Is it usual that a narrow aperture gives a darker picture in aperture priority mode?

Question

I have a Nikon D60 camera here with a Nikkor 35 mm 1:1.8G lens. I don't have any experience with any other digital SLR, so excuse the beginner question.

When using aperture priority mode, a narrower aperture gives a very noticeable darker pictures. The picture becomes so dark that I usually feel the need to increase the exposure compensation setting when using a narrow aperture.

I would expect the automatic shutter speed adjustment to give pictures with approximately the same brightness, regardless of what aperture setting is used.

Is the behaviour I see usual? Or does it indicate a problem with the camera's metering? If it is usual, is there a technical reason for it (e.g. some limitation in how the metering is done)?

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Here are two test photos to demonstrate what I mean: one and two. The first one was shot with an aperture setting of 3.5, the second one with 18 (you can see this in the EXIF info). The camera did not reach the fastest possible speed during this test and the metering was set to "matrix" to minimize the effect of me moving my hand between the two shots.

Answer 1

Ok time to cut through a little of the confusion:

• Technically there should be no difference in exposure when using a smaller aperture in one of the automatic modes as the camera should vary the other parameters to compensate. You might see a difference in the extreme corners due to vignetting at wide apertures, but this would make the small aperture shot brighter.

• In this case shutter speed decreased in the small aperture shot, meaning in this case metering is not [entirely] to blame. There was a 1/3 stop discrepancy between shots however.

Lets dig into the details a little...

The recorded parameters for the two shots were

1/1250s f/3.5

and

1/60s f/18

ISO was 100 for both. Exposure is proportional to shutter time over f-number squared. This gives a 1/3 stop disparity in the camera settings (this is probably a result of the camera's meter, however I'd be willing to forgive this minor variation as the composition did change a bit).

However if you look at the images, taking a reading from the highlighted side of the building bottom centre gives:

RGB = [134, 138, 135]

and

RGB = [74, 73, 72]

assuming a roughly linear tonecurve (unless you shot RAW we can't assume otherwise) this gives a 0.9 stop difference. So clearly there is something else going on here.

f/18 isn't exactly a third of a stop down from f/16, it's just rounded to 18 for convenience. In any case we don't know how accurately the camera can control the aperture when it's that small. Likewise you would expect some variation in shutter speed. As the shutter gets worn the curtains can get out of sync, giving you considerable variation, especially at high speeds.

If we say the actual settings used were

1/1100s f/3.4
1/65s f/19

Then suddenly we get a 0.9 stop difference in the expected exposures. There other factors that could be involved

• Slight haze passing in front of the sun can drop the exposure slightly whilst not being noticeable to the human eye.

• Nonlinear tonecurves and image processing in camera to produce the JPEG

Normally these things cancel out, but if they all line up one way you can get sizeable swings in exposure.

The lesson to bring from this is that it's probably normal, if it happens consistently it may well be something out of calibration, but I wouldn't worry about it, the camera's meter is not infallible, a better approach [] with digital is just to check the histogram after each shot.

Answer 2

The ISO for both pictures is the same: 100
F-stop are f/3.5 and f/18: i.e. 4 2/3 stops
shutter speed is 1/1250 and 1/60: i.e 4 1/3 of a stop.
So the exposure value for the two pictures are not the same...
DSLR's calculate exposure using maximum aperture may be that is why you have a darker picture when stopping down the aperture.

Answer 3

Is it usual that a narrow aperture gives a darker picture in aperture priority mode?

No, it is not usual.

does it indicate a problem with the camera's metering?

It might, or there may be a problem with the specific lens. You can find out by trying a different lens on the D60 and also trying the 35mm on a different camera. If you don't have a friend with a Nikon DSLR camera, you might ask a friendly camera store.

is there a technical reason for it (e.g. some limitation in how the metering is done)?

The scene in your sample shots could be a very bright day requiring fast shutter and small apertures at low ISO. A neutral density filter could help by reducing the amount of light to a level where the mid point of the available speed/aperture/sensitivity is used. Try taking a pair of shots somewhere darker and see if the same problem persists.

Answer 4

What METERING MODE are you using? Cameras like the 60D usually offer a variety of metering modes, which affect how a scene is processed in order to determine a proper exposure. Evaluative/Matrix metering should produce a correct exposure for both the f/3.5 and f/18 shot. If you are using a center weighted, partial, or spot metering mode, or some form of AF-point linked metering mode, then you could indeed encounter differing exposures when using a semi-automatic camera mode like aperture priority.

Find and set your metering mode to evaluative/matrix, and see if that fixes your problem. If not, then there might be an issue with the hardware. The D60 is getting pretty dated now, and its metering hardware may not be as capable and exact as in more modern DSLR gear. I would suspect, however, that you have simply selected a different metering mode...probably an AF-point linked spot meter mode. Changing back to Matrix should fix the problem.

Answer 5

I an not familiar with the Nikon D60, but...

Perhaps the camera is reaching a minimum shutter speed, and will not lower the speed below a certain level?

This may be be intended to prevent camera shake?