Pentax 645Z Review

Image Quality Overview

The Pentax 645Z is all about resolution.  With its 51-megapixel sensor measuring 32.8 x 43.8mm, it packs about 1.5 times the resolution of the largest available full-frame sensor with a slightly larger pixel pitch. If we were to trim the 645Z's sensor down to full-frame size, it would be left with 30.5 megapixels.  Trim it again to APS-C and we'd be at around 13.5.  Therefore, we expect it to deliver sensitivity comparable to some of today's best sensors, albeit at a much higher resolution.

Those 51 megapixels give use an effective file resolution of 8256 x 6192 pixels in RAW and full-size JPEG/TIFF mode.  If that's too much for you, you can choose to have your JPEGs or TIFFs saved in 36MP or 21MP resolution instead, saving space and speeding up desktop previews.

Suppose we wanted to crop the landscape photo below as follows:

We would still be left with a 10-megapixel file big enough for 12x8 print at 300 DPI. Click on the thumbnail for the full-size version:

Cropped version of the photo above (click for full size)

Based on its size and sensor type, it should be clear that the 645Z is best for landscape or studio photography, though it can certainly be used for other applications as well.

File Formats

As we've hinted at in the performance section, the 645Z can shoot in 14-bit RAW (Adobe DNG or proprietary PEF format), uncompressed 8-bit TIFF, or compressed JPEG. JPEG mode includes low (one star), medium (two stars), and high (three stars) quality settings.  The in-camera Custom Image profile is applied to the latter two formats, and they can also be used with lens corrections and lower resolutions.  For best image quality, however, RAW is still preferable over TIFF due to the increased color depth/dynamic range.

Bokeh and Depth of Field

Compared to smaller-format lenses with the same field of view, 645Z lenses produce a shallower depth of field, since a longer focal length is required.  Also, keep in mind that as of the writing of this review, there are no Pentax 645 lenses with an aperture faster than F2.8.  This is something you should consider based on your specific shooting needs.

4:3 Image Format

Like compact digital cameras, certain mirrorless formats, and film 645 cameras, the 645Z captures images with a 4:3 aspect ratio.  This means that photos will appear more "square" than typical photos from 3:2 formats like APS-C or 35mm film.  This opens up new creative possibilities, but if you're a fan of 3:2, the 645Z has more than enough pixels for you to crop.  If you crop a 645Z photo to 3:2, you'd still be left with about 45.4 megapixels!

Hand-Held Use

While we kept our 645Z on a tripod most of the time, we did have a chance to test it hand-held as well.  Although it is possible to achieve usable hand-held photos, and we have succeeded in doing so (in our first impressions and on the sample photos page in this review), the vast majority of our (unpublished) hand-held test photos were underwhelming.  Unless you're able to maintain a very fast shutter speed, image quality can deteriorate very quickly when compared to a stable tripod shot.  We know that many dedicated users will carry around and hand-hold a 645Z, but do we recommend it?  Certainly not.  If you're looking for a high-resolution camera designed to be hand-held, go with a 36-megapixel full-frame Nikon or Sony, or a 24-megapixel Pentax APS-C body if you don't need the high-ISO performance.  Ultimately this is for you to decide, so that's all we'll say on the topic.

Astrophotography

From an image quality point of view, the 645Z is well-suited for astrophotography thanks to its high resolution and low noise at ISOs such as 1600 and 3200.  The great high-ISO performance is especially important for this camera, since medium format lenses typically have maximum apertures no faster than F2.8 or F4, depending on focal length.  With the right technique, it is possible to create some impressive deep space photos with this camera.

We only ran in to a few small quirks while using the 645Z for this purpose.  First of all, we regret to say that slow shutter speed noise reduction (a.k.a. dark frame subtraction) cannot be disabled.  This effectively doubles the capture time of any exposure above 30 seconds, as the camera will need to process the image for exactly as long as the original exposure was in order to "subtract" the noise introduced by sensor heat.  While not a deal-breaker, this was rather inconvenient for us while we were out capturing star trails.  Edit: as of November 20, 2014, the v1.20 firmware update added the ability to disable slow shutter speed noise reduction.  Low-light photos may still require longer processing than regular photos, but no longer will we have to wait for twice the lenght of the original exposure!

Another issue we experienced was difficulty focusing with the 25mm F4 lens.  Due to the low magnification and relatively narrow maximum aperture of this lens, even live view magnification wasn't bright enough for us to accurately gauge the focus setting.  We therefore had to resort to trial-and-error for our star photos.  This wasn't as much of an issue with the 55mm F2.8 lens, however.  In retrospect, having a Flucard while shooting would have allowed us to more accurately check our results while still out in the field.

Finally, even though the 645Z supports the O-GPS1 GPS unit, it lacks the handy astrotracer feature, since there is no in-camera shake reduction system to move the sensor.  Thus, stacking (or an external star tracker) is required for deep space photos.

Despite not having achieved perfect focus with the 25mm lens, we were able to get a rather detailed galaxy photo from just five 6-second frames shot wide-open at ISO 3200.  We used DeepSkyStacker (freeware) to stack the frames after scaling our RAWs down to 4000x3000 resolution in Photoshop.  With the 645Z's full-size files, the stacker ran in to memory issues.

Stacked galaxy photo (galaxy goes from the lower-left to the upper-right corner; it is bisected by a cloud)
Click for full-size TIFF

Due to bad monsoon season weather, we are unfortunately unable to present you with anything more impressive than the above, as we had to abruptly end our only fruitful star shoot.

Throughout the Next Many Pages...

Instead of commenting more on the 645Z's image quality here, we have dedicated the next 6 pages to various tests.  Those tests will be accompanied by a host of real-world sample photos, all of which include original RAW downloads.  Then, we'll be able to make a verdict about the 645Z's image quality.