Constant aperture lens....what f stop on camera??

[ooooops0]

Hi,
due to my new work I have access to some pretty cool new photo gear including some great lens's. One I was playing with is the canon ef 70-200mm f2.8 l is usm. On a 5D mk3.
My question is this.....if the lens is fixed at f2.8, what effect will changing the F stop on camera have? trying to get my head around it. As my tests shots were at F8 and look great.

thanks in advance.

[bitsnpieces]

Here's a small article I wrote more recently about apertures and stuff:
http://d3studio.com.au/photography/4...owall=&start=2

But generally speaking, by using a larger aperture (smaller number f2.8) and smaller aperture (larger number - f8), what you're doing is changing the amount of depth that will be in focus on the Z Plane (the close to far and vice versa plane).

So, large aperture, great blurry backgrounds (too large and you may miss what you're trying to focus on if you don't get the right spot), and smaller aperture, more things in focus (too much and it'll all just look bland).

[ameerat42]

Preface
Sounds like a bit of confusion of terms....

The "correct, usual" term is "Fixed f-stop" lens.

F-stop means
(Focal length of lens)/(Aperture)

Aperture means "opening of lens" in (usually) millimetres.

Now you're talking about a 70-200mm zoom lens - that is, one which changes its focal length AND which has a CONSTANT f-stop.

If you -
set your lens at f=70mm and f/2.8, its aperture will be 25 mm.

If you then (see a distant bird and) zoom out to f=200mm but do not do anything else, then the (f-stop of) f/2.8 remains CONSTANT, but
the aperture of the lens changes according to:
Aperture = (focal length) / (f-stop)
ie,
Aperture = 200/2.8
which is approximately 71.4 mm.

In other words, the lens aperture diaphragm "opens up" to the new aperture to accommodate the "constant f-stop".

I hope this is not too unclear. If so, pls ask for clarification.

Your question

...if the lens is fixed at f2.8, what effect will changing the F stop on camera have?...

Answer:
The f-stop will remain set to whatever you change it to while the actual aperture changes to suit the focal length you use.

(I'd better stop!)

Am.

[ooooops0]

OK, sorry for confusion here. I know the effects of aperture etc, but what I am trying to work out is; the lens has fixed F stop of 2.8, BUT the camera can still be set at a variety of F stops, so will you still get different depth of field? based on the camera settings?

[ameerat42]

Well, can you use a load of hot air?

You're saying that there is NO aperture adjustment for that lens?

If that's true, then changing the f-stop in camera will have NO effect on the lens, but I suspect it may trick the camera into giving it more or less exposure time/ISO setting.
But that's if you have it set OFF Manual. On Manual, I would suspect NOTHING would happen.

What's the lens name/ID?

[bitsnpieces]

I don't quite understand what you mean - the constant f/2.8 just means no matter what setting or focal length you use, it has the capability to still run at f/2.8.

And by changing the aperture, it'll affect the depth of field, no doubt.

So when you say change the camera settings? Are you saying, you want to affect the depth of field, without changing the aperture? The only thing that will affect that is a combination of your physical distance to the subject, and focal length.

If that's not what you mean, then i'm at a lost here. Sorry.

[ooooops0]

Well, can you use a load of hot air?

You're saying that there is NO aperture adjustment for that lens?

If that's true, then changing the f-stop in camera will have NO effect on the lens, but I suspect it may trick the camera into giving it more or less exposure time/ISO setting.
But that's if you have it set OFF Manual. On Manual, I would suspect NOTHING would happen.

What's the lens name/ID?

the lens is in my post; canon ef 70-200mm f2.8 l is usm

- - - Updated - - -

I don't quite understand what you mean - the constant f/2.8 just means no matter what setting or focal length you use, it has the capability to still run at f/2.8.

And by changing the aperture, it'll affect the depth of field, no doubt.

So when you say change the camera settings? Are you saying, you want to affect the depth of field, without changing the aperture? The only thing that will affect that is a combination of your physical distance to the subject, and focal length.

If that's not what you mean, then i'm at a lost here. Sorry.

yeah I'm just confusing myself I think. I use it in full manual mode, but i'm used to variable lens's that change with zoom. These change the F stop on the camera body. But with this lens I had it set at F8 on camera body, but the lens is fixed at 2.8.

