Constant Aperture Zoom: Explain it Like I'm a 4 Year Old

[Designer Dale]

I have several zoom lenses, two from Tamron. One is my first lens, the 28-300 f/3.5-6.3. The other is the most excellent 28-75 f/2.8 end to end.

I understand the 3.5 to 6.3 change. The max aperture is 3.5 at the short end. As the lens lengthens, the physical aperture stays the same but less light reaches the sensor because it has farther to travel. This has the effect of making the max aperture 6.3 at 300mm.

Now we come to the constant aperture question. The lens lengthens from 28-75 mm, but the max aperture stays at 2.8. How day do dis?

Dale

 

[harcosparky]

Let's say you have two zoom lenses.

First lens is a,
70-200 f/4 ( this is an f/4 constant aperture )

Second lens is a,
70-200 f/3.5 - 5.6 ( this is NOT a constant aperture )

With a constant aperture zoom, the aperture stays the same throughout the zoom range. This means if it is f/4@70mm it will also be f/4@200mm.

With the non-constant aperture lens it will be f/3.5@70mm and change to f/5.6@200. The change will occur at various zoom lengths.

 

[ppc_michael]

The f-stop is a ratio between the focal length and the aperture diameter, right?

So if you increase your focal length (zoom in) but your aperture diameter remains the same, that means the f-stop increases since it's a fraction and the numerator is getting larger but the denominator remains the same. So that's what's happening with your f/3.5–6.3.

So in order to maintain a constant f-stop throughout, the diameter of your aperture has to increase at the same rate as your focal length to keep the same ratio.

 

[VirtualRain]

I think the key to this is to keep in mind that...

f-number is the focal length divided by the "effective" aperture diameter (not actual physical diameter)

Then, knowing that, this is the best explanation I came across on how lens design can leverage this...

There are a couple of related misconceptions about lens design, and once you understand them the whole thing about constant vs. variable aperture zooms makes more sense. The first is that the size of the aperture in question is the actual size of the physical aperture in the lens. The related misunderstanding about constant aperture zooms is that they have to change the physical aperture size as they zoom in order to maintain their constant f-number.

Actually, the f-number depends on the size of the "virtual aperture", that is the apparent size of the physical aperture as it's seen through the lens. If the lens has a positive front group*, like most normal and longer lenses, the positive group works as a magnifier, so the virtual aperture is larger than the physical aperture. If the front group is negative, like retrofocus lenses, the negative group makes the virtual aperture smaller than the physical aperture.

When you zoom a zoom lens, it works by changing the magnification of the front group, the rear group, or both. If it changes only the front group, the change in the focal length of the lens is exactly the same as the change in the magnification to the aperture, so the f-number remains constant as you zoom. If you change the magnification of the rear group, the change to the lens as a whole will be smaller than the change to the front group, so the f-number will change as the lens zooms. If you made a lens that worked by changing only the rear group, the change in aperture would be as big as the change in focal length. Some lenses actually do that; AFAIK it's restricted to very long telephoto zooms. In practice, most lenses do the majority of their zooming with the front group.

As I understand it, there are a few constant aperture lenses that do change the physical aperture. The designers wanted to make a constant aperture lens for practical or marketing reasons, but the best optical choice involved a design that had some zooming with the rear group. In that case, it's possible to cheat by mechanically linking the physical aperture to the zoom mechanism, but it's very rare to do so.

 

[Designer Dale]

Ok. So does this mean that the diameter of the aperture is physically larger at the 75 mm end of my 2.8?

To be 2.8 at 28 mm and 2.8 at 75 mm the aperture must be larger on the long end.

Edit: VR: So in your article my 28-300 zoom with the rear elements and the 28-75 zooms with the front elements. Constant zoom lenses are always sharper and variable aperture is used on lower cost lenses. Are front zoom elements harder to manufacture?

Dale

 

[MSM Hobbes]

Check out the following:

http://www.jeffwignall.com/tutorials/alensapertureprimeri.html
http://www.jeffwignall.com/tutorials/lensapertureprimeriv.html
http://www.cambridgeincolour.com/tutorials/camera-lenses.htm

That may help hopefully somewhat.

 

[VirtualRain]

@Dale... No, the physical size of the aperture is staying constant, but the effective size of the aperture as the lens is zoomed is increasing in direct proportion to the zoom amount, thus keeping the ratio and therefore the f-number the same.

On your variable aperture lens, the aperture is also staying the same, but it appears smaller as the lens is zoomed because there is some magnification going on in the rear group of lens elements.

 

[Designer Dale]

Let me clarify a bit here. I understand variable aperture zoom.

