HD Pentax DA 20-40mm F2.8-4 Limited WR on Sony A7R

HD Pentax DA 20-40mm F2.8-4 Limited WR on Sony A7R

Does the price of this fine crafted Pentax beauty match the performance?

The lens is light weight and compact, not much different sized than an 18-55mm kit lens. The metal knurled rings for focus and zoom are very nice feeling. The marked distance scale is a nice touch since few lenses still use markings.

The hood is small and a might wonder if it serves any practical purpose since it basically is a lens cap holder. Some of the worst flare causing light is that of a directly overheat street lamp or son that shines across the front element, this often leads to streaks across the frame. This hood is effective against preventing that sort of flare. Most users will probably leave it equipped the entire time with any filter underneath.

The lens is light at only 283 g, and has 9 elements in 8 groups, with 1 extra low dispersion element. It has a good minimum focus distance of 0.2 m with a max magnification of 0.2x. It uses 9 rounded blades for a circular aperture at large openings (1 stop down). The 55mm filter threads are standard on many f/4 lenses.

No longer does Pentax using the term SMC for its multi-layer coating. They have since switched to marketing term HD for their multi-coating. It also uses Pentax’s DC focus motor. This helps keep size down, and should help keep reliability high.

This lens is pricey for a 2x standard zoom, does it live up to the high standards? (A7 used in crop mode for most shots, but do have it tested full frame and 25 mm.)

SAM_2273 SAM_2274
SAM_2269 SAM_2271
20 mm 40 mm
SAM_2270 SAM_2272
30 mm 40 mm (no hood)

Optical Performance

20 mm

SMCP2040f284Ltd_20mm_A7r_files001
SMCP2040f284Ltd_20mm_A7r_files002
SMCP2040f284Ltd_20mm_A7r_files003
MTF20 Chart maxes at vertical resolution of the sensor which is 3200 LW/PH for A7R in crop mode. Anything above that is meaningless. The MTF50 chart is set at the value where the best lenses perform in the center, or about 2500 LW/PH on the A7R.
Note: 4k video is 2160 LW/PH
Note: HD video is 1080 LW/PH

30 mm

SMCP2040f284Ltd_30mm_A7r_files001
SMCP2040f284Ltd_30mm_A7r_files002
SMCP2040f284Ltd_30mm_A7r_files003
MTF20 Chart maxes at vertical resolution of the sensor which is 3200 LW/PH for A7R in crop mode. Anything above that is meaningless. The MTF50 chart is set at the value where the best lenses perform in the center, or about 2500 LW/PH on the A7R.
Note: 4k video is 2160 LW/PH
Note: HD video is 1080 LW/PH

40 mm

SMCP2040f284Ltd_40mm_A7r_files001
SMCP2040f284Ltd_40mm_A7r_files002
SMCP2040f284Ltd_40mm_A7r_files003
MTF20 Chart maxes at vertical resolution of the sensor which is 3200 LW/PH for A7R in crop mode. Anything above that is meaningless. The MTF50 chart is set at the value where the best lenses perform in the center, or about 2500 LW/PH on the A7R.
Note: 4k video is 2160 LW/PH
Note: HD video is 1080 LW/PH

25 mm Full Frame

SMCP2040f284Ltd_ff25mm_A7r_files001
SMCP2040f284Ltd_ff25mm_A7r_files002
SMCP2040f284Ltd_ff25mm_A7r_files003
MTF20 Chart maxes at vertical resolution of the sensor which is 4912 LW/PH for A7R. Anything above that is meaningless. The MTF50 chart is set at the value where the best lenses perform in the center, or about 3850 LW/PH on the A7R.
Note: 4k video is 2160 LW/PH
Note: HD video is 1080 LW/PH

Comments on the Results

For APS-C context in crop mode the lens is very good or excellent at all settings. The weakest is probably 20mm f/2.8, but surprisingly this lens holds its quality even at the telephoto end of the range. Corners are good at all settings as well.

Distortion is fairly noticeable barrel of about 2.5% at 20mm fading to fairly minor barrel distortion of about 0.5 % at 40mm. No pincushion here.

Lateral CA is fairly strong at 20mm and fades towards 40mm.

Flare resistance: Superb, a strong point of this lens. My DA 21mm f/3.2 would get green orbs in the frame if the sun was in the frame, I can’t hardly see any flare with the lens in the frame or just out of the frame.

Field curvature: Minor, there is some near the extreme edges, but typical focus center should be fine.

