f. stop changes automatically??

Using an EOS 20D in manual mode, if I set the f stop at 4, it frequently changes to 5 or 5.6 when I snap the picture. It's frustrating to not be able to use the the lowest f stop. What am I doing wrong? Or what can I do to get the opening to stay at what I've set?

What lens are you using? If it is a variable aperture lens then maybe you are setting it and the wide end and it is changing when you zoom out.

This will occur if your lens is a 4-5.6 lens, such as the 17-85 IS USM. The lens is f/4 at its widest and f/5.6 when it is fully zoomed in. Partially zooming in will change the aperture gradually until it reaches f/5.6.

If you set it to 4 it'll try and keep it that low as you zoom in but it's obviously limited by how physically wide it can keep it... it'll return to 4 as you zoom back out.

Edit: Having said that, that's why people pay good money for a lens which is a constant aperture such as the 24-70 2.8... That lens won't change aperture on you no matter what zoom position it is in. Quite handy.

Just moving the composition of a shot around, for instance, panning across a scene from something that is a little more backlit than somewhere else in the scene will change the f stop as well, especially when in Tv mode. I was seeing this over the weekend.

The situation that I was running into was a limb blowing in the breeze would sway one way and the sunlight would hit my subject a little more than when it was blowing back the opposite direction.

Valid point but the OP said it was in Manual.

Yep... I missed that during the first read of the OP's message. There are times when I could use a delete option for my own posts.

At least, I do know from your validation of my point is that I was not in fact going crazy this weekend. I was actually seeing that.

That lens will hold the maximum aperture of f/2.8 or below through the entire zoom range.

My Tamron 28-75mm will also hold the f/2.8or below.

That's why it is f/2.8.

your 17-85mm will hold f/4 and below @ 17mm and f/5.6 and below @ 85mm

Take a hard look at the Canon 24-70 f2.8L if you are replacing your 17 - 85. It's a few $$$ less than the 70 - 200 and might make a better "walk around" lens.

define your "walkaround". i consider a walk around lens where the camera and the lens won't be too obvious or too large to handle and won't get in the way of what you have to do. the 70-200 f2.8 IS is really large, so I definitely don't consider that a walk around lens..... unless you like walking around with a huge lens. lol...

the 17-85 is a walkaround and the 24-70 seems a bit large for a walkaround. a 17-40 or 16-35 or even the 24-105 IS is small enough to be walkaround.

That's a great decision! See what you need first then if you can, try it. Specialy with the more expensive lenses.

Even though I do not repent buying the 70-200mm (I have the f/4 version, not the f/2.$ ) I thought it would serve me for some birding (city birding at least). Talk about being wrong... only later I understood that birding would only be conviniente from 400mm upwards (maybe 300mm, but...).

Isn't there a "safety shift" feature or something like that where in manual if you over or under expose a shot, the camera automatically adjusts the settings?

In these days of auto exposure, what is so useful about constant aperture? It seems to me that constant aperture just means that the wide end isn't as fast as it could be. On my 24-70, the aperture blades shut a bit when zooming to wide to stay at 2.8.

Auto exposure has been around for 30 years, but cameras still have an "M" option. Sometimes M is best.

So it's nice to have a lens that stays where you set it. That said, if you have a 4-5.6 lens, it will stay where you set it if you set it at 5.6 or higher.

I don't think you understand.
It's F5.6 at the long end and F4.0 at the short end. It has nothing to do with zooming, just the focal length you've picked. At intermediate focal lengths it has an intermediate aperture.

kwsanders wrote:
Yep... I missed that during the first read of the OP's message. There are times when I could use a delete option for my own posts.

At least, I do know from your validation of my point is that I was not in fact going crazy this weekend. I was actually seeing that.

Delete option should be there for your own posts. Just edit and delete...

It's saved me many times.