Between The Sprockets A photography blog

Cheap Lenses

When I first started taking photos I often wondered why some lenses cost so much more than others.  Sure, they were "faster" (had a wider aperture, allowing more light in), but I couldn't see why some were 3x -4x the cost of others (even with similar apertures).  It turns out there's many reasons, but there's one that's easy to see in the end result, it's called chromatic aberration.

Chromatic aberration is a type of distortion where the lens fails to focus all colours to the same point on the film / sensor.  What you end up with is parts of your picture having halos of colour.  See if you can spot it in the first image here (cropped from a larger picture, taken with a Konica Minolta 5D and Minolta 70-300mm 4.5-5.6 D lens):

It's not easy to spot, especially at this size, so here's a zoomed in shot:

See the blue / violet on the bird's shoulder?  That's chromatic aberration.  The lens has failed to focus the violet light to the same point as all the other colours to make the white (shade of grey in this case) colouring of the bird.

Most lens manufacturers have ways of dealing with this, but it's not cheap.  This is where the more expensive lenses come from.  In the Minolta (now Sony) range, these normally have the "APO" badge which stands for Apochromatic.  This is the name of a method of using multiple lens elements to reduce chromatic aberrations.

If you would like more information, I can suggest the following Wikipedia links.  They are very complicated, but all the info you ever wanted is there.

Chromatic Aberration
Apochromat