Wondering something about aperture

What are the physical differences between, say, a lens with a minimum fstop of 2.5 and one of 1.4? What are the primary manufacturing processes or design choices that allow a lens to open up wider? What are the physical limits on such things and why?

It makes sense to me that a zoom lens would have a different maximum aperture as you extend the length of the barrel, but I realized I don't understand why two lenses with similar physical sizes would have such different maximum apertures.

Which two lenses do you have in mind?
--
Phil

The "f/number" is just the ratio between the diameter of a lens and its focal length. A 50mm f/2 lens is an "f/2" lens because the effective diameter of the optics is 25mm (50mm focal length / 25mm aperture = f/2).

A 100mm f/2 lens has an effective optical diameter of 50mm. So there you have two lenses, both f/2, that have very different lens diameters.

Right, but lets say you have two 25mm prime lenses, one is an f2.5 and one is an f1.4, in effectively similar casings. If I'm understanding this correctly, the only obvious difference between these two would be that one manages to open is shutter wider than the other, correct?

So is the tradeoff for manufacturer's one of being able to get the shutter open and closed quick enough and with a wide enough hole to achieve a minimum shutter speed and maximum aperture respectively, or are there other primary factors at play?

slipster216 said:

So is the tradeoff for manufacturer's one of being able to get the shutter open and closed quick enough and with a wide enough hole to achieve a minimum shutter speed and maximum aperture respectively, or are there other primary factors at play?

Click to expand...

The shutter is in the camera, not in the lens. You're thinking of the aperture blades, which don't have to operate with shutter-like speed.

Typically an f/1.4 lens will have way more glass in it than an f/2.8 lens, to accommodate more light. But sometimes there are mitigating factors. For example, if you compare the Pana-Leica 45 f/2.8 and the Oly 45 f/1.4 (or is it 1.8?) the fact that the Pan-Leica is a macro lens will enter into the comparison, since macros typically have more complex optical designs.

--
Phil

slipster216 said:

Right, but lets say you have two 25mm prime lenses, one is an f2.5 and one is an f1.4, in effectively similar casings. If I'm understanding this correctly, the only obvious difference between these two would be that one manages to open is shutter wider than the other, correct?

Click to expand...

The f1.4 lens will have a much larger effective diameter, thus require larger optical elements and (probably) a larger diameter case. It may also require a more powerful focusing motor to drive larger elements.

F number equals focal length divided by effective diameter. So the 2.5 lens requires an effective diameter of 9.6mm, while the 1.4 needs 17mm. Remember this is the effective diameter of the lens at the aperture -- the front element will likely need to be larger. Also, the faster 1.4 lens may require more complex optical design (more elements, more expensive optical materials, or more complex element shapes) to achieve the same image quality across the full sensor area. People paying for a faster lens will most likely expect premium image quality, so the faster lenses are often built to higher standards, which adds to the price.

slipster216 said:

So is the tradeoff for manufacturer's one of being able to get the shutter open and closed quick enough and with a wide enough hole to achieve a minimum shutter speed and maximum aperture respectively, or are there other primary factors at play?

Click to expand...

Shutter speed is a different, though related, matter.

--
After 40 years of Canon and Nikon I'm now using a camera named after my toaster.

Silver Mirage Gallery:
http://www.silvermirage.com

slipster216 said:

lets say you have two 25mm prime lenses, one is an f2.5 and one is an f1.4, in effectively similar casings. If I'm understanding this correctly, the only obvious difference between these two would be that one manages to open is shutter wider than the other, correct?

Click to expand...

Assuming they had a similarly sized housing, the difference would be most apparent in the glass itself. Larger apertures need bigger glass.

slipster216 said:

So is the tradeoff for manufacturer's one of being able to get the shutter open and closed quick enough and with a wide enough hole to achieve a minimum shutter speed and maximum aperture respectively, or are there other primary factors at play?

Click to expand...

Not sure I understand the question you're asking. A wider aperture allows a faster shutter speed, not a slower (or as you said, minimum) speed.

The trade-off between two lenses of such different maximum apertures is light gathering ability and cost.

Ummm....Gato, better chuck that calulator. 25mm/2.5 = 9.6mm? You've got me wondering how you possibly got that instead of the 10mm an 8 year old would come up with in her/his head in three seconds or less for the effective front element diameter!

For the f/1.4, my calculator comes up with 17.9mm (rounded), but I'm too lazy to put pen to paper to check it out.

Pete

slipster216 said:

Right, but lets say you have two 25mm prime lenses, one is an f2.5 and one is an f1.4, in effectively similar casings. If I'm understanding this correctly, the only obvious difference between these two would be that one manages to open is shutter wider than the other, correct?

Click to expand...

Here's a 50mm f/1.4 lens:



...and here's a 50mm f/2 lens:



The overall diameter of the casing is the same because the lens ends up having to fit on the same camera, and for the Pentax lenses I've shown they've all been designed to accept the same 49mm diameter filters. But notice how much smaller the actual glass is on the f/2 lens. It's only 25mm across (50mm / 2 = 25mm) versus 35mm (50 / 1.4 = about 36mm).

So it's not just the aperture blades that are different, the faster lens (the f/1.4 lens in this comparison) need to have all of the individual optical elements manufactured with a larger diameter as well. That means more glass, more polishing, closer tolerances across all of the optical surfaces to maintain sharpness at the focal plane, etc. That's why faster lenses are more expensive (and often less sharp).

slipster216 said:

What are the physical differences between, say, a lens with a minimum fstop of 2.5 and one of 1.4? What are the primary manufacturing processes or design choices that allow a lens to open up wider? What are the physical limits on such things and why?

Click to expand...

focal length/aperture diameter.

That is all.

slipster216 said:

It makes sense to me that a zoom lens would have a different maximum aperture as you extend the length of the barrel, but I realized I don't understand why two lenses with similar physical sizes would have such different maximum apertures.

Click to expand...

Because the lenses have different diameters limiting how lagre the maximum aperture can be.

Maximus Decimus Tedolphus