I've been taking a class in portrait photography and one of the questions that came up in discussion was the difference between a "prime focus" and a zoom lens.
There's a reason for this.
aberationsAn ideal lens focuses all the colors on the same plane. A real lens can have both spherical aberations (not focusing on a plane) and chromatic aberations.
An ideal lens (on the left above) will focus all points from the same distance on a single plane. It will also focus all of the colors in the same place.
Real lenses fall short of idea. The focal plane isn't actually planar (spherical aberration) and the colors don't focus in the same place (chromatic aberration).
Chromatic aberration occurs because the index of refraction (relative velocity of the light) in glass depends on the wavelength of the light. Fortunately, the index also depends on the composition of the glass. So "achromatic" lenses which are made of two (or more) lens elements of kinds of glass shaped to correct for the differences in index of refraction have been used since the 1850's (if not earlier). Mind you, early photographic processes were orthochromatic and most sensitive to blue light so chromatic aberrations were less critical than with modern detectors.
My old film lenses had a special mark for Infra-Red focusing. After finding the best focus with visible light, you could adjust the focus if you were using IR-sensitive film.
This is also why a small concave mirror works better as a fire starter than most lenses - all the reflected light is focused on the same place which results in a much hotter spot.
Spherical aberration requires careful design of the shape of the lens and lens elements to bring the focus into a plane. The analytical approximations from my old optics book have been replaced by numerical ray-tracing and similar computational methods. Computer aided manufacturing has brought the cost of high-quality lenses into the range that most of us can afford.
Applying these corrections to a lens at a single focal length is relatively straightforward. Not trivial, but straightforward because there's only one target, the best image a a fixed length from the lens.
A zoom lens has multiple configurations of the lens elements and the corrections have to work reasonably well in all of those configurations. That means that the design will have made compromises. There is, typically, an optimal zoom where the image is sharpest. My A7III came with a kit zoom lens (28-70mm) and the best image quality about the middle of the zoom range. The lenses that came with my old Nikon are best at the lowest zoom. The lens needs to have more elements than a typical prime focus lens. Each element absorbs and scatters some of the incident light. Hence the lens is both heavier (more elements mean more glass) and slower than the equivalent prime focus lens.
Usually, you can use "chimp zoom" and move closer or farther away to frame the image the way you want. There are times when you can't and that's where the zoom excels. A zoom can also have much smaller physical dimensions (my 600mm lens is a great deal shorter when it's not zoomed out which allows me to put it in a pack and carry it to where the animals are.)
January
February
(1)
March (1)
April
May
June
(2)
July (2)
(2)
August (2)
September
October
November
December
|
January
February
March
April
May
(2)
June (2)
(15)
July (15)
(14)
August (14)
(13)
September (13)
(8)
October (8)
(10)
November (10)
(11)
December (11)
|
(9)
January (9)
(3)
February (3)
(2)
March (2)
(3)
April (3)
(3)
May (3)
(3)
June (3)
(9)
July (9)
(6)
August (6)
(4)
September (4)
(4)
October (4)
(4)
November (4)
(4)
December (4)
|
(4)
January (4)
(2)
February (2)
(2)
March (2)
(3)
April (3)
(2)
May (2)
(1)
June (1)
(1)
July (1)
(2)
August (2)
(2)
September (2)
October
(1)
November (1)
December
|
January
February
March
April
May
June
July
August
September
October
November
December
|
(2)
January (2)
February
March
April
May
June
(1)
July (1)
(1)
August (1)
September
October
November
December
|
January
February
March
April
May
June
July
August
September
October
November
December
|
January
February
March
April
May
June
July
August
September
October
November
December
|
January
February
March
April
May
June
July
August
September
October
November
December
|