A question about aperture changes along a zoom lens

[shahmatt]

Does the maximum aperture of a zoom lens change linearly with range?

For example, in a 28-300 lens of max aperture f/2.8 to f/4.9. Does that mean that at a range of 100mm, the maximum aperture is f/3.36 (if you calculate linearly)

[Graystar]

Nah, usually the bulk of the variable max. aperture shifts take place in the first third to half of the zoom len's focal length.

I followed your procedure on my A710 P&S camera and this is what I got. The increases in f/stop seem fairly linear and in step with changes to focal length. I guess P&S lenses and DSLR lenses scale differently?
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[DonSchap]

I'll be honest here, I do not have a 28-300mm f/3.5-6.3 because I use an 18-200(250)mm f/3.5-6.3 instead, due to its use on a APS-C sensor camera.

The 28-300mm is better used on a full-frame or 35mm film camera.

It is my understanding that they operate nearly identically when it comes to this aperture/focal length relationship and it is definitely NOT linear (except from 100-300mm, where it is flat).

Please see graphs below. :cool:

[shahmatt]

I lent my S6500FD to a friend for a few days so I can't really check right now. From what I remember I think the relationship on my zoom lens was closer to linear. But I just thought it would be worthwhile posting to find out if it was the case for all zoom lenses.

I'm sure consumers assume that aperture/range relationships are linear throughout a zoom lens. Do companies publish the information anywhere? I know they don't for consumer cameras, but how about DSLR lenses?

[fabi]

For our Fuji it looks like this:

28 mm - f/2.8+
31 mm - f/2.9+
32 mm - f/3.0+
34 mm - f/3.1+
39 mm - f/3.2+
42 mm - f/3.3+
45 mm - f/3.4+
53 mm - f/3.5+
58 mm - f/3.6+

69 mm - f/3.7+
97 mm - f/3.8+
130 mm - f/3.9+
147 mm - f/4.0+
177 mm - f/4.1+
192 mm - f/4.2+
206 mm - f/4.3+
220 mm - f/4.4+
234 mm - f/4.5+
247 mm - f/4.6+
260 mm - f/4.7+
270 mm - f/4.8+
286 mm - f/4.9+

[shahmatt]

Thanks fabi. Here are your numbers in graph form. Looks like a steep increase in f value from 28mm to 60mm, and then a sudden drop in rate. The rate is nearly linear from 180 to 300. I guess the zoom lens for the S6500 operates with two mechanisms, one for the 28 to around 60mm range, and another from 60 to 300mm. This graph still looks better than Donshap's attachment earlier in this thread. Not too bad at all!

Can users of other ultrazoom cameras such as the Canon S3, Panasonic FZ8, Sony H5 etc also post their findings? It would be interesting to compare.

[shahmatt]

Actually Donshap, I didn't want to isolate the discussion to a DSLR or P&S lens. I am grateful for any information. And I guess I just wanted to know if Aperture/Focal length variations were considered by consumers before buying a lens.

I'm a bit surprised that your graph is for a DSLR lens though. I mean we pay more for these lenses right? The P&S lens graphs for my S6500FD and the A710IS posted by Graystar show better apertures throughout the range than the example DSLR lens variation you have posted. How can that be, since these lenses are more expensive? What model DSLR lens is this graph for btw?

[shahmatt]

Isn't the curve a bit dissapointing then? I wonder what the curves for Sigma lenses look like.

[DonSchap]

Okay, mounted on a Nikon D80 ... which provides an even more detailed Aperture reading ... I just hammered out this curve based on the results taken from a TAMRON AF28-300mm f/3.5-6.3 XR Di LD Aspherical (IF) lens, which was fresh out of the box and mounted on a brand new camera body.

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After plotting, as I suspected, it revealed the apparent aperture curve of what I had found on an 18-200mm pretty closely. 50% of the focal length changes and then it settles at f/6.3 for the rest of the ride to 300mm.

Here is the TAMRON AF18-200mm f/3.5-6.3 XR Di-II LD Aspherical (IF)

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And finally, an older TAMRON AF28-200mm f/3.8-5.6 Aspherical

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These plots are hardly linear ... and you kind of wish they were, but it's not going to happen with these DSLR lenses, due to the natural effect that happens when their changing the actual lens length during zoom. P&S lenses are much shorter and don't suffer from the same type of dramatic extension issues.

This is a manageable effect with experience and, in some cases, if your camera body has some type of image stability feature. The reasoning is that because you can obtain a longer exposure time from the camera, due to reduced camera-shake, you can effectively extended the usability of these darker f/6.3 shots. I've personally experienced this with the SONY A100 ... which, unlike cameras such as the Canon or Nikon DSLRs, it's "Super SteadyShot" feature offers an f-stop of two of light compensation, when handheld.

