| review: Takahashi FS-60C / Pentax SMC XL Projector for Minolta Dimage 7i Digital Camera (appr. € 30,-) |
The Dimage's built in zoom lens has a maximum focal length of 200 mm (aquivalent to 35 mm film).
This is not enough for high resolution shots of the moon and other objects.
The Dimage's lens cannot be unscrewed so the only chance to increase magnification is eyepiece projection.
Eyepiece projector based on 10 mm aluminium plate
Projector's Maginfication
I used my Takahashi FS-60C together with a Pentax eyepiece of 14 mm focal length.
This combination is giving a magnification of
M proj = 355 / 14 = 25.3 x
Total Maginfication
The Dimage's front lens is quite large.
This is introducing vignetting to the image.
To keep vignetting as low as possible the built-in zoom lens must be set to minimum focal length.
This setting is 28 mm and can be estimated to give a magnification of
M dimage = 28 mm / 50 mm = 0.56 x
These estimations are based on the fact that a 50 mm is giving a 1x magnification.
The overall magnification and final effective focal length can hence be estimated to
M eff = 0.56x * 25.3x = 14
FL eff = 14 x 50 mm = 700 mm
A eff = 700 mm / 200 mm = 3.5
This is giving an amplification factor 3.5 in respect to the built-in tele of 200 mm.
The whole system is built to be able to focus with an additional Barlow lens as well and hence the overall magnification can be increased by factor 2 or more if needed.
Camera distance can be adjusted for focusing the refractor or even an added Barlow lens
The Dimage's lens cap is nicely covering the rubber cap of the Pentax eyepiece
Another way to calculate the amplification factor is counting the pixels of an imaged full moon with and without the projector.
A eff = 526 / 140 = 3.7
The estimation with the 50 mm lens giving 1x seems to work fine.
Vignetting
Vignetting is a big problem with all DigiCams having a large diamter front lens like the Dimage 7i.
The bigger the field lens of the used eyepiece the lower the vignetting should be.
The Pentax field lenses are among the largest I know about.
But still there is vignetting.
The rectangular image of the Dimage is cropped by that to fact to a diameter slightly larger than the Dimage's image height.
Serious vignetting. The Dimage was not properly centered yet to the optical axis of the projector
If you need a rectangular, unvignetted picture then you will end up with 1600 x 1200 pixels when using Dimage's highest resolution of 2560 x 1920 as an input image.
That might not be enough for professionel reprints but it is still very nice for my purposes.
After optimizing the lens to lens distance and the centering of the optical axises the best result achieved looked like that:
Optimized alignment and lens-to-lens distance. The shadow on the right is not vignetting. The image was taken through a window with a black net against insects behind it.
Conclusion
The Dimage 7i has a fine built-in APO zoom lens.
The Tak's optics as well as the one of the Pentax eyepieces' is reference quality.
So it is not very surprising that the resulting images are very sharp and with low abberation.
The major goal with this setup will be various moon rises with common terrestrial objects in front of the huge moon's disk.
Search photos taken with the Dimage 7i
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