Peter West Carey Photography

Tag: metering

When You Change Focus, You Change Exposure – Photography Experiments
You might not know this, but your camera is a little neurotic. Trust me. Or don’t, and try this little experiment so I can prove it to you.

Focus

The premiss here is your camera has a bunch of light metering points. These points don’t cover your entire field of view when looking through the viewfinder (or back of the camera) and, unlike your focus points, you can’t see them. But they are there and they measure how much light is hitting each and every one of them, then your camera uses that information to figure out which Aperture, Shutter Speed and ISO to use.

One thing most people don’t realize is a little change in focus can cause a little change in metering. If you are shooting in Manual you can ignore this. This is for the rest of us who shoot in Program or Aperture or Shutter Priority.

Here’s the experiment you can run:

To prove my point and to help you understand the ramifications this feature has on your photos, let’s set up a simple scene. You’ll need a dark object, maybe your black camera bag, and a white object, like a piece of paper but with a little writing on it to help your camera focus.

1) Set the objects on a table, side by side.
2) Make sure the objects are well lit, either by an overhead light or the sun, which counts as an overhead light, I guess.
3) Change your camera’s focus mode. As all cameras are a little different, you will be looking for a mode which allows you to take manual control over which focus points are used. IMPORTANT! We are not looking for manual focus here, just manual selection of the focus points. Big difference. Consult your boring camera manual if need be.
4) Set your camera to Aperture Priority and set the aperture to the smallest number you can achieve.
5) Set your camera’s ISO to an appropriate level given the amount of light you have. 400 should work just fine.
6) Set your camera on the table or on a tripod so it will not move. This part is important because we don’t want our metering to change because of a framing change, only because of changing focus.
7) Frame your scene so the middle focus point is right in the middle, so a little of both the dark object and white object are covered. Make sure there is not much else other than your subjects in the field of view.
8) With that middle focus point selected, make note of your camera’s shutter speed.
9) Now select a focus point that is squarely surrounded by the dark object. Again, make note of the shutter speed. Did it change?
10) Lastly, select a focus point completely over the white object and also note the shutter speed.

DONE!

What you should have seen is a slight change in the shutter speed. Or maybe your camera is more sensitive and the change was larger. Either way, my point is to be aware of how this will affect your photography.

If you shoot on anything except Manual or Auto (meaning Program, Aperture or Shutter Priority) your camera’s metering will come into play. It is important to understand that the metering mechanism is affected by where your camera is focusing. While this is certainly not as drastic as using Spot Metering, as demonstrated in my previous post about Metering Modes, it is still a real effect that needs to be understood.

For Your Own Contemplation
- How will this affect you in the real world?
- What might you do to compensate for the difference in metering? (Hints are here and here)
Questions? Pop ’em like Pez in the comments section below or email me at peter@peterwestcarey.com.

Photography Basics – A 43 Day Adventure, and its companion 40 Photography Experiments, are series written by professional photographer Peter West Carey. The series are designed to unravel the mysteries of photography, helping you can take better pictures. Subscribe here to receive all the updates and bonus material. Your comments are always welcome.

If you enjoy the series, consider learning photography first-hand on a professionally led international photo tour in Nepal or Bhutan!
March 16, 2021
Use Your Histogram Info For Better Photos – Photography Basics

What is a Histogram?

Boring Answer (from Wikipedia): An image histogram is a type of histogram that acts as a graphical representation of the tonal distribution in a digital image.^[1] It plots the number of pixels for each tonal value. By looking at the histogram for a specific image a viewer will be able to judge the entire tonal distribution at a glance.

While true, and full of more cross links than I can ever muster, what is it really?

Sexy Answer: A Histogram is your secret key to well balanced exposures sure to blow the minds of duchesses and dukes alike.

Mildly Serious Answer: It shows the tone and intensity of light across your camera’s total dynamic range so you know if things are going horribly, horribly wrong.

