Originally posted by 8540tomg Why are primes better than zooms? Good question and I’ll give it a shot. I think it is because a zoom lens is all about compromises through its entire focal range. In order to keep down size, price and still make a good optic a lot of compromises have to be made.
Older zooms of 20-30 years ago were pretty sad when compared to their prime counterparts in terms of IQ. This is still the case with many cheap zooms of today although they have come a long way. Premium zooms such as The Pentax DA* series and Canon L series certainly approach the IQ of a prime lens. I haven’t done a study but I suspect a quality 300mm prime will still outperform a zoom at that particular focal length because it is optimized to do so.
I’m sure Wheatfield and Ben Edict will have some thoughts on this one. Am I out to lunch on this one guys or do telephoto primes still rule?
You pretty much summed the reasons up! From a technical perspective it is also quite easy to understand, why a zoom usually is not as good as a quality prime (we shouldn't forget, that there are also very poor prime lenses...):
Basically a zoom lens consists of a primary lens with a fixed focal length. After that we have a lens group which is called the "variator". This varietor group is in principal a sliding teleconverter. The distance between this varietor and the the other lens elements and especially between the variator and the sensor/film plane determines the magnification. As the variator slides front to back and vice versa, the magnification varies.
After the varietor there are usually some correcting elements, because the variator has limitations: the higher the magnification gets, the smaller the field of view will be, introducing vignetting. This needs correction. Also the shifing of the focal length would lead to a shift of the focal plane, which we do not want. Therefore there is some kind of projection element (a relay group) at the rear of a zoom lens, which makes the fixed focal plane possible. And ofcourse more lens elements also may introduce more residual colour abberrations. Also, we shouldn't forget, that a teleconverter (aka the variator) works best at a certain distance to the primary lens and to the focal plane. Any other position will lead to increasing loss of contrast and sharpness at the corners. The farther away the tc is from its optimum position, the smaller the useable center of the image circle will be.
Modern zoom lenses are more complicated, especially if they introduce internal focusing etc., but the basics are enough to understand , why zooms usually have lower IQ. Also the travel of the variator must not neccessarily been a mechanical travel, but can be achieved by moving different elements against each other, such achieving the "travel" by projection variations:
1. the primary lens group has a fixed fl, usually about the shortest fl, the zoom lens delivers
2. the variator magnifies the image of that primare lens group
3. in doing so, all abberrations and other shortcomings of the primary group will be magnified, too (just like using a tc)
4. the variator group introduces loads of problems, as it works far out of its optimum distance from lenses and focal plane.
This explains easily, why "super-zooms" (18-200 etc.) have such limited quality: the primary lens group has a comparetively short fl and the variator needs to travel through a large distance, with a large degree of magnification variation. A short fl for the primary lens means, more distortions and higher amount of spherical abberration (mostly corrected by using aspherical elements somewhere in the optical train), which will be highly magnified by the variator. Also the varietor has to travel far more with a short primare lens, to achieve a resulting long fl, so that it works very far from its optimum position - this explains the usually break-down of performance at the longest setting.
Ben