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 (12 comments 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

A 20mm lens can see much more of the scene than a 50mm lens, so the meter will choose settings that are different.

Try to find a very large neutral-gray surface with neutral lighting. Compose a shot where the gray fills up the 20mm lens. Now do the same with the 50. Exposure should be the same.

Another experiment is to switch your d80 over to spot metering mode. Choose something in the middle or near middle of your scene. Now take pictures with both lenses. You should end up with similar exposure settings. Notice that some areas of the 20mm image are over or under exposed. You could also do this experiment in manual mode, forcing the same settings.

Definitely play with the manual ("M") exposure mode on your d70 and the spot meter option. Program mode and matrix metering is great for getting good exposure every time, but you can do some interesting things if you move away from the perfect images.
posted by b1tr0t at 6:29 AM on December 2, 2008

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

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

Cowbell's last statement might be correct - that the manufacturer hasn't rated the lenses properly. The second statement is wrong though, focal length is already taken into account when the F numbers are computed. A teleconverter changes the effective f-stop precisely so that F numbers can still be compared between lenses of different focal length.
posted by b1tr0t at 6:59 PM on December 3, 2008