Exposure ? on constant aperature telefoto lens

[Snurk71]

Have a question about the exposure I'd get using a constant aperature telefoto lens, like the canon 70-200, f/2.8. If I were shooting from a constant position, set my shutter speed appropriately for 200mm (and set a constant aperature and ISO), and then flipped back and forth shooting the same subject at 70 and 200 - would the exposure of the shots be the same or would one be lighter/darker than the other?

Not sure if I'm explaining that well or not. Basically I want to be able to shoot the same subject wide and close and have the same resulting exposure on the wide and close shots.

Thanks

 

[photo_chick]

In theory they should be the same exposure value if only the focal length is changing. Even though we say a lens has a constant aperture, the size of the opening isn't really constant at all. It's diameter will change based on focal length / aperture = diameter. so f/2.8 at 200mm is physically wider than f/2.8 at 70mm. It has to change because less light gets down the barrel at longer focal lengths and the actual aperture numbers are based on the amount of light let in.

That's the theory anyway. In practice I find my 70-210 f/4 to be a little darker on the long end at an equal exposure value.

 

[Gianna'sPapa]

I think I know what you're asking. If you use Aperture Priority and set it to f2.8 and set your ISO then as you zoom, the shutter speed probably will change depending on the lighting. The camera will compensate for the lighting by adjusting the shutter speed to provide a proper exposure. If it didn't then you would have an underexposed or overexposed image. Is this what your asking?

Photo chick, I didn't mean to step on your post I got interrupted as I was writing.

 

[zackiedawg]

If you're shooting in Manual mode, and controlling the shutter, ISO, and aperture yourself, then yes - in theory it should be the same exposure. Variations may occur for other reasons - such as the amount of vignetting at different focal lengths affecting corner darkness, or the fact that the zoomed-in shot has significantly more highlight or more shadow filling the frame than the wider shot (say a boy in a white baseball jersey on a mound throwing a ball - from 70mm that boy might be 10% of the composition, so blown out whites wouldn't really show up as noticeably, but at 200mm he may be 25%-30% of the composition, and the shot may LOOK much more overexposed.

As soon as you go to any mode which allows the camera to do the metering, then you could end up with very large differences, due to how much light or dark is in the metering area to be read by the camera.

 

[Snurk71]

Thanks, guys. That does answer my question (and is what I thought the answer was, but I wasn't sure). My hope/intent is that the wide and close shots will relatively have the same exposure so that I won't have a high variability in the exposures to adjust in post processing. I know I'll probably have some vignetting on the close shots - but I can live with a little vignetting.

Gianna'sPapa - I'll be going full manual instead of aperature to make sure all things stay constant so I don't suffer what zackiedawg described could happen.

 

[photo_chick]

Photo chick, I didn't mean to step on your post I got interrupted as I was writing.

No worries... I don't think you did.

I often take like 20 minutes to write one and by then 5 other people have posted. LOL

 

[artistico]

If you're shooting in Manual mode, and controlling the shutter, ISO, and aperture yourself, then yes - in theory it should be the same exposure. Variations may occur for other reasons - such as the amount of vignetting at different focal lengths affecting corner darkness, or the fact that the zoomed-in shot has significantly more highlight or more shadow filling the frame than the wider shot (say a boy in a white baseball jersey on a mound throwing a ball - from 70mm that boy might be 10% of the composition, so blown out whites wouldn't really show up as noticeably, but at 200mm he may be 25%-30% of the composition, and the shot may LOOK much more overexposed.

As soon as you go to any mode which allows the camera to do the metering, then you could end up with very large differences, due to how much light or dark is in the metering area to be read by the camera.

I agree ... sometimes you must careful with your manual modes stettings !!!!!