The Lens Turbo is a very interesting little item to review with regards to image quality, as it’s very hard to pin down exactly. Does it do what it promises? Yes and no. There are both very positive things about the image quality when using this converter, and some big negatives as well. I’m going to review this a little differently than normal because of the nature of how the Lens Turbo behaves.
Image Quality Positives:
- Images with slower aperture lenses, or fast aperture lenses when stopped down are quite sharp across the frame, save for the extreme corners. I had no issues with the MD 135mm f/2.8 when wide open, or the MC Rokkor 50mm f/1.4 when stopped down a fair bit. Images were sharp and contrasty and looked great.
- Bokeh appears to be predominantly unaffected by the Lens Turbo, retaining the same qualities of the lens attached.
- Image center is sharp even at very wide apertures (f/1.0 when used with the MC 50mm f/1.4), and it’s acceptably sharp across most of the frame at further focusing distances.
- You truly can get that full-frame look with fast lenses.
Image Quality Negatives:
- When used with fast-aperture lenses at closer focus distances, the image sharpness falls off dramatically away from the center, such that even the mid-frame area is notably softer than with the bare lens and a standard adapter. This is especially true when focusing on flat field subjects due to the point below.
- Field Curvature appears to be exaggerated. If you focus using the edge of the frame for an isolated subject at the edge of the frame, sharpness isn’t quite as bad on fast lenses (though still not nearly as good as with the bare lens). However, the added field curvature means that if you focus using the center of the frame and recompose, or shoot a flat field subject that fills the frame, the edges become frankly terrible.
- The Lens Turbo introduces a fair bit of barrel distortion.
- Chromatic aberration is exacerbated at the edges of the frame.
- Can introduce some odd flare artifacts in certain circumstances.
- Some added spherical aberration is present at wide apertures.
So, ultimately, where do we stand with this thing? Well, first off, as expected, there is some optical compromise when you are going for the $130 knockoff rather than the original, very well designed Metabones SpeedBooster.
However, for the price, despite degradation of resolution with faster lenses outside of the image center, I think the Lens Turbo is a useful little device as long as you don’t set your expectations too high. While images with fast lenses aren’t particularly sharp, they are, for the most part, quite usable and adequate for smaller print sizes and general shooting.
If you planned to shoot with the Lens Turbo for a large portion of your work or for professional work, I’d recommend spending the extra money on a SpeedBooster (provided they make it for the mount you’d like to convert), but for occasional use when you want that full-frame depth of field, it’s useful. I don’t often need the depth of field provided by ultra-fast lenses on full frame, as the depth of field from faster lenses on APS-C and Micro 4/3 provides plenty of isolation while keeping the subject sharp. However, for those times I want that super shallow look (especially in the normal focal range), I can grab this and get a decent shot.
Ultimately, the Lens Turbo provides usable images with that full-frame look at a relatively low price. It’s not a great buy, and it’s not a perfect set of optics, but it’s not a bad buy either. The current $130 price range on eBay is right about where I’d put the worth on the Lens Turbo – if it were more expensive, the cons to image quality would be much harder to ignore.
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