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constant aperture through zoom range
I am looking at purchasing a new Tamron high end zoom lens which offers a constant aperture of f/2.8 right through the zoom range (70 - 200).Could somebody please explain this function please...

Comments (14)

F-stop is the ratio of aperture to focal length. If the focal length changes (zooms), but the aperture stays the same size, then the f-stop changes. This is the case for most low to mid-priced zooms. Its simply cheaper and easier to produce. Youll usually lose 1.5 to two stops (or more) over the zoom range of the lens. In practice this means youll have to use slower shutter speeds as you zoom to the long end of the range..

When you get to the high-end you get constant-aperture zooms that increase or decrease the aperture opening as the lens zooms so that the f-stop stays the same over the zoom range. This is a highly desirable feature if youre shooting sports or wildlife and need high shutter speeds at long focal lengths! This is also expensive and takes a lot of room. Constant aperture lenses are usually huge and heave compared to their non-constant cousins..

Note that while lens manufacturers call these behemoths constant-aperture lenses they are, in fact, constant F-stop lenses. The aperture changes as the lens is zoomed to maintain the same ratio (F-stop) as the lens is zoomed..

I have a Nikon 70-200 2.8, and I love it! Shooting wide open at 200mm is great when shooting birds and such..

Gro..

Comment #1

This feature is mostly found on more expensive lenses. Basically, it allows you to set the f-stop to a single value and have it stay there throughout the zoom range..

On a 5.6 - 6.3 range lens, it might be 5.6 at the wide end but 6.3 at the zoom end..

It is a nice feature to have, but you pay for it!My Blog: http://secretsofphotography.blogspot.com/..

Comment #2

Elwood wrote:.

This feature is mostly found on more expensive lenses. Basically, itallows you to set the f-stop to a single value and have it stay therethroughout the zoom range..

On a 5.6 - 6.3 range lens, it might be 5.6 at the wide end but 6.3 atthe zoom end..

It is a nice feature to have, but you pay for it!My Blog: http://secretsofphotography.blogspot.com/.

And they weigh a lot more..

They also usually perform better at larger apertures. For instance, let's say you have enough light for f/5.6. Your cheap f/5.6 lens is probably pretty soft at 5.6. The expensive f/2.8 lens will probably be very sharp at f/5.6 or even f/4...

Comment #3

Jetjet wrote:.

I am looking at purchasing a new Tamron high end zoom lens whichoffers a constant aperture of f/2.8 right through the zoom range (70- 200).Could somebody please explain this function please..

With respect I think you have it the wrong way round (or perhaps it is just the way you typed your question). It would be logical to identify a need and then ask what lens you need to solve it or equally to ask why the lens is much more expensive and what facilities it has to justify the price..

For outdoor daylight shooting in all but poor light or when photographing high speed action a 70-200 f4-f5.6 lens will be quite OK, will cost a lot less and will be a lot less heavy to carry around. You can bump up the ISO to 1600 (or beyond) to get occasional poor light shots (I see you have a D300).

If you are photographing indoor sports or theatre then even an f/2.8 lens may not be sufficiently fast and you may have to consider a fast prime like the 85mm f/1.8..

I am not sure why you would want the 70-200 in addition to the 80-400 that I can see from another thread that you already have without a specific use in mind..

You might be better off with something much lighter than the 80-400. I wonder whether a Sigma 50-150 f/2.8 might have any use being significantly lighter? But it will depend on your intended use..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #4

Simply- You have a maximum of f/2.8 through the entire zoom. Variable zooms reduce max f stop on the long end requiring longer shutter or higher ISO...

Comment #5

Jetjet wrote:.

I am looking at purchasing a new Tamron high end zoom lens whichoffers a constant aperture of f/2.8 right through the zoom range (70- 200).Could somebody please explain this function please..

Constant f-ratio (or f/stop) of 2.8 means the aperture at any given focal length (any part of the zoom range) can be as wide as f/2.8, where f is the focal length being used. The aperture (diameter of entrance pupil in the lens) isn't constant, but the f-ratio (or f-stop) is, which means you get constant light per unit area of the sensor, so metering isn't changed..

With a lens that's say f/4-5.6, it means that the widest aperture at the short end of the zoom range is f/4 and the widest aperture at the long end of the zoom range (and a bit back from there) is f/5.6, which needs more light for the same brightness scene..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #6

Re my current needs for lenses, I do have the 80-400 mm lens and D300 body which I use for my newly found love for taking bird shots .I am very pleased with some of the results so far but I am very aware of the fact that even though this camera yields good results with ISO set at say 800 you have to have good daylight,especially with flight shots.I used this lens with a D80 body and did get some good results at airshows but had strong sunshine lighting.I therefore am looking for a suitable lens where I can autofocus quickly when the conditions are not particularly bright and still have a reasonably good 'reach'...

Comment #7

I'd go with something with f4 myself. 70-200 f2.8 zooms can be really heavy, like 3 or more pounds. Then you'd probably want IS or a tripod, either one adding to the price tag...

Comment #8

Unfortunately, a 70-200 doesn't have the reach for birding or airshows. Few zooms that reaches 300 or 400mm are much faster than what you have..

One way to get a faster 300 or 400mm is with a fixed focal length lens like the 300mm f/2.8, maybe with a 1.4 teleconverter to get 420mm f/4. Nikon's is $4500..

Another is Nikon's 200-400 f/4. $5000..

Sigma has some options, but those are still $3000+..

Jetjet wrote:.

Re my current needs for lenses, I do have the 80-400 mm lens and D300body which I use for my newly found love for taking bird shots .I amvery pleased with some of the results so far but I am very aware ofthe fact that even though this camera yields good results with ISOset at say 800 you have to have good daylight,especially with flightshots.I used this lens with a D80 body and did get some good resultsat airshows but had strong sunshine lighting.I therefore am lookingfor a suitable lens where I can autofocus quickly when the conditionsare not particularly bright and still have a reasonably good 'reach'...

Comment #9

I agree that a 70-200 will not have the reach for birding or airshows. You may want to consider combining an f/2.8 70-200 with a 1.4 x tele convertor. That would give you a 98-280mm f/4 and greater flexibility..

I rather think that you are better served by just accepting the penalty of bumping up the ISO a notch and using your existing equipment..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #10

Thanks for all your advice guys,it would be nice if one could take lenses out for trial runs,regards to all..

Comment #11

Note that while lens manufacturers call these behemothsconstant-aperture lenses they are, in fact, constant F-stop lenses..

Not really..

The aperture changes as the lens is zoomed to maintain the same ratio(F-stop) as the lens is zoomed..

In fact, in the typical design of a constant aperture lens, the lens aperture does not change at all. The classic constant aperture design lens is made so that all the optical changes of zooming and focusing happen in front of the lens iris or aperture. With this approach, the exit pupil never changes size or location, and from the viewpoint of the film, there truly is a constant aperture - hence the name. However, from the viewpoint of the subject (in front of the lens), the entrance pupil appears to change size and/or location - even though the actual physical aperture doesn't change at all..

Jay Turbervillehttp://www.jayandwanda.com..

Comment #12

Conatant f/stop means exactly what it says. You get the same f/stop at wide angle max as in max tele setting. Lens with two numbers have the larger for WA with the smaller number for TEL...

Comment #13

Jetjet wrote:.

Thanks for all your advice guys,it would be nice if one could takelenses out for trial runs,regards to all.

You can do that. Most larger cities have a shop offering rentals of pro gear, and there's mail services as well...

Comment #14

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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