Why is actual exposure changing as I go from one extreme to the other in aperture priority mode?

Question

I just got my first DSLR, Nikon D5200 and after reading various tutorials and understanding basics of Exposure Triangle, I was out for testing my knowledge to get a practical view of how things get different with different settings.

Now that I'm back, I'm stuck in a odd situation.

Please have a look at https://i.sstatic.net/vacrg.jpg. All the 4 pictures have 18mm focal length and are shot in Aperture Priority mode.

Here's the EXIF data of the 4 pictures (all at ISO 100):

• 1st : f22, 1/60
• 2nd : f14, 1/200
• 3rd : f9, 1/500
• 4th : f 3.5, 1/3200

The odd situation is, in small aperture (large f number) the exposure is bright while in large aperture (small f number) the exposure is underexposed. How can it be so?

Technically I thought all the pics should have the same exposure, as in A mode, camera changes the shutter speed along with change of aperture to maintain a balanced exposure. And, as you can see it did change the shutter speed to maintain the ratio.

How did this happen?

Answer 1

The second and subsequent pictures should be darker. You increased the aperture size by 1 1/3 stops from f/22 to f/14 but reduced the shutter speed by 1 2/3 stops from 1/60 to 1/200. So the second picture should be 1/3 stop darker than the first.

How did this happen when you only changed the aperture and the camera automatically changed the shutter speed? One of several things might have happened.

• There is no such thing as perfect exposure. For a given meter reading the correct shutter speed for a specific scene and a selected aperture will probably fall somewhere between two available shutter speed settings. For instance, in your photo with f/22 (which is actually f/22.6) as the selected aperture the meter may have said the correct shutter speed should be approximately 1/69. But your camera has no 1/69 second shutter speed! It must choose either 1/60 or 1/80. It chose 1/60. When you stopped down to f/14 (which is actually f/14.3), It may have wanted a shutter speed around 1/181. Since there is no 1/181 shutter speed available for your camera, it chose 1/200 rather than 1/160.
• The clouds behind you may have shifted just enough to make the scene darker at the time the picture was taken than at the time the meter reading was taken. If the meter reading was almost exactly between two available shutter speeds, then even moving your head to cover slightly more sky while revealing slightly more of the dark top of the building directly behind you may have been enough to sway the meter from rounding up instead of down. Not to mention that your framing was slightly different in each photo, which likely affected the meter reading. If you include less of the darker buildings in the frame and more of the brighter sky, the meter will set a lower exposure value because the overall brightness of the scene has changed.
• The difference between the theoretical aperture values a camera/lens is set for and the actual aperture value that setting gives you is not exact. The same is true of shutter speeds. This is even when the actual target number of f/22.6 for a setting labeled f/22 is taken into account. The design of the iris in that diaphragm may give you an opening that equates to f/22.4563793.

For the third photo you increased aperture by 1 1/3 stops from f/14 to f/9 while increasing the shutter speed 1 1/3 stops from 1/200 to 1/500. The exposure is the same as the second photo as can be seen in the photos.

For the fourth photo you increased aperture by 2 2/3 stops from f/9 to f/3.5 while increasing shutter speed by 2 2/3 stops from 1/500 to 1/3200. The exposure should again be the same. But the last photo does appear a tad darker (but probably by not even a 1/3 stop difference) than the second and third photos. What might be the explanation?

I'm going to go out on a limb here and guess that you are using an 18-55mm f/3.5-5.6 kit lens. At 18mm and f/3.5 you are at the very end of the lens' focal length and aperture ratings. But those numbers may be slightly rounded to the nearest whole stop/focal length number. In fact, it is very likely that they are rounded.

f/3.5 is the lens' stated maximum aperture, but it is only that wide at 18mm. If you are zoomed in at all, the change in focal length will reduce the f-number for the same size hole in the diaphragm. And even at 18mm the f-number is probably rounded to the nearest 1/3 stop, so f/3.5 might actually be more like f/3.7 and 18mm may be more like 18.5mm. So when you open all the way up to f/3.5 you are probably not quite getting f/3.5, especially at the lens' widest focal length that may be rounded down to 18mm.

Answer 2

In each of the 4 photos, the amount of sky and foreground has changed.

The sky is very bright and everything else is much darker. This makes for a very challenging exposure for any camera. The camera metering had to decide between the light and dark areas and come up with a guess as to what the correct exposure should be. Your camera actually did a very good job considering the variations in the each scene.

If you want to repeat your test, use a tripod with completely static objects.

Answer 3

Exposure value, EV, is log2(Fnumber^2/Time_in_secs). That is, if the aperture is set to f/2, the Fnumber is 2; and suppose time is set to 8 secs, log2(2^2/8) = -1 EV. Closing the aperture by 1 stop, that is increasing the Fnumber by sqrt(2), we are getting Fnumber = 2.8. Decreasing the shutter speed to compensate, also by 1 stop, we are getting 16 secs, and the formula above results in the same EV number, -1 EV.

For your shots the EV numbers are: 14.83 15.26 15.31 15.26 That is, the difference in the exposure in the first shot and in the three others is close to 1/2 stop. Given the light in the scene did not change...

The exposure experiments are better done when the light does not change, say, indoors, with incandescent lights (not fluorescent, due to flicker).

Keep the area under the focus point constant if experimenting with matrix metering, as the metering system gives the priority to the brightness under the focus point. Keep the framing the same. Watch that Active D-Lighting (ADL) is Off, it may cause the results to be all over the place. Any "Auto" may also skew the results.

Drop the exposure triangle, exposure does not depend on ISO. Accepting exposure triangles folks start to think that the reason for the noise is high ISO, not the insufficient amount of light.

Also, exposure has little to do with image brightness, change the monitor brightness and what now? - does exposure change? No. Exposure depends on shutter speed, lens aperture (T-stop, actually) and scene luminance at the time the image was taken.