Learning What Your Smartphone Knows

On the previous page we looked at three things we must do to take a good picture.  On this page, we're going to take a deeper dive and understand what your smartphone knows and how you can do this too.

The First point is to frame your picture - deciding on your subject(s) and how much to include in your background.  In this regard, there is not a lot of difference between what we do when using a smartphone or a dedicated camera.  What a smartphone does is automatically detect your subject(s) and background and then optimize exposure, color balance and focus. 

The Second point is to have correct focus where we want it - and often place our background out-of-focus.  In a smartphone the subject(s) and background are detected automatically, and if picture is shot in "portrait" mode then digitally blur the background, which helps to make the subject(s) standout.  

When using a dedicated camera we have two options.  Based on camera, lens and aperture, we can produce the out-of-focus blur directly in the image when we push the shutter button, or we can using photo-editing software to digitally blur the background, analogous to what can automatically be selected in smartphones.

If we want to increase the amount of out-of-focus blur (often called reduced depth-of-field) in the original picture then we need to consider five things - the sensor size (this is the physical size, not the number of megapixels), the focal length of the lens, the aperture (f-stop) used to take the picture, the distance to the subject and the distance between the subject and the background.  That seems like a lot to keep in mind, so let's break it down.

Sensor size - As mentioned, a smartphone uses a relatively small sensor.  (The iPhone 13 sensor dimensions are 7.6 x 5.7mm.) The dimensions for a full frame sensor in a dedicated camera are 36 x 24mm which is over 4x larger.  In actuality, the sensor itself does not determine the degree of out-of-focus blur, but it directly determines the focal length of the lens used for any given picture. A sensor that is 4x larger in either dimension means that the lens will need to have an absolute focal length 4x longer.  I use the term absolute focal length because often focal lengths are specified in terms of their full-frame equivalent which is scaled by the ratio of the sensor size to full-frame. 

Focal Length - Doubling the focal length of the lens results in four times the degree of out-of-focus blur.  This means a long focal length telephoto lens - when shot at the same distance to the subject - give increased out-of-focus blur than a wide angle lens.

Lens Aperture (f-stop) - Opening the aperture (reducing the f-stop) allows more light through the lens.  This is because it accepts a broader (more divergent) set of light rays to enter the lens.  When a broader set of light rays are out of focus, the degree of out-of-focus increases.  

The Distance to the Subject and Background.  The first consideration is how close the subject is to the camera - with subjects that are closer creating the potential for increased out-of-focus blur. The second factor is the distance between the subject and the background.  A larger separation results in greater out-of-focus blur. 

In summary - to increase out-of-focus blur - select a camera with a full-frame sensor and select a lens that makes shooting with a wide aperture - ideally f/2.8 or less at 50mm.  "Faster" lenses which allow for apertures of f/2.0 and below will give even more depth of field control.  I very much enjoy shooting with my Leica Noctilux 50mm f/0.95 which allows me to get an out-of-focus background even when my subject is 50 feet away and at the same time using a normal field of view.  Of course, the challenge shooting with a large amount of out-of-focus blur is that your depth-of-field is very shallow, meaning you must focus very carefully to get the subject sharp and in focus.

We can, of course, shoot our picture with smaller apertures (higher f-stop values) and add out-of-focus blur when we edit our pictures in photo-editing software.  This is what a smart phone is doing for you automatically.  The good news is that such editing capabilities has become much easier.  Artificial Intelligence (AI) tools built into the software can automatically detect subjects and background and allow you to edit them and sharpen or blur them independently.  

However, the artistic nature created by software tools for the out-of-focus blurring (also true for smartphone portrait mode) do not offer the artistic characteristics that can be achieved producing these effects in camera.  You can find many reviews that cover photographers love for various lenses, each with it's own out-of-focus characteristics or so called BOKEH.

Third is proper exposure.  This is harder than you think.  The human eye is remarkable at automatically adjusting not only for varying light levels but also the color of the light - often referred to as temperature (K) or white balance. What makes things even more challenging is the degree that light intensity and color can change in any given scene. 

Smartphones have developed several techniques to adjust to variations in light intensity and color across an image.  While much of these "under the hold" elements are part of each manufacture's proprietary technology here's some basics which we can then use ourselves to improve both our picture taking and photo-editing techniques.

The first step is to ensure that (in general) we don't over expose bright areas of our pictures that where we still want to make sure we can see colors and details.  A very good example of this is taking the picture of a sunset.  If the sun is still in the sky and in the image, then it is very, very bright, the highlighted clouds close into the sun are are also very bright.  If we want to capture these, then we need to make sure that the exposure is targeting this part of the image.  Of course, this will mean that much of the rest of the image will be darker than we might like. 

To account for such situations, there are various auto-exposure modes that can be selected from in a dedicated camera.  This include "center-weighted", "spot", "multi", and "highlight".  These work to asses the scene and give you an optimal exposure. You may find that in some situations you need to help out.  In my work, I often use center-weighted, but I realize that my subject may not be in the center, so for a brief moment, I aim the camera so the subject is in the center. I half press the shutter button to "set the exposure" then recompose the image to my liking and press the shutter button the rest of the way to take the picture.

Also realize that there is often no one exposure setting that will give me optimal exposure for everything in the scene.  When this is the case, remember it is easier to recover color and details from dark areas than from bright areas.  This means, when in doubt, set the exposure targeted more for brighter areas in the picture, then recover the shadows when editing.  Because smartphones are able to automatically detect the subject(s) and background, they are are also able to automatically brighten the shadows and decrease the highlights in the image.  You can apply this same treatment as well in photo editing.  As mentioned above, with AI tools it is easy to automatically select subject(s) or background, or the sky and edit them to give the exposure balance you wish.  For more on this topic, look at sections on Lightroom editing.

There are simply times when there both bright and dark areas and there's no one exposure to capture everything.  Because of this smartphones are programmed to actually make multiple exposures and then combine them together - automatically.  This can also be done with a dedicated camera.  We call this mode HDR for high dynamic range.  You can set the camera to automatically take several pictures with a single click, each with a progressively longer (or shorter) exposure.  Then in your editing software, these can all be merged together and then edited to see details and color from deep in the shadows to bright highlights as well.  More on this in the photo editing pages.

The basics to taking a good picture is to understand how to frame your subject and background, achieve the focus you wish and establish optimal exposure. 

Of course, the real question is how to I get a great picture?  That goes beyond framing, focus and exposure and requires that you produce great pictures with Natural Radiance.