a newbie question:

[thereforeiamx]

i don't get what the point of saying something like "f/4-5.6" in the name of a lens. its f/stop range will obviously be much greater than that, so why do we need to put something like that in the name?

 

[Battou]

i don't get what the point of saying something like "f/4-5.6" in the name of a lens. its f/stop range will obviously be much greater than that, so why do we need to put something like that in the name?

That is the lenses maximum aperture, and yes. In this case the aperture is variable pending the zooms focal leingth. in other words at one end of the zoom minimum aperture is 4 and at the other it is 5.6. The lens likely goes up to 32 but minimum aperture is not necessary and not noted. You will find that prime lenses have only one number for example 50mm f/1.4 or 55mm f/2. The multiple ones like you mentioned is unique to (some but not all) zoom lenses where elements are moving and changing the way the aperture effects the light.

 

[dxqcanada]

The specification of the Maximum aperture size is important for photographer's that require lenses with greater light gathering ability for low light photography.

I shoot a lot of wildlife photograph's, so I will tend to look for lenses that will permit me to shoot under lower lighting conditions ... so labeling the maximum aperture on the lens is important to me.

ie. 70-200mm f/2.8 vs 70-200 f/4.5-5.6

 

[Battou]

The specification of the Maximum aperture size is important for photographer's that require lenses with greater light gathering ability for low light photography.

I shoot a lot of wildlife photograph's, so I will tend to look for lenses that will permit me to shoot under lower lighting conditions ... so labeling the maximum aperture on the lens is important to me.

ie. 70-200mm f/2.8 vs 20-200 f/4.5-5.6

I knew I forgot something.

 

[thereforeiamx]

You will find that prime lenses have only one number for example 50mm f/1.4 or 55mm f/2.

so those belong to the fixed lenses that don't have the ability to zoom, hence their focal length and aperture remains the same, right?

and also, by "f/4-5.6," that technically means that when the lens is zoomed out at its widest (would you say widest or longest?) focal length of (in this case) 10mm, the widest aperture possible would be an f/stop of 4? and when it is zoomed in at 20mm, the widest aperture possible is f/5.6?

 

[dxqcanada]

The Maximum aperture can vary on a Zoom lens, depending on design.
Some zoom lenses are designed that they compensate as the lens is zoomed and will maintain the same maximum aperture.
Most zoom lenses will have a varying maximum aperture.

Correct.

ie. 10-20mm f/4-5.6
max aperture at 10mm = f/4
max aperture at 20mm = f/5.6

ie. 10-20mm f/4
max aperture at 10mm = f/4
max aperture at 20mm = f/4

 

[AlexColeman]

The 50 you mentioned is what is called a constant aperture lens, and is useful, because it doesn't change, and usually indicates a better lens( if it is a zoom).

 

[thereforeiamx]

so from what i'm getting, the 50mm f/1.4 example given before must be a fixed lens/non-zoom?

so not all constant aperture lenses are fixed AND not all fixed lenses are constant aperture lenses? or are there fixed lenses with fluctuating max. apertures too?

 

[tsaraleksi]

All fixed/prime lenses only have one maximum aperture, but there are zoom lenses, generally higher-end, that have a single max aperture over the entire zoom range. The advantage, of course, is that you don't have to worry about it changing as you zoom, which is important for lots of types of shooting. The disadvantage is that it means the zoom will be a lot larger/heavier.

 

[dxqcanada]

The 50mm f/1.8 is an example of a fixed focal length lens.
Fixed focal length lenses have a single maximum aperture.

Zoom lenses (ie 70-200mm) have variable focal lengths.
Variable focal length lenses may have a varying maximum aperture.
Constant maximum aperture on a variable focal length lens increases the price, due higher costs in the design/manufacturing.

** Ah too slow ... I am just repeating tsaraleksi ...

 

[dtornabene1]

so those belong to the fixed lenses that don't have the ability to zoom, hence their focal length and aperture remains the same, right?

and also, by "f/4-5.6," that technically means that when the lens is zoomed out at its widest (would you say widest or longest?) focal length of (in this case) 10mm, the widest aperture possible would be an f/stop of 4? and when it is zoomed in at 20mm, the widest aperture possible is f/5.6?

These folks are all correct and no need for me to repeat on aperture; it seems you have it. Be careful, however, in your understanding of focal lengths and aperture in reference to "widest" and "zoomed out".

The lower the number the higher the aperture. Aperture is simply number give to the ratio of the lens width and its length. (Actually it is more complicated, but this will do for now) For example, a lens that has a maximum aperture of 2 means fully open, the length is twice the glasses opening.

That is why a lower number refers to a higher aperture. When we say a lens is wide open, it is at its maximum aperture, in-other-words the shutter leafs are open all the way to let the most amount of light in.

When we refer to focal length however, it is the other way around. Zoomed out would refer to a longer focal length, so in your example 20mm. Focal length is the distance from the lens to the point of focus.

While I can get too technical, let me just say this, you are on the right tract. Longer focal lengths refer to a higher power lens. Aperture, measured using F-stops, is used to measure how much light is let in.

-Nick

 

[Steph]

The 50 you mentioned is what is called a constant aperture lens

No. It is called a prime lens (which means a lens with a single focal length).

 

[pugnacious33]

What everyone said here is accurate. I'd also add that the larger a lense's maximum aperture, the more expensive the lens. For most, the largest aperture is THE most important factor when purchasing a lens, not only because it will determine how well it will shoot in low light, but for portraits, it will determine how much of the background you will be able to eliminate, since aperture determines depth of field. Take a look at adorama.com and you'll notice some 85mm 1.2 lenses. These are primarily portrait lenses. Also, notice how expensive they are. If you plan on shooting portraits, one of these will greatly improve your flexibility and creativity.

 

[ksmattfish]

Very basically the math behind aperture size is: focal length divided by aperture diameter = f/#

That's not entirely accurate ( Aperture - Wikipedia, the free encyclopedia ), but it's close enough to help understand what's going on unless you are making your own lenses.

With a cheaper zoom the aperture stays the same size as you adjust the focal length, so the math in the above formula changes; hence designations like f/2.8-4.

 

[Chewbecca]

No. It is called a prime lens (which means a lens with a single focal length).

Exactly.

And to the original poster, just because you have a 50mm f1.8, does not mean that the aperture has to stay opened to f1.8.
You can stop the lens down (close the lens) to various levels.
You can stop to f2, f2.8, f3.5, f5.6 etc...

When you have a zoom lens with a constant aperture of, say, f2.8, that means it can stay open at f2.8 no matter what focal length you are at.
But you can also stop it down to f4, f5.6, f6.3, f7.1, etc...
You don't HAVE TO remain at f2.8, basically.

You probably know this, but I wanted to throw that out there just in case you didn't know.
That, and I'm half awake and in a hurry.