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I have been reading about Focal length and Fast lenses when I came across this lens which has 85mm focal length and F1.4 as FStop.

https://www.sony.co.in/electronics/camera-lenses/sel85f14gm

Generally lenses with 1.4mm Fstop have focal length below 35mm but here its 85mm!! As we calculate FStop as FocalLength/diameter this implies that this lens must be having huge diameter to achieve 1.4 Fstop.

Looking at cost this lens is very expensive, so must be a premium lens.

Can someone please explain how having a large diameter makes it a premium lens.

EDIT : This question is not a duplicate of ones mention in comment, it focusses on specific aspect as to why a certain feature makes a lens premium. Also it is not specific about lens SEL85F14GM, it is just an example.

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As we calculate FStop as FocalLength/diameter this implies that this lens must be having huge diameter to achieve 1.4 Fstop.

Larger than a lot of lenses, but not "huge" in human terms - 85mm / 1.4 = 60mm - or about the size of the palm of your hand. But this isn't a small lens - it weighs the best part of a kilo.

Can someone please explain how having a large diameter makes it a premium lens.

As you've noted, having a large diameter is a prerequisite to being a fast lens; being a fast lens has two advantages:

  1. The lens can let in more light, and is therefore useable in lower light conditions (or in the same light conditions at faster shutter speeds or with a lower ISO).
  2. The depth of field at wider apertures is lower, so the lens can ensure that objects away from the focal plane are more blurred than a lens with a narrower aperture. This is generally considered an advantage in portrait photography, and the page you've linked to makes it clear this is the primary selling point for the lens ("The ultimate portrait prime").
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  • Thanks a lot for explanation. What I still don't understand is that why would someone go for 85mm Fast lens. At 85mm focal length, I am basically approaching towards telephoto lens where I don't require fast speed as its not low light and depth of field will not have much significance for telephoto shoots. Am I missing something? – Lokesh Oct 8 at 9:19
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    "its not low light" - there are plenty of low-light telephoto applications. Gig/event photography is possibly one of the most common, although 85mm is probably a bit short there. "depth of field will not have much significance for telephoto shoots" - 85mm is a very typical focal length for upper-body/headshot portraits, and depth of field is very significant there. – Philip Kendall Oct 8 at 10:06
  • For telephoto, having a faster lens is quite relevant when shooting handheld - any shaking is amplified at that range. And yes, for concerts speed is significant - dim lighting that changes constantly, and depending on genre the musicians move a lot. The 70-200mm F2.8 is the common go-to lens, since primes are often too unflexible. – Dynat Oct 9 at 6:29
  • @Dynat I rarely use my 70-200/2.8 for shooting concerts unless it is in a venue with theatrical type lighting. For smaller clubs and bars f/2.8 is too slow. There my go to lenses are primes: 135/2, 85/1.8, 50/1.4, and 35/2. – Michael C Oct 9 at 8:04
  • @MichaelC Sure, it depends on the venue. Clubs/bars have less distance and lighting than medium/large festival stages, while the latter can have a time/space limit speaking against a collection of primes. Either way, 85mm is way too short for certain situations, which Lokeshs comment was about. – Dynat Oct 9 at 9:38
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Can someone please explain how having a large diameter makes it a premium lens.

The point is also indirectly linked - manufacturers usually create slower entry level lenses with budget specs, like Canon's 50mm 1.8 - dirt cheap, good image quality, but feels like a toy. It has a cheap plastic body, that's nowhere near what they use for the good lenses.

Fast lenses become worthless on the market without premium build quality and specs, so this increases the price in addition to simply requiring more material to achieve the aperture. The advantages of having a larger aperture in the first place were already mentioned by Philip Kendall.

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Why a bigger diameter lens can gather more light. Consider, on a dark night, you can see the glow of a burning match off in the distance. Your friends standing near you can also see this distant glow. Translated, the light from the match radiates outward in all directions. Your eyes only receive a fraction of its radiated light. For your friends to see its light, some of it must also play on their eyes. If this is true, do you suppose we could use a funnel-like devise and collect or accumulate more of the matches’ light rays? We can! Capturing and accumulating light, and then projecting an image is the job of the camera lens.

Opticians know that big diameter lenses capture more light then small lenses. this principle is used by astronomers. The largest lens ever made is 60 inches (1,524mm) in diameter. Larger is better but the glass lens is thick in the center and thin at the edges. It is believed that a large such lens will crack under its own weight. We can make bigger, but these are mirror lenses, supported from the rear.

Anyway, the bigger the diameter of a lens the more light it can capture. However, the lens must project an image of the outside world onto the light sensitive surface of film or digital sensor. The longer this projection distance, known as focal length, the more magnified the image will be.

Now everything has its pluses and minuses. When we make the focal length long, we get more magnification (telephoto effect). However, with this increased magnification comes a loss of image brightness. This loss can be severe. We need a way to know just how sever this light loss is.

f-numbers to the rescue: The f-number is short for “focal ratio”. When we compare one lens to another, as to image brightness, we use the f-number as a way to make this comparison. We measure the focal length (magnifying power) and the working diameter of the lens called aperture, both in millimeters and divide.

As an example, a 50mm focal length with an aperture of 25mm equal 50 ÷ 25 =2. We call this focal ratio f/2. This method gives us a simple way to compare lenses. Suppose someone has a camera with 500mm telephoto lens with an aperture diameter of 250mm (a big lens). The focal ratio is 500 ÷ 250 = 2 written f/2. Both the 50mm with aperture 25mm and the 500mm aperture 250mm, deliver the same image brightness. In fact, any lens operating at the same f-number as another, will deliver the same image brightness’.

The camera lens is adjustable as to its working diameter. We or the camera’s software make this adjustment. This system mimics the human eye which makes involuntary diameter adjustments based on vista brightness. The colored portion of our eyes, called the iris after the Greek goddess of the rainbow, make this adjustment by altering the pupil diameter.
The f-number system seems complex. It is based on an increment of change that doubles or half’s the amount of light that traverses the lens. The f-number set from brightest to dimmest: 1 – 1.4 – 2 -2.8 – 4 – 5.6 – 8 – 11 – 22 – 32

Going right cuts the light in half. Going left doubles image brightness, each a 2X change. The f-number intertwines lens aperture (working diameter) with focal length (magnifying power). When we enlarge the aperture diameter we increase the lens’s working area of entry. This sequence sets an increment of adjustment. That increment is a 2X change. The f-number unit is what we use to control exposure via the lens setting and it also controls depth-of-field. The smaller the aperture setting (going right), delivers a greater span of depth-of-field.

To let in more light, the lens must have a larger diameter. It is more costly to make such a lens because the curve of the glass (figure) must be more accurate especially at the edges.

Nobody said this stuff was easy!

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  • "If this is true, do you suppose we could use a funnel-like devise and collect or accumulate more of the matches’ light rays? We can! Capturing and accumulating light, and then projecting an image is the job of the camera lens." You are conflating a telescope (or teleconverter) which creates a larger and potentially brighter (and upright) virtual image with a camera lens that creates a real (and upside down) image on the sensor. – user95069 Oct 8 at 19:41
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    @ user95069 All camera lenses work this way. This includes camera lenses, telescope lenses, camera lenses with teleconverter, In other words, the larger the working entrance, the more light gathered, the brighter the image. A camera lens operating at f/1 is a awesome capturer of light, It has an aperture diameter that is equal to the focal length. All such lenses project an upside down image, It takes another lens or prism to invert the image. For terrestrial telescopes and binoculars this added component is a requirement. Not so for the camera upside down is OK. – Alan Marcus Oct 9 at 1:23

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