Same exposure @ different zoom levels...

[iskoos]

You point your camera to take two random shots (say a lake scenery at a state park).

You are shooting at aperture priority mode (say f/9.0) your ISO is set to 100 and camera uses 1/250 to obtain the correct exposure.
For the first shot you use wide angle end of the lens(something like 18mm) and 50mm for the second shot.
Since all the settings, camera angle and location are same for both shots, and you use the same lens (say f/2.8 throughout the zoom range) you would expect the camera to use the same shutter speed for both shots. But it doesn't.
I did this test a few times outdoor today and camera shutter speed changes as I zoom in and out. The change is not a drastic change but it changes.
And I cannot explain why. Could someone?

 

[Gaerek]

What lens are you using?

Going off of what you have in your sig (the 18-55), that lens has an aperture that varies through the zoom range. At 18mm, it's max aperture is 3.5. At 55mm, it's max aperture is 5.6 (I think). So let's say you set your max aperture at 18mm. As you zoom in, the camera will automatically adjust the aperture value. Since it's in Ap mode, it will vary the shutter to compensate.

If you're using a different lens (one with a constant aperture) then I'm not sure why it would do something like that.

EDIT: I didn't notice the 17-85 earlier, but it also has a variable aperture. I think it is 4-5.6?

 

[Dominantly]

He said he was shooting at f/9, which wouldn't change with the zoom.

 

[Dominantly]

I'll take a stab at it, when you zoom in, you are changing the detail in the viewfinder, which is what your metering system is judging to create your proper exposure. If you are in a matrix or evaluative metering mode, it will still be evaluating a different ratio of the same scene, and will adjust your shutter to compensate. The change would probably be minimal (depends on your subject though).

 

[Gaerek]

He said he was shooting at f/9, which wouldn't change with the zoom.

Hmm, was confused then when he mentioned the f/2.8. Never mind my other post.

Something else to consider, however. What metering mode were you in? If you were in evaluative, for example, the amount of light in the whole scene will be changing as you zoom in or out.

This is an example to illustrate what I'm saying, not necessarily a real situation. Imagine zoomed out, you're taking a picture of a black car, surrounded by say a white sand beach. As you zoom in on the car, more of the black car is in the scene, and less of the white sand is in the scene. The camera's meter will have to change the exposure for that.

Did I just make sense? It's hard for me to explain, but I hope that made sense.

EDIT: Also, if you were in one of the other metering modes, if you weren't on a tripod, it's very likely that the exposure 'sweet spot' is going to move. If you're in spot metering mode, this could be a huge difference. In some of the others, it may be minimal.

Also, Dom beat me to it

 

[Dominantly]

I beat you!

 

[Gaerek]

I beat you!

:cry:

 

[Josh66]

He said he was shooting at f/9, which wouldn't change with the zoom.

It does, but it doesn't.

The f/number doesn't change, but the physical size of the aperture does. Either that is what's causing it, or the slight differences in metering are doing it. Or both.

edit
Now that I think of it, the physical size of the aperture shouldn't really matter. It's gotta be the meter 'seeing' the scene differently.

 

[Gaerek]

He said he was shooting at f/9, which wouldn't change with the zoom.

It does, but it doesn't.

The f/number doesn't change, but the physical size of the aperture does. That's what is causing this.

True, but I'm pretty sure the camera will compensate for this by opening the aperture unless he's at the max aperture.

 

[Dominantly]

He said he was shooting at f/9, which wouldn't change with the zoom.

It does, but it doesn't.

The f/number doesn't change, but the physical size of the aperture does. Either that is what's causing it, or the slight differences in metering are doing it. Or both.

Word.
To find this, you would take your focal length, and divide it by your f/stop.

Fo sho.

 

[Derrel]

I'll take a stab at it, when you zoom in, you are changing the detail in the viewfinder, which is what your metering system is judging to create your proper exposure. If you are in a matrix or evaluative metering mode, it will still be evaluating a different ratio of the same scene, and will adjust your shutter to compensate. The change would probably be minimal (depends on your subject though).

I have found the above to be true. At wider angle settings, more ground or sky is often included in the metered area than when the lens is viewing a narrow angle of the scene.

 

[iskoos]

Alright, I went through all the replies and I am pretty sure I got my answer
It must have been the metering mode and what the camera sees in the viewfinder. It totally makes sense. At wide angle, camera sees the subject and lot of surroundings; when zoomed in, it sees less of that surrounding. These 2 factors should be the reason for the change in the shutter speed.

My metering was evaluative correct. I was shooting with EF-S 17-85 lens which has f stop of 1:4-5.6. I was shooting at f/9.0 but I went a head and said that consider I was using a lens that has constant low f stop of 2.8. I said so because I knew somebody would jump in and say on some lenses relative aperture changes with the zoom. I just wanted to eliminate that but I failed

I know aperture(diameter) changes with the focal length and (this is due to keep the same f stop=relative aperture) and It is the relative aperture which is focal lenght/aperture diameter that changes things. So as long as I see the same f stop value in the viewfinder, it doesn't matter what focal lenght I am using. Though this is the theory I guess. In the real world, there are other factors as explained above. BTW thank you for those explainations (posts #4 and #5)

Well after all this, can I say that if I was shooting a plane horizon over an empty sea and a clear sky right after the sunset. And the camera was on a tripod and the horizon line was in the middle, then in that case I would probably not see any change on the shutter speed. Am I right?

 

[Dominantly]

Meh, can't say for sure.

You can always change your metering mode, to maybe spot, and then keep the spot of the area you want to set your exposure off... that way it will stay the same.

OR you could shoot it in manual mode and stop of the camera from choosing all this stuff on it's own; that's my recommendation.

 

[iskoos]

Understandable. I will try the same thing with spot metering and see how it comes out...

 

[Dominantly]

Maybe I'm sending you down a frustrating path with that one.

How about look at what your settings are while in Ap mode. Then put the camera in manual, enter those settings in, and shoot away.