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  1. #1
    captainwookie's Avatar
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    Changing the f/stop during exposure?

    I’ve seen this technique casually mentioned in a book, but the writer did not elaborate on its purpose or use. It was to change the f/stop of the lens during a long exposure. The concept was interesting to me, but I have no idea why I would do this, or what the effect would be. Has anyone ever herd of this before?

  2. #2
    rbarker's Avatar
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    Assuming the camera was rigid enough to accomplish this without introducing camera shake, I could see the technique being used to create soft edges in some portions of the foreground or background due to the resulting change in DOF - essentially a creative effect to further focus viewer attention within the image. When doing multiple exposures for commercial shots, for example, it's sometimes effective to rack the focus in and out slightly during a secondary exposure while the main lights on the product have been turned off.
    [COLOR=SlateGray]"You can't depend on your eyes if your imagination is out of focus." -Mark Twain[/COLOR]

    Ralph Barker
    Rio Rancho, NM

  3. #3
    captainwookie's Avatar
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    Well, unfortunately it was in a library book so I can’t look back at it. It was just one of those things I read that made me go, “huh?” and stuck with me. Someday, I'll have to give it a try.

  4. #4
    Les McLean's Avatar
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    Many years ago I tried a simlar process, instead of using a long exposure and changing the fstop I used multiple exposures and changes the fstop several times during the time I made the exposures. The end result was that the image had soft edges as has been suggested by rbarker. I also experimented with moving the point of focus as I changed the fstop. The idea of the experiment was an attempt to introduce a sense of movement into the image. Some of the images looked similar to those produced when a zoom lens is zoomed during the exposure.
    "Digital circuits are made from analogue parts"
    Fourtune Cookie-Brooklyn May 2006

    Website: www.lesmcleanphotography.com

  5. #5
    David A. Goldfarb's Avatar
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    And just to add to what Ralph and Les have said--if you want to use two different apertures to control diffusion it's easiest usually to do this as a multiple exposure. You could, say, make 25% of the exposure at a wide aperture or with a soft focus or diffusion attachment and 75% of the exposure at a small aperture.

    You see this a lot in product advertisements and sometimes with food shots, where the object is sharp but has a kind of a glow, but you could also use it in a landscape or anything where the subject isn't moving (unless you want to record the motion, like Les does with his multiple exposure technique).
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  6. #6
    Mongo's Avatar
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    I've used this technique as Les has described it (multiple exposures using different apertures). It's effective and interesting with the right subject, but every time I've done it the end print has been a little too "gimmicky" for me. I was basically looking for a way to emulate a soft-focus lens in some forest shots...I almost got there but never quite got what I wanted. It's a technique I'll probably re-visit at some date in the future with the hope of perfecting it, but for now I have about a dozen other things that are more important to me to work on.

    Having said this, if you shoot roll film with a camera that will allow multiple exposures, give it a shot sometime. You'll only be wasting one frame, and you might find a technique you like in the process.
    Film is cheap. Opportunities are priceless.

  7. #7
    Cheryl Jacobs's Avatar
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    This image was printed while changing both the f-stop and the focus during exposure.
    Attached Thumbnails Attached Thumbnails dancess.jpg  

  8. #8
    captainwookie's Avatar
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    A nice photograph, though it does kind of remind me of stuff I saw on a web site about photographing ghost.

    Anyway, thanks for all the explanations. At least now I have a better understanding of what changing f/stops will do. I have to give it a try someday.

  9. #9
    Eric Rose's Avatar
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    Mongo, I had the same challenge and the technique I used to get the results I was after is to make two exposers on the same frame. Each naturally at 1/2 of the required exposure. One shot would be at a reasonable f-stop for sharpness and the next using a silk stocking or nylon over the lens. Depending on the highlights and SBR the effect can be quite mystical.
    www.ericrose.com
    yourbaddog.com

    "civility is not a sign of weakness" JFK

    "The Dude abides" - the Dude

  10. #10

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    Quote Originally Posted by Eric Rose
    Mongo, I had the same challenge and the technique I used to get the results I was after is to make two exposers on the same frame. Each naturally at 1/2 of the required exposure. One shot would be at a reasonable f-stop for sharpness and the next using a silk stocking or nylon over the lens. Depending on the highlights and SBR the effect can be quite mystical.

    Not unlike the approach that Canon had with one of the early EOS Rebel's the Rebel II/IIS I think..it had a SF mode where it would make one image in focus, the repeat the shot with the AF lens just out of focus. Was quite good, not sure why they dropped the function.
    Mike C

    Rambles

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