tfttf532 – KODAK filed for bankruptcy

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Today is Q&A time! Jonathan asks about bokeh and prime lenses, Chris wants to know how to archive photos for long term storage.

Also KODAK has just filed for Chapter 11 bankruptcy, so Chris will take a closer look at that.kodak

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  • Thomas Becker

    Differences in bokeh between the viewfinder and a photo…

    Last year I notice the effect your questioner asked as well. An easy experiment to see it…

    Take a lens faster than f/2.8, mount it on your camera, put the camera in aperture mode, and set the aperture to an f/number greater the f/2.8, say f/8. While looking through the viewfinder, press your camera’s focus preview button. You will see two affects – the image will get darker as the aperture closes down and anything that was out of focus will become somewhat sharper. For a bonus, if you look at a point source of light while the lens is out of focus, you will see the infamous circle of confusion of the light become smaller. Now repeat the above, flicking the focus preview on and off for wider and wider apertures. You will notice that both the view’s brightness will change less and the circle of confusion size or bokeh will change less. At some point, the change stops altogether. On my camera (a Canon T1i), this affect actually stops at f/2.8. If I open my lens to any aperture wider than f/2.8, flicking the focus preview causes no changes. This implies that there is an aperture somewhere in the light path from lens to eyeball that has an effective aperture of around f/2.8.

    Clearly it’s not the lens since taking a photo at f/1.8 differs from one at f/2.8. It’s also not the lenses in the viewfinder since I would expect that at a minimum, the light intensity of the viewfinder to change when flipping the focus preview. This leaves the focus screen.

    Now take a view through the viewfinder at a point source while setting the focus of the lens as close as possible. With the point source far away, you’ll see the circle of confusion, but its interior appears textured in a regular pattern. This comes from the fact that the focus screen is actually not ground glass as is commonly thought, rather it is manufactured with micro-prisms on it. I’m not exactly sure how these are shaped, but given that we see the texture in the circle of confusion, it is also clear that not all the light falling on the focus screen actually gets through it. It turns out the light not getting through is light that comes in at a steep angle, ie light coming from the outer parts of the lens beyond f/2.8.

    Because of this effective aperture of the focus screen of f/2.8, what we see with the focus preview is not necessarily what you get in a photo. Also what we see through the viewfinder is not necessarily equivalent to a wide-open lens, though usually it is because our lenses usually aren’t faster than f/2.8. We see an image equivalent to the smaller aperture of the lens or the focus screen. For a Canon T1i, this appears to be around f/2.8.

    See http://www.kenrockwell.com/canon/slrs/5d/focus-screens.htm for more info.

    Hope this helps explain your guest’s question.

    Thomas

    P.S. Someone on one of your Happy Shooting workshops that you podcast asked the same question. I’d love it if you could pass this on to him as well.

  • http://www.lightpicture.ch Manuel

    Hey Chris,
    PocketCris and Episode??? Why you not call it just “Appisode”?! ;)

    Cheers: Manu