Exposure for beginners.

Started Dec 24, 2020 | Discussions thread
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Exposure for beginners.

Here we go, in Q&A form:

What is exposure?

You don't need to know. Really -- you don't. You can take great photos all day long without knowing anything about exposure.

But I really want to know -- I hear about it all the time and sounds important. What is it?

Exposure is the amount of light per area projected on the sensor. For a given camera, a higher exposure means more light is projected on the sensor and a lower exposure means less light is projected on the sensor.

That's it? So higher exposures are brighter and lower exposures are darker?

Higher exposures are typically represented lighter and lower exposures are typically represented darker, but that's a matter of how the photo is processed, either in camera or on your computer. You can make a low exposure night scene look like it's day time or make a high exposure day scene look like it's night time. But the exposure is simply the amount of light per area projected on the sensor -- like I said, you don't need to know.

So why does everyone make it seem like it's important?

Higher exposures, for a given camera, will result in less noisy photos, but possibly have more blown highlights. Lower exposures will result in more noisy photos, but possibly less blown highlights.

So exposure is all about noise and blown highlights?

Yep. However, what matters more is how the exposure is achieved.

What do you mean? How do you "achieve" an exposure?

The exposure is determined by:

  • The scene luminance (how bright the scene is).
  • The f-number of the lens.
  • The exposure time (how long the shutter is open).

Changing the scene luminance (e.g. flash photography), f-number, and/or exposure time will result in a change in the exposure (thus the noise and/or blown highlights) unless they are adjusted in such a way that the changes cancel out.

I don't follow. Can you give an example?

Sure. Let's say you set the camera in P-Mode Auto ISO and it chooses f/4 1/50 ISO 100. Changing the ISO setting manually in P-Mode will result to changes in the f-number and/or exposure time.

For example, if we change to ISO 400 (4x as much -- two stops higher), the camera might choose f/5.6 1/100, which will result in 1/4 as much (two stops lower) exposure, and thus a more noisy photo. It will also result in a deeper DOF and, if there's motion in the scene, less motion blur.

Alternatively, let's say you set the f-number to f/2.8 (one stop wider aperture, thus one stop more light) and the exposure time to 1/100 (half -- one stop less -- exposure time, thus one stop less light). The one stop wider aperture and one stop shorter exposure time cancel out, so the exposures remain the same. Hence the noise (since the total amount of light remains unchanged), but the DOF would be more narrow due to the wider aperture and the risk/amount of motion blur would be less due to the shorter exposure time.

What do you mean by "1/4 (two stops lower) exposure"?

Setting f/5.6 results in an aperture with half (one stop less) the area than f/4. This results in a deeper DOF and half as much light projected on the sensor.

An exposure time of 1/100 is half as long (one stop less) as 1/50, which results in less motion blur (if there is motion in the scene) and half as much light projected on the sensor.

Each of these halvings (one stop decreases) in light result in a net of 1/4 (two stops less) light on the sensor, and hence a more noisy photo.

Why do higher f-numbers result in more DOF and less light? Why do shorter exposure times have less motion blur?

The diameter of the aperture (the optical opening in the lens) is the quotient of the focal length and the f-number. For example, the aperture diameter at 100mm f/2 is 100mm / 2 = 50mm and the aperture diameter at 100mm f/4 is 100mm / 4 = 25mm.

Wider apertures result in a more shallow DOF and a greater aperture area, thus more light passing through the lens for a given scene and exposure time. Shorter exposure times means that elements of the scene that are in motion move a shorter distance over the shorter time, thus less motion blur.

Does the difference in noise matter?

That depends on a lot of things. You could also ask if the difference in DOF matters or the difference in motion blur matters. It all depends on the scene and your aesthetics.

So, can't I just put the camera in Auto Mode with Auto ISO and let the camera do it's thing?

You absolutely can. However, there will come a time (probably sooner rather than later) that you find the DOF, motion blur, and/or noisiness not to your liking and want to do something about it.

OK, let's say it comes to that. How do I do it?

