Lens review

Sony DT 35 mm f/1.8 SAM

3 June 2011
Arkadiusz Olech

4. Image resolution

When it comes to the testing equipment designed for smaller frames the Sony company doesn’t spoil us. We own a Sony A100 but we decided that camera is too old for testing contemporary optics. We don’t even mean resolution here because, after all, a CCD 10 Mpix class detector, very good at low ISO settings, managed to differentiate between good and bad lenses without any problem. We think rather about the autofocus, which, in the case of A100, is very weak, especially if you use contemporary standards of assessment.

To sum up there is no body with an APS-C/DX sensor on the current offer of the Sony company which would be able to fulfill our requirements concerning a good testing camera. Models with an immobile mirror, like the A55, show significant differences in the MTF50 function values between the vertical and horizontal component and it makes it very difficult to present credible result and assess astigmatism in a good way. Some A3xx and A5xx models don’t show that difference but they don’t feature a fully-fledged mirror lock-up function for a change which is indispensable when it comes to the tests of optics with longer focal lengths. They don’t have a very useful function of autofocus microregulation either.

As we didn’t want to put off the next tests we decided to turn a blind eye a bit. The Sony A580 became our choice. A very well-made CMOS sensor with the resolution of 16 million of pixels is its unquestionable advantage. What’s more important to us it gives more or less the same MTFs like a similar detector used in the Pentax K-5 and the Nikon D7000; it also has the same results as the Canon 50D and the Olympus E-3, used in our tests as well. Because of that results we obtained on all of aforementioned bodies allow us to compare the lenses directly with each other. It is a huge advantage – it made us turn a blind eye to the lack of a fully-fledged initial mirror lifting in the A580. Fortunately this model allows to lift the mirror for 2 seconds. That time is long enough to prevent lowering the resolution results in a significant way of lenses with focal lengths on the level of 50 mm. Certainly it wouldn’t be appropriate for tests of optic devices with focal lengths from 100 mm upwards and such tests won’t be conducted on it. The Sony’s DT 35 mm f/1.8 SAM focal length is so short, though, that there should be no problems with a test on this body

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At the beginning it is worth mentioning that in the case of tests conducted on APS-C/DX sensors with 15-16 Mpix the decency level is situated near 35 lpmm and the best fixed-focal lengths can reach results on the level of 52-53 lpmm. Let’s check how the DT 1.8/35 fares in comparison.

Sony DT 35 mm f/1.8 SAM - Image resolution

In the centre of the frame the lens doesn’t give us any reason to complain. Quite the opposite in fact – we can only praise it. First of all, the maximum relative aperture is fully useful as we get there values as high as 43 lpmm – it provides good image quality. On stopping down the situation gets only better – its maximum, on a splendid level of a tad above 52 lpmm the lens reaches by f/4.0 and f/5.6.

The edge of the frame is a bit worse. The f/1.8 and f/2.0 apertures don’t guarantee getting pictures of good quality. Only on stopping down to f/2.8 or a bit higher allows us to go near to the decency level.

The results the Sony 1.8/35 showed in the frame centre and on the edge are very similar to those of the Nikkor 1.8/35. In both cases we deal with optics with similar properties – definitely praiseworthy, taking into account the moderate price.

Sony DT 35 mm f/1.8 SAM - Image resolution