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DLP LightCommander lens concern

Hi Tech support,

I would like to use the DLP LightCommander for a projection and capturing application. I am planning on switching the 50mm f1.8 lens that came with the light commander to a Nikon 18-55mm f3.5-5.6 DX(non full frame) and I would like to know some information regarding using this lens on the light commander. 

These are the following concerns:

Since the light commander uses a 1024x768, 0.55inch sensor chip, what is the difference in crop factor when using the 18mm end of the Nikon 18-55mm f3.5-5.6 DX(non full frame) lens comparing to the Nikon 50mm f1.8. And also, what is the crop factor when using the original 50mm f1.8 lens.

On the TI form, http://e2e.ti.com/support/dlp__mems_micro-electro-mechanical_systems/f/387/t/122748.aspx#438481, it was stated that  "You can use any F-mount lens, although we recommend lenses with at least F 2.5 or better."  I've notice that the Nikon 18-55mm f3.5-5.6 DX(non full frame) would have a largest aperture of f3.5; what would be the effect when using the Nikon 18-55mm f3.5-5.6 DX(non full frame), or is this lens even compatible with the DLP light commander.

Regards,
Wayne


 

1 Reply

  • Wayne,

    Welcome to the DLP&MEMS E2E.

    First - the Nikon 18-55mm f3.5-5.6 DX lens will work fine on the LightCommander. My only concern is the relative dimness of the lens when compared to the f/1.8 of the supplied 50mm lens.

    The crop factor really has meaning only in the context of comparing the field of view (FOV) of a lens used on a DX camera to full-frame 35mm camera. The Nikon DX sensors are APS-C (23.6 x 15.8 mm), whereas a "full-frame" sensor is 36 x 24 mm. This means that a "full-frame" sensor has a diagonal about 1.5x larger than a DX sensor. This results in a DX sensor having a field of view about 1/1.5 times that of the "full-frame" sensor for a given lens focal length. This is where the "crop factor" terminology arises.

    If a 50 mm lens is used for a "full frame" sensor and then a DX sensor, the FF sensor will see a wider FOV than the DX, entirely because the DX sensor is smaller. Nothing changes in the image plane for the lens. It is just that the larger sensor will see more of the image plane, and the smaller sensor will see less of the image plane.

    The FOV of any sensor varies with the focal length of the objective lens. For a DX sized sensor, the FOV is somewhat wide for 18 mm, and almost 3X narrower for 55 mm. The same will be true for a FF sensor, but the FF sensor will see wider at both focal lengths. The main difference between FF and DX lenses is how big an area they are designed to cover. A lens designed for FF must cover a circle of at least 44 mm, whereas a lens designed for DX must cover at least a 29 mm circle (the circle is equal to the sensor diagonal).

    Now, with respect to the LightCommander, there is no sensor, but rather a DMD forming an image to be projected. However, the same FOV issues apply - in exactly the same way as with a camera. The LC DMD - DLP5500 has a diagonal of 0.55 inches, or 13.8 mm. This means that the DLP5500 is a even smaller than a DX camera sensor. However, if you look at the placement of the DMD in the LightCommander, you will notice that the DMD is offset somewhat from the optical axis of the lens. This is done so that the LC will project its image above the horizontal plane (desirable when the LC is sitting on a table). However, the size and placement of the DLP5500 is still such that it is adequately covered by a DX lens.

    The projected image will vary in size (throw ratio) as the lens is zoomed from 55 to 18 mm - by a factor of 3. To emphasize something from the above discussion, the image for the 18-55mm lens will be exactly the same size when set to 50 mm as it is when the fixed focal length 50 mm lens is used.

    The only difference which will be apparent between the lenses is that the 50 mm f/1.8 lens will produce a considerably brighter image on the screen than will the 18-55 mm zoom lens - because at 50 mm it will only have an aperture ratio of perhaps f/5 when set wide open. Therefore it is very important to make sure that the aperture ring on the 18-55 mm lens is set for maximum opening, or else the projected image may become darker than desired.

    I hope this discussion helps.

    Best regards,

    Pascal