Pentax 645Z Review

Performance and Burst Mode

On this page, we will be taking a look at the burst performance and general speed of the Pentax 645Z.  Thanks to a the new PRIME III processor, the new sensor and an improved shutter mechanism, the 645Z is able to not only shoot bursts at 3FPS (up from 1.1FPS on the 645D), but also offers a smooth live view mode and HD video recording capabilities.

All of our tests were performed with a 32Gb SanDisk Extreme Pro 95Mb/s SDHC card (90Mb/s max. write speed), and our JPEGs were captured at full resolution and high quality; our RAW files format was set to DNG.  The ISO was set to 100.

First, let us take a look at the time the 645Z requires to carry out common tasks. 

General Performance

When comparing this performance to that of the Pentax K-3, we can conclude that Pentax engineers have done a very good job of adapting the PRIME III engine in the 645Z.  The camera does take a bit longer to process and save its files, but considering their 51-megapixel resolution, we really can't complain.

One thing to note is that if you plan on using JPEG mode for bursts, performance is greatly improved by leaving distortion correction off.  Other lens corrections are applied considerably faster, and thus result in minimal overhead.

Burst Mode Performance

The 645Z offers a number of file storage modes, and we have tested its performance in each mode.  For this test, we shot a well-lit and varied indoor scene.  Note that actual burst performance will vary based on the complexity of the scene and the ISO.

Continuous Hi mode was used for all of our burst tests*.

"Sequential" Image Storage Performance

In sequential mode, the camera only writes to one SD card and treats the other card as overflow space.

As a side note, you still cannot specify the primary card via the menu. If you want the camera to switch cards, you must power down, remove the unwanted card, power cycle the camera, power down again, and then put the card back in.

Legend: The first column indicates the total number of files captured at maximum FPS (*note that the camera continues to shoot at around 1FPS after the buffer is full).  The second column indicates the total duration of image capture.  The third column shows the number of seconds that elapsed from the capture of the last frame to the time the SD access light went out.  The final column is a rough estimation of the amount of time each file took to save and process (for comparative purposes only).

This mode offers the best overall speed and buffer size.  In JPEG mode the camera is quicker than many older K-mount DSLRs.  A burst of 36 files was captured and written to the memory card just 27 seconds (12 + 15) after first pressing the shutter button.

Continuous RAW bursts are considerably shorter, but can flexibly be used as needed during a shoot as the camera is quick to save all the files.  A burst of 13 DNG files took 22 seconds from start to finish.

"Simultaneous" Image Storage Performance

In simultaneous mode, the camera uses the second SD card as backup space and stores two copies of every file captured.

Compared to sequential mode, the number of files that can be captured in each burst is about the same. However, the writing time nearly doubles, which shows that the camera hardware cannot save to both cards simultaneously.  For this reason, the camera feels somewhat sluggish in this mode.

"Split" Image Storage Performance

The final image storage mode saves your RAW files to one card and your JPEGs to the other.

The writing performance is roughly the same as shooting RAW+ in sequential mode and faster than RAW in simultaneous mode, but the buffer holds slightly fewer images.

Findings

Based on our test data, we find that sequential mode offers the best overall file storage performance and buffer size.  Simultaneous and split storage should therefore only be used when your assignment requires it.

Another observation is that TIFF mode is unusually slow (due to large filesize; see below) and therefore should be avoided.

Shooting at Lower Resolutions

Shooting smaller JPEGs at lower qualities can improve burst performance slightly.  However, since overhead is introduced when the camera goes to process the sensor data and create each JPEG, and since JPEG files are already rather small, the actual performance gain isn't considerable.  Therefore, unless you want to conserve storage space or speed up desktop file editing, we recommend shooting at maximum resolution and quality.  The nature of the scene can have a more significant impact on performance than your JPEG settings.

Card Write Speed

To find out roughly how quickly the 645Z can write its files, we captured a series of complete bursts and timed the camera's reaction.  Note that we used a different (more uniform) scene for this test, hence the larger JPEG burst count.

In addition to this test, we repeated all 3 bursts tests above using a Sandisk Extreme 45Mb/s card and obtained identical results as with the 95 Mb/s card.

We can therefore conclude that the 645Z's burst performance is limited more by its CPU and buffer rather than the SD card rating.  Even though the camera can take advantage of UHS-I speeds, the high specs of our Extreme Pro card brought no benefit apart from faster desktop file transfer (which shouldn't be overlooked, however).  The Extreme Pro card offered marginally faster playback performance in multi-thumbnail mode, but has no practical significance.

Required Card Speed for Optimal Performance

Cards with a write speed slower than 45 Mb/s will offer sub-optimal burst performance with the 645Z.  Cards slower than 30 Mb/s will even limit you in JPEG mode.

Video Demonstration

Below is a video of a sequential-mode JPEG and RAW burst test.

Note that while the camera is writing data to the SD card, its menus are locked, but additional photos can still be captured using the viewfinder and primary shooting controls.  As the buffer is cleared, additional photos can be captured even before all the files have been written.

File Size

The 645Z's 51-megapixel resolution means that its image files occupy a considerable amount of disk space.  For landscape applications, expect JPEGs in the 25-30Mb range and DNGs/PEFs in the 60-75Mb range.  However, as you crank up the ISO, the RAW file size increases (since random noise worsens compressibility).  RAW file size starts skyrocketing at ISO 3200 and above.

The table indicates sample RAW file sizes for a daytime landscape scene.

Verdict

For a high-resolution camera, the 645Z offers very good burst performance and buffer size.  While we would have liked to see a larger RAW buffer, JPEG performance is more than satisfactory for high-resolution action photography.  The 645Z's burst framerate of 3FPS, like its ISO range, is class-leading.

There are only two key issues to make note of this section.  The first is TIFF performance, which is unusually slow due to the fact that TIFF files are well over 100Mb at 51-megapixel resolution.  The 645Z simply cannot write files quickly enough to make using this format practical.

The second is the simple fact that the 645Z is a memory hog.  Expect to blow through 32Gb and 64Gb cards in a day, and be sure to stock up on a few terabytes of external drives or plenty of cloud storage space.