What I need is an explanation of how constant aperture zoom works. And "I don't know" is an answer.

Example: The second link in one of the replies only describes what constant zoom is. How does it work.

Virtual Rain: See the edit to my earlier post. And thanks. You make sense.

Dale

 

[VirtualRain]

Edit: VR: So in your article my 28-300 zoom with the rear elements and the 28-75 zooms with the front elements. Constant zoom lenses are always sharper and variable aperture is used on lower cost lenses. Are front zoom elements harder to manufacture?

Dale

I've read that it is much more complex to design a front element zoom that will keep the effective aperture constant. So I guess that's why we only see this feature on high-end lenses.

Also, I'm not sure your 28-300 would use exclusively rear elements to zoom, it may combine both, but certainly there is some magnification going on after the aperture plane.

 

[Designer Dale]

One of the issues with this question is that it deals with two related terms: f/stop and aperture. f/stop is a ratio of light reaching the shutter and aperture is the mechanical opening that controls that light.

The following was gleaned from a Pentax forum and put in my words. I'll use my 28-75 f/2.8 here.

At 75 mm, the aperture opening required for f/2.8 is larger than the aperture opening needed to provide f/2.8 at 28 mm. In a constant zoom, the aperture is linked to the lens zoom either electronically or mechanically. If this lens is set to f/2.8 at the short end and zoomed out to the long end, the diameter of the aperture increases. The diameter for 2.8 is larger at 75 mm than it is for 28 mm.

And this is why you only find the constant f/stop feature on more expensive lenses.

Please excuse me for using the term "shutter". I'm not sure that DSLR cameras have shutters. I'm old school.

Feel free to comment.

Dale

 

[VirtualRain]

One of the issues with this question is that it deals with two related terms: f/stop and aperture. f/stop is a ratio of light reaching the shutter and aperture is the mechanical opening that controls that light.

The following was gleaned from a Pentax forum and put in my words. I'll use my 28-75 f/2.8 here.

At 75 mm, the aperture opening required for f/2.8 is larger than the aperture opening needed to provide f/2.8 at 28 mm. In a constant zoom, the aperture is linked to the lens zoom either electronically or mechanically. If this lens is set to f/2.8 at the short end and zoomed out to the long end, the diameter of the aperture increases. The diameter for 2.8 is larger at 75 mm than it is for 28 mm.

And this is why you only find the constant f/stop feature on more expensive lenses.

Please excuse me for using the term "shutter". I'm not sure that DSLR cameras have shutters. I'm old school.

Feel free to comment.

Dale

If I understand what you're saying, I believe it's a bit flawed.

Agreed that f2.8 at 28mm is smaller than f2.8 at 70mm. However, there is typically no electrical or mechanical linkage between the zoom and the iris on any lens (fixed aperture or otherwise). It's an optical property that enables a fixed aperture zoom.

On any lens, the iris is always opened to it's maximum. On a constant f-stop lens, as the focal length changes, the magnification before the iris increases, and thus the apparent size of the aperture changes, effectively increasingly the aperture optically, and thus the f-stop, but not physically changing the iris opening.

However, there were exceptions (see last paragraph of my quote in post #4), but that's not now most constant aperture zooms are done today.

 

[Cliff3]

Now we come to the constant aperture question. The lens lengthens from 28-75 mm, but the max aperture stays at 2.8. How day do dis?

Dale

Simple, the lens has a larger diameter to support the larger aperture at the longer focal length.

The diaphragm is kept as large as possible until the moment the shutter is pressed, then at that point in time it is closed to a size appropriate to the exposure setting chosen in the camera. It is not directly coupled to the zoom, nor to focus which can also affect focal length.

(edit: if your camera body has a depth of field preview button you can trigger this behavior on demand)

 

[Ruahrc]

Please excuse me for using the term "shutter". I'm not sure that DSLR cameras have shutters. I'm old school.

DSLRs definitely have shutters.

 

[mtbdudex]

my 4 year old speak: 70-200 f2.8 L mkII; big, heavy, stays same length no mater the zoom, tack sharp, and people get outta way when I swing the beast around, it commands R-E-S-P-E-C-T.

It's actually kinda cool to look inside at the optics as you zoom 70-200,

 

[Designer Dale]

my 4 year old speak: 70-200 f2.8 L mkII; big, heavy, stays same length no mater the zoom, tack sharp, and people get outta way when I swing the beast around, it commands R-E-S-P-E-C-T.

