The same stripe with contrast strongly enhanced:
The above serie of exposures was taken with my Nikon D70s with Nikon AF-S DX 55-200mm 1:4-5,6 G ED, @ 200mm, F=32.
When I shot one of my first mosaics with my Nikon D70s and the 55-200mm DX lens (@200m), I was very disappointed with the result. The brightness of the single images was not constant. Of course I had fixed time, aperture (position 'M') and white balance. Here is the result, stitched with PTGui 5.6, blended with PTGui blender:
First I thought the reason is the JPEG-conversion that the camera does. So I shot the next mosaic in RAW and converted the images all with the same settings. But the result was the same: The brightness changed from image to image.
What is the reason ? To find out this, I did the following test: fixed the camera on the tripod and shot a serie of images from the blue sky, all in the same direction. Then I cropped an area of 200 x 850pixel from the center of each image, mirrored it horizontally to get a stripe of 400 x 850 pixel. Then I assembled one stripe beneath the next. This now looks like this:
The same stripe with contrast strongly enhanced:
The above serie of exposures was taken with my Nikon D70s with Nikon AF-S DX 55-200mm 1:4-5,6 G ED, @ 200mm, F=32.
The serie below was taken with the same camera and lens, but with aperture fully open (F=5,6):
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-contrast.jpg)
All images have the same brightness.
My conclusion now was that the changing brightness is caused by the aperture. The aperture does not close precisely from shot to shot.
I repeated the test with other lenses on my D70s:
Nikon AF-S DX 18-70mm 1:3,5-4,5 G ED
Nikon AF Fisheye 10,5mm 1:2,8 G ED
Nikon AF 85mm 1:1,8
All these lenses gave similar changes in brightness from shot to shot. Some lens more, some less.
And what I found out too: the effect depends on the temperature of the camera+lens. The colder, the bigger the differences. At room temperature (+18°C) all images had the same brightness with the 55-200mm lens.
Is it only my camera that is defective?
I did the test with my Nikon Coolpix 5000 (a point-and-shot camera). All images had exactly the same brightness!
Then I asked Bernhard Vogl to do the same test with his D70.
He shot a serie of images with the Nikon AF-S DX 18-70mm 1:3,5-4,5 G ED and another serie with the Nikon AF 17-35mm.
With the AF-S DX 18-70mm there were no changes in brightness, but with the Nikon AF 17-35mm @ 35mm, F=8 the serie shows like this:
The effect is less than with my D70s, but it is still present.
Next I want to borrow a Canon 300D from another friend to do the same test.