How To Decide What Aperture To Use

Inside: Are you struggling with what aperture to use?  This post provides two questions to help you decide.

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At six weeks old my third baby had begun to smile in earnest and I knew I HAD to capture it.

I settled the baby in her bouncer with a pretty blanket draped over the seat to create a nice background and enlisted my husband to help me coax out a baby smile.

My camera settings were ready and I felt my heart beat faster as my husband chatted with the baby in sing-song tones.

The moment came – her daddy’s voice made her face light up with the sweetest grin and I snapped away.

I reviewed my images on the back of my camera and saw a beautiful photo of my baby’s smile.

Success!  A keeper!

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…or so I thought.

Later when I downloaded my photos to my computer I admired my capture.

The exposure looked perfect and her grin made me giddy.

I imagined how much everyone would love this photo on her birth announcement.

I zoomed in…

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Wait, the eyes.

My heart sank.

Her left eye looked blurry.

My excitement shifted to deep disappointment.

What happened? Where did I go wrong?

I had no idea.

As a new photographer I’d read the rule of thumb:

choose your aperture based on the number of subjects in your photo.

That was easy enough to remember.

So I’d chosen an aperture of f/1.8 because I had one person in my photo. Shouldn’t that have worked?

Why didn’t it work?  How do you know what aperture to use?

The fact is it depends.

The aperture you should use depends on how many people or subjects are in your photo.

But other factors also need to be considered as you decide what aperture to use.

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Aperture, also known as f/stop, is one of the legs of the exposure triangle.  

Aperture will help you create beautiful, blurry backgrounds in your photos. 

Related: How To Use Your DSLR Camera. 

This photo was taken at f/4.0.  Notice how the subject is sharp and the mountains in the background are blurry.

A photo taken at a lower f/stop setting is said to have a “shallow depth of field.”  This means that a very small “slice” of your image is in focus.

Think of it like a pane of glass running on a vertical plane through your image.  A lower f/stop setting will create a “thin” pane of glass through your subject.

This imaginary window pane, or in-focus “sliver” of the photo is called the “depth of field.”

A photo taken at a higher f/stop setting has a “wide (or deep) depth of field.”  This means that a larger “slice” of your image will be in focus.

The pane of glass running on a vertical plane through your image will be wider.

Need more help understanding aperture?  Check out this photography quick start guide and cheat sheet.  It includes a quick exercise to help you better understand your camera settings.

Scroll through the photos below and see if you can spot what I changed from the first to the second photo.

Hint: It wasn’t my aperture setting.

In the first photo notice only the toes are in focus.

In the second image notice how all of both feet and part of the legs are in focus, even though I didn’t change the aperture.

Do you know what changed from the first photo to the second?

Click on the photo to see a larger version:

In the example with my daughter’s baby photo taken at f/1.8 the shallow depth of field could not encompass both eyes.

To fix the problem I could’ve changed my aperture to f/2.8 or f/3.2 to capture both of her eyes in focus.

or

I could’ve taken the photo from further away to get both eyes in focus without adjusting my f/stop setting.

In the example of the toes above I took a few steps back but didn’t change my settings. 

I cropped in closer on the second image (in Lightroom) to show the increased depth of field created when I moved further from my subject.

But you can see here how far back I was compared to the first photo, which was not cropped.

The next time you need to decide what aperture to use ask yourself two questions:

1.  How many people are in my photo? 

As a rule of thumb, use one aperture stop for every person in the photo.  For 5 people, for example, choose at least f/5.

If the people are on different planes, such as in rows, you may need to raise your f/stop number more to get everyone in focus.

The number of people in your photo is a good rule of thumb, but also remember to ask yourself…

2.  How close am I to my subject(s)?

If you’re close to your subject use a higher f/stop to increase your depth of field and get all the important parts of the subject’s face in focus.

Remember to zoom in on the camera’s LCD to check your focus!  If I’d zoomed in on those photos of my daughter I’d have realized I needed to adjust my aperture setting or back up a bit to get both eyes in focus.

NOTE:  your lens and the type of camera you have also impact a photo’s depth of field.  Check out this helpful article to better understand how these other factors affect the depth of field in a photo.

Practice with different lenses if you have more than one. Different lenses will affect your depth of field based on their focal length.

If you’re new to photography, don’t be afraid to make mistakes! Embrace them because they’ll teach you a lesson you’ll remember.

Use the questions above when deciding what aperture to use.  You’ll avoid download disappointment and increase your keepers.