Hotspot problem - bad gg ? at

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Old June 23rd, 2006, 05:36 PM   #1
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Hotspot problem - bad gg ?


Im trying to make "working" adapter, but there are problems with hotspot when I have f/2 and higher. How can I find what is wrong, bad gg or bad macro.

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Ivan Brasen is offline   Reply With Quote
Old June 23rd, 2006, 06:41 PM   #2
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do you have a condenser?
video : xl2 / letus35xl / bogen 503
photo- canon 1dmkII - bronica etrsi
Andrew Todd is offline   Reply With Quote
Old June 23rd, 2006, 10:12 PM   #3
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With the 200mm f4.5, what you have is about as good as it may get.

All images suggest to me the centre axis of the SLR lens is about 1.5mm off-centre to right relative to the camcorder lens centre axis. This could be more or less depending on how large the frame you are using on the groundglass.

I am only guessing the error based on my use of a 24mm wide frame off the groundglass.

I would not concern myself about trying to adjust offset centres because camcorders themselves are not perfect for keeping to centres and the centres of your adaptor may be already spot on.

The same effect can happen if the SLR lens axis and camcorder lens axis are not parallel.

You would detect this by zooming back from the groundglass and looking at the vignetted image. If it is slightly oval shape, not circular shape, then you need to look at correcting to true parallel or making sure the centre axis is true for both SLR and camcorder.

You may also see soft focus on one edge. The groundglass will also do this if it is not square-on to centre axis and is the more likely cause of a soft edge.

Except for the hotspot you mention, I find nothing otherwise displeasing about your images.

My personal preference would be to look at making your image frame off the groundglass smaller to get inside the hotspot and not try to fix something that really needs little fixing. If you have some camcorder zoom-in range to go closer, this would be my choice.

If you have no more zoom-in to go, then you might need to look at a stronger close-up lens (macro).

If it is good enough for P+S Technik to use a smaller image of 21mm, then I am happy to go that way.

Alternatively, you could use condenser lenses as suggested and stay with the size of image you are now using. There are enough success stories about condensers posted here to convince me it can work. The Movietube is built around a condenser pair.

From the look of your 50mm f1.4 images, I am guessing your frame on the groundglass is in the ballpark of 32mm wide. (If you are using this size image or larger, then you should look at making the centres correct not ignoring it as I suggest.).

If a groundglass is too transparent, the hotspot problem is aggravated. Sharpness is better. Brightness and contrast are better. Bokeh can be worse depending on the lens. The image can look more like video than film.

If you go in closer to a smaller frame to avoid the hotspot, you lose some sharpness. If you make the groundglass more opaque with a coarser finish, then there is a lesser sharpness, more light lost, less contrast, better film look and you may not need to go closer to avoid the hotspot.

So much of this is about personal preference. I choose to go with a slightly more transparent groundglass because I want to be able to intercut with direct-to-camera images. So I use the smaller area off the groundglass.

Don't take too much notice of my comments as I am not an optical engineer.
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