Lenses with variable aperture

[KatiePurple]

Hi-
I tend to shoot in manual most of the time because I tend to have a particular objective when I'm taking photos. All of the lenses that I use have a fixed aperture (50mm f/1.4, 70-200mm f/2.8, 105mm f/something...I lent it to a friend) macro. Am I correct in my understanding that if a zoom lens has a variable aperture, I would have to reset the aperture every time I zoomed? That seems to make using a variable aperture lens impossible, at least for me, because I can't imagine getting so many things done between shots (probably important to note that I tend to shoot kids, pets, and sports, not things that stand still!). Or am I understanding this all wrong?

Thanks!

 

[zombiesniper]

I think you may be mistaking your lens markings. The lenses you’re describing are listing the maximum aperture not that they only have one or a fixed aperture.

Lenses that list two apertures are usually listing their max at the lowest and highest focal length.

So you could use you f2.8 at f8 if you wanted.


Sent from my iPhone using Tapatalk

 

[Soocom1]

first, read this:

What is a Constant Aperture Lens

Second:
If I am understanding what your saying, if your shooting totally manual with a manual lens on any camera (AF or not) then the aperture is a separate item. Variable aperture lenses are designed differently than fixed apertures and typically found on modern AF type cameras.

 

[ac12]

Fixed aperture like the 70-200/2.8, the aperture stays at f/2.8, if you set it there.

Variable aperture like an 18-140/3.5-5.6.

• The maximum aperture will change as you zoom. So at 18mm it would be f/3.5, but at 140mm it would be at f/5.6.
  
  • So it is variable between the ranges indicated f/3.5 to 5.6.
• BUT if you set the lens to f/5.6 (or higher), it will be constant at that aperture at any zoom position.

Note that this applies ONLY to the lenses where the aperture is controlled by the camera.
If you use a manual lens, where the aperture is on the lens, things are different.

If you are using the camera's meter to determine exposure (manual or automatic), the meter is compensating for the variable aperture as you zoom. So you just follow the camera's meter, and you should be fine.

 

[zulu42]

In response to "if a zoom lens has a variable aperture, I would have to reset the aperture every time I zoomed"

No, you don't have to reset the aperture every time you zoom.

If you have, for example, an 18-55mm f/3.5-5.6 lens, the maximum aperture at the wide 18mm end is f/3.5. the maximum aperture at the telephoto 55mm end is f/5.6.

If you're shooting at 18mm, wide open at f/3.5, then you zoom in to 55mm, the aperture value becomes f/5.6 - without any input from you

 

[Designer]

Or am I understanding this all wrong?

The term "fixed" is throwing you off. It is not fixed in the sense that it is always at one aperture, and never anything else. It just means that the maximum aperture for that lens does not change when you zoom.

Your 70-200 f/2.8 could, if you want it to, keep that aperture when you are zooming through the entire range. These lenses are more expensive than the "variable" zooms.

The term "variable" applies to the maximum aperture available at each position of the zoom ring.

Your camera will still set the aperture to what you select as you take the photograph. It cannot, however, exceed the maximum aperture for any particular lens, or whatever is the maximum for that zoom setting.

In practice, you will hardly notice any difference, as your camera sets the aperture just before you capture the image, and you don't even see it change.

The less expensive zoom lenses may cramp your style some, if you really wanted a wide-open aperture, and the lens simply cannot give it to you.

 

[petrochemist]

Although her wording isn't perfect I can't see why everyone thinks Katie has things so severely wrong. Her lenses all have a constant maximum aperture - Primes always do & the 70-200/2.8 can manage f/2.8 throughout. Her question makes good sense in this case. If she'd used the word constant instead of fixed it would have been clearer, but to me that's a fairly minor niggle.

I think all my lenses with aperture rings give the indicated aperture except when zoomed in further than that aperture is possible. So it's only if the lens is set to below the maximum f/no for the longest focal length that exposures might need to be re-set. FWIW all my many adapted zooms are variable max aperture types.
With TTL metering the actual f-stop isn't too important for the OP's typical subjects (DOF isn't critical, & the exposure is automatically corrected)

I do have several lenses that have a fixed aperture, these are all primes (mirror lenses, pin holes, or body cap lenses) with the exception of a zoom telescope that came with a camera adapter (effectively ranging from 1000mm f/16 to 4000mm f/64 - zooming on a camera isn't really worth it as diffraction looses any extra detail).
None of these have any form of aperture control and are very different from the sort of lenses she's talking about.

