so if I understand f-stops correctly...

[Apollos]

at least in terms of aperture...

then a AF-S DX NIKKOR 18-105mm f/3.5-5.6G ED VR lens' maximum aperture does not actually physically change, however the f/ number increases due to the increased ratio of hole (aperture) size to focal length.

In other words, a f/2.8 "hole" is not the same physical size at 50mm as an f/2.8 "hole" is at 200mm ...right?

this would explain the expense in lenses like Nikon's 70-200mm f/2.8 because the lens must have a function to increase/decrease the actual aperture diameter as focal length changes.

or am I completely off base and should I kill myself?

 

[chuxter]

at least in terms of aperture...

then a AF-S DX NIKKOR 18-105mm f/3.5-5.6G ED VR lens' maximum aperture does not actually physically change, however the f/ number increases due to the increased ratio of hole (aperture) size to focal length.

In other words, a f/2.8 "hole" is not the same physical size at 50mm as an f/2.8 "hole" is at 200mm ...right?

this would explain the expense in lenses like Nikon's 70-200mm f/2.8 because the lens must have a function to increase/decrease the actual aperture diameter as focal length changes.

or am I completely off base and should I kill myself?

You are a little "off base", but not enough to perform hari kari.

Read this:

http://en.wikipedia.org/wiki/F-number

Your 18-105mm lens does change the max aperture as you vary the zoom, but not as much as a fixed max f/# lens does...nor as predictably (there is no spec that tells us exactly the f/# at specific intermediate FLs.

Perform the math:

• 18mm / 3.5 = 5.14mm diameter
• 105mm / 5.6 = 18.75mm diameter

If that lens was a fixed f/3.5, the result at 105mm would be:

• 105mm / 3.5 = 30mm diameter

--
Charlie Davis
Nikon 5700, Sony R1, Nikon D50, Nikon D300
HomePage: http://www.1derful.info

"If ever a time should come, when vain and aspiring men shall possess the highest seats in Government, our country will stand in need of its experienced patriots to prevent its ruin."
-Samuel Adams, 1776

 

[Graystar]

at least in terms of aperture...

then a AF-S DX NIKKOR 18-105mm f/3.5-5.6G ED VR lens' maximum aperture does not actually physically change, however the f/ number increases due to the increased ratio of hole (aperture) size to focal length.

In other words, a f/2.8 "hole" is not the same physical size at 50mm as an f/2.8 "hole" is at 200mm ...right?

this would explain the expense in lenses like Nikon's 70-200mm f/2.8 because the lens must have a function to increase/decrease the actual aperture diameter as focal length changes.

or am I completely off base and should I kill myself?

The internal working of modern lenses is very complex. You understand what is effectively going on...and that’s good enough. Don’t worry about what is actually going on in the lens.

For example, it’s my understanding that the iris does not change in a 70-200mm lens while zooming. Rather, the optical relationship between the various groups of glass within the lens change. It’s really not worth trying to understand as long as you understand the effect.

.

 

[sacundim]

at least in terms of aperture...

then a AF-S DX NIKKOR 18-105mm f/3.5-5.6G ED VR lens' maximum aperture does not actually physically change, however the f/ number increases due to the increased ratio of hole (aperture) size to focal length. In other words, a f/2.8 "hole" is not the same physical size at 50mm as an f/2.8 "hole" is at 200mm ...right?

That's right, but you're running into terminological issues by putting it that way. Photographers, for better or worse, use the unmodified term "aperture" to refer not to the physical size of the hole independent of focal length (as you're doing here), but rather to an aperture ratio like f/2.8, f/5.6, etc. (where, mathematically speaking, "f" is a variable that stands for any focal length). This is different than the convention that people use for, say, telescopes, where "aperture" refers to the actual diameter of the "hole."

So put conventionally, the maximum aperture (i.e., aperture ratio ) of that lens does change with focal length. (I.e., as you change the focal length, the ratio of the focal length to the maximum hole size at that focal length changes.)

As I've found, most photographers get very confused if you use the word "aperture" to mean the size of the physical hole, even if you qualify this as "physical aperture" or such. So, I expect that a substantial number of photographers will insist that you're wronger than you are.

 

[jertzeedon]

So put conventionally, the maximum aperture (i.e., aperture ratio ) of that lens does change with focal length. (I.e., as you change the focal length, the ratio of the focal length to the maximum hole size at that focal length changes.)

I knew I shouldn't have read this thread...lol

 

[Apollos]

So put conventionally, the maximum aperture (i.e., aperture ratio ) of that lens does change with focal length. (I.e., as you change the focal length, the ratio of the focal length to the maximum hole size at that focal length changes.)

I knew I shouldn't have read this thread...lol

right...aperture RATIO changes...so the relative size of the hole compared to focal length changes...but the physical diameter of the hole opening does not...even if that concept if completely immaterial to a photographer. right?

haha...I just understand things better when I know, at least to a certain degree, how they work.