Lots of ways. There are different shooting modes on the camera that allow you do vary different settings independently. For example, if you're in A mode (Av mode for Canon) and you set the ISO yourself, then every time you raise or lower the f-number, the camera will correspondingly lower or raise the exposure time (changing risk/amount of motion blur) maintaining the same exposure (and thus the same noise).

For example, if you set f/4 ISO 400 and the camera sets 1/200 for the exposure time, then if you change to f/5.6 (one stop higher) at the same ISO setting, the camera will set 1/100 (one stop longer) for the exposure time, maintaining the same exposure.

If instead you raise or lower the ISO setting in this mode, the camera will correspondingly raise or lower the exposure time. Since the f-number remains constant in this mode, the exposure will be changing in a way that higher ISO settings will correspond shorter exposure times and thus lower exposures, and hence more noisy photos.

For example, if you set f/4 ISO 400 and the camera sets 1/200 for the exposure time, then if you raise the ISO setting to 800 (one stop higher), the camera will set 1/400 (one stop shorter) for the exposure time, resulting in an exposure one stop lower, and thus a more noisy photo.

Sounds complicated.

It does, doesn't it? Here's a quick guide (keeping all other exposure settings equal):

  • Lower f-number → higher exposure, and vice-verse.
  • Longer exposure time → higher exposure, and vice-verse.
  • Greater flash power → higher exposure, and vice-verse.

Then understand that:

  • Lower f-numbers result in a more shallow DOF. and vice-verse.
  • Longer exposure times result in a greater risk/amount of motion blur, and vice-verse.
  • Greater flash power does more to alter the lighting of the scene, and vice-verse.
  • Different exposure modes make it easier/more convenient to make various changes to various exposure parameters (f-number, exposure time, flash power).

OK -- I'll think on that. What about ISO?

Mostly, people use the ISO control in various shooting modes as an indirect means to changing the f-number, exposure time, and/or flash power. However, you can control any and all independently from the rest.

We want the lightness of the resulting photo (in the EVF and/or the LCD playback) to be more or less how we want it, so if it's too light or dark, we can also fix the ISO setting and use EC (exposure compensation) to compensate.

What's this EC thing?

Let's say that you're shooting P-mode and the camera chooses f/4 1/50 ISO 400, but the photo is too dark. Then if you fix the ISO setting at 400 and use EC+1, the camera will use a one stop higher exposure (by switching to either f/2.8, 1/25, or some combination thereof) and the resulting photo will not only be twice as (one stop) lighter, but be made with twice as much (one stop more) light, and thus less noisy.

On the other hand, if you're using Auto ISO and the camera is already at it's widest aperture (lowest f-number) and the camera doesn't want to use a longer exposure time, then when you apply positive EC, it will instead increase the ISO setting, which will simply lighten the photo by a stop, but the exposure will remain the same, so the noise will also remain the same.

What if I don't want to change the f-number or exposure time, but just want the photo to be brighter?

Then you can use M mode, fix the f-number and exposure time to what you want, and adjust the ISO setting to whatever gives you the lightness you want.

Don't higher ISO settings make the photo more noisy?

Inasmuch as setting a higher ISO setting results in a lower exposure (higher f-number, shorter exposure time, and/or less flash power), yes. But it's not the ISO setting, per se; rather, it's the lower exposure that results in a more noisy photo. If the exposure is unchanged, and you just raise the ISO setting to make the photo lighter, the higher ISO setting will actually result in a slightly less noisy photo -- don't ask.

What about Equivalence? I heard that an f/1.4 lens on mFT is really an f/2.8 lens.

That's another discussion. But, on the quick, no, an f/1.4 lens is an f/1.4 lens no matter what system it's mounted on. However, the effect that f/1.4 has will vary from format to format, just like the effect of the focal length will vary from format to format. Still, like I said, that's another discussion. For now, just get used to the camera you're using and don't worry about how it compares to other formats.

OK, thanks, I guess. I still don't get it.

No worries. When it's a broad topic, it's hard to explain all at once. It's much easier to understand when it's a more narrow question. Then, when you've understood a bunch of narrow questions, the bigger picture becomes a bit more clear (pun intended).

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