Why do we recommend faster (more expensive) lenses over 'kit' lenses?
(faster = larger maximum aperture. eg. f/1.7 or f/1.4 on a 50mm lens; f/2.8 on medium length lenses; f/4 on say a 400mm lens.
The reason wider apertures (more open) are referred to as faster is that you need a faster shutter speed (for a constant ISO setting) to get the same exposure as a slower (more closed) aperture.)
Most kit lenses can do very well and produce great images, but they have limits and this post discusses those limits and why you may want to consider better/faster lenses.
Please refer to the water analogy in Aperture, Shutter Speed, ISO etc. - Explanations
Simply imagine you have a wall with a hole in it and a bucket on the other side.
Light is water, aperture is the size of that hole, shutter speed is how long you push water through it, and ISO is the water pressure.
Correct exposure is when the bucket is nearly full (not overflowing).
If the hole is 2cm and the water is on for 10 seconds (at a constant pressure) a certain volume will flow.
If the hole is closed to 1.4cm (half the area) you will need the water to flow for 20 seconds to get the same volume.
A camera lens is similar in that you need an amount of light to hit the camera sensor to obtain a correct exposure.
A larger aperture allows more light onto the sensor, and you require a faster shutter speed to than if you had used a smaller aperture (larger f/stop number).
The key issue is that when a given lens is at its maximum aperture (wide open) the shutter speed required may still be too slow to avoid camera shake (blur) especially when hand held.
Increasing the ISO sensitivity can help, but introduces noise.
Your can also purchase a lens with a larger maximum aperture, a.k.a. a faster lens.
These faster lenses cost more due to the larger physical lens size and the precision manufacturing required.
They also (usually) have much better Image Quality (IQ) than 'kit' lenses due to better coatings and other techniques used in design and manufacturing.
As you tend to keep lenses and change camera bodies over time, we recommend the investment in good glass rather than kit lenses.
One major side effect of a larger aperture is shallower Depth of Field (DoF).
But remember there are more factors than affect DoF than just aperture.
- aperture - more open ... less DoF
- subject distance - closer subject ... less DoF
- focal length - longer (mm) ... less DoF
- focal plane size - more sensor area ... less DoF
So keep all these factors in mind when out taking photos and purchasing lenses.