 

[Soocom1]

Ok. for those who are experienced enough, you already understand this.
for those unfamiliar, Ill give it a shot to explain.


A Fixed aperture is just that. Fixed. It doesn't move with the movement of the lens elements for focusing reasons. In lens construction, a fixed aperture is exactly that. At a fixed distance from the image area (image plain/ film plain) Thats what gives you the f number to begin with.

Zoom lenses with a variable aperture means that there is some movement of the aperture mechanism forward and backward when the lens elements move. Variable apertures have some advantages and some disadvantages. Mostly the extreme zoom end out.

The "F" number for those uninitiated is the size of the actual hole vs. the distance to the image plain.

Aperture - Wikipedia

So in a variable aperture lens, again, the actual mechanism of the aperture itself is physically moving. hence why F3.5-F5.6 or whatever it is.

 

[ac12]

Ok. for those who are experienced enough, you already understand this.
for those unfamiliar, Ill give it a shot to explain.


A Fixed aperture is just that. Fixed. It doesn't move with the movement of the lens elements for focusing reasons. In lens construction, a fixed aperture is exactly that. At a fixed distance from the image area (image plain/ film plain) Thats what gives you the f number to begin with.

Zoom lenses with a variable aperture means that there is some movement of the aperture mechanism forward and backward when the lens elements move. Variable apertures have some advantages and some disadvantages. Mostly the extreme zoom end out.

The "F" number for those uninitiated is the size of the actual hole vs. the distance to the image plain.

Aperture - Wikipedia

So in a variable aperture lens, again, the actual mechanism of the aperture itself is physically moving. hence why F3.5-F5.6 or whatever it is.

We have a difference in definition here.

• One definition (that I am familiar with) is constant aperture zoom, where the f-stop remains constant, as you zoom.
  • This would be the 70-200 f/2.8
• Another definition is fixed (size) aperture zoom, where the physical diameter of the aperture remains fixed as you zoom.
  • This would be the 70-300 f/4-5.6

I will be using the f-stop formula: FL / aperture in mm = f-stop
and the aperture size formula: FL / f-stop = aperture size in mm
And for convenience I am using easy to calculate numbers in the example below.

On a constant aperture zoom, the f-stop remains constant, as you zoom.
The size of the aperture will change as you zoom, to maintain that f-stop.
Following shows the aperture size (diameter in mm) change, with a constant f-stop of f/4.

A 200mm lens at f/4 has a 50mm aperture. 200 / 4 = 50mm
A 100mm lens at f/4 has a 25 mm aperture. 100 / 4 = 25mm
A 50mm lens at f/4 has a 12.5mm aperture. 50 / 4 = 12.5mm​

On a variable aperture, the f-stop changes as you zoom.
The size/diameter of the aperture does not change, so as you zoom, the f-stop will change. Hence, variable aperture.
This is done in part to reduce cost. The mfg does not have to design and put in a cam to adjust the aperture as you zoom the lens. Following shows the f-stop change with a constant size aperture of 25mm.

A 200mm lens with a 25mm aperture is f/8. 200 / 25 = f/8.
A 100mm lens with a 25mm aperture is f/4. 100 / 25= f/4
At 50mm the with a 25mm aperture is f/2. 50 / 25 = f/2
​

In practice when designing the lens, other things come into play to affect the actual aperture (f-stop).
example 70-300 f/4-5.6

At 70mm / f/4 = 17.5mm aperture, and at 300 / f/5.6 = 53.6mm aperture.
Because at 70mm a 53.6mm aperture would give you a f/1.3 aperture, which is clearly not the case.​

The interesting thing is.

• A constant aperture lens is actually "throwing away" lens speed at shorter focal lengths.
  If you look at the numbers above, at shorter focal lengths, the lens is reducing the size of the aperture, to maintain that f-stop. If you did not reduce the size of the aperture, the larger aperture size would give a faster f-stop.
• A variable aperture lens makes the use of all the light coming through the lens, and does not "throw away" any of that light.

 

[ac12]

A slightly different viewpoint may help to understand the two types of lenses, metering.

A constant aperture lens maintains the set aperture whatever the focal length.
This was CRITICAL in the days of using an external handheld meter.
If your meter said f/5.6, you set the lens to f/5.6.
If the aperture varied by focal length, then it would be f/5.6 at only ONE focal length, and incorrect at all other focal lengths, and the exposure would be incorrect. And on slide film with very little exposure tolerance, this was important.