Histograms Deconstructed

Total dynamic range, as mentioned before, is the number of stops of light your camera sensor can handle. These days it’s about nine stops of light from the darkest to the lightest, on average. The histogram for that type of image looks like this:

And can be accessed on all DSLRs, mirrorless and a lot of Point and Shoot cameras if you hit display or info. button enough times when viewing an image. What that metering is showing you (this version is actually from Adobe Lightroom and not the back of the camera, so your histogram may show more of less information, especially the color information and the shot information) is how the light is falling and with what intensity.

On the left are the darkest blacks (RGB color of 0,0,0) and on the right are the brightest whites (RGB 255,255,255). If the line that is the histogram does not come back down to zero, the bottom basline of the graph, before it hits one side, that is called clipping.

It’s a lot like clipping in American football; it’s not nice. And that means there was light data beyond what the camera captured. Lost data. Lost quality.

The example above is fairly well balanced. It ends before getting to the extreme sides and thus, has no overexposure and no underexposure. Hooray for our team.

Helpful Histograms

How can histograms help? For one thing, the screen on your camera is small and not able to show the finite detail in exposure you might need. It’s not the same as your computer screen, which has more range.

So the histogram is a way to see a statistical representation of where light falls. Let’s look at an example of a ferry boat in Washington.

This first photo was shot on Auto, so the camera surmised this is the proper exposure, with its accompanying histogram below:

You can notice there is already some clipping. The darks/shadows area of the histogram on the left is not down to the baseline bottom and ends quite high on the left side. This is evident in the photo (which is admittedly harsh, with more dynamic range than my camera can handle) by the fact that there is, to put it simply, some really dark shadows.

Most cameras these day will show you just which areas are too bright and too dark by flashing the over/under-exposed areas or coloring them red like this:

Shifty Histograms

If we were to under expose or over expose the picture on purpose (see yesterday’s Exposure Compensation), the histogram shifts.

The histograms below are examples of the same image above, over exposed by +1, +2 and +3 stops:

Some things to note here. When changing exposure, it’s not like the light is always symmetrical. See how that mound in the middle at first moves right (indicating it’s going towards over exposed) and then it changes to being all bunched up?

Second, take a look where the light falls on the left side, the shadow side. You can see it creeping to the right as the exposure is biased more and more overexposed. This shows you you have room to spare on that side (left side). And that main area close to the right in the ‘proper’ exposure? It quickly went off the deep end of overexposed with major clipping.

Now let’s under expose things one stop at a time.

Things get bunched more and more to dark side and there is a lot of clipping. Below is an example of the image under exposed by 3 stops.

How Histograms Can Help You

How can this help you? It can help you by taking some of the guess work out of exposure. In tricky situations it lets you know how much latitude you will have to adjust the exposure brighter or darker without losing information.

For example, here’s a shot of the moon rising behind some trees in Western Australia, along with its histogram.

And now a “bad” moon overexposed by +2.25 stops and histogram:

See how much clipping there is and how much loss of detail in the moon? And notice that we don’t care about the black clipping in this case because that is all silhouette and the blackness of space?

It’s important to note here that the histogram is just giving you information. It’s a messenger and should not be despised nor shot. In this case, the histogram is not evil, the exposure is. Don’t go hating on histograms when they tell you things are wrong. Fix them!

For this moon, I can also move the exposure -1.25 and underexpose a bit and see what happens to the image and histogram:

While the moon is darker, it is not beyond the range of the sensor and that means we have more room to play with the image in a computer once home. The histogram shifted left but still well within range of acceptable.

Truth be told, both the slightly underexposed version and the first version will come out just fine with modern computer help.

Putting It All Together

Histograms are very helpful when shooting from a drone as there is often glare on the screen, making exposure estimates hard. Trust your Histogram, let go!

If there is clipping on one side, try moving the exposure the other way, via Exposure Compensation. If there is clipping at both ends, you may need to used a graduated neutral density filter to cut down on light in certain areas, otherwise, you simply have to accept that digital cameras do have a finite capability that can be surpassed, currently.

An option here is to use well thought out and executed High Dynamic Range techniques. These use a number of overexposed and underexposed images that are merged, typically in a special program or in-camera on a phone, to increase the overall range of the finished image. But I’m not going to get into that now. That’s for another post.

Right now, I need to find a bunny and my library card.

