Pentax K-S1 Review

Autofocus

The Pentax K-S1 has two separate focusing systems: phase detection autofocus when shooting through the viewfinder, and contrast detect autofocus when using live view.  The viewfinder system is generally faster to focus and suitable for moving objects, while the live view system is slower but more accurate.  On this page, we will cover both systems in detail.  We will also discuss the various manual focusing aides included in the camera.

Viewfinder Autofocus

The K-S1 inherits the 11-point SAFOX IXi+ phase detect autofocus system from the K-50 and the K-30 before it.  This AF system has 9 cross-type points in a 3x3 grid in the center of the frame, with an additional point on either side.  This AF system isn't cutting-edge for sports or action photography, nor is it as sensitive in low light as Pentax's flagship DSLR, but it gets the job done during everyday shooting and is therefore by all means satisfactory for this type of camera.

While shooting through the viewfinder, you have the option of using all 11 points, just 5 points, or a single point (that can be moved around if/as needed).  If the K-S1 is your first DSLR, we recommend the 11-point mode, as this allows the camera to focus on what it thinks is most likely to be your subject.  It isn't always right, though, and thus most seasoned users prefer using only the central AF point, or pinpointing the location of their subject when contrast/ambient light is low.

The K-S1 features viewfinder AF point illumination, so you always know which AF point is being used.

We are happy to report that the K-S1 has been fitted with an improved screwdrive motor, which means that the camera can deliver focusing speeds up to 50% faster than the K-50 when paired a screwdrive lens (such as the 18-55mm kit lens).  We measured identical focusing speeds for SDM/DC lenses on the K-S1 and K-50.

For see our K-50 review for more technical information on SAFOX IXi+.

Live View Autofocus

Our analysis of the K-S1's live view AF system brings more good news: the camera is very quick to focus with most lenses and its performance is on par with the Pentax K-3 flagship.  The camera really only struggles in low light or when using a lens with long focus throw, such as a telephoto. In both these cases, the viewfinder autofocus makes up for it.

Live View Screen in Auto Area AF

Focus Locked

Pressing the Menu button while in live view brings up the live view sub-menu:

Live view sub-menu

Among other settings, this menu allows you to select the contrast detect AF mode:

You'll find a total of six different modes:

• Face Detection
  Highlights faces on the screen and attempts to focus on them
• Face Detection Assist
  Face Detection; LED lights in the grip light up based on the number of faces detected
• Tracking
  Attempts to stay locked on a fixed object as you pan; does not work for moving objects
• Auto Area AF
  The camera automatically focuses on objects within a 5x3 central grid
• Select
  Same as Auto Area AF but allows the user to choose the size and position of the AF area
• Spot
  Always focuses on a small area in the center of the frame

If you plan on shooting many portraits and family photos, sticking to Face Detection is a good idea.  Otherwise, we recommend using either Auto Area AF or Select mode, depending on whether or not you plan on shooting difficult subjects, or just snapshots. 

Note that in Face Detection/Select/Auto Area AF mode, the camera can lock on to multiple AF areas and report them all in focus.  The same logic does not apply to the viewfinder autofocus, which always picks a single point.

Spot AF should be avoided during everyday shooting as it is slower than Select mode with multiple areas.

Below are screenshots of all the AF area sizes available in Select mode.  The default for Auto Area AF and Face Detection is in the upper-left:

Manual Focusing

The K-S1 is close to perfection when it comes to manual focusing, at least for an APS-C DSLR.  Here's what it brings to the table:

• Large viewfinder
  The optical viewfinder is as big and as bright as it will get in a camera of this class
• Catch-in-focus
  With the correct menu settings and while focusing manually, the camera will not fire until it detects that the subject is in focus
• Focus confirmation
  You will hear a beep and see an LED light up in the viewfinder when the camera detects that the subject is in focus
• Image magnification
  You can enlarge the live view display up to just over 100% to make sure your subject is in focus
• Focus peaking
  The camera adds white outlines around objects that are in focus while in live view

Live View Magnification

While in live view, pressing the OK button will enlarge the image.  You can then zoom in and out using the scrollwheel.  (if the OK button pulls up an AF point prompt, hold it down for 2 seconds first to switch to magnification mode)

When image sharpness is critical, this is the single most convenient feature for ensuring that you've nailed the focus.  It would have been perfect if the camera didn't reset the magnification as soon as the AF button is pressed.

Focus Peaking

The screenshots below (borrowed from our Pentax K-3 review) illustrate how the latest-generation Pentax focus peaking system works:

Notice how outlines appear around edges that are in focus.  This makes it easy for the user to quickly appreciate what is currently sharp. 

This feature becomes even more powerful when combined with magnification.  Below is a close-up view of a focus peaking on with 4x zoom:

There's still a little bit of room for improvement here: Pentax could add different color options and line thicknesses for the outlines so that they can be changed based on the colors in the image.

Noise

We have to examine one final topic, and that's noise.  Simply put, Pentax screwdrive lenses are noisy, especially when used in live view on a camera with a fast AF motor like the K-S1.  The kit lens is long overdue for an upgrade to silent focusing.

The video below shows the sound and speed of the K-S1's autofocus with the 18-55mm kit lens:

Compare this to the autofocus with a 18-135mm DC lens with a built-in silent AF motor:

We therefore strongly recommend that you replace the 18-55mm lens with a silent-focusing lens such as the 18-135mm or Sigma 17-70mm if your budget allows it.

Focusing Verdict

The K-S1 delivers satisfactory focusing performance through the viewfinder and quite impressive speed and accuracy in live view.  It also makes precise manual focusing easy thanks to focus peaking and image magnification.

Not unexpectedly it lags behind the K-3 flagship when it comes to viewfinder AF points and low-light sensitivity.

Overall, the K-S1's autofocus is an improvement over the K-50 in two key scenarios: when shooting in live view with any lens, and through the viewfinder with a screwdrive lens.  We therefore believe that most users will be happy with it.