It's actually kinda cool to look inside at the optics as you zoom 70-200,

Thanks for something that I can understand! I spent a lot of time looking down the barrel of my 28-75 trying to understand what was going on inside. It looked pretty cool. I really couldn't pin anything down, though. Either the diaphragm was adjusting as I zoomed or the front element was making it look like that through magnification. I checked other photography forums and found this question asked repeatedly with many different explanations. I even feel embarrassed for asking it. We need a lens engineer to truly explain this. But that wouldn't be like I'm a 4 yo...

Thanks too all for your contributions. It seems that there are several ways for lens builders to do this. That makes everyone right !

If your new toy stays the same length, then the lens groups move inside the lens.

Dale

 

[jabbott]

A more accurate term would be "constant f-stop" or "constant f-number". One way to explain this concept is by using the aperture area formula:

area of light entering a lens = pi * (focal_length / 2 / f_stop)^2

So at 28mm focal length and f/2.8 aperture, the area of the aperture is 78.5 mm^2. However at 75mm focal length and f/2.8 aperture, the area of the aperture is 563.5 mm^2. So when the focal length increases on a constant f-stop lens, the iris is staying significantly more open during the exposure in order to keep the f-number constant.

Variable aperture lenses also do this as their focal length increases, but not by as much. For example, if the 28-75mm lens you mentioned went from f/2.8 at 28mm to f/4.0 at 75mm instead, the respective aperture areas would go from 78.5 mm^2 at 28mm focal length to 276.1 mm^2 at 75mm focal length. So the aperture on such a lens would open 2x less than the constant f/2.8 lens above. This is easier to engineer so it costs less.

If you truly had a "constant aperture" lens, one where the iris did not move at all for example, you would have an equal multiple difference in f-stops from the low focal length to the high focal length. Using the same focal lengths and initial f-stop as above, such a lens would go from f/2.8 at 28mm to f/7.5 at 75mm (a 2.67x difference in both the focal length and f-number). I shudder to think of using such a lens (horrible pun, I know).

 

[jabbott]

If I understand what you're saying, I believe it's a bit flawed.

Agreed that f2.8 at 28mm is smaller than f2.8 at 70mm. However, there is typically no electrical or mechanical linkage between the zoom and the iris on any lens (fixed aperture or otherwise). It's an optical property that enables a fixed aperture zoom.

There is definitely an electrical and mechanical link between the zoom and the iris on modern lenses. The lens defaults to full open aperture (no iris closure) while using the viewfinder so that you can have the brightest possible view. When you fully depress the shutter release button however, the camera electronically tells (or has told) the lens what f-stop to use. The lens then uses its own microelectronics to determine how far it should retract its iris based on its current focal length, which it then retracts mechanically during exposure. Here's a fun experiment to try in good lighting: set your camera to Av (aperture priority mode) and set it to the highest f-stop possible (like f/22). Now set it to burst mode capture. Then turn your camera around, look at the inside of the lens and hold the shutter down for a couple seconds. You'll notice that the iris only does its magic for the duration of the exposure. You can see a beautiful example of this with the Canon 7D here. For more fun experimentation, try the steps above at different focal lengths and apertures.

 

[VirtualRain]

There is definitely an electrical and mechanical link between the zoom and the iris on modern lenses. The lens defaults to full open aperture (no iris closure) while using the viewfinder so that you can have the brightest possible view. When you fully depress the shutter release button however, the camera electronically tells (or has told) the lens what f-stop to use. The lens then uses its own microelectronics to determine how far it should retract its iris based on its current focal length, which it then retracts mechanically during exposure. Here's a fun experiment to try in good lighting: set your camera to Av (aperture priority mode) and set it to the highest f-stop possible (like f/22). Now set it to burst mode capture. Then turn your camera around, look at the inside of the lens and hold the shutter down for a couple seconds. You'll notice that the iris only does its magic for the duration of the exposure. You can see a beautiful example of this with the Canon 7D here. For more fun experimentation, try the steps above at different focal lengths and apertures.

Yeah, good point, the lens definitely knows the focal length and uses that in various ways, but I was referring to the effects of zooming on the maximum f-stop of a constant aperture zoom. BTW, cool video. It's amazing how fast those aperture blades move.

 

[jabbott]

Yeah, good point, the lens definitely knows the focal length and uses that in various ways, but I was referring to the effects of zooming on the maximum f-stop of a constant aperture zoom. BTW, cool video. It's amazing how fast those aperture blades move.

Thanks -- it was a interesting video to find. As for zoom having effects on the maximum f-stop (which I assume you mean minimum f-number), it should just be how far the aperture opens up, as governed by the formula I posted earlier. The more zoomed in the lens is, the more the aperture needs to open to achieve the same exposure, all other things (e.g. shutter speed, ISO speed) being equal. This is because the field of view is smaller, so you are gathering less photons during the same amount of time. Any 4 year olds left? Bueller? ...Bueller?

