Aperture Tips?


TPF Noob!
Mar 19, 2009
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I'm using the XSi kit as it says in my sig. I've owned the camera for about 9 months now and I think I understand all of the functionality.

I realize that at a "fully open" (so low f-spot right?) aperture I get the most shallow depth of field and most light, and vice versa.

What I'm curious about is, how do I make a decision between let's say 7 and 29? Any tips, or general rules?

It gets more difficult to see the differences in the viewfinder after around 10, so I'm wondering what the high f-spots are for?

Thanks, and pardon my noobish ways. :)
You can view the scene if your camera had a Stop Down Preview (as you mentioned ... the screen gets dark). If lighting is good enough you should be able to see the scene in the viewfinder.

In the old days, most lenses had DoF markings on the lens so you can see the focus distance within a particular f stop. Most of the newer lenses do not have this anymore.

There are no general rules ... if you do not have one of the options above, it would come from experience.
There's also a sharpness issue to remember. Most lenses are sharpest somewhere around f/8. For your particular lens, tests show it is sharpest around f/5.6. So unless you're photographing a really bright scene or need the gigantic DOF (the latter being highly unlikely), you would want to choose f/7.1 (there is no f/7) over f/20+.
What I'm curious about is, how do I make a decision between let's say 7 and 29? Any tips, or general rules?

The easy answer is: Whatever you like.

That my friend is one of the things that make this hobby (or profession) so fun, and enjoyable.

And it is ENTIRELY up to you, creatively, to make your decisions based on your "vision".

The book "Understanding Exposure" really rides this theme well, and is highly recommended. There are MANY ways to take photos, and everybody has their own creative vision.

So it is up to us, individually, to decide how close to stand to our subject, how wide to open the aperture, what ISO to use, to shoot vertically or horizontally, to shoot color or black & white, etc., etc.

The fun is in the journey, my friend.
The F-Stops are up to you, simply a tool to achieve a specific effect.
Thank you everyone for the responses. Astrostu, thanks for the link.. I'm reading it now and trying to figure out what it all means.

I'll try and post some pictures this weekend.
Here's a pretty easy Rule of Thumb.. (at ISO 100)

For most photography on a good day: First set your Aperture at f-8 (or a stop either side) and choose a shutter speed that gives you good exposure.

If you want to stop some action or movement: First set your shutter speed fast enough (say 1/250 and up, depending on the action), then choose the aperture that gives you good exposure.

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