I'm not here to impress people by how much I know about photography but simply to tell the facts or truths regarding what TS wants to know.
1. Why are lenses with big/wide aperture more expensive than smaller ones?
This has to do with the cost to make such a product.With respect to machining the aluminium lens barrel to closer/precise/exact tolerances especially where the lens elements will be rest/sit on (plane of reference).
The design of the lens elements and choice and use of different types of glass to fulfill design objectives relevant to lens function which is transmission of light rays.
Grinding and shaping of lens curvature to precise tolerances.The precision
and quality of the machines used and the skill of the production operator.
You simply cannot employ a foreign worker and expect to produce good lenses
even with cnc (computer numerically control) machines from day one, it takes years to learn and practice. That's why they are known as craftsman in Japan and not simply a production worker.There is a sense of pride and value
appreciated by society.
2. Quality control procedures at every step of production in terms of man hours and testing to conform to minimum standards.
3. Facts of optical science. Light consists of different colours and because of this each colour is of different wavelengths.A simple analogy is taking different length of sticks and putting them in a cup (plane of reference)
one will notice not all sticks protrude at the same height.
This is the primary function of lens elements to zero in these light rays just like you zero the gunsight of a gun much like on film or image sensor plane. This is what makes a sharp image of contrast and colour. If it is a zoom lens it is much harder to do! More moving parts and variables.
4. F-stop vs photography.
Does it mean if you use a lens with wide f-stop you will make the best photos? The answer is sadly no! It all depends, F-stop is about how fast light is recieved vs ISO and depth of field of the subject.The difference between a canon and third party lens is quality.All lenses are sharp at the center but soft at the edges in general but a branded one is better as it is designed to be so. Basically it's marketing strategy of third party brands, they know people want large apertures but for a given price it is a compromise.So generally people who use third party lenses don't use it at widest f-stop because they know it is not as sharp unless it's the difference of getting the photo or not.Same as if the widest f-stop is F4 it will not be as sharp as a narrower aperture. so whatever lens you use it pays to know at which f-stop it is sharpest( it usually get sharper 1-2 stops from the widest).It does not mean the narrowest aperture is best because you will encounter the diffraction problem beyond a certain f-stop and just more depth of field but not more sharpness per se.
5. F-stop vs portrait photography. this you must really understand!
A sharp photo is nice to look at but wide f-stop is also a double edged sword so to speak. If you look at the side profile of the human head which feature
protrudes the furthest, forhead, nose right? A wide aperture has very shallow depth of field so the auto focus may focus on this even when you use face recognition feature.As a result you may get blurry eyes and nothing is more irritaing to see eyes that are out of focus.So it pays to check if this is the case if you do a head shot.
Secondly this is also what most people don't know because they see only portraits of young beautiful women with smooth skin all the time. Of course
digital photography is forgiving of smooth skin but what about the skin of mature women? Digital image sensors are too high resolution and you can be sure these women will not like what they see of themselves digitally.You still want very pin sharp photos of older women? You may not get paid after shooting them..haha. That's why film is more forgiving of older women as well as younger ones because the film is curved and therefore has a natural distortion much like soft focus! So the sharpest lens is not always good for older skin.
Best regards.
