The Canon 100-400mm has max aperture of 4.5 at 100mm and max aperture of 5.6 at 400mm. The min aperture isn't usually specified, probably because it's not as important..

You'll find out that lenses with wider max aperture cost a lot more, and are more desirable. Wider apertures gives you:.

1. Higher shutter speed, which reduces camera shake and motion blur.2. Longer flash range, because higher shutter speed doesn't cut down the flash3. More background blur for subject issolation.4. Better focus in low light.

Smaller aperture, on the other hand isn't too valuable. First of all, it creates diffraction which makes the image less sharp. Second, if focused at the hyperfocal, everything from there to infitinty would already be in focus. So there's really not much desire to have apertures that are too small...

F4.5 to f5.6: that means at the zoom lens' low end the widest the aperture opens is 4.5 and that at the zoom lens' high end the widest the aperture opens is 5.6. This is a variable aperture zoom lens..

The tighter you set the fstop the more you risk diffraction reducing picture sharpness. Diffraction is light scattering off the blades of the lens diaphram that forms the circle the light passes throught to reach the camera's digital sensor or film. Some think, for many lenses, that f8 is the "sweet spot" for a tight aperturemeaning very little diffraction. With my canon L lenses I find using f14 to f16 does not produce images suffering from diffraction.thezero..

And alsopeople say for instance, "Stop down to f/8 for best results." Sothat means number eight is a good number for a lot of shots in goodlight?.

You'll find out that newbies say this statement more often. Pros say "I usually shoot wide open". Here's the truth behind this statement. Each aperture has a different sharpness. Some lenses have a "sweet spot" where the sharpness peaks. For example, the kit lens that comes with the Canon 400d has best sharpness at F8, while the Canon 50mm f1.8 starts it's sweet spot at F4..

Recommending F8 blindly is like saying driving at 65mph is the best speed. Try that in a parking lot, and you'll hit a light pole. The aperture affects too many things: exposure, depth of field, shutter speed, sharpness, contrast, ambient vs flash balance, etc. Sharpness is not the only important aspect of photography...

May I expand on that?.

You should be thinking when you take a picture about what you want. You have a choice in that you can chose the shutter speed and then have little choice about the aperture or else you can chose the aperture and have little choice about the shutter speed if you want the exposure to be correct..

You can also select the ISO sensitivity and that complicates things further and noise comes into it etc: so I'll ignore it..

So for a landscape you might want everything in sharp focus and a lot of depth of focus, in which case you use the sweet spot and focus carefully (or rather on the hyperfocal distance - another complication I'll ignore). Having decided that the aperture is important the shutter speed will be forced on you. Or the opposite, if you take a portrait the subject may not want all the "smile lines" etc showing. So the last thing you want is extreme sharpness. So open out to f/12 or whatever's widest and focus on the eyes precisely and the rest of the face will be slightly out of focus and, with luck depending on the zoom, the subject will be happy. (This, btw, is why people spend lots and lots of money on a good portraiture lens.



On the other hand, you might be shooting kids playing some rough or fast game and want a high shutter speed and so you need to be able to get the exposure right even though the light is coming in for a minute fraction of a second. So you use the aperture to get the right exposure by letting in a lot of light..

BTW (2) you are lucky to have a camera that allows adjustment of the aperture; not so long ago so-called "pro" cameras were little more than expensive P&S's with little or no choice. What it cost me to find that out nearly 10 years ago doesn't bear thinking about..

Regards, David..

I appreciate these great replies..

David, I think you should be a teacher. I'm glad you decided to expand on it..

Dylan, I like that m.p.h analogy. That's what makes photography so challenging, eh. I keep remembering to myself someone's tag here about how the only failure in failure is if you fail to learn. I've found that I'm pretty fair with composition, but now I'm finding that I want to make a certain photo and I don't know enough to make my camera do what I want it to do when I want it to do it. Accidental good pictures are no longer satisfying!.

Okay, thanks again for your responses. They are all together very helpful for me today. I'm in Wyoming at Devil's Monument and taking photos morning, afternoon, and evening. It's TOO HOT to stay out all day so I have to do it incrementally..

I'm going to use ISO less. I found it rather magical and was going up to 1600, but methinks I should be experimenting with shutter and aperture more..

Take a walk along Chatcolet Lake in Idaho:http://www.bubbleshare.com/myalbum/39314/1052027..

I'm going to use ISO less. I found it rather magical and was goingup to 1600, but methinks I should be experimenting with shutter andaperture more..

If you're aiming for sharp and clear shots - you should generally aim to keep the ISO down at 100 - maybe 200..

The higher you go, the more noise your final product contains. It's not too hard to remove most of the noise PostProcessing, but you'll never get the same clarity - and why make extra work for yourself!.

Http://www.instantphotos.co.nz..

The f number of a lens is the maximum size of the aperature at a given distance. A 28-135mm f/3.5-5.6 lens allows you to set the aperature as large as f/3.5 at 28mm, but as you extend the distance to 135mm the widest possible aperature will be f/5.6. Lens such as 24-70mm f/2.8 will let you keep the wide f/2.8 the same no matter howe long you set the lens to. This is a lot more versatile, but it also costs a lot more money. The important thing to remember is that you can set it to any f-stop smaller than the maximum, but you can't set it for a larger aperature. The aperature's main use is to set the depth of field of your images.

This makes the subject stand out and kind of grab your attention. But for a landscape picture you want a small aperature that will keep everything in focus all of the way to the background.Magic_Puzzle..

Thanks - flattery will get you places. BTW, I see choosing an aperture or a shutter speed as simple (you don't have much to chose from anyway) and leaving it at ISO 100 all the time woks pretty well. What's hard is choosing the right exposure....

I mentioned the hyperfocal distance and said that it would complicate matters. Then I spotted something in another thread and though I'd mention it..

Go to this spot and there's a DoF calculater..

Http://www.dpreview.com/learn/?/Glossary/Optical/Depth_of_Field_01.htm.

Pick a suitable aperture and distance for the camera at it's widest and it will work out the DoF and also the hyperfocal distance. This is very usefull and if you just know this you'll find it handy..

The joy of it is that at that focal length and that aperture all you have to do is focus on something the distance away that you picked and you'll get a huge DoF. Choosing something like f/4 or f/56 at the widest setting of the zoom and focusing on something like 3 metres away ought to be OK but try it..

Regards, David..

Yeah, I'm having trouble with DOF. I took a real pretty picture of a doe and her fawn...the doe is a few feet behind the fawn and the fawn is in perfect focus but the doe is blurry. It would have been a perfect picture...they're both looking at the camera with pretty eyes. Boo hoo.Take a walk along Chatcolet Lake in Idaho:http://www.bubbleshare.com/myalbum/39314/1052027..

This might be helpfull - I've not explored it completely..

Http://www.better-photographs.com/index.html.

Regards, David..