[joseph_me]

I was thinking about this for a while: let's say I have a manual lens, 70-300mm f/4-5.6. That means at 70mm the max aperture is f/4, and at 300mm the max aperture is f/5.6 (Right?).

What would happen to the lens, the body, or IQ if I manually set the lens at 300mm and aperture ring at f/4? Would the camera explode or the lens spontaneously combust? (kidding!)

I took some pictures and compared it but could not see much difference - though that might be due to my untrained eyes.

I also looked it up online and read an article that said it could damage the mechanism for the aperture because it will try to open beyond what the focal length allows.

What do you think?

[alamo5000]

The focal length is a function of how far the element is from the sensor (if I remember correctly). Aperture is a function and related to the size of the glass. Imagine one is measuring length and the other is measuring width (such as the filter diameter). There is a formula to figure it out but I don't know it. Unless the lens is physically larger in size you can't get more aperture out of it.

[jatrax]

What would happen to the lens, the body, or IQ if I manually set the lens at 300mm and aperture ring at f/4?

Nothing. Go ahead and try it. The aperture blades stay the same but because the rear element is farther from the sensor the effective amount of light reaching the sensor is reduced.

Put the lens on f/4 and watch as you zoom it out, nothing changes with the blades. You do realize people used manual lenses all the time before fancy electronics? If doing anything like you suggest would actually harm the lens or even make it not work properly someone would have done it long ago.

[UncleVanya]

A variable aperture zoom is slower typically at the long end and it isn't possible to open it too far. The f4 on the lens collar is in fact f/5.6 when zoomed to 300mm because the aperture diameter did not change in size and the focal length did. On electronic lenses this is communicated via the camera so you never see this difference in the aperture ring setting and the actual f/stop. The same thing happens when you add a manual teleconverter to a manual lens the f/stop listed is inaccurate because of the converter.

[stevebrot]

Focal length is the distance from the rear nodal point (often, but not always the surface of the rear element)l to the focal plane when focused at infinity. The f-number is the focal length divided by the diameter of the lens entry pupil (the absolute aperture...often, but not always the diameter of the diaphragm iris opening) and is always written as the fraction f/(number).

You are correct that if the absolute aperture remains the same and the focal length increases, the relative aperture (f-number) will increase proportionally. The concept of relative aperture is very useful in that it allows easy calculation of exposure value (EV) that can be applied for consistent exposure across lenses regardless of focal length. This incredibly helpful when using a hand-held meter to determine exposure settings.

Unfortunately, there are two problems:

1. F-number only strictly applies when focused to infinity. At close distances the effective focal length may be much longer than that stamped on the side of the lens.
2. The mechanical "stops" that determine the size of the absolute aperture on a traditional zoom lens with aperture ring are not typically coupled to reflect changes to focal length as the lens is zoomed

As a result, the amount of light actually striking the film or sensor may vary quite a bit as a lens is zoomed and as focus is changed from far to near. The latter is readily observed with a 1:1 macro lens pointed at a blank target. As the focus ring is racked from far to near the viewfinder image becomes progressively darker and the meter display reflects the dimming numerically. The same may be observed when moving to a longer focal length with a zoom lens.

Enter through-the-lens metering. That allows for seamless correction of both the focus and focal length issues...assuming you are using manual settings or aperture-priority automation. For shutter-priority or programmed automation with a zoom lens, the aperture control must "know" the current focal length or the lens must be designed to maintain constant relative aperture regardless of focal length*. The second approach is the more desirable, but requires larger glass elements and more complicated designs.


Steve

* Explaining how this is done requires more words than I am willing to donate this evening...

[promacjoe]

the aperture number is a mathematical relationship between the focal length and the usable diameter of the object lens, the front/1st element. Specifically the focal length divided by the usable diameter of the lens. for instance, a 300mm focal length lens at F 5.6 will use approximately a 52mm usable diameter. but a 75 mm focal length lens at F 4 will only need a 18.75mm usable diameter. Some of the differences because the front element is so close to this second element / group. the second element can only see so much of the first element. the aperture itself does not have any affect on how much the second element sees of the first element. if the usable portion of the front element is less than stated by the aperture, the aperture will not have any effect on the photo. As you zoom out, the aperture will start to have an effect. but setting the aperture wide-open cannot damage your lens or camera. the only way the lens could damage your camera is if the aperture lever were stuck. in which case your lens would be defective.

[zapp]

Some zooms are actually designed so that the aperture blades move slightly to adjust to the correct value as the barrel is moved in and out. I imagine his is to keep the aperture controlled at precisely defined f stop points across the zoom range so that they can be used in aperture priority mode with accurate results on mechanically coupled bodies. For electronically coupled lenses, the correct exposure compensations could presumably be made on the fly by calculating using the electronically reported zoom distance.

Blades move slightly? Sounds very tricky.
Ever since we use TTL metering the number we read off the lens are just numbers... the electronic readings are alos just numbers, easier to read out/repot... I had a misaligned aperture ring on a 2.8 zoom ones. The camera always reported 2.4. This makes no difference in everyday life.

[Lowell Goudge]

Some zooms are actually designed so that the aperture blades move slightly to adjust to the correct value as the barrel is moved in and out. I imagine his is to keep the aperture controlled at precisely defined f stop points across the zoom range so that they can be used in aperture priority mode with accurate results on mechanically coupled bodies. For electronically coupled lenses, the correct exposure compensations could presumably be made on the fly by calculating using the electronically reported zoom distance.

It is not the blades but is achieved due to the lens design and placement of baffles in or near the aperture mechanism.

But you are otherwise correct there are many constant aperture zooms.

This was and still is an important point specifically as it relates to flash photography