Lenses with large apertures...

Forgot to mention the interesting conflict/confusion about focusing...

Focusing is more CRITICAL at wider apertures because there is so little room for error...that room/range of error is called Depth of Focus or Depth of Field (DoF) which refers to the RELATIVE amount in focus within a range in front of and behind the POINT of focus, a range that is based on a print/enlargement of about an 8x10 on a 35mm camera...

The wider the aperture (F2 vs F8 ), and the closer you are to the subject at the same aperture, the less the DoF....the smaller aperture/greater distance, the more DoF....

[But DoF is ONLY an APPEARANCE of "in focus" because as you enlarge the image (as you are getting closer) you reduce the range of the DoF....only the POINT of focus will remain in focus....conversely, the SMALLER the image relative to an 8x10, (it looks as if you are farther away from the subject) the LARGER the range of focus of the DoF.... On a rebel/xxD the same rule holds but the base/standard for measuring the DoF is approximately 10x12 because the smaller the sensor/film frame the more is "in focus"]

Back to the focusing problem...if the aperture is made smaller the DoF increases and any focusing "error" of locking on to the point of focus perfectly is less noticed...but at the wider aperture the point of focus is more easily discerned, so errors are also more easily discerned, but if it is "spot on" the image will seem sharper because the focused point is more isolated from the foreground/background that is out of focus...on the other hand, some photographers will use a smaller aperture to get EVERYTHING in focus...

It is why many images are sharper when taken with a zoom set at 100mm/5.6 than at 100mm/2.8...

The challenge in low light situations to getting the point of focus accurately is giving the lens/body mechanism enough leeway (DoF) to get it right but not so much DoF that you get no isolation (if that's what you want)...

So with instances of error in using wide apertures, the problem is two-fold: either the focus mechanism is too wide to lock on the point of focus, (e.g. the focus box is as large as the whole face but you only want the eyes in focus) or the photographer isn't up to the task of aiming the camera correctly to allow the AF to get the job done...yes sometimes it is the archer and not the arrow....AF is not entirely automatic...

And finally...wider aperture allows you to use a faster shutter speed to reduce camera shake, a big buggaboo in image quality....but the smaller DoF also reduces the likelihood of perfect focus, also detrimental to IQ

...what do I do?...alway, well almost always, stop down 1 click from wide open...it's a compromise that has worked for me...

Hope this hasn't given you a headache...

Jack McEntire wrote in post #8546196
Thanks again for the fast replies, I think I understand it a lot better now!

I'm wondering though, why are lenses with shorter focal lengths often cheaper than those with longer ones? For example, the [SIZE=2]Sigma 50mm f2.8 is a lot cheaper than the [SIZE=2]Sigma 105mm f2.8. Why is this, and what advantages/disadvantag​es do these lenses have in comparison to each other? I'm thinking of buying one of the two and I'm feeling a bit stuck, so any advice would be much appreciated!

Longer focal lengths generally require more glass. More glass = higher cost.

there are 3 lenses that are the easiest to make...35mmF2.8, 50/F2 and 135mmF2.8 ...okay 200mm F4 too...virually all others are more complex...as it is to make the above lenses wider

Jack McEntire wrote in post #8545968
Is that so? I was thinking that that must be the case... I can't see why those lenses command such ridiculous prices, what real advantages do they have?

yes, DoF shrinks with smaller f-stops. That doesn't mean it shrinks to 1 inch. DoF is influenced by f-stops, subject-camera distance, focal length, and a bunch of other factors you can worry about later. Shooting at 200mm and f/2 doesn't automatically mean the DoF is very small since you still have to consider the distance to the subject.

The advantage is background separation and the extra shutter speed to freeze motion.

Jack McEntire wrote in post #8546196
I'm wondering though, why are lenses with shorter focal lengths often cheaper than those with longer ones? For example, the [SIZE=2]Sigma 50mm f2.8 is a lot cheaper than the [SIZE=2]Sigma 105mm f2.8. Why is this, and what advantages/disadvantag​es do these lenses have in comparison to each other? I'm thinking of buying one of the two and I'm feeling a bit stuck, so any advice would be much appreciated!

Shorter focal lengths are not cheaper than longer ones. However, for reasons that I'm not quite clear on, 50mm is the cheapest lens to produce (for 35mm film, at least). As you get wider or longer, the cost increases, especially wider, since wider lenses are more vulnerable to optical aberrations.

...and then there's construction (build quality) and the age of the design to consider.

How about this way of looking at cost - A 50mm F/2 lens has a roughly 25mm in diameter front elelment. A 500mm f/2 lens woudl have a 250mm in diameter front element. Glass does cost money.