f stops

[SteelDirigible]

What I was wondering is why some lenses will say, like, f/2.8 while others will give a range, of say f/2.8-4.5

What brought this up is me looking at some of the sigma 17-50mm lenses,

These are around $400 and I think are basically the same with the latter improving the macro capability
Sigma Lens: Zooms - Sigma 18-50mm f/2.8 EX DC - SLRgear.com!

Sigma Lens: Zooms - Sigma 18-50mm f/2.8 EX DC Macro - SLRgear.com!

but when I went to search it at adorama I came across this:
SG1850284NKK Sigma 18-50mm f/2.8-4.5 DC OS HSM for Nikon AutoFocus Lens Kit, with Tiffen 67mm UV Filter, Lens Cap Leash, Professional Lens Cleaning Kit

The only differences I see are the price and them showing either a single f/2.8 or f/2.8-4.5
but I thought that even the ones just saying f/2.8 go much higher.
What's the differences in these, and why is one twice the price of the other? It woul dbe nice to replace the kit lens with one that is faster.

and after a little more quick research, I'm seeing this is called variable aperture. what is it, and why are these cheaper?

 

[Light Guru]

Lenses are listed with their widest aperture. Some zoom lenses do not keep the same widest aperture at all focal lengths. So a lens that says 2.8-4.5 means that when zoomed out you can use a 2.5 aperture but when fully zoomed in you only get a 4.5 aperture. A lens that can keep a 2.8 aperture at all zoom distances is going to be more expensive.

 

[480sparky67 66]

........... A lens that can keep a 2.8 aperture at all zoom distances is going to be more expensive.

As well as be physically larger and weigh more.

 

[Dao]

In general, when the lens manufacturer make a ZOOM lens that has a wide constant max aperture, it usually performs better optically and hence the higher cost.

 

[alvarez57]

Ok, I'm a newbie to the forum and to DSLR (last SLR was a Minolta XE-7 35mm). I have a Canon60D with basic EFS18-135mm f 3.5- 5.6. How does this work in terms of f stops and DOF compared to a 35mm SLR -i.e. f1.4-16.0 ? Have we lost the DOF in these Digital zooms as opposed to the ole 35mm? I miss my DOF!
Thanks.

 

[480sparky67 66]

You still have DOF. You just don't have the scale stamped on the lens.

 

[Derrel]

Sigma Lens: Zooms - Sigma 18-50mm f/2.8 EX DC Macro - SLRgear.com!

but when I went to search it at adorama I came across this:
SG1850284NKK Sigma 18-50mm f/2.8-4.5 DC OS HSM for Nikon AutoFocus Lens Kit, with Tiffen 67mm UV Filter, Lens Cap Leash, Professional Lens Cleaning Kit

Those are two TOTALLY different lenses. The second lens has variable maximum aperture settings (ranging from f/2.8 at wide-angle to a mere f/4.5 at the longer focal length settings), and the 2.8~4.5 lens has Optical Stabilizer or OS.

 

[flyin-lowe]

Ok, I'm a newbie to the forum and to DSLR (last SLR was a Minolta XE-7 35mm). I have a Canon60D with basic EFS18-135mm f 3.5- 5.6. How does this work in terms of f stops and DOF compared to a 35mm SLR -i.e. f1.4-16.0 ? Have we lost the DOF in these Digital zooms as opposed to the ole 35mm? I miss my DOF!
Thanks.

The 3.5-5.6 is the lowest f Stop you can get to a the different zoom ranges. For example if you are shooting wide at 18mm f 3.5 is as low as you can go. If you zoom in to 135mm then 5.6 is as low as you can go. You can go a lot higher, you could shoot f16 at 18mm if you wanted, it just shows you the lowest it will go at each end of the lens range. Like stated above that is why the 2.8 lenses are so much more. My 75-300 can only go down to 5.6 when zoomed all the way out to 300mm. The 70-200 2.8 can stay at 2.8 when zoomed all the way out to 200 mm. I hope that doesn't confuse you more.

 

[alvarez57]

Ok, I'm a newbie to the forum and to DSLR (last SLR was a Minolta XE-7 35mm). I have a Canon60D with basic EFS18-135mm f 3.5- 5.6. How does this work in terms of f stops and DOF compared to a 35mm SLR -i.e. f1.4-16.0 ? Have we lost the DOF in these Digital zooms as opposed to the ole 35mm? I miss my DOF!
Thanks.

The 3.5-5.6 is the lowest f Stop you can get to a the different zoom ranges. For example if you are shooting wide at 18mm f 3.5 is as low as you can go. If you zoom in to 135mm then 5.6 is as low as you can go. You can go a lot higher, you could shoot f16 at 18mm if you wanted, it just shows you the lowest it will go at each end of the lens range. Like stated above that is why the 2.8 lenses are so much more. My 75-300 can only go down to 5.6 when zoomed all the way out to 300mm. The 70-200 2.8 can stay at 2.8 when zoomed all the way out to 200 mm. I hope that doesn't confuse you more.

Thanks! No actually it DOES clear the confusion. So, according to what you said, I can shoot, say at 50 or 85mm using the Av function (or manual) and set it up to f11 as long as the light conditions are favorable, no?

 

[480sparky67 66]

f/11 will be f/11 no matter the focal length chosen. It will be the same for all the f/stops up to the smaller of the two listed for the lens. So if a lens is labeled f/4-5.6, then f/5.6 at the longest focal length will still be f/5.6 at the shortest. Only when you start setting the aperture wider than the larger number listed will there be a difference.

 

[alvarez57]

Thanks Sparky, I will try again the "fence" photo to play with the DOF. I tried before and the results were a very, very narrow DOF... but it may have been also that I used the wrong focus mode... you see? Here I do miss my ole 35mm that I did EVERYTHING manual!

 

[KmH]

To summarize:

A zoom lens that has a range of maximum apertures printed on it or used in it's description, like f/3.5-5.6 is called a variable aperture zoom lens. If the lens has a focal length range of say, 18-105 mm, the widest aperture at 18 mm would be f/3.5. As the lens is zoomed to a longer focal length the maximum aperture changes automatically. At 50 mm the lens max aperture may now be f/4, and at 75 mm f/4.5, until just before 105 mm the max aperture will be f/5.6. In other words, you cannot use f/3.5 at a focal length that is much longer than 18 mm. However, you can use f/5.6, or smaller apertures, at any focal length the lens has available. Note: f/5.6 is a smaller aperture than f/3.5.

The other type of zoom lens only has onemaximum f-stop number and is called a constant aperture zoom lens. A 70-200 mm f/2.8 lens, can be set to f/2.8 at any focal length between 70 mm and 200 mm.