The number specifies the MAXIMUM aperture. So at the 18mm end, the max aperture is f3.5, but it can also go all the way down to f22 (or thereabouts). At the 55mm end, the max aperture is f5.6, which can also go down to f22...
See what the previous poster said..
Most zooms do this: as you zoom in the image is expanded more, yet the amount of light that can enter the front lens element is fixed. The result is that the image gets dimmer as it expands, effectively a reduction in aperture. The common 18-55s are all f/3.5 at the wide end and 5.6 at the tele end; the common 55-200 zooms likewise decrease from f/4 to f/5.6 as maximum aperture as you zoom in (but will go down to about f/22). This is the price you pay for having a lens that is light and portable. Genuine fixed-aperture zooms like the Sigma 70-200 f/2.8 (where the aperture does not change) are bulky and expensive..
Best wishesMike..
I can see your confusion if you are used to using a P&S type digital camera. As the other posters have mentioned, the figures you are seeing are maximum apertures. The minimum apertures are rarely stated because the vast majority stop down to f/22, and some stop down to f/32 or f/45. Some specialized lenses, such as the Canon MP-E 65 mm macro lens, only stop down to f/16; but this very specialized lens..
Brian A...
Usapatriot wrote:.
Http://www.digitalfotoclub.com/...4671792&rf=froogle&dfdate=8_24_2007.
The camera in the above link, the f-stops on the lens go from 3.5 to5.6, isnt that incredibly low DOF?.
What lenses do f2.8-f22 or something like that?.
The apertures shown for the Pentax kit lens in your link represnet the maximum apertures at the wide and tele end of the lens, f/3.5 at the 18mm end and f/5.6 at the 55mm end, as the other posters have explained. This lens will go to f/22 at all focal lengths..
Zooms which start at f/2.8 are typically professional level, constant aperture zooms, which maintain the f/2.8 maximum at all focal lengths.....the Pentax DA* 18-50/2.8 or the Sigma 18-50/2.8, However, these lenses are pricey...the Pentax being around $800 and the Sigma $500 or so. Sigma also makes a variable aperture 17-70mm with an f/2.8-4.5 aperture, which is in between the chaeper 3.5-5.6 and the constant f/2.8 lenses. All of these though will stop down to f/22 or better..
JohnPentax *ist-D, K100D, Fuji F20/31fd, Oly Stylushttp://www.pbase.com/jglover..
Hugowolf wrote:.
I can see your confusion if you are used to using a P&S type digitalcamera..
I don't, as the vast majority of P&S cameras also have variable max aperture zooms..
The minimum apertures are rarely statedbecause the vast majority stop down to f/22, and some stop down tof/32 or f/45. Some specialized lenses, such as the Canon MP-E 65 mmmacro lens, only stop down to f/16; but this very specialized lens..
The minimum aperture isn't part of the name because it's rarely important. If it is important to you, you'd look at the full specs..
Lens makers allow a lens to stop down as far as it can and still produce decent results. For 35mm systems, f/22 is about as far as you need to go, as that's about the diffraction limit for the system (using "standard" CoC). Long telephoto and macro lenses are sometimes designed to go a stop or two into diffraction..
And you don't need to go to an oddball lens like the Canon MP-E to find a lens that doesn't stop down to f/22, many very fast lenses (f/1.0, f/1.2, f/1.4) are limited to f/16..
Seen in a fortune cookie:Fear is the darkroom where negatives are developed..
John Glover wrote:.
This lens will go to f/22 at all focal lengths..
True, but minimum aperture is also variable. Helps keep the design simple and the costs down. It will go to f/38 at 55mm..
Seen in a fortune cookie:Fear is the darkroom where negatives are developed..
I don't know if the answers have been clear so I will put my comment here. Those numbers (3.5 - 5.6) refere to the largest apreture opening of the lense letting more light in. The smaller the number, the larger the apreture opening. You can adjust the opening to a larger number if you choose. up to F 22 or something.Will..
Nickleback wrote:.
Hugowolf wrote:.
I can see your confusion if you are used to using a P&S type digitalcamera..
I don't, as the vast majority of P&S cameras also have variable maxaperture zooms..
Ah yes, but how many buyers look at that data. The aperture range is also very limited on mosst P&S cameras..
And you don't need to go to an oddball lens like the Canon MP-E tofind a lens that doesn't stop down to f/22, many very fast lenses(f/1.0, f/1.2, f/1.4) are limited to f/16..
Thank you, I was forgetting about lens like the 50 mm f/1.2.
Brian A...
Usapatriot wrote:[snip].
The camera in the above link, the f-stops on the lens go from 3.5 to5.6, isnt that incredibly low DOF?.
Just to correct a misunderstanding in your question: f-stops relate to aperture size. It is explained here (from the Learn/Glossary section of this site):http://www.dpreview.com/learn/?/Glossary/Exposure/Aperture_01.htm.
DOF (depth of field) is something quite different, although it is affected by the f-stop. It is explained here:http://www.dpreview.com/learn/?/Glossary/Optical/Depth_of_Field_01.htmChris R..
So if I want maximum DOF for say, a landscape shot I would have to zoom in to 55mm, but it would not make a difference in sharpness if I shot the photo at f 5.6 at 28mm? Since everything is infinity in my view.Fuji Film S9100s9100/s9600 Flickr Group:http://www.flickr.com/groups/37994085@N00/..
So if I want maximum DOF for say, a landscape shot I would have tozoom in to 55mm, but it would not make a difference in sharpness if Ishot the photo at f 5.6 at 28mm? Since everything is infinity in myview..
Wrong way around... zooming in reduced depth of field, so does opening up the lens. For maximum d-o-f you want a wide angle and a small lens aperture like f/16..
You can see how it works in this useful on-line calculator for depth-of-field:.
Http://www.dofmaster.com/dofjs.html.
Best wishesMike..
Usapatriot wrote:.
So if I want maximum DOF for say, a landscape shot I would have tozoom in to 55mm, but it would not make a difference in sharpness if Ishot the photo at f 5.6 at 28mm? Since everything is infinity in myview..
If you shot at f/38 you'd be so deep into diffraction nothing would be sharp. DOF also depends on focal length and distance to subject. You have almost as much DOF at 18mm f/22 as you have at 55mm f/38. And for most landscapes at wide angle, f/11 or so is just fine..
Seen in a fortune cookie:Fear is the darkroom where negatives are developed..