A Simple Equivalent Focal Length Test
On the Converter Lenses Overview page, we suggested: if the on-camera lens and a converter lens both can yield the same focal length, use the on-camera lens instead of a converter for better image quality. This suggestion is based on a simple fact that any additional glass put on top of the on-camera lens has the potential to reduce the optical quality of the system. For convenience, many of us prefer not to remove a converter lens even though we know the on-camera lens can do a better job. So, let us see the difference with a simple example.
The following image was shot at the 420mm focal length, F5.6, 1/4 sec and +2/3 stop exposure compensation. Moreover, SHARPNESS and SATURATION are set to LOW. This flat newspaper subject is chosen because it is tough to many lens and can easily expose lens problems. The comparison will focus on the center portion and the upper-left portion as marked. Then, a Olympus TCON-14B is mounted on camera at the same location, and the camera lens is zoomed back to about 7X in order to capture essentially the same area.
The following shows the center portion. It is clear that the on-camera lens delivered a sharper image and the definition of each letter is very clear. The contrast of the on-camera lens result is also higher.
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| On Camera Lens at 420mm | Olympus TCON-14B at 7X |
The following shows the upper-left portion. The image by the on-camera lens is not as sharp as its center portion; but, it is still shaper with higher contrast than the one using the TCON-14B converter.
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| On Camera Lens at 420mm | Olympus TCON-14B at 7X |
So, our conclusion, given this test, is that if the camera lens can do the job, don't use a converter. Some may claim that the difference is usually not noticeable. This is true to some degree. Shooting flat subject with fine texture is very tough to many lenses, and, hence, you may consider this as nearly the worst case. In daily shooting, your scene may not have very fine details and, as a result, you won't see much difference. However, if you are aiming for maximum image quality, remove the converter if the camera lens can do the job.