[coldrain]

The graph just represents Don plotting points on a graph just by guess work.
I mean, an f6.3 lens at f4.9?


There is NO difference between DSLR lenses and compact digital lenses.

Thing is.... with lenses with a variable f-number, the aperture actually stays the same thoughout the zoom range. The aperture is the size of the "hole" through wich the light goes. What changes is the f-value.

So, why does the f-value change with a changing focal length?

f2.8 is acually f 1/2.8
f = aperture size / focal length

So, when the focal length increases, and the aperture size stays the same, the f-value becomes smaller. So... f= 1/3.5 can turn into f= 1/6.3, just by increasing focal length.

And it depends on how you plot the focal length range, on whether the f-values are going to be linear.

More expensive lenses can actually change the aperture while zooming, to keep the f-vaue the same (like: a constant f2.8).
This will mean that even when you zoom, the amount of light reaching the sensor/film will stay constant.

So... to recap:
Lenses with a variable f-value have a constant aperture.
Lenses with a constant f-value have a variable aperture.

[Rhys]

Isn't the curve a bit dissapointing then? I wonder what the curves for Sigma lenses look like.

I should imagine all zoom lenses with varying apertures are a bit like that. It's interesting to see it all plotted though. I must attempt a plot with my 17-85, 28-75, 18-55, 70-300 and 17-35 when I feel the urge.

[DonSchap]

I should imagine all zoom lenses with varying apertures are a bit like that. It's interesting to see it all plotted though. I must attempt a plot with my 17-85, 28-75, 18-55, 70-300 and 17-35 when I feel the urge.

Knowledge is truly power in your photography, Rhys. Happy charting. :D

[Razr]

Panasonic DMC-FZ20 - 6-72mm, f/2.8 throughout the zoom range (36-432mm in 35mm)
How is that "variable aperture" lens?

OLYMPUS (4/3rds) 35-100 f/2 zoom lens (70-200mm in 35mm): same question?

[coldrain]

Panasonic DMC-FZ20 - 6-72mm, f/2.8 throughout the zoom range (36-432mm in 35mm)
How is that "variable aperture" lens?

OLYMPUS (4/3rds) 35-100 f/2 zoom lens (70-200mm in 35mm): same question?
This thread never was about those lenses, now was it?

And anyway... those two lenses ARE variable aperture lenses. They just have a fixed f-value!

And the only way to get a fixed f-value throughout the zoom range is to have a variable aperture in the lens.
The aperture is the hole through which the light passes, and that gets varied in size when you zoom. The word aperture gets used for f-values, but it actually is not the same.

[shahmatt]

Thanks coldrain. This is very enlightening stuff. My thread title is a bit misleading I guess. What I'm more interested in is the f value vs focal length, rather than aperture. Oh well, too late to change it now.

[DonSchap]

Another popular lens is the SONY DT 18-70 f/3.5-5.6

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This one has the steepest slope I have run into, yet. At 34mm, you are at f/5.6 Maximum aperture. A mere 16 mm shift from f/3.5-5.6.

Even a $150 70-300 is brighter than that. It would actually be an improvement.

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Of course, with two seperate charts, it is kind of hard to determine that, but combined ... it becomes quite clear.

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Now, this is informational ... but it is concerning lens in two different classes ... normal and telephoto. You probably should have both, anyway.

Let's take it one step further and see what happens when you overlay the 18-200mm vs the 18-70mm ... for a general appreciation of just how much more light the 18-200 allows for a longer focal length.

Well, isn't this chart revealing? It would appear that the TAMRON AF18-200 f/3.5-6.3 XR Di-II LD Aspherical (IF) is a "brighter lens" and gives you extended range. It does not have as nearly as steep a slope as the 18-70mmm does and you'll shoot brighter images with it:

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Remember: Aperture is not a gradual change in light, it is in definitive steps, so I have taken the liberty to show it as such. This is actually how the camera sees these lenses.

Through the center of each plot, there is a trend line that shows the actual slope. The 18-70mm is, obviously, much steeper, which translates to harsher on light through the lens.

Okay ... if you are not convinced ... assume each lens is set to 40mm and it's widest available aperture ... according to the graph ... what MAXIMUM aperture is available on each lens?

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A: 18-70mm = f/5.6
... 18-200mm = f/4

Yes, that means that the 18-200mm will be full f/stop "brighter" than the same 18-70mm shot. In other words, your light doubles. In my opinion, that appears to be a pretty impressive difference in performance, for the same shot, especially indoors, where you would most likely use this focal length selection.

