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I have a Canon 600D with a Canon 70-200 f/4 L lens. I mostly shoot during the day, in bright daylight, and I love sharp images of people with a blurry background. Now to achieve that, I keep my aperture as wide as possible(f/4), and with a low iso (100-400). This means that my shutter speed often goes faster than 1/1600 or even 1/2500 to keep the exposure right! May this have a negative effect of my IQ?

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In general, shutter speed can affect IQ in several ways:

  1. Directly, by interacting badly with the natural pulsing frequency of lighting (particularly long-tube fluorescent lights and some cheap LEDs and CFLs), resulting in uneven lighting in different parts of the image (with a rolling electronic shutter) or inconsistent exposure between shots (with a global electronic shutter or mechanical shutter).
  2. Directly, by interacting badly with the timing of a flash.
  3. Indirectly, by forcing an aperture that isn't sharp on your particular lens.
    • Wide apertures have shallow DOF, so if the lens exhibits back/front-focusing, it can be soft.
    • Narrow apertures can be less sharp because the lens becomes diffraction-limited.
  4. Indirectly, by forcing a higher ISO (which adds noise).
  5. Aesthetically, by freezing motion that you might prefer to have blurred.

In your particular case, #1 doesn't apply, #2 applies only a fill flash, and #3 doesn't apply because you have enough light to use a relatively low ISO. However, they're all worth keeping in mind when you're shooting indoors, in cloudy weather, etc.

  • "by forcing an aperture that isn't sharp on your particular lens". Could you explain this further? – Tindra Sep 19 '16 at 2:28
  • If you make the shutter speed faster, then to get the same exposure, you have to either increase ISO or open the lens wider. Not all lenses are sharp wide open. – dgatwood Sep 19 '16 at 18:31
  • And also narrow apertures can be soft because of diffraction limiting. – dgatwood Jan 5 '17 at 0:55
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Fast shutter speeds like 1/2000 1/4000 or even 1/8000 have no effect on image quality.

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What can affect the photo is that wide open your lens may not be at its best. Usually it is recommended to stop it down one or two stops for maximum sharpness.

Also, if you hit the shutter speed maximum you risk overexposing.

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