| review: Minolta Dimage 7i Digital Camera (appr. € 1000,-) |
There are a lot of great reviews about the Dimage 7i under daylight conditions.
A german language site dedicated to the Dimage 7 is here.
A review in english language can be found here.
This review will focus on the abilities of the Dimage 7i when used under low light conditions such as astronomical objects or night scenes.
7i features for low light imaging
- "bulb" exposures up to 30 seconds
- automatic dark frame subtraction for exposures longer than 4 seconds
- manual focus with magnification close to the final image resolution
- manual control of f-stop
- optional remote control for exposing
- filter thread for coupling the 7i to a telescope
- high quality apochromatic built-in lens
- high zoom range from wide angle to tele
- optional external power supply
Built-in lens and image processing
Some reviews reported problems with the 7i when imaging low contrast scenes.
Of course this is crucial for astronomical objects.
I cannot confirm this flaw.
The following example is a cloudy, hazy sky where not too much detail was visible with the naked eye.
Low Contrast Scene
Other authors reported this behaviour when a high contrast object was surrounded by low contrast features.
Again I cannot confirm this disadvantage.
Low Contrast Scene mixed with high Contrast Object
Moon low, just above Horizon with built-in lens at 200 mm
These pictures are right out of the camera.
No further processing was applied.
The EV setting of the camera was set to -0.3 and the color saturation to +1.
Sharpness control was set to neutral (no additional sharpening).
Power Consumption
Several authors reported enormous power consumption - others did not.
My experience with 2000 mAh batteries and a high quality computer controlled recharger is that the 7i is just fine.
I can use the camera for about 3 hours in typical operation.
The Olympus 2100 UZ and most of the pocket digital cameras I know are worse.
If the camera is constantly exposing in bulb mode for 30 seconds in cold conditions a set of batteries is lasting for about an hour.
In this case - typical for using it with a telescope - the external power supply is highly recommended.
Long exposure noise
The 7i is is very promosing when it comes to long exposures.
For every 30 seconds shot the Dimage 7i is taking a dark frame and is subtracting that from the light frame.
The Orion Nebula was taken with the 7i riding on a telescope to prevent star trailing.
It is the median of 5 frames with 30 seconds of exposure each.
The picture is showing a 500 pixel wide detail from the 2560 x 1920 image at the original resolution.
Orion Nebula with built-in Lens
The whole image is showing the three belt stars of Orion with the Great Nebula at the lower center:
Constellation of Orion - Orion's Belt, 1 single exposure of 30 seconds
I will definetely do long exposures with much more frames at a later time to exploit the 7i's full potential.
Star Trails
With 30 seconds of exposure and the camera fixed to a tripod you will see star trailing with all the lens settings.
If you combine a lot of these pictures the star trails will elongate to the typical arcs caused by the earth's rotation.
I expected the Dimage 7i to produce dotted lines because after every 30 seconds exposure it takes an automatic 30 seconds dark frame.
Here is an example.
Star Trailing around North Pole, 7 exposures of 30 seconds average combined
Increasing Magnification
The built-in lens is a fair tele for everyday use, equivalent to a 200 mm with 35 mm film.
For astronomical objects like nebula, galaxies or planets the tele is not long enough.
An amplfication of 5x to 10x or more to the 200 mm is needed.
This can be achieved by coupling the 7i to a telescope with eyepiece projection.
A homegrown eyepiece adapter for the Dimage 7i
1) Focuser with T-Thread (M42x0.75)
2) Quick Connector for T-Thread, rotatable
3) Eyepiece holder for T-Threads, holding a 25 mm Plossl covered by the extension tube
4) 50 mm plumbers ring for water pipes
5) Extension tube for T-Thread, the length is so that the eyepiece is almost touching the first camera lens
6) Adapter to change T-Thread to the 49 mm filter screw of the camera
7) Plate to give more stability and not to overload the zoom objective of the camera
Unfortunately the first lens of the 7i is that big that most of the eyepieces will introduce serious vignetting.
Here is a test picture with a Sonnar 4/300 telephoto lens in combination with a Pentax SMC XL eyepiece of 14 mm.
To reduce vignetting the 7i was set to 28 mm wide angle which is somehow counter productive, of course.
The vignetting (not visible in that detail) is still severe at the corner of the frames.
The Moon was very low, just above the horizon.
The color rendering of the 7i is very close to the orange look with naked eyes at that time.
Moon low, just above Horizon, with Sonnar 4/300 and eyepiece Pentax SMC XL 14 mm
Conclusion
The Dimage 7i is not only a good daylight camera.
The first tests are motivating to see what else this camera can do.
I am expecting good night shots and even useful deep sky shots and hopefully good shots of lightnings too.
Search more photos taken with the Dimage 7i
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