 

[Designer Dale]

Thanks -- it was a interesting video to find. As for zoom having effects on the maximum f-stop (which I assume you mean minimum f-number), it should just be how far the aperture opens up, as governed by the formula I posted earlier. The more zoomed in the lens is, the more the aperture needs to open to achieve the same exposure, all other things (e.g. shutter speed, ISO speed) being equal. This is because the field of view is smaller, so you are gathering less photons during the same amount of time. Any 4 year olds left? Bueller? ...Bueller?

Thanks. This supports one of my original premises that the diaphragm in a constant zoom is able to open wider than it's marked maximum. It's "stopped down" throughout the zoom range to keep the amount of light reaching the sensor constant for the set f/. My 28-75 f/2.8 is designed to be 2.8 at 75 mm and the diaphragm stops down to provide 2.8 at 28 mm.

There are several ways to do this, however. The diaphragm can be placed on a sliding rail that moves forward as the lens zooms. This provides the correct light ratio without changing the diaphragm openings physical size. I have a crude diagram around here. It's from Photography on the Net.


Dale

 

[VirtualRain]

We are regressing here.

There's no change in the mechanical opening in a constant aperture lens. The iris doesn't move, it doesn't open wider, it remains the same. It's all in the optical properties.

As the magnifying front element/group moves relative to the iris, when you zoom it magnifies the aperture as seen from the front of the lens thus increasing the relative aperture and therefore maintaining the constant f-number. Constant aperture lenses are designed so that the entry pupil varies in direct 1:1 ratio with the focal length of the lens as you zoom.

If you have a zoom lens, you can see this at work. Simply take the lens off your camera and look into the front of the lens and notice that as you zoom it, the circle allowing light through appears to increase. The only difference between constant aperture and regular zoom lenses is that with constant aperture, the input pupil magnifies in 1:1 ratio with the focal length increase. With a regular zoom, the ratio is not 1:1 and thus although the aperture is still getting magnified, it's not getting magnified enough and the effective f-number decreases with increased focal length.

To recap, it's taking advantage of, and designing for a particular magnification effect, that makes a constant aperture zoom possible. It's not a mechanical/electrical property.

 

[jabbott]

We are regressing here.

There's no change in the mechanical opening in a constant aperture lens. The iris doesn't move, it doesn't open wider, it remains the same. It's all in the optical properties.

As the magnifying front element/group moves relative to the iris, when you zoom it magnifies the aperture as seen from the front of the lens thus increasing the relative aperture and therefore maintaining the constant f-number. Constant aperture lenses are designed so that the entry pupil varies in direct 1:1 ratio with the focal length of the lens as you zoom.

If you have a zoom lens, you can see this at work. Simply take the lens off your camera and look into the front of the lens and notice that as you zoom it, the circle allowing light through appears to increase. The only difference between constant aperture and regular zoom lenses is that with constant aperture, the input pupil magnifies in 1:1 ratio with the focal length increase. With a regular zoom, the ratio is not 1:1 and thus although the aperture is still getting magnified, it's not getting magnified enough and the effective f-number decreases with increased focal length.

To recap, it's taking advantage of, and designing for a particular magnification effect, that makes a constant aperture zoom possible. It's not a mechanical/electrical property.

The 1:1 ratio is correct, and I have the math to explain it. The aperture area formula I wrote about before uses the aperture diameter as one of its variables:

aperture area = pi*(d/2)^2 where d = aperture diameter = (focal length/f_number)

Going back to the 28-75mm f/2.8 lens, the aperture diameter at 28mm is 28/2.8 = 10mm, and at 75mm, the aperture diameter is 75/2.8 = 26.8mm. The zoom ratio for the lens is 75/28 = 2.68x, and the ratio that the aperture diameter opens is 26.8/10 = 2.68x. So we have two ratios we are comparing together as another ratio: the zoom ratio to the aperture diameter ratio. For this lens, it is 2.68:2.68 which can be reduced to 1:1.

I may have worded one of my earlier posts somewhat vaguely, so I will try to clarify a bit. The iris works to further reduce the aperture (for example if you wanted your camera to take a photo at f/8 instead of f/2.8), while the entrance pupil is what governs the amount of light that is let in at full "wide open" aperture. The entrance pupil is what automatically changes as a factor of zoom, not the iris - although the amount the iris must close during shutter actuation does still depend on the focal length and f-number. At wide open aperture, the iris should not have to close at all because the entrance pupil is already creating the proper aperture. Hope this helps.

 

[Policar]

Nevermind.