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Checking Manual and Autofocus of Modified Camera:
Replacement with Baader filter: Since the Baader replacement filter used for the 450D modification is the same thickness as the original Canon IR filter that was removed, the manual and auto focus operation of the modified camera through the viewfinder remains unchanged, but it is good to check anyway.
Replacement with other filter or optical glass: If a different thickness replacement filter or optical glass is used instead to replace the orignial IR filter, calculations would need to be made to correct the focus point to that of the original unmodified camera. I did not measure the thickness of the original Canon 450D IR filter but it is much thinner than the 300D IR filter. This is because of the stacked multi-filter arrangement of the self cleaning filter/imaging chip assembly that can be seen Here. I was told by Alpine Astronomical that they may suply a clear glass repacement in the future if there is enough interest. The calculations for determining the amount of shimming that would be required if a replacement filter or optical glass of a different thickness from the original IR filter are discussed Here for the Canon 300D. The thickness of the original IR filter of the 450D would need to be measured for these calculations. The location of the shims needed to be added or removed for the 450D are shown in Step 35 Here.
Live View Focusing:
The Canon 450D has a "live view" feature that uses the image recorded directly by the camera's CMOS imaging chip. Using "live view" focusing acheives the best focus if it can be done accurately. The "live view" image can be displayed for focusing on the camera's display or a computer display using a USB connection. "Live view" focusing works well for astronomy since the object being imaged is stationary and can be magnified (zoomed in) to check for precise focus . There are two "live view" focusing modes on the 450D, "live view manual focus" and "live view auto focus". For true "live view auto focus" operation, the camera's custom function setting Fn-8 (AF during Live View shooting) should be set to setting "2. Live mode". The Canon owner's manual states Live View Auto Focus can fail to achieve focus for the following: "subjects in low light", "night scenes or points of light", "extremely small subjects", or "subjects strongly reflecting light". That does not sound promising for use of Live View Auto Focus for astronomical subjects but testing will tell.
Traditional Focusing:
Traditionally, focusing of SLR and DSLR cameras has been through a viewfinder by eye. A diagram of a typical DSLR camera can be seen on Leon Goodman's site Here. The light path from the camera lens to the viewfinder's focusing screen needs to be the same as the light path to the imaging chip in order for manual or autofocus to be accurate. There is the rub....if the light paths differ, focusing will be innacurate. Early versions of the Canon Rebel such as the 300D have an Autofus adjustment screw for correcting any discrepancy in the focusing light paths. It appears that the Canon 450D and the 40D do not have such AF adjustment screws. The only way I see at this time to adjust for proper focusing when not using the live view mode would be to disassemble the camera and re-shim the CMOS imaging chip. The location of the shims are in Step 35 Here.
Focus Check: To check if the modified camera is achieving perfect focus, the camera's focus can be easily tested. For testing my camera after the modification, I used the procedure by Bob Atkins on photo.net. Mr. Atkins also provides a focus test chart Here if you wish to use it. Instead of using the chart, I used the markings of a level as shown below:
You may want to check the focus of your camera before doing the modification and if focus is out of the Depth of Field, return it for repair.
For the test setup I used:
For the two images below, manual focusing and autofocusing through the viewfinder was used to focus on the line corresponding to the number "7" on the level:
For the two images below, manual focusing and autofocusing in "live view" mode was used to focus on the line corresponding to the number "7" on the level:
The test images above show that good focus has been maintained in the modified camera.
The Live View Manual Focusing mode in remote shooting from a computer has been working well for achieving good focus.
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