variable max aperture lenses question

[slipper]

Hi, i'm new here i apologize if this question has been answered before. Is it really that much cheaper to make say a 4.5-5.6 zoom lens instead of just 5.6? I use a 100-400 and all the time i just use it at 5.6 or smaller so the aperture won't change as i zoom in and out. Does making it a fixed 5.6 significanty increase the price of the lens?

 

[Loswr]

Depends on how you look at it. It wouldn't be more expensive to make a 100-400mm f/5.6 than the current lens, but it would be a LOT more expensive to make a 100-400mm f/4.5. If the element diameters are sufficient to support 400mm f/5.6 (i.e. can fill a 71.4mm iris diaphragm with light), then at shorter focal lengths they can support a wider f/stop. Probably most people would prefer to have the extra 2/3 stop of light at no additional cost, especially when autoexposure (Av or Tv) can compensate for the variable aperture, although I certainly agree that it's a pain for M mode.

 

[PeterJ]

Just a related question out of general interest because at the moment I've only got fixed aperture lenses. In Av mode if you select say f/4.5 and zoom the lens to where it has to shift to f/5.6 does it 'stick' there or revert back to f/4.5 when possible?

 

[MazV-L]

Just a related question out of general interest because at the moment I've only got fixed aperture lenses. In Av mode if you select say f/4.5 and zoom the lens to where it has to shift to f/5.6 does it 'stick' there or revert back to f/4.5 when possible?

Just tried it out with the ef-s 17-85mm f4-5.6 is on my 350D IR, and I can tell you on this particular lens if you have the aperture set at f4 then zoom out to 85mm to take a photo (f5.6) then zoom back in to 17mm the camera will automatically revert to f4 aperture setting ( at least definitely in Manual and Av settings)

 

[branden]

Just a related question out of general interest because at the moment I've only got fixed aperture lenses. In Av mode if you select say f/4.5 and zoom the lens to where it has to shift to f/5.6 does it 'stick' there or revert back to f/4.5 when possible?

Just tested this with an EF-S 18-135mm f/3.5-5.6 IS lens on a Rebel T3, and it "sticks" at f/3.5 even if you zoom in and shoot at f/5.6 -- when you zoom back out to 18mm the aperture will be f/3.5 again automatically.

Edit: beat by 58 seconds

 

[dr croubie]

... but if you're at the long end, and change the aperture to anything and back to f/5.6, when you zoom back to the wide end, the aperture will stay at f/5.6 and not go back to f/3.5

 

[Rampado]

I can´t understand why he thinks a fixed 5.6 is better than 4-5.6... makes no sense...

 

[Loswr]

I can´t understand why he thinks a fixed 5.6 is better than 4-5.6... makes no sense...

Because if you shoot in manual mode, with a variable aperture zoom the exposure will change as you zoom, so you must change your settings to compensate - and that's a pain.

 

[Rampado]

I can´t understand why he thinks a fixed 5.6 is better than 4-5.6... makes no sense...

Because if you shoot in manual mode, with a variable aperture zoom the exposure will change as you zoom, so you must change your settings to compensate - and that's a pain.

It´s not a pain... In fact, It´s very simple to avoid... If you shoot in manual and knows your lens is 4-5.6, and you preffer a fixed f5.6 lens, just let it in f5.6 in the wide end...

again, makes no sense someone complain about something this simple...

 

[Loswr]

...and that's exactly what the OP does. It doesn't bother me, personally. The 100-400mm is my only variable aperture zoom, and I use it almost exclusively on my 7D, which has a functional Auto ISO in M mode.

 

[Edwin Herdman]

To actually answer the OP's question: "Is it really that much cheaper to make say a 4.5-5.6 zoom lens instead of just 5.6?" I assuming you meant "instead of just f/4.5" - a constant f/5.6 lens would just be equivalent to the variable aperture (f/4.5-5.6) lens's minimum, smallest, aperture. The maximum aperture is f/4.5.

That very interesting question has been asked before. Most people never receive a satisfactory answer, alas.

Another way of asking it is this: "When you make a fixed f/2.8 (or whatever) lens, isn't some of the front glass 'wasted' @ the shortest focal length? It seems like it should be since we know that the f-number is the focal length (i.e. 300mm) divided by the entrance pupil (i.e., about 107mm for a f/2.8 lens); if the focal length selected is shorter, the entrance pupil doesn't need to be as big."

Ahh, simple lenses. First, Great Bustard says this:

It all comes down to understanding the differences between the three apertures: the physical aperture, the virtual aperture (entrance pupil), and the relative aperture (f-ratio).

The/relative/ aperture (f-ratio) is the quotient of the focal length and the diameter of the virtual aperture (entrance pupil), where the virtual aperture is how large the physical aperture appears when looking through the FE (front element).

So, in a constant aperture zoom, the physical aperture stays the same size, but as the lens zooms, the size of the virtual aperture changes in such a way that the relative aperture stays constant as well.

What Bustard hasn't said is that the "way" the lens changes is made possible by a magnification group (or possibly groups) within the lens, which rgmoore explains.

Basically, the key is the magnification of internal groups: When you change the virtual aperture, the amount of actual glass used at the front of the lens (for example) does not need to change.