting, never the shutter speed setting. At the time his explanation made sense, but I forgot now why he said that. Can you please refresh my memory on why it's not worth using the shutter speed setting?

Thanks and I hope you're doing well,
Jennifer R

Reply:

Thanks and yes we are doing well.

Shutter priority can cause your camera to make mistakes at high and low light. Aperture priority does not. The mistake is over or under exposure.

For example, on a bright sunny day, if you are in shutter priority and you set your shutter speed to 1/2 a second to blur a waterfall, your camera would set your f-stop (aperture) to the highest number it can, say F32. However even at ISO 100 and F32, the shutter speed will be too long and the image will be over exposed. If you use aperture priority, the image would be properly exposed, but the waterfall would not be blurred because the shutter speed would be near 1/1000th of a second.

Another example, on a cloudy day at Starved Rock in any of the canyons, where it is very dark, if you are in shutter priority, and you set your shutter to 1/1000th of a second to absolutely freeze the waterfall, your camera would set your f-stop to the lowest number it can, say F3.5. However even at ISO 800 and F3.5, the shutter speed will be too short and the image will be under exposed. If you use aperture priority, the image would be properly exposed, but the waterfall would not freeze action because the shutter speed would be near 1/2 of a second.

The important thing here is to control the image to your liking. Use aperture priority to control depth of field and blur. Depth of field is how much is in focus, ie is the foreground and subject and background in focus.

Use a high F-stop to get a lot of depth of field and a slower shutter speed, like when shooting a landscape and you want everything in focus. Use a low F-stop to get little depth of field and a faster shutter speed, like when shooting a flower and you want the background out of focus to isolate the flower.

Mike