Maximum allowable lens apertures

I posted this question in one of the Sigma 17-70 f/2.8-4.0 threads, but I think it got buried away within that thread. Let me start a new one here...

Why don't lens manufacturers make all zoom lenses f/2.8 (without changing any of the optics - just mechanically allow for larger apertures), and let the user decide if the quality is acceptable or not when shot wide open? This would be similar in essence to camera manufacturers allowing ISOs in the 6400+ range - sure, the quality is horrible above that range, but at least users have the option of going there.

I know it's not physically easy or cheap to make any lens f/2.8 well. However, from what I understand, lenses with non-constant apertures simply have a chip inside the lens that electronically tells the camera "do not use an aperture larger than X when at focal length Y" (this might actually be done mechanically by the lens itself, but I doubt it).

So why can't camera manufacturers simply lower X (which is probably just an 8-bit integer stored in a look-up table and could easily be changed), and let the user decide if it's good enough quality or not?

I suppose there's an eventual limit to the opening of the diaphragm. However, take the case of the 18-55mm f/3.5-5.6 lens (D indicates diameter of the diaphragm):

18mm @ f/3.5; D = 5.1mm
18mm @ f/2.8; D = 6.4mm
55mm @ f/5.6; D = 9.8mm
55mm @ f/2.8; D = 19.6mm

So, unless I'm missing something, the kit lens could certainly do 18mm @ f/2.8 without requiring physical re-design. Note that I'm not saying the lens will do this well, but why shouldn't we have the option of using it, just like we can use ISO 12,800 if we want. A focal length of 55mm with f/2.8 might not be possible, depending on if the diaphragm can physically get that large.

If I had a enter a guess, I would probably say a) marketing/product differentiation, and b) a large class of beginner users would shoot in that range expecting great results and become very disappointed. So the next logical question is then, why don't they disguise these wider apertures as they do with ISOs (e.g. HI/LO settings)?

The non constant aperture zooms are a result of optical design. The longer the focal length the larger opening you need to collect light.

Look at the size of the 200 2.8, 300 2.8 and 400 2.8 (all non-IS to be fair) and note how much larger the lens has to get to keep that aperture.

Also, take your lens at the wide end and hold the Depth of field preview button down while looking through the lens. Point the viewfinder at a bright source to allow you to see inside. Now zoom the lens to the long end and note that the apreture blades do not change position.

manttium wrote in post #10215189
If I had a enter a guess, I would probably say a) marketing/product differentiation, and b) a large class of beginner users would shoot in that range expecting great results and become very disappointed. So the next logical question is then, why don't they disguise these wider apertures as they do with ISOs (e.g. HI/LO settings)?

You are missing something. Making a f/2.8 is difficult and costly as well. It's not as easy as just making the aperture bigger. You need to get the light in there as well which means that the front element also has to increase in size as do all other pieces of glass. The more optical glass in a lens, the higher the cost.

manttium wrote in post #10215189
Why don't lens manufacturers make all zoom lenses f/2.8 (without changing any of the optics - just mechanically allow for larger apertures), and let the user decide if the quality is acceptable or not when shot wide open?

You won't sell many lenses if they all cost more than US $1,000.

Canon's EF-S 17-55mm f/2.8 IS costs US $1,100. The EF-S 18-55mm f/3.5-5.6 IS costs US $170.

The reason so many people use DSLR's in the 21st century is because the prices have plummeted in recent years. Cameras that once cost US $25,000 now cost less than US $700.

Price points count as much as performance when companies try to achieve the sales that keep them in business. If all lenses are f/2.8, you're pricing most consumers out of the market.

manttium wrote in post #10215189
However, from what I understand, lenses with non-constant apertures simply have a chip inside the lens that electronically tells the camera "do not use an aperture larger than X when at focal length Y" (this might actually be done mechanically by the lens itself, but I doubt it).

This is true, but not accurate. Aperture is a ratio, not a physical measurement. So as you zoom the lens, the ratio changes. When you get to a specific point in the zoom range that chip lets the body know it will only be able to produce up to a given aperture, and the body lets you know this by changing the display. When set to wide open, then zoom, the aperture does indeed change. But the aperture blades do not move.

As to why not all be a constant 2.8... its the same for any item on the market that has more than one version if it. Such as, why doesn't Cadillac only sell the CTS-V, what is the point of the "plain" CTS. Why dont, all coffee makers have a timer. Why is there 2 sizes of spoons in silverware? Why are there different sizes of lawn mowers? Why does Canon/Nikon make more than 1 body?... Ill stop here....

It is simply because 1 item of anything does not fit everybody. Whether it is price, weight, size, color, or performance, we all have different needs. And manufactures knowing this, will market items to fit those needs.

silvrr wrote in post #10215606
Look at the size of the 200 2.8, 300 2.8 and 400 2.8 (all non-IS to be fair) and note how much larger the lens has to get to keep that aperture.