- - - Updated - - -

OK think I get it here.
I just found it confusing that they say its fixed 2.8;;;;;;;;I thought it stayed at 2.8 constantly BUT according to this explanation it can be set at anything?

off another site;
Of course,
you can set the aperture at anything from f4 to f22 (I believe) and have it
stay there. A wide aperture lessens your depth of field, so that
backgrounds can be pleasantly out of focus, diminishing distracting details
behind your subject. So, the 24-70mm f2.8L can, but doesn't have to, stay
at a maximum aperture of f2.8 throughout its zoom range, but the 28-135
f3.5-5.6 IS will be at a maximum of f3.5 at 28mm, f4 by 50mm, f4.5 at 70mm,
and f5.6 at 100mm. So you can keep the background more out of focus at the
long end with the 24-70 than you can the 28-135.
I know it's confusing, I have a "fixed aperture" 400mm (old Canon) FD mount
Spiratone "Baseball Bat" telephoto that is permanently set at f8.
By the way, small sensors like in the S40 give the lenses a greater depth of
field, so apertures don't make as much difference as they do with the larger
sensors of DSLRs, or so it seems.

if only they used the word MAXIMUM 2.8
quote;
For a lens that is (for example) a 24-70mm 2.8 L, the "2.8" simply indicates
that the maximum aperture of 2.8 is available throughout the zoom
range...which is highly preferable to most consumer zoom lenses that end up
giving you a reduced max aperture as you zoom toward the tele end.

[swifty]

I think you answered your own question with your last post.
The constant aperture refers to the ability to use a constant maximum aperture throughout the zoom range.
It doesn't limit you to only use the max aperture, f2.8 in your example. You can set any aperture you desire up to a maximum of f2.8 for the entire zoom range.

[ameerat42]

Good! I must say, though, NOWHERE in any of the sites Canon/DPR/Etc does it spell out unequivocally that this is a CONSTANT F-stop lens.
They use lots of other terms - like "Brindled cat", "mongrel dog", etc. - And don't worry, somewhere I saw "constant f-stop" uke: - the worst of the lot!

So, I'd say my first reply covers it. This is just icing on the cake

- - - Updated - - -

...
The constant aperture refers to the ability to use a constant maximum aperture throughout the zoom range...

Oh, not ewe two, Swifty-y-y!

[swifty]

Oh, not ewe two, Swifty-y-y!

LOL. Whether the terminology is misleading or not, colloquially the term constant zooms are often used to describe the fixed maximum aperture.
So I'll rephrase then. There is a fixed maximum aperture value that remains the same throughout the zoom range. It is not constantly that value, you set your aperture to the desired value up to maximum fixed value.

[ricktas]

the fstop noted on any lens is the largest fstop (largest aperture - smallest number) that the lens can do.

So 70-200 f3.5-f5.4 means that at 70mm the largest is f3.5 and at 200 it is f5.4,
so 70-200 f2.8 means that it can be f2.8 across the entire zoom range.

Now remember these are the MAXIMUMS, it doesnt stop you using a smaller aperture (larger fstop) at all, it just means these are the maximums you can use.

So I am confused about where you found the wording FIXED f2.8 reference in relation to the Canon lens?

[ooooops0]

I used term constant in title but some threads and on this thread someone called it FIXED in their replies. ??

the fstop noted on any lens is the largest fstop (largest aperture - smallest number) that the lens can do.

So 70-200 f3.5-f5.4 means that at 70mm the largest is f3.5 and at 200 it is f5.4,
so 70-200 f2.8 means that it can be f2.8 across the entire zoom range.

Now remember these are the MAXIMUMS, it doesnt stop you using a smaller aperture (larger fstop) at all, it just means these are the maximums you can use.

So I am confused about where you found the wording FIXED f2.8 reference in relation to the Canon lens?

[ameerat42]

Yep. Usually referred to as "constant". I'm afraid I used "fixed" instead, which is somewhat less used
from what I have seen.

Main thing, though, is to apply either one to "f-stop", not "aperture". This is particularly so to help less experienced members get the
terminology right (or at least as far as possible) to help build ideas.

Am.

[fillum]

"Fixed" aperture is generally used for those lenses that don't have any way of changing the aperture - mirror (reflex) lenses for example. (From memory, some of those cheap 'super teles' that appear on ebay also have fixed apertures).

Something else to note is that maximum aperture and focal-length are (by convention) stated with focus at infinity. These values can change as the point of focus comes closer to the camera. For example some macro prime lenses rated with a max aperture of f/2.8 might actually have a max aperture of around f/4 at 1:1 magnification. For most users this rarely presents a problem in practice.



Cheers.