Recently, I hit this issue where the textures are blurry in the render ONLY when using Mental Ray. This is because MR does some serious sampling to soften the image.
Select the texture node in the hypershade for your material. Under the attribute editor for the shader on the model select "filter type" and set it to "off".
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This will cancel out the extreme sampling that mental ray does by default.
For the over all image sampling you can change the settings in the render settings panel.
Mental Ray>Quality tab>Multi-pixel filtering.
The default setting when you select the "production quality" preset in your render settings is shown in the picture on the right (above). The left picture result is done by turning filtering off for the texture in the atributes, and changing Multi-pixel filtering to Mitchell and the filter size to 4x4 in render settings. (see stills above)
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