Sony A7 with Pentax Full-frame Glass

Pentax K-3 and Sony Alpha 7 Image Comparison

The K-3 and the Alpha 7 both boast 24 MP sensors so it would be natural to compare the two cameras with matching lenses so as to illustrate the difference (or lack thereof) in image quality.

Daylight Shooting

For the first set of images we used a tripod and the cameras set to ISO 100. In Adobe Camera Raw (ACR) and Photoshop CC we made these adjustments:

  • Pentax:
    • ACR: Increased exposure by 1/3 stop for the first image. No correction for the second image. Converted using the Adobe Standard profile.
    • CC: Unsharp mask 120%, radius 0.9, threshold 0
  • Sony:  
    • ACR: Increased exposure by 2/3 stop. Increased white balance by about 1000 K to match that of the Pentax. Converted using the ACR Camera Standard profile.
    • CC: Unsharp mask 120%, radius 0.9, threshold 0

To get approximately the same field of view from the two cameras we used an smc Pentax-FA 31mm F1.8 Limited lens on the Pentax and an smc Pentax-FA 50mm F1.4 lens on the Sony. To get approximately the same depth of field we used F5.6 on the Pentax and F8 on the Sony.

The first row of samples show the entire frame. Focus was on the Saguaro cactus on the middle. Second row shows 50% of the frame so as to make download faster. Focus in this case was on the palm tree in the back.

Click on a thumbnail to browse a larger version or follow the link to download a full-sized JPG created from the RAW file. 

JPGs developed from RAW, ISO 100
Pentax K-3, ISO 100
smc Pentax-FA 31mm F1.8 at F5.6
Sony Alpha 7, ISO 100
smc Pentax-FA 50mm F1.4 at F8

Link to full size JPG as developed from RAW

Link to full size JPG as developed from RAW

Pentax K-3, ISO 100
smc Pentax-FA 31mm F1.8 at F5.6
Sony Alpha 7, ISO 100
smc Pentax-FA 50mm F1.4 at F8

Link to full size JPG as developed form RAW

Link to full size JPG as developed from RAW

The thumbnails and even the larger images you can browse above are virtually identical.  To appreciate the differences you need to download and pixel peep the full-size images. In our view the K-3 has a slight edge at low ISO, but that could be due to inaccuracies in manual focusing with the Sony. We used contrast detect auto focus (i.e. live view) with the Pentax.

We also found that Pentax K-3 jpg images right out of the camera were perfectly useable, the Sony images, however, needed some tweaking of white balance (which tended to vary with the aperture used) and exposure.

High ISO Tests

We next ran some high-ISO tests indoors in mixed daylight/tungsten light, again with the cameras tripod mounted. To get approximately the same field of view form the two cameras we used an smc Pentax-FA* 28-70mm F2.8 set to 50mm on the Pentax and an smc Pentax-FA 77mm F1.8 Limited on the Sony. This not being a lens test we permitted ourselves to use a zoom on one camera and a prime lens on the other. To get approximately the same depth of field we set the aperture one stop smaller on the Sony.

No post-processing of the RAW images other than lifting exposure about 1/3 stop in ACR for the Sony images.

The scene

 

RAW 100% crops without noise reduction in post processing

Pentax K-3, ISO 6,400

Sony Alpha 7, ISO 6,400

Pentax K-3, ISO 12,800

Sony Alpha 7, ISO 12,800

Pentax K-3, ISO 25,600

Sony A7, ISO 25,600

JPG from camera, 100% crop
Pentax K-3, ISO 6400

Sony Alpha 7, ISO 6400

Pentax K-3, ISO 12,800

Sony A7, ISO 12,800

Pentax K-3, ISO 25,600 Sony A7, ISO 25,600

The images converted from RAW: At ISO 6,400 the Sony has the edge whereas from ISO 12,800 the Pentax pulls ahead of the Sony. This is surprising, unless the K-3 is applying some noise reduction to the RAW image on the sly. The Sony A7 tops out at ISO 25,600, whereas the Pentax K-3 goes up to ISO 51,200.

Regarding JPGs straight out of the camera at high ISO at default settings: The Sony image at ISO 6400 has less noise and more contrast than the Pentax image, the level of detail is about the same.  At ISO 12,800 and 25,600: The Sony shows less noise and less detail and the Pentax has more noise and more detail. The JPG development settings can be tweaked in the camera menus, of course, and different results than the defaults shown here can be obtained.

Interestingly, at ISO 12,800, the JPGs converted from RAW with no noise reduction in post processing are better than the JPGs straight out of the camera with camera-default noise reduction applied.

Low Light Performance

Since the High ISO tests above didn't reveal any benefit of the full frame Sony sensor over the Pentax K-3 APS-C sensor, we gave the Sony another chance and ran a second test, outdoors this time.

The scene (K-3, ISO 6400, JPG)
The scene (A7, ISO 6400, JPG)

The A7's auto white balance is off in low light, we have not corrected that in these images. The A7 also tends to overexpose at night. Since we did  bracket the shots we are able to pick shots that match closely exposure wise (2/3 stop negative EV compensation for the A7) and we fine tuned the exposure in ACR:

RAW, 100% crops, no post processing
Pentax K-3, ISO 6400 RAW, no post processing

Sony A7, ISO 6400, RAW, no post processing

Pentax K-3, ISO 12,800 RAW, no post processing

Sony A7, ISO 12,800, RAW, no post processing

Pentax K-3, ISO 25,600 RAW, no post processing Sony A7, ISO 25,600, RAW, no post processing

At ISO 6400 the A7 shows less noise and the noise present is small grained and uniformly distributed. Nevertheless, the Pentax K-3 image looks better (more contrast, looks sharper).

At ISO 12,800 we have a draw and at ISO 25,600 the A7 pulls slightly ahead.

The 6 RAW files of the above examples can be downloaded here (180 MB).

Conclusion of the Image Comparison

Our conclusion is that there is no reason to go full frame with the Sony Alpha 7 over a Pentax K-3 unless you need the 36MP resolution of the Alpha 7R. At high ISO shooting the results are mixed; in some samples the Sony A7 comes out best, in other samples the Pentax K-3 is best. The differences are small, however, and will vanish with proper post processing.

The A7 may deliver more dynamic range and better colors according to sensor tests such as DxOmark, and this advantage may be discernible in tough lighting- but for everyday shooting, the gap is very small, as you've seen, and none of our tests were able to bring any advantage of the A7 sensor to the surface.


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