Different exposures of the same scene

December 2, 2008 5:32 AM

Exposure difference between two lenses at the same aperture? I just noticed that using the programmed exposure on my d80 with two different lenses at the same aperture produces different shutter speeds - even though the scene is the same.

First i measured the exposure with my Nikkor 50mm 1.4 at an aperture of 2.0. This yielded a shutter speed of 25.
Next i did the same with my Sigma 20mm 1.8 at aperture 2.0. This gave me a shutter speed of 15.

Can someone explain this difference to me?

posted by FidelDonson to Technology (10 answers total)

Did the light change? You could be looking at the same scene but with different lighting you would possibly need a different shutter speed.

Likewise, what metering mode were you using? If spot metering, were you metering off of the exact same thing?
posted by theichibun at 5:54 AM on December 2, 2008

You're dealing with two different lens focal lengths, which means that the framed scene will be different. And that could mean that the camera's metering sensor is detecting a different pattern of light, and making a slightly different pick for shutter speed. If you are using a spot metering mode, the same thing could be happening, or some other slight difference in the light transmission charectaristics of the lens is leading to the difference.
posted by baggers at 6:02 AM on December 2, 2008

It depends on the scene lighting and the metering mode you use. If you want a consistent and smarter method of determining exposures, buy a handheld light meter. Then you can expose for the light which you deem is important and not let the camera use its complex algorithms to water down a scene.
posted by JJ86 at 6:14 AM on December 2, 2008

Best answer: I think baggers is right; the most likely cause is just that the scene is different. Smaller/bigger field of view bringing less/more light.

I'm not positive, but my gut says that the focal length also effects the amount of light getting in, all other things being equal. (Like a teleconverter).

The lenses may also absorb differing amounts of light even if their apertures and focal length are the same if there are many more glass elements in one versus the other. (example)
posted by cowbellemoo at 6:42 AM on December 2, 2008

FWIW, 1/15 sec and 1/25 sec aren't all that different.
posted by JimN2TAW at 6:48 AM on December 2, 2008

50mm / 2 = 2.5cm diameter aperture. (focal ratio = focal length/aperture diameter)
20mm / 2 = 1cm diameter aperture.

In other words, holding the focal ratio constant (which is what you call the aperture setting), a longer lens has a larger light-collecting area. You say the same aperture, but you really mean the same focal ratio.
posted by kiltedtaco at 8:04 AM on December 2, 2008

kiltedtaco: What you say is true, however, a shorter lens is also collecting more light; a 20mm lens shines light from much more of the scene onto the sensor than the 50 does. The reason why we do this math in the first place is to eliminate that factor and make a term we can talk about that reliably predicts exposure independent of focal length.

So I agree that everyone else that on a featureless white wall you should be getting the same results, and it's only because something dark or light is coming into or out of the metering region.

It's also possible that the glass sigma on the absorbs a little more light than the nikon. It does have a lot more elements in it.
posted by aubilenon at 8:28 AM on December 2, 2008

I disagree with this: "1/15 sec and 1/25 sec aren't all that different" -- that's nearly a full stop, which is certainly significant.
posted by BaxterG4 at 11:28 AM on December 2, 2008

cowbellemoo writes "The lenses may also absorb differing amounts of light even if their apertures and focal length are the same if there are many more glass elements in one versus the other. "

If is definitely a factor. Back when coatings were in there infancy is wasn't unheard of to pick up a 1/3 or 1/2 stop on otherwise identical lenses when they went from no coating or single coating to multi coating.
posted by Mitheral at 4:31 PM on December 2, 2008

Response by poster: I did use spot metering on a large grey-white surface that filled the entire frame with both lenses. The lighting did not change in any perceptible way - at least not enough for it to change the exposure 2/3 of a stop - which imo is a significant difference.

Cowbellemoos answer seems to account for the difference the best.
posted by FidelDonson at 2:26 AM on December 3, 2008