Focus performance: Manual only on my A7R, sorry can’t test this. Adapted to Sony I found it a little annoying to use, I kept grabbing the wrong thing to try to zoom or focus. This is aggravated by the adapter which adds an aperture dial, so there are too many things to try to grab and turn. I kept trying to grab the mounting ring to adjust zoom or aperture. On a Pentax camera I don’t think this would be as much of an issue, although the zoom ring or focus ring can be a bit hard to find in a hurry.

Measured Focal length at 20 mm and 1:53 is about 21 mm.
Measured Focal length at 40 mm and 1:53 is about 41 mm.

For full frame, it has full coverage at 24 or 25mm. Notice that the corners aren’t great, but it is usable, especially if you do a slight crop. This will net you a slightly wider field of view, about 16 mm on APS-C.

Pros and Cons

Effective Resolution 20 mm

HD Video = 2 MP, 4k Video = 8 MP

Aperture Weighted Center Partway Corner
f/2.8
11.1 MP
12.5 MP
9.3 MP
10.7 MP
f/5.6
14.4 MP
16+ MP
13 MP
12.4 MP

Effective Resolution 30 mm

Aperture Weighted Center Partway Corner
f/3.2
13.1 MP
15.8 MP
10.6 MP
10.3 MP
f/8
14.3 MP
15.9 MP
13.3 MP
11.1 MP

Effective Resolution 40 mm

Aperture Weighted Center Partway Corner
f/4
13.1 MP
14.7 MP
12 MP
10.1 MP
f/8
13.7 MP
15.3 MP
12.7 MP
10 MP

Effective Resolution 25 mm 24x36

Aperture Weighted Center Partway Corner
f/4
20.5 MP
30.3 MP
13.6 MP
2.5 MP
f/8
24.5 MP
32.3 MP
20.4 MP
5.3 MP

Note the different scaling on the full frame chart, 13.6 MP is bad for a 36 MP full frame, but excellent for a 16 MP APS-C. It is relative.

Bottom Line

The lens should be excellent on a K-5IIs camera, which should closely match the A7R in crop mode. Neither uses an AA filter and both are about the same resolution. Based on what I see here, the 24 MP K-3 might strain the resolution more, especially at 20mm f/2.8 or 4.

On the K-5IIs at least, I see this as a solid replacement for the 21mm, 35mm, and 40mm limited lenses. I don’t think you will give up anything in image quality nor flare performance. There is always the size trade off of course, so depends on what you are going for, but this would be an awesome backpacking lens.

Is the lens a tad expensive? Sure, if you look at it as only a 2x standard zoom of modest aperture. You are paying upwards of a grand (US) for a high quality kit lens that is hardly larger aperture than the 18-55mm kit lens, which is f/4.5 at 40mm. From the perspective if one was going to buy the 21, 35, and 40mm limited lenses: it is a bargain.

Bottom line is it will perform at least as well, if not better than most of your old 20, 24, 28, and 35mm f/2.8 primes. Does that make it expensive? I will leave that up to you to decide.

Buy on Amazon: HD PENTAX-DA20-40mm F2.8-4ED Limited DC WR Black

Buy on BH Photo: Pentax HD Pentax DA 20-40mm f/2.8-4 ED Limited DC WR Lens (Black)

Gallery

Default light room settings from RC 5.3.

Gallery:
Full size samples are available here: http://viking79.zenfolio.com/p981043569

ff25_f28
Above: 25mm wide open on full frame, uncropped.
ff25_p2
Above: 25mm stepped down on full frame, uncropped

crop20mm_f28
Above: 20mm APS-C Crop f/2.8

crop20mm_p1
Above: 20mm APS-C Crop stepped down.

crop40mm_f40
Above: 40mm APS-C Crop f/4

40mm APS-C Crop stepped down shot is bad, the A7R likes to introduce shutter blur making this type of shot extremely difficult without forcing shutter speed slow and tripod or using faster than 1/250th.

The gallery images are © 2014 Eric Tastad, and may only be used for personal evaluation of the lenses. Click the play button and click “visit gallery” in the upper right to go to Zenfolio, to download full size images click on an image, hover over an image, and hover over menu, download, and click all originals.

2 Comments

  1. wpu911 says:

    how do you adjust aperture?

  2. admin says:

    With the adapter (has ring and a pin moves to open/close). I figured out approx. Settings using my test chart and shutter speed vs a known lens.