Some questions regarding lenses.

[Chrisku13]

I feel like I have gotten a basic understanding of how a lens correlates with photography down, but had a few questions I haven't seen much information on.

1. I am interested in wide angle photography, but was curious as to what point the images start to become distorted. I forget the term for this, perhaps one of you could refresh my memory. I know subject matter is a factor in how bad distortion will appear, but does lens quality also affect this? And I know some programs say they can reduce this effect, but is it really possible?

2. Also, does a prime lens usually yield better image quality than a zoom lens at the same focal length? I've heard that this used to be the case, but the gap between the two has narrowed.

3. Also, on some lenses' reviews I noticed that they said the lens performed better if it wasn't stopped down to it's maximum aperture. Is this usually the case?

I hear many different things about lenses nowadays, and just wanted to see what some of you had to say regarding these questions.

[AlexMonro]

There are several types of distortion which can appear in wide angle shots. Many wide angle lenses have some barrel distortion, which causes straight lines near the edge of the frame to appear bowed outwards. However, good quality lenses are designed to minimise this.

There is also the distortion that causes objects closer to the camera to appear unusually large - e.g. big noses in close up portraits. This isn't really distortion, just an effect of perspective, and is apparent because the field of view is wider than what the eye is used to seeing, at normal print viewing distances. If you have a large print, and hold it very close, the distortion will be less apparent.

Finally, there is the distortion of fish eye type lenses, which is often regarded as a special effect feature. There is some software which can correct this and produce a normal rectilinear image, but by neccessity, this crops a lot of the frame.

A prime lens is still likely to produce a slightly better image than a zomm of equivalent focal length, because zooms need many more glass elements, and, despite the effectiveness of modern coatings, each glass to air interface is going to introduce some reflections. However, it's certainly true that the gap has narrowed, particularly for top quality lenses.

A prime will still be a lot lighter to carry, and is likely to be smaller and less expensive.

Many lenses tend to perform best at around the middle of the aperture range. However, there is a fundamental law of physics which can reduce the sharpness of a photograph at very small apertures. This is called diffraction, at the aperture at which it starts becoming noticeable depends on the sensor / film size of the camera. It's around f/11 - f/16 for most DSLRs, f/16 - f/22 for 35mm film.

There's a detailed discussion of diffraction here (http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm).