Field Report DA 20-40 Ltd, f2.8/f4

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Richard B.
Richard B. Veteran Member • Posts: 5,802
Field Report DA 20-40 Ltd, f2.8/f4
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I have been intrigued by this lens since it came out. I was initially disappointed by the PF review that criticized poor edge performance at the 40mm end of the zoom range. It seemed a puzzle to me that Pentax would want to sell such a lens. Then I started to see some nice sharp looking looking samples posted online by other photographers that appeared to contradict the PF assessment. I already owned the 40mm and 21 mm Ltd primes, so I really didn’t have much incentive to pursue the matter any further. However, the recent black Friday sales and B&H’s offer of a $300 discount, proved too much and I bought one.

I took a few initial shots to compare the zoom against my existing DA primes. My initial reaction was that the 20-40 zoom matched the primes for contrast and tonality and was capable of capturing the nuances of light that you would expect in a Ltd lens. Also, great control of flare and very little purple fringing, as expected. I had heard people comment about field curvature at the 20mm end. While there is some mild barrel distortion at 20mm, I saw very little evidence of field curvature in day-to-day shooting at that focal length. I would say the performance at 20mm is very close to the 21mm DA Ltd. At the long end though, there did appear to be some softness at the edges and corner, similar to the PF review. The example below compares the DA 40mm Ltd at f5.6 to the 20-40mm zoom at f11. The prime is clearly sharper in these edge crops.

I also checked the corners on the 20-40mm, as you would expect these to be even worse on a lens with poor edge performance. However, they appeared a good deal sharper than the middle of the edge (crop below). The foreground corners were also much closer to the lens and in looking through other photos I had taken, I could see a similar pattern at 40mm, with sharp foreground edges, but softer background on the edges. I tried a depth of field test. I oriented the shot of the building below so that the building edge receded further into the distance across the frame. See full frame of shot below.

Full shot (using right focus point)

Using center point focus, the foreground wall on the extreme left is sharp, but the distant section of the wall is quite soft. I had deliberately used f5.6 to reduce DOF and emphasize this affect.

100% crops using center focus point

I then took the same shot but chose the rightmost focus point in the VF, rather than the center focus point. This had the affect of moving the focus point much deeper into the shot. There was quite difference. The distant part of the wall was now acceptably sharp, with the foreground remaining sharp. I also tried the same shot with the micro-focus set to focus as deep as it could go and obtained a similar result. I would conclude from this that the DA 20-40mm Ltd exhibits some field curvature at 40mm.

100% crops using right focus point

I subsequently took another set of landscape shots at relatively wide apertures (f4 and f5.6) but pushed the focus point deeper into the shot. I was very happy with the results. The DA 20-40 Ltd is actually pretty good at f4, including edges and corners, and excellent at f5.6, as long as you pay attention to where you set the focus point. I would speculate that with the resolution testing PF did on the DA 20-40 Ltd, the corners on their test shots were simply outside the DOF.

A recent favorite below, 40mm @ f5.6.

If anyone is interested there are full size samples in my DA 20-40 Ltd Samples gallery .

The gallery is arranged newest to oldest. I have left some of the older shots in there to show the potential impact of the field curvature using just center point AF. For the more recent shots I focused deeper into the picture and reframed for the final shot. The landscapes are all tripod mounted with a 2s delay shutter delay, ISO 80. The travel shots from around the UK were all handheld at ISO 400. The indoor museum shots were ISO 1600. All the shots have the EXIF intact. You are welcome to download the full size images, but please no further distribution.

Final thoughts. The 20-40 is a very nice lens that lives up to its Ltd designation. It will remain on my K-5iis as my main walk-around and travel lens. However, since the DA 40 Ltd prime has a wider shooting envelope at that focal length, and I already own both the 21mm and 40mm, I will likely stay with that pairing as part of my main landscape photography kit. However, if I had purchased the DA 20-40mm first, I could see it would be difficult justify buying those two primes later at a later date.

To steal some text from another recent thread in the forum…. Oops Pentax have done it again, another great lens!

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 Richard B.'s gear list:Richard B.'s gear list
Pentax K-5 IIs Pentax smc DA 15mm F4 ED AL Limited Pentax smc DA 17-70mm F4.0 AL (IF) SDM Pentax smc DA 55-300mm F4.0-5.8 ED Pentax smc DA* 60-250mm F4.0 ED (IF) SDM +7 more
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