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hope it's not a stupid question but being a newie i would like to know if you buy a lens that says F 1.8 does that mean you can get better DOF (more blur) than a lens that says F 3.5.

in other words what i am trying to ask is what does the F 1.8 etc etc mean on lenses? is it the largest aperture it opens up to? therefore the more blur you will get with DOF?

hope i don't sound too stoopid!!!!

Yep, you got it.
It would have been stupid if you didn't ask it.

You have it correct, except that more blur means less DOF (depth of field). With more DOF, more of the photo (in front and behind the bit you focused on) will appear to be sharp.

You get more DOF with f/22 than with f/16. More with f/16 than f/4, etc. With more DOF, you get less blur.

But the DOF is based on what you set the lens to, rather than what it is rated.
i.e. a 300mm f/2.8 will have the same DOF as a 300mm f/5.6 if both lenses are at f/16.

so i won't get very much blur (which is what i am after) with my standard kit lens with the 400D and Canon EF 18-55mm f/3.5-5.6 Lens
Canon EF 75-300mm f/4-5.6 III Lens is this correct?

It will all depend on how far the background is from the focus point. You can still get a good amount of blur at f/16, especially with a lens at 300mm, if the background is far enough away.

The DOF is fixed for each focal length and each f/stop. It is the situation that really governs what will happen.

The F-stop in the lens description is the maximum (widest) aperture that the lens supports. So a 50mm f/1.8 can give you f/1.8 (very wide) to maybe f/16 (check the lens specs for minimum aperture).

Using a wide/big aperture gives you:
- Very little depth of field (smaller area in focus)
- Very fast, allows for high shutter speeds - useful in low light

Stopping down and using a smaller aperture gives you:
- More depth of field (more in focus)
- Slower shutter speeds. Sometimes useful for achieving artistic blur or panning.

A lens which opens up wider just gives you more options to play with. A fast lens requires more glass to give you the wider aperture so it will be heavier and cost more. Conversely, smaller aperture lens = less glass, less cost, lighter.

Also, consumer zooms often list 2 apertures, Eg 70-300mm f/4-5.6. The wider aperture represents what the lens is capable of at the wide angle (in this case 70mm f/4). The other represents telephoto (300mm f/5.6).

More expensive/professional zooms may have a constant aperture across the whole range. Eg, 70-200mm f/2.8.

Hope this helps.

excellent reply. just what i needed to know thanks very much.

Read this (from our tutorials):

http://www.dslrusers.net/files/PHOTOGRAPHYINANUTSHELL.pdf

Then read this:

http://www.dslrusers.com/files/Exposure101.pdf

These articles should give you all the help you need.

Here is a couple of examples of older manual focus Lenses with depth of field scales on them. Unfortunately modern lenses seem to be discarding these very useful tools.

These are Takumar SMC K mounts - the left a 28mm F2.8 and the right a veritable 50mm F1.4.

You can read the focus area from the scales on the lenses. For example on the wide angle at F8 every thing from infinity to just below 1.5 metres is in focus while on the 50mm at F8 everything from infinity to just under 5 metres is in focus. In the these days of auto focus these readings don't seem to be used that much and as a consequence the scales are being left off a lot of new lenses.

These readings were very useful for adjusting F stop to give good Bokeh or the best Shutter speed for the situation.

Takumar generally always highlighted the F8 stop in orange or red as these generally was the best design location for the lens and where most of the image subject would be in focus.

The little red tick to the left of the orange diamond is the infared focus point.