Lazy J Loc: Pacific Northwest

I have been looking at some 24-70mm f/2.8 lenses (Canon/Sigma) and am confused.

Specifications indicate "Aperture Range: f/2.8 to f/22". However, descriptive comments state "a constant f/2.8 maximum aperture affords consistent performance throughout the zoom range as well as greater control over depth of field". These seem contradictory to me?

What am I missing or not understanding? I believe it is referring to the end of the lens staying in a fixed position as opposed to extending in and out. But I am not understanding the difference.

Can someone enlighten me? Thx in advance!

RWR Loc: La Mesa, CA

The set aperture remains constant throughout the zoom range.

blackest Loc: Ireland

f2.8 is the maximum aperture available and f22 the minimum. Many zoom lens offer a larger maximum aperture at the shorter focal lengths than the longer perhaps f4:f5.6 a stop less light at the longest focal length. Better engineered zooms can keep the aperture constant and the exposure constant.
through the zoom range.

rwilson1942 Loc: Houston, TX

"I believe it is referring to the end of the lens staying in a fixed position as opposed to extending in and out."
Nope, has nothing to do with lens extension. The f2.8 to f22 part refers to the aperture range, from a maximum (largest opening) of f2.8 to an minimum (smallest opening) of f22.
The "constant f/2.8 maximum aperture" just means that the aperture does not change as you zoom.
Some zooms list a maximum aperture of something like f3.5-f5.6.
On those lenses the maximum aperture changes as you zoom.

tinplater Loc: Scottsdale, AZ

In simplest terms it means you can shoot at f2.8 at 70mm.

Mac Loc: Pittsburgh, Philadelphia now Hernando Co. Fl.

You can use f/2.8 at all focal lengths.

Charles 46277 Loc: Fulton County, KY

It sounds to me as if you are right to be confused. If the widest aperture is f2.8 at a long zoom setting, that same size hole would be a much bigger f-stop when set at a short setting. Perhaps they have engineered it so that when the lens is short, the aperture will not open as big as it does when the lens is long--but why do that (and lose the benefit of faster apertures)?

Sure, you would not have to worry about changing f-stops when you zoom, but who would not rather have an f2 or f1.4 lens at the shorter settings, if it is f2.8 at the longer settings?

Or, perhaps the lens f-stop dial changes the size of the hole to keep constant f-stops when using aperture priority and smaller stops. For instance, if you set the aperture at f16 on aperture priority, this f-stop could be kept constant by design, at any zoom setting.

But the fact remains that a given size hole (the largest one on the blades) will be different f-stops at different zoom lengths. This cannot be engineered away.

CHG_CANON Loc: the Windy City

The f/2.8 lenses maintain f/2.8 at all focal lengths, if desired by the photographer, when zooming between 24mm and 70mm. I can't speak for the Sony, but the Canon model physically extends when zooming. The v II of this lens also provides a lock so the lens does not extend when carried vertically. The lock cannot be used except when the lens if fully retracted at 24mm. The purpose is for transit, not photography.

martinfisherphoto Loc: Lake Placid Florida

If you are comparing the pricing as well you will notice the ones that can hold a constant aperture of F/2.8 thru out the zoom range are sometimes twice as expensive, not to mention larger and heavier. Make sure your purchasing the correct one if you go thru with a purchase..

jerryc41 Loc: Catskill Mts of NY

Photographic terms are confusing, especially when you get into numbers. By "constant," they mean that if you are using f/2.8, the lens will stay at f/2.8 as you zoom to 70mm. Less expensive lens reduce the aperture as they zoom. So "constant" doesn't mean it's always f/2.8.

Be sure to check out the new Tamron 24-70mm. A reviewer used the word "great" five times.

https://digital-photography-school.com/field-test-thoughts-tamron-24-70mm-f2-8-di-vc-usd-g2/

Screamin Scott Loc: Marshfield Wi, Baltimore Md, now Dallas Ga

It's saying it's a fixed maximum aperture as opposed to a variable aperture. In other words, the aperture opening remains constant at whatever you set it at instead of changing as you zoom out.

Notorious T.O.D. Loc: Harrisburg, North Carolina

True, the Canon 24-70 is external zoom but the Canon 70-200 f/2.8 is internal zoom and the lens length remains constant. The aperture is a maximum of f/2.8 throughout the entire zoom range for either lens. F/2.8 to f/22 is the range of the aperture to close down in the 24-70 lens, largest aperture to smallest aperture. Lenses that are f/4.5-f/5.6 are lenses that have a wider aperture at the shorter end of the zoom range and a lesser maximum aperture at the longer end of the zoom range. At some point in the zooming the aperture becomes smaller or larger in maximum size depending on which way the lens s being zoomed. Internal vs external zoom and constant vs variable aperture are both design considerations and impact the size, weight and cost of the lens design.

Best,
Todd Ferguson

Charles 46277 Loc: Fulton County, KY

"By "constant," they mean that if you are using f/2.8, the lens will stay at f/2.8 as you zoom to 70mm."

Jerry, that is clearly the intended meaning--but how is that possible? For numerical simplicity, suppose a lens has maximum aperture of f4. That means at 100mm the zoom has an aperture of 25mm across, yes? Then you zoom to 50mm, and the same f4 must be half that size, or 12.5mm across. This can be engineered. But the puzzle is that if it opens to 25mm across at 100mm in the first place, why can't you use that opening at the 50mm zoom and have an f2 maximum aperture lens instead of f4? If I am right, for any zoom the size of the opening (at any f=stop) will be different at 50mm and 100mm zoom. That cannot be changed. If it can open wide at f4 at 100mm, then it ought to be able to open wide at f2 for 50mm--that would have the same sized hole. Sure, it could stay at f4 for either setting, but it could open to f2 only at 50mm because that is the size hole it is. It still seems to me that it could keep constant apertures through zooming only at higher f-stop numbers, with a mathematical certainty.

Note--when a system has shutter preferred automatic, and if it is set at the largest aperture for short focal length, it cannot open wide enough for the same f-stop at long focal length--and you would get underexposed pictures. The camera will tell you this--mine will say it is one or two stops under, when I zoom out. With aperture preferred setting, the camera can give correct aperture settings depending on the zoom setting, and they will not be constant. My camera will not let me set a larger aperture for a given focal length than the lens can set. The aperture setting dial will only go to the maximum aperture setting possible, and it does nothing if you keep turning. If I zoom to 300mm, I cannot set aperture at 2.8--the camera dial will only go to f4.5 in that case. It knows what it is doing, even if I don't. Of course, if I set aperture at f4.5, it will be constant at that aperture for all zoom settings--it can do that.

jerryc41 Loc: Catskill Mts of NY

See? I told you that things get complicated when you start talking numbers.

Charles 46277 Loc: Fulton County, KY

I failed college math and they let me take Greek instead. That is why everything I say sounds like Greek to everybody.