I did a quick test today with the Sony Nex-5n, LA-EA2 lens adapter, and the Sony DT 35mm f/1.8 Lens. I then compared the Sigma 30mm f/2.8 EX DN lens and the Sony 50mm f/1.8 OSS lens to see how each of these lenses performed in comparison. I was in pretty decent light, so the camera had a relatively easy time.
In theory the Phase detection capable LA-EA2 lens adapter should make the focusing faster as the contrast detection auto focus is slower by design. In addition to that the focus motor of the particular lens is ultimately going to determine the max focus speed as the AF motor can only go so fast. I wish I had a SSM lens so I could show you how fast that is in comparison to these lenses. It’s at least twice as fast going from memory.
Just so you know, I shot wide open (f/1.8 and f/2.8) with all these lenses to make the focusing as hard as possible as it relates to depth of field. Note, when using the LA-EA2 lens adapter the Aperture is automatically stopped down to f/3.5 in video mode. So for the Sony DT 35mm f/1.8 lens, even though it was set to f/1.8 the camera automatically switched it to f/3.5 when I started recording video. Why exactly Sony does this is still unknown, but I was told it’s so beginners wouldn’t blow out the scene do to the 1/4000sec shutter speed limitation in combination with fast apertures. Personally, I think they did this in order to save the camcorder market and professional grade equipment. Who knows really?
The narrower the depth of field, the harder it is for the lens to focus. This makes sense as the “high contrast” area the camera is looking for is only visible for a fraction of a second with a super narrow depth of field. In other words the camera will tend to search front to back for something with high contrast. If it doesn’t find anything, it will just continue to hunt. You may have run into this issue in low light shooting wide open ;) Best bet is to switch to manual focus when this happens. When the depth of field is large, most of the scene will be in focus, so finding a high contrast area is much easier for the camera and the auto focus performance is usually much better.
Nex-5n AF Testing Video
I was in pretty decent light, so the camera had a relatively easy time. I’m going to do a low light low contrast test tomorrow!!
As you can see from the video all three lenses performed pretty good in my opinion with the A-Mount DT 35mm f/1.8 and LA-EA2 lens adapter combo performing the best. The focus transitions were smooth and the af locked on as soon as sharpness was achieved.
The Sigmas 30mm f/2.8 EX DN lens always locked on, but hunted a little bit back and forth just before it locked focus. Almost like it was making sure it locked on or something. I actually did tried this lens twice in the video to make sure it did that hunting dance both times, and it did.
The Sony f/1.8 OSS Lens performed better than the Sigma and almost as good as the LA-EA2 combo!! It transitioned a bit different though from one focal point to another. Super smooth transitions which this lens is know for, and it makes for a better viewing experience as it’s almost an ease in effect.
The Optical Steady Shot of the 50mm lens stands out to me as the most in the video however. I was hand holding with all the lens combinations and you can clearly see how shaky the camera is. Once I put the E-Mount 50mm f/1.8 OSS lens on, it smooths right out on the video. Check it out again if you didn’t notice, as it’s really drastic how well the OSS works.
One other odd thing I noticed was both the Sony lenses focused automatically as I moved the camera from spot to spot, but the Sigma 30mm lens did not. I had to actually press the shutter re-lease to get the Sigma to re-auto focus. I’m not sure why this happened though, as I just tried it again and it worked just like the Sony lenses. Perhaps some kind of user error, but I don’t think so. A SAL (SonyAlphaLab) reader (Eli), actually reported an issue with the sigma lenses failing to focus all together and having to re-boot the camera to fix it. Perhaps it was something similar to that issue, but I’m honestly not sure. I can’t seem re-produce it.
In any event, I hope you all got something out of this video auto focus test, and please let me know what you think. I can do more tests for you if you want, just let me know and please be specific I’ll have some time this evening to try a few things if anybody wants.
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