A question about aperature.. I have a question that I'm hoping someone can explain to me. Why is it that some zoom lenses (IE an 18-200 VRII lens

thrasher Loc: Norwalk, CT

I have a question that I'm hoping someone can explain to me. Why is it that some zoom lenses (IE an 18-200 VRII lens - Nikon) has a variable aperature (f 4.0 -5.6) while others don't (IE a Sigma 70-200 f 2.8) why is there a difference????

Nikonian72 Loc: Chico CA

FAQ: Why does my aperture change from f/3.5 to f/5 when I zoom from wide-angle to telephoto?
http://www.uglyhedgehog.com/t-26509-1.html

ALL zoom lenses change aperture from shortest focal length to longest focal length. Some manufacturers do not advertize the change. Just think how big (& expensive) the glass & diaphragm (aperture) would be in a 200-mm f/2.8 lens, zoom or not.

thrasher Loc: Norwalk, CT

Ok. That much I kinda assumed. But what about lenses that don't have "variable aperature"???

CaptainC Loc: Colorado, south of Denver

Not sure what your mean by "what about" them. The constant aperture lenses are considerably more expensive since they have to be designed to allow the same amount of light to pass regardless of the focal length change. That is one reason you do not find zoom factors much more that 3X or so with the fixed aperture lenses. Making a 28-300 with a constant aperture would be really costly.

JR1 Loc: Tavistock, Devon, UK

Nikonian72 wrote:

FAQ: Why does my aperture change from f/3.5 to f/5 when I zoom from wide-angle to telephoto?
http://www.uglyhedgehog.com/t-26509-1.html

ALL zoom lenses change aperture from shortest focal length to longest focal length. Some manufacturers do not advertize the change. Just think how big (& expensive) the glass & diaphragm (aperture) would be in a 200-mm f/2.8 lens, zoom or not.

Not all zooms change ap that is why they are stated as f2.8 through the range, don't know where you go that from

http://www.dpreview.com/lensreviews/sigma_70-200_2p8_n15

http://www.beyondmegapixels.com/2011/04/photography-101-fixed-aperture-explained/

thrasher Loc: Norwalk, CT

Ok. Now I get it!

Nikonian72 Loc: Chico CA

JR1 wrote:

Not all zooms change ap that is why they are stated as f2.8 through the range, don't know where you go that from

I understand the theory, but have a different experience in field use. I will explore this a bit more.

Also, for a 200-mm focal length lens (zoom or not) to have an f/2.8 aperture, that aperture will have to measure 71-mm in diameter. That is nearly 3-inches across.

elie Loc: France

ALL zoom lenses change aperture from shortest focal length to longest focal length. change. .[/quote]
wrong-there are zoom lenses having one maximum aperture
througout the entire zoom range. I guees that requires more complicated enginearing,hence more expensive.

Julian Loc: Sarasota, FL

Nikonian72 wrote:

FAQ: Why does my aperture change from f/3.5 to f/5 when I zoom from wide-angle to telephoto?
http://www.uglyhedgehog.com/t-26509-1.html

ALL zoom lenses change aperture from shortest focal length to longest focal length. Some manufacturers do not advertize the change. Just think how big (& expensive) the glass & diaphragm (aperture) would be in a 200-mm f/2.8 lens, zoom or not.

I am not sure I understand your generalized statement about all zoom lenses changing aperture. My 70mm-200mm VR Nikon maintains a constant aperture through its entire zoom range; its front element, which defines its light gathering power measures approx 72mm, equivalent to focal length/aperture = 200/2.8. This is confirmed by setting the camera to aperture priority at f/2.8 and zooming thru the entire range without any changes in shutter speed. Did I misundrestand you?

EnglishBrenda Loc: Kent, England

Thanks JR. 'Beyond Megapixels' is another really helpful site to learn from. One thing that it did not explain is that when a lens is marked 3.5 - 5.6 but goes up to F32 it not marked 3.5 - 32. Is it because at 3.5 to 5.6 it is at its best?

Julian Loc: Sarasota, FL

Nikonian72 wrote:

JR1 wrote:

Not all zooms change ap that is why they are stated as f2.8 through the range, don't know where you go that from

I understand the theory, but have a different experience in field use. I will explore this a bit more.

Also, for a 200-mm focal length lens (zoom or not) to have an f/2.8 aperture, that aperture will have to measure 71-
mm in diameter. That is nearly 3-inches across.

Nothing unusual about a front element measuring almost 3" across. My 24mm-70mm f/2.8 ED measures 72mm and uses a 77mm filter. The 70mm-200mm Nikon VR, as stated previously also measures 72mm.

GHK Loc: The Vale of Eden

Nikonian72 wrote:

JR1 wrote:

Not all zooms change ap that is why they are stated as f2.8 through the range, don't know where you go that from

I understand the theory, but have a different experience in field use. I will explore this a bit more.

Also, for a 200-mm focal length lens (zoom or not) to have an f/2.8 aperture, that aperture will have to measure 71-mm in diameter. That is nearly 3-inches across.

I'm not sure that you've got this right. It's a difficult point, but true telephoto construction means that the optical centre (or nodal point) of the lens is in front of the first lens element, so you 71 mm is only effective at this point and it is necessary to postulate the existence of a virtual diaphragm in thin air. Such a diaphragm would not be constrained in diameter.
GHK

Pepper Loc: Planet Earth Country USA

Trudy wrote:

Thanks JR. 'Beyond Megapixels' is another really helpful site to learn from. One thing that it did not explain is that when a lens is marked 3.5 - 5.6 but goes up to F32 it not marked 3.5 - 32. Is it because at 3.5 to 5.6 it is at its best?

The 3.5-5.6 youre referring to is the maximum or smallest, depending on how you look at it, f stop or the largest aperture setting available at each end of the lenses focal length. So if the lens is say a 70-300mm zoom the smallest f stop or largest aperture available at 70mm is 3.5 and then 5.6 at 300mm.

jimmya Loc: Phoenix

thrasher wrote:

I have a question that I'm hoping someone can explain to me. Why is it that some zoom lenses (IE an 18-200 VRII lens - Nikon) has a variable aperature (f 4.0 -5.6) while others don't (IE a Sigma 70-200 f 2.8) why is there a difference????

I agree about some makers don't advertise the smallest aperture but the simple fact is, the longer the lens the less light can get in thus the higher f/stop.

elie Loc: France

Pepper wrote:

quote=Trudy Thanks JR. 'Beyond Megapixels' is ano... (show quote)

----------3.5 or5.6 that is the largest aperture opening,letting in more light,so it should'nt be referred to as" smallest".
f22or32 are the smallest opening. why it's not indicated-that's like you'll say:you can put in this container objects measuring up to 20 inch.-you don't have to add it will take in smaller ones!