Guess you know which "kit" lens to go with, now, eh?

[Razr]

This thread never was about those lenses, now was it? Not really about any particular lens either: now was it?
As per here:
"Does the maximum aperture of a zoom lens change linearly with range?"
I am a history and English major, not an engineer, and as the OP does not sufficiently describe what he means by "linearly", I do however recall the question being around as long as zoom lenses have existed.
There have been many such arguments regarding whether or not the aperture actually changes in constant aperture lenses.
Answer this: when we take a variable aperture lens (any make, any model) and set the aperture at f/2.8 in either "A" or full manual operation, does the variable lens so set becomes a constant aperture lens: YES___ NO___? (Trick question, answered below).

And here:
"For example, in a 28-300 lens of max aperture f/2.8 to f/4.9. Does that mean that at a range of 100mm, the maximum aperture is f/3.36 (if you calculate linearly)" Again, no particular lens brand is mentioned in the question which leaves my submission as valid then as now.

And anyway... those two lenses ARE variable aperture lenses. Tricky answer. I had to read your declarative statement over for its content before I found the fatal flaw in your argument:
True, they can be (in automatic operation) "Variable aperture" lenses.
But yours is a really valuable lesson in "antics with semantics ".
What you are really saying is they can operate from f/2.8 to f/22, thus, according to your clever sophistry; they are "Variable aperture" lenses.
But just as true that in full manual and "A" operation, they are, for the sake of clarity in answering the OP question, "constant aperture" lenses; case closed.
They just have a fixed f-value! DUH!
And the only way to get a fixed f-value throughout the zoom range is to have a variable aperture in the lens. Your "antics" are taking on the taint of reductio ad absurdum (Disproof of a proposition by showing that it leads to absurd or untenable conclusions.)
Of course I agree (disingenuously) that a constant aperture zoom lens must have a "variable aperture", but I submit-so too do "constant aperture" prime lenses: have a "variable aperture" feature that is: DUH!
The aperture is the hole through which the light passes, and that gets varied in size Only in relationship to its need to maintain a constant light value at any given focal length.
That is: no matter the size of the actual hole (aperture) at any given focal length, the light value passing through the hole is CONSTANT-got it?
****In “A” or full manual modes, every lens, prime or zoom, is a constant aperture lens.

[shahmatt]

Coldrain. I just had a thought.

According to your definitions, aperture is the hole that light comes through, and f value is given by aperture size/focal length. This is a linear relationship.

Since, from the graphs presented in this thread, f value does not change linearly with focal length, can I conclude that aperture (size of hole) changes as well? Because that's the only way you can explain the parabolic curve.

[coldrain]

That is: no matter the size of the actual hole (aperture) at any given focal length, the light value passing through the hole is CONSTANT-got it?
****In “A” or full manual modes, every lens, prime or zoom, is a constant aperture lens.
No.

Th f-value is the amount of light reaching the film/sensor.
And... from f = aperture / focal length you can see that the light going through a 5mm hole at 100mm will be half of what will pass through a 5mm hole at 200mm.

Why? Simple really. Imagine a wall full of 345 40 watt light bulbs.
With a 10mm wide angle lens you may for instance get the whole wall in view, and all the light of all 345 light bulbs that is directed towards the lens will reach the sensor. A LOT of light.
Now, what happens when we take a photo with a 300mm tele lens, from the smae distance, of the same wall. All of a sudden, we only get the light of only one of those light bulbs into the lens. Hmmm.. a LOT less light, yet the subject is the same.

Suppose the aperture (the hole) is 5mm. The f number for 10mm would be: 5 / 10 = 1/2. So.... an f2 lens.
And with 300mm?
5 / 300 = 1 / 60. So... f 60 !!!

So.. yes, a lot less light will reach the sensor with 300mm and the same size hole. As the light bulb example already predicted.

[coldrain]

Coldrain. I just had a thought.

According to your definitions, aperture is the hole that light comes through, and f value is given by aperture size/focal length. This is a linear relationship.

Since, from the graphs presented in this thread, f value does not change linearly with focal length, can I conclude that aperture (size of hole) changes as well? Because that's the only way you can explain the parabolic curve.
Linear?

Going from 10 to 20mm is only 10mm... yet it doubles the focal length (and halves the f-value)!
Yet... going from 110 to 120mm is also 10mm.. yet it only is a very small difference in focal lenght, and will not change the f-value in the formula much either.

[shahmatt]

Oh yeah. That's true. I guess the curve is normal then.