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?
In general, shutter speed can affect IQ in several ways:
- 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).
- Directly, by interacting badly with the timing of a flash.
- 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.
- Indirectly, by forcing a higher ISO (which adds noise).
- 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.
Operating a mechanical shutter close to the limit of its speed can cause loss of sharpness due to diffraction since then a significant amount of the exposure time is spent near at least one of the shutter curtains (or the shutter leaves, depending on type of mechanical shutter). Electronic shutter does not suffer from this particular problem.
Depending on how IQ is defined, the following may also be a concern: a focal plane shutter becomes a rolling shutter at fast speeds (see this slow motion video) and is then susceptible to various rolling shutter distortions. For instance, here's an animation of a rolling shutter capturing a rotating disk:
(Image credit: cmglee, CC BY-SA 3.0)