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How do I correctly adjust the aperture and focus on my camera with the help of Digiflow?

When I first set up my camera I had a lot of trouble getting the focus right - I'd set the aperture first and then focus the camera. Occasionally though I'll not get it quite right - the camera would not quite be in focus or the image would either saturate or be too dark. I'd like to know how to properly focus the camera and make sure that my image will not saturate while still using the full range of the camera.

I have a camera that is already connected to the computer and I can view the image live though DigiFlow (a windows program used for image analysis by fluid dynamics experimentalists). The camera I am using senses light intensity of each pixel (but not colour) and I think is a standard type used in labs (it is not an SLR or similar, though I think this might work fine for other cameras too, so long as you were able to connect them to the computer). It has manual focus, zoom and aperture.

[Affiliation: My supervisor wrote the program, I am not affiliated in the sense that I will not have any positive repercussion to me putting up questions and answers on SO, I just know the person who made it. I can ask him any questions I like but other people can't - I wanted to record when I found out how to do something and try and build some kind of online help community, firstly among my own lab and also for people who use the program in other labs.]

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Are you affiliated with DigiFlow in any way? –  ChrisF Jun 8 '12 at 11:15
    
My supervisor wrote the program, I am not affiliated in the sense that I will not have any positive repercussion to me putting up questions and answers on SO, I just know the person who made it. I can ask him any questions I like but other people can't - I wanted to record when I found out how to do something and try and build some kind of online help community for people who use the program in other labs. –  tannoreth Jun 8 '12 at 12:02
    
Can you please expand your question a bit? Adding details such as camera type, the interface between camera & the computer, the OS goes a long way in making a question make accessible. Thanks –  Sathya Jun 8 '12 at 12:04
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@tannoreth here's the problem I'm seeing, you need to be more clear in your statement of the problem and give a few "things I've tried" in order for this to work. I think what you are trying to do is great, the execution just needs a bit of tweaking. Take a look at some other good questions around here and try to frame your question in a similar manner. Then tailor your answer to the specific problem you describe, rather than just a general "do xyz" –  wax eagle Jun 8 '12 at 12:28
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So the important thing is to focus the camera first with a very wide aperture, then set the aperture second. By doing this you reduce the depth of field so when you adjust the focus of the camera you can get it very finely tuned (as with a narrow depth of field any slight error in the focus will be more obvious).

Set the aperture second - this is where DigiFlow comes in. There is a function in DigiFlow that can help with this as it can be used to show the saturated pixels at twice the brightness of all other pixels, making them easier to see.

Focusing the Camera

This applies to the sort of camera you would use for experiments (it should preferably be on a tripod so you know you aren't going to move it afterwards). The trick is to reduce the depth of field so that it will be easier to see when the correct focus is set up.

  1. Turn off any projectors or high intensity light sources.
  2. Open the aperture all the way.
  3. Put something in the tank and focus the camera on this.
  4. Now turn on any light sources and adjust the aperture as appropriate.

Setting the Aperture Using Digiflow

Correctly setting the aperture ensures that the full range that the camera is able to sense is used.

  1. Turn on appropriate light sources.
  2. Open up a live view.
  3. Set the colour scheme to 'left single cycle - aperture'. (View > Colour Scheme or shift+ctrl+B). This sets the intensity of all pixels to half observed, unless that pixel is saturated, making it easier to see saturated pixels.
  4. Set the aperture such that a few pixels are just saturated.
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