Lens f stop question

[Kaptekarev]

My question pertains to f-stop designations on lenses. What does the f2.8 portion of this lens designation 28-70 f2.8L mean?

I understand that an f-stop is a fractional designation (larger the f = small hole) of camera aperture, but I can’t understand how that relates to a lens callout.

If this question can’t be answered here, please refer me to some reading material on this topic.

[Longwatcher]

This one I think I can answer,

The f# is the best setting you can achieve from that particular lens as far as letting light into the camera.

Thus a 50 f1.2 will work in lower light conditions then a 50 f1.8.

The 28-70 f2.8 should allow you to use a faster shutter speed (at the expense of depth of field), then the 28-80 f3.5-5.6. Thus justifying the expense of "L" glass, if nothing else does.

You may never take pictures at f2.8, but it is there when the lights are not good enough to use 3.5 or worse 5.6.

[CyberDyneSystems]

Roughly,. the larger the aperture (lower the f stop number) The more light that the lens will let in. Usually this does not mean that the lens can't be stopped down to a stop of a much higher number,. but the lower number as advertised is the maximum opening the lens will achieve.

As a general rule,. the lower the number listed, the better the lens as it will let in more light. Some of the Canon "L" lenses such as the one you mention will maintain an f2.8 aperture throughout its entire Zoom range. This is a relatively new technology,. and makes for a remarkably flexible lens. Truly some of the best there is.

Most zoom lenses however will give you two f stop numbers. One for its widest zoom setting and one for the extreme telephoto. Invariably the telephoto end will have a higher f stop, as you increase zoom,. less available light is let in. Again this is why the f 2.8 "L" lenses are so sought after.

[justme_dc]

That is a great question. This link should give you all the info you need.
http://www.mir.com.my/rb/photography/fototech/apershutter/aperture.htm

Good Luck to ya.

[CyberDyneSystems]

ha ha we posted at the same time! :)

[Kaptekarev]

Finally!

That makes sense. Thank for the great info.

[Yance]

One thing to also keep in mind is that the sizes of the apertures for different lenses are not equal. CyberDyne touched on the concept about zoom lenses having different maximum apertures for the different focal lengths. The reason for this is that wider lenses require a smaller aperture size to let in the same amount of light than a longer lens.

F Value (f-Number, f-Stop)
The aperture settings of a lens are called f-numbers or f-stops. An f-number represents a ratio between lens focal length and the effective diameter of a given aperture. Because it is related to focal length, the f-number is also called the relative aperture. The f-number equals the focal length of the lens divided by the entrance pupil of the aperture. Aperture settings are marked so that each position changes the amount of light passing through the lens by a factor of 2: the light is either doubled or reduced by one-half. That is, a high number represents a smaller aperture, one that stops twice as much light as the previous aperture. Conversely, a lower number represents a larger aperture, one that increases light twice as much as the previous number. The speed of a lens is the f-number of its maximum effective diameter - i.e. when the aperture is wide open.

from Sigma lenses (http://www.crkennedy.com.au/sigma_II/Introduction/Lens_Knowledge/lens_knowledge.html)

[Webster]

f-stops are written as f/8, f/11, etc. because that represents exactly what it means. That is to say, for a lens with a focal length of 55mm, f/11 equals 55mm divided by 11 = 5mm, which is the size of the aperture. In order for a zoom lens to maintain a constant f-stop, the aperture must change as the focal length changes. In order for a zoom lens to have the same maximum aperture across it's entire range, it has to be physically capable of providing an even smaller f-stop number for the shorter focal lengths than they allow to be used.

[mtscomasp]

webster wrote:
...a focal length of 50mm, f/11 equals 55mm divided by 11 = 5mm...

Hmmmm? How did 50mm become 55mm? Is that a typo or do you add 5mm always?

[Webster]

Thanks for pointing that out. Not exactly a typo, more like mind drift. Anyhow, I edited the post and fixed it.

[droosan]

Other aspects of a large maximum aperture (like f/2.8 or bigger) are:

* Faster focusing in low light conditions because the camera has more light to find focus with.

* Brighter Viewfinder.

Also remember when comparing apertures that the amount of light let-in by the lens corresponds to the square of the f-stop. This means that a f/2.8 zoom isn't just twice as good as a f/?-5.6 zoom. It is four times as good (at the long end, which is where you generally need as much light as you can get.)

The downside is that the amount of glass needed increases with the cube of the f-stop. This means the f/2.8 zoom might approach 8x as heavy, and correspondingly, 8x as expensive.