But with TTL (Through The Lens) metering, the camera's light meter automatically adjusted for any changes in the aperture/light transmission through the lens when you zoomed, so you just match the needle to the notch or run it on auto. The need to absolutely know the f/stop at a particular focal length disappeared. And variable aperture lenses became practical.

 

[Dacaur]

Wow you all got something totally different from that op than I did....
So here is my answer.
On a variable aperature lense, the maximum aperature gets smaller (higher number) as you zoom in, so as an example, canons 18-55 is stm lense, has a 3.5-5.6 aperature. It's only 3.5 at 18-23mm, then it moves to f4, then at 32mm it moves to f4.5, then to f5 at 37mm, and at 47mm it goes to f5.6

So first, to answer your question, no, you don't need to reset your aperature as you zoom, because your camera will automatically change your aperature as you zoom if you zoom to a level that doesn't allow the aperature you set, so if you set your fstop below whatever that lenses "upper" maximum aperature is, f5.6 in my example, you would need to adjust your shutter speed as you zoom in and out.
Basically, if have your aperature set at f3.5 at 18mm, then zoom to 40mm, your aperature will automatically change to f5, so you would need to adjust your shutter speed to get the same exposure. Then you zoom back to 33mm, your camera remembers you manually set your aperature to 3.5, so the aperature will automatically go down to f4, (which is as close as it can get to what you set at that zoom level), so you need to adjust your shutter speed again.

On the other hand, if you set it to f5.6, it will stay at f5.6 no matter what you zoom to. If you set it to f4, then zoom out to 18mm, it will stay at f4.

Basically, when using a variable aperature zoom when you don't have time to adjust your shutter speed all the time, either set your aperature to the "upper" maximum aperature or higher, or use aperature priority. If that's not acceptable, get a lense with a fixed aperature....

 

[wfooshee]

No way of knowing if the OP has returned, but to get away from the technical definitions and just answer the question, all lenses are marked with their maximum aperture, the highest amount of light they can pass. A zoom lens that's marked with an aperture range has a maximum aperture that's different at each end of the zoom range. That's done mainly because it's easier to build, thus less expensive.

If your aperture setting is is a higher number than the stated number, then the aperture does not change as you zoom. Example, using a 70-300 4.5-5.6 zoom: if I'm using f:8, it doesn't matter what my zoom setting is, the aperture is f:8. But if I set the lens wide open at 70mm, which would be f:4.5, once I zoom, the aperture changes, approaching f:5.6, the wide-open setting for 300mm. If I'm auto-exposing, the camera compensates. If I want f:4.5, I have to stay at 70mm, that aperture is not available at longer focal lengths.

If your zoom lens only has one aperture marking, like the 70-200 2.8, then f:2.8 is available throughout the zoom range. You'll find that zoom lenses with the maximum aperture available at all focal lengths are generally more expensive, sometime significantly more expensive, partly because of the constant maximum aperture, and partly because such lenses are generally faster anyway, which is more expensive. For zooms with variable aperture, the difference in aperture from minimum focal length to maximum is generally one stop or less.

 

[Soocom1]

No you are correct.
Where I blew it was differentiating the diff. between Prime and Zoom.
Remember also that Telephoto in the most basic sens is a foreshortened prime.
(Think 1000mm here)

So yes I aquiest to blowing it on that def.

But i would also point out that in a 'genuine' telephoto (old definition) is a lens who's focal distance is longer than the lens itself.
So the aperture is still fixed.

my bad.

Zoom lens regardless has a diaphragm that does in fact move with the elements.

 

[ac12]

I think the term "fixed" aperture is more appropriate to primes, and specifically a prime with a fixed (non-adjustable) aperture, like a mirror lens or a telescope.

But these words are subject to interpretation, and that varies by country and person's background.

As for @Soocom1 , part of the confusion is based on one's background, and where one would look.
I look at the f/stop, an engineer type would look at the aperture mechanism itself.
So both interpretations are correct, but from different points of view.
And in my own life, I will flip flop to different points of view for many things.

 

[Soocom1]

Well remember that until the screw type zoom lens came out, it was a push-pull type zoom. They were an exotic item back in the day and the concept of a variable aperture had not yet been formed.
It was when AF type cameras came out that it changed.

That lends to the original OP point on manual lenses like the Rokinon, Samyang, and others that do not have the AF motors in them. In many aspects the lens structure remains like the older primes and the aperture is physically situated at one and only one point.