This really hits home the first time you physically see a 400/2.8 lay on the counter next to a 400/5.6. The 5.6 looks like a kids toy!

I think you guys are missing the point a little bit here. I certainly understand the "aperture" is a ratio - how do you think I got the diaphragm diameters numbers? By using the definition of f-stop (N = f/D), where N is the f-stop, f is the focal length, and D is the effective diameter.

My point is that, taking the kit 18-55mm f/3.5-5.6 lens, the maximum diaphragm opening is found at 55mm and f/5.6, giving D=9.8mm. Why can't I use D=9.8mm at a focal length of 18mm, giving an f-stop or "aperture" of f/1.8? Sure, the optics will be terrible, but I can't see any physical limitation here. I 100% agree that if you want things to look good at f/1.8, then you would need a larger front element and more sophisticated optics. That's the not the point of this discussion. The point is why aren't we given the choice to go there, if the lens is physically capable of doing so?

As I originally surmised, the best guess is product differentiation. But why don't we at least have the option of going to large apertures, just like we have the option of going to high ISOs on consumer cameras, with the understanding that if we want better results, we have to pay more for bigger and better optics?

Well, you can't because as part of getting to 18 mm, without the physical diameter of the aperture diaphragm changing, the apparent aperture ("entry pupil") shrinks. Try zooming the lens while looking in the front of your camera and watch the aperture size change. The physical opening that's your aperture diaphragm isn't changing size at all; what's changing is how much the lens elements in front of that opening are actually magnifying it.

I see - so there's a distinction between "entry pupil" and diaphragm (or "exit pupil"). What really matters in the calculation (i.e. the parameter D) is the diameter of the entry pupil, *not* the diaphragm. The material I had been previously reading did not make this distinction, hence the confusion.

I did a quick test as a sanity check for my own understanding - the the diaphragm does not change diameter as you zoom the kit lens, yet the effective entrance pupil appears to get larger due to the refractive optics in front of it. In other words, the optics make the "virtual image" of the physical aperture appear larger at longer focal lengths. To make a lens have a lower aperture at a given focal length, the only option is to increase the size of the diaphragm, which is constant for cheaper lenses.

I don't have a constant-aperture lens with me right now, but if you were to zoom such a lens from focal length 1 to focal length 2, would you see the physical diaphragm size change (i.e. looking in from the back of the lens)? According to N=f/D, it should get bigger as you zoom out. Alternatively, looking in from the front of the lens, the size of the "effective aperture" (entry pupil) should stay the same.

Thanks for clarifying this BTW, I appreciate it!

Chck thiS out. It may help.
http://www.uscoles.com​/fstop.htm

TheNewGuy01 wrote in post #10218042
Chck thiS out. It may help.
http://www.uscoles.com​/fstop.htm

Unlikely, as the author also seems to be unaware that it is the entrance pupil diameter, not the iris diameter that is important.

Maximum aperature is Zero. But at zero everything is out of focus.

manttium wrote in post #10215189
However, from what I understand, lenses with non-constant apertures simply have a chip inside the lens that electronically tells the camera "do not use an aperture larger than X when at focal length Y" (this might actually be done mechanically by the lens itself, but I doubt it).

You have this all backwards.

The simplest design of a zoom lens (which is used to produce zoom lenses that are most "affordable") has a constant size "hole" for the light to get through. Before the days of electronic control of the iris (the device that adjusts the "hole" inside the lens), photographers had to always manually compensate their exposure settings as they changed the focal length of a typical zoom lens. This is because with a constant "hole" size and a varying focal length, the f-stop value (which is a ratio of hole to focal length) changes as the focal length is adjusted. Thus, "constant-aperture" zoom lenses were developed for the more demanding photographer. These typically have cams that internally adjust the iris so that the resulting aperture (f-stop) value does not change as the focal length is changed.

The bottom line is that constant-aperture zoom lenses are MUCH more expensive to create than conventional zoom lenses.

With cameras using electronic control of the iris in lenses, the net effect is that if the photographer chooses an f-stop value that is a larger number than the widest aperture at the longest focal length, the camera will keep the f-stop constant for all focal lengths. This sounds complex, so I will give an example.

Let's assume that you have a zoom lens that is a 50-150mm f/4-f/5.6. This lens cannot have a smaller f-stop number than f/5.6 with the focal length adjusted to 150mm. If you choose to use f/8, the electronic camera can adjust the lens' iris opening so that the actual aperture works at f/8 over the entire focal length range. This is a definite improvement over yester-year's manually adjusted lenses. However, if you had the focal length set to 50mm and the f-stop set to f/4, the automated camera can do nothing about the fact that the actual f-stop changes as the lens' focal length is increased from 50mm.