Focus length and Aperture setting

[blackhatgreyhat]

I have been learning a few things and recently realized that my zoom lens is not giving me the wide aperture at longer focus lengths. I have a 18-135mm lens and after about 36mm I do not get the option to select f/3.5 which is the widest on this lens. Is this the same across all zoom lens or there are lens that give f/1.8 at all focus lengths? How can I know from specifications which lens give constant widest aperture at all focus lengths?

I am considering trading in this lens to a better zoom lens hence the question.

[Black Pearl]

If you look at your lens (any lens for that matter) it will show the focal length in MM's and the maximum aperture. If there are two numbers for the aperture that means as you zoom to the longer end the aperture reduces in size - the numbers get larger.

You can see the difference below with your lens on the left and a fixed aperture f2.8 lens on the right:



There is only one f1.8 zoom lens I can think of in common production - the Sigma 18-35mm f1.8
More typically they will be f2.8 or f4.

[Mike Buckley]

Zoom lenses are made that provide the maximum aperture throughout all focal lengths. They will be both more expensive and heavier than comparable zooms that don't provide that capability. Look in the specs for the indication, such as "constant aperture throughout zoom" or "aperture of f/1.8 throughout zoom," or something similar.

The corollary is that a zoom lens that does not allow the constant aperture will be indicated in the specs as "maximum aperture f/4 - f/5.6" or something similar. That indicates that f/4 is the largest aperture at the shortest end of the zoom and that f/5.6 is the largest aperture at the longest end of the zoom.

[Manfred M]

The first thing to understand is that the aperture setting is nothing more than the focal length divided by the input aperture of the lens, so if the input aperture is fixed, then the maximum aperture will decrease as the focal length you use increases. This is what you are seeing with your lens.

There are a few lenses that are a few fixed aperture zoom lenses, but they tend to be the high end lenses that tend to cost around $2000 or more. They tend to have a very restricted magnification factor and rarely exceed 3x. They also tend to be no faster than f/2.8.

[mknittle]

I wouldn't worry about it, just shoot with it in as many different situations as you can and go from there. You may find a completely different reason for wanting your next lens

[blackhatgreyhat]

Those are some great points. I have seen apertures mentioned in both ways depending on the manufacturer you look at - f/1.8 or f/4-5.6 and they were with restricted apertures at larger focal length. I guess I wanted to confirm that most lenses in the market have same behavior.

I would in that case spend time understanding how the aperture-focal length settings work rather than spend a fortune on high end lens just to get a 3X magnification. Also I hadn't looked at aperture in relation to focal length as GrumpyDiver mentioned and will try to learn that through experiment.

[xpatUSA]

I have been learning a few things and recently realized that my zoom lens is not giving me the wide aperture at longer focus lengths. I have a 18-135mm lens and after about 36mm I do not get the option to select f/3.5 which is the widest on this lens. Is this the same across all zoom lens or there are lens that give f/1.8 at all focus lengths? How can I know from specifications which lens give constant widest aperture at all focus lengths?

I am considering trading in this lens to a better zoom lens hence the question.

The Imaging Resource site makes it clear in the listing for a manufacturer, for example:

http://www.imaging-resource.com/lens...ws/#zoomLenses

And the page for a specific lens tells the rest, for example my Sigma 17-50mm (constant f/2.8):

http://www.imaging-resource.com/lens...os-hsm/review/

I especially like their live blur diagrams:

http://www.imaging-resource.com/lens...blur/canon-7d/

You can vary the f-number and the zoom setting and see how sharp they are across the whole frame. Beats MTF diagrams any day.

[george013]

Those are some great points. I have seen apertures mentioned in both ways depending on the manufacturer you look at - f/1.8 or f/4-5.6 and they were with restricted apertures at larger focal length. I guess I wanted to confirm that most lenses in the market have same behavior.

I would in that case spend time understanding how the aperture-focal length settings work rather than spend a fortune on high end lens just to get a 3X magnification. Also I hadn't looked at aperture in relation to focal length as GrumpyDiver mentioned and will try to learn that through experiment.

Just like Manfred says. The diameter of the aperture is a physical unit, the f-number a ratio between aperture diameter and focal length.
But even with a fixed length it's not always constant. If you take a Nikon and a macro lens, you'll notice the f-number can't be set when in close distance. The Nikon will show you the so called "effective f-number". That's the ratio aperture diameter and image distance. Which is 2x the focal length with a magnification of 2.

George

[rpcrowe]

The variable aperture lens allows the widest aperture at the shortest focal length and the narrowest aperture at the longest focal length. As an example, a Canon 28-135mm f/3.5-5.6 lens will give you an f/3.5 aperture at 28mm and a f/5.6 aperture at 135mm. The lens will stop down from f/3.5 as you increase the focal length. The specific aperture at each focal length will depend on the lens. Quite often, researching the focal length/aperture combinations of a lens is possible using Google.
http://www.the-digital-picture.com/R...-USM-Lens.aspx


Most often, the variable aperture lenses are lighter and less expensive than the fixed aperture lenses. However that is not always the case. The Canon 100-400mm f/4.5-5.6L IS II lens is, as its title indicates, a variable aperture lens. However, this lens is fairly heavy and is quite expensive but, is an excellent piece of glass.

Fixed aperture lenses such as the 70-200mm f/4L IS have the same aperture at the shortest focal length as at the longest. In this case, the aperture is f/4 at any focal length. Quite often, the fixed aperture lenses have a shorter focal range than the variable aperture lenses. Usually in the area of a 1:3 ratio.