Bokeh doubt (50mm vs 24mm)

Discussion in 'Photography Beginners' Forum' started by Meredoth, Nov 26, 2017.

  1. TCampbell

    TCampbell Been spending a lot of time on here!

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    As Keith points out... Bokeh refers the the quality (not quantity) of out-of-focus blur. I can pick on the Canon EF 50mm lenses as an example...

    Previously Canon’s “nifty fifty” was a 50mm f/1.8 lens that only had 5 aperture blades. A de-focused point of light will blur to the shape of the aperture blade. So if the aperture is 5 blades that aren’t well-rounded, you get pentagonal shapes. With this particular lens, everything blurred to that shape and the over-lapping texture gave it a quality that wasn’t very smooth (sometimes described as “jittery” bokeh or even “nervous” bokeh).

    Canon’s current “nifty fifty” was redesigned, now uses the STM focusing motor, and uses 7 aperture blades and is a bit more well-rounded. Out of focus blur is closer to a circle than a pentagon and the quality of the out-of-focus blur is substantially improved over the previous generation.

    More aperture blades and having well-rounded aperture blades is a factor that influences bokeh... but qualities of the optics themselves also control this. If you imagine the curved front surface of your lens with the very center as representing the “North Pole” with lines of longitude moving away from center, and lines of latitude circling around the center. But in photography the lines that radiate away from the center (like bicycle spokes) are called “sagital” lines and the lines that go around the center (like concentric rings) are called “meridonal” lines. It turns out some lenses blur more strongly in one direction than in the other.

    Lomography makes a lens they call the “Petzval” which is meant to be something of a reproduction of an old lens (it’s got a brass barrel, completely manual, and instead of aperture blades that constrict and retract it comes with a set of metal plates that slide into the side of the lens — each having a different size hole. These are called “Waterhouse stops” and it’s how they control the aperture size). But the stops are perfectly round drilled holes. But it turns out the bokeh quality of this particular lens creates a swirl effect because the lens blurs more strongly in the meridonal direction than in the sagital direction. This is just one example of how the optics themselves ... and not the shape of the aperture ... control the quality of the blur.

    So again... “bokeh” refers to the quality of the blur... not how strongly it blurs.

    Depth of field controls the strength of the out-of-focus blur (regardless of whether or not you like the quality of the blur.)

    Factors that influence the strength of the out of focus blur are:

    1) close subject - distant background (so the focus distance to those elements of your shot are not similar).
    2) long focal length lens - very long focal lengths naturally produce a shallower focal ratio. Very short lenses (ultra-wide lenses) have very very broad depth of field and you wont get strong out-of-focus blur.
    3) low focal-ratio - using a low f-stop (large aperture opening) causes light to use more of the “glass”. Narrow focal ratios cause light to go only through the center of the lens. It turns out you’ll get stronger out-of-focus blur (significantly stronger) with a low focal ratio (provided it’s also not a short focal length).

    A 24mm lens will have a low amount of out-of-focus blur and it’s probably not what you want.

    A 50mm lens will have more out-of-focus blur (provided you shoot at a low focal ratio such as f/2 or f/1.8).

    If you want STRONG out of focus blur, consider moving to a longer lens... such as an 85mm f/1.8 ... or a 135mm f/2 ... the 70-200mm f/2.8 generates strong bokeh at the 200mm end. I have a 300mm f/2.8 lens which generates so much sweet creaminess in the image that it’ll give you diabetes. ;-)


     
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  2. Meredoth

    Meredoth TPF Noob!

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    It's like magic!!! :D But I did not run outside with an empty roll of paper. Though I used my own hand at home to see it. hehehe

    Thanks for the in-depth explanation! Already ordered the 50mm STM, hope to get it till Friday. It will give me, with the 24mm, nice flexibility for what I need, I suppose. After that, maybe saving some money for a Sigma lens. *.*
     
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