Next Up: The Rule Of Thirds

Questions? Pop ’em like Pez in the comments section below. or email me at peter@peterwestcarey.com.

Photography Basics – A 43 Day Adventure, and its companion 40 Photography Experiments, are series written by professional photographer Peter West Carey. The series are designed to unravel the mysteries of photography, helping you can take better pictures. Subscribe here to receive all the updates and bonus material. Your comments are always welcome.

If you enjoy the series, consider learning photography first-hand on a professionally led international photo tour in Nepal or Bhutan. More information can be found at Far Horizon Photo Tours.

October 18, 2018
Metering Modes: How Your Camera Sees Light – Photography Basics
Let’s jump right into the guts of your camera and how it makes exposure decisions.

Metering Modes

What are Metering Modes? Your eye and brain are fantastic at figuring out what is the correct exposure for any scene and adjusting to it. You squint without thinking and in dark situations your pupils dilate to bring in more light (they do that at the eye doctor as well but that’s just because of those wicked drops).

Your camera has to try to do the same thing: judge the amount of light in a situation and set the ISO, Shutter Speed and Aperture appropriately to make a proper exposure.

NOTE: Metering Modes come into play when you are shooting in any Camera Mode other than manual.

The problem is, a camera is not nearly half as cool or as complex as your brain and eyes. So the camera manufacturers have come up with some methods for metering light and trying to figure out the best settings.

Different Modes for Different Situations

For most DSLRs and mirrorless cameras there are three different metering modes in common: Matrix or Evaluative, Center Weighted and Spot. In all of these modes (and others, depending on manufacturer) the camera uses a special sensor to evaluate light hitting it, as if it were hitting the main sensor.

As mentioned before, light is measured in a camera in stops. From one stop to the next is either half as much light or twice as much light. The dynamic range of most cameras, that is, the range of stops captured in a single image, is maybe 8 stops. Cameras improve all the time and now top 13 stops.

The dynamic range of the human eye in a single instance is estimated at 14 stops, with an overall dynamic range of around 25 stops.

As the camera is limited in range (we’ll ignore HDR for now), it has to pick what gets favored for exposure. Maybe a scene has a dynamic range of about 15 stops and your camera is limited to 7 stops. What to do? Metering will pick what’s important and select an exposure based on that. Let’s look at the modes.

The Three Main Metering Modes

Matrix/Evaluative Mode is going to attempt to meter 80% of the scene, give or take a bit. This mode works well for a wide range of subjects especially if there is a lot going on and you aren’t sure where you want to meter. It has its limits as it doesn’t cover the corners (usually not a big deal) and may try to balance a scene with a wide range in a manner not to your liking. This mode is the king of compromise.

Center Weighted (sometimes shown without the spot in the very center) starts to narrow things down a bit. In a lot of cameras the light reading is taken from the center 13% of the frame. A number of cameras even have the ability to change this percent to three different values. Anything not in the general center area will be ignored as far a light metering is concerned.

Spot Metering is handy when you know exactly what you want exposed properly and it is relatively small. Spot metering is a circle of about 2-3% of the frame (cameras will often show an actual center spot in the viewfinder to aid in selecting the area to meter). This can be handy when scenes are back-lit or there is a lot of brightness, contrast and other action going on, but maybe only one element is important to the exposur

Metering Modes in Real Life

Now lets take a look at how these metering modes can affect your image. I have set up a likely scene on a chair using my daughter’s friends Lamoin and Hopster.

In all these scenes I set the ISO to 100 and the aperture to f/8 to hold them constant. Only the shutter speed will change and it will only change because the camera is sensing differences in the light.

It will sense differences in the light because it will use different modes and we’ll start at the top with Matrix or Evaluative.

ISO 100, f/8, 2 Seconds

The shutter was open for two seconds. The camera metered for the entire scene and faced with a tough choice (lots of lights and darks) it picked something it thought was middle ground. If it could think. It can’t. Ha ha ha ha. Only Hopster can think. Ok, enough nervous laughter, next is Center Weighted.

ISO 100, f/8, .6 seconds

That superimposed oval is about the area the sensor metered (and a good mathematician will tell me it’s far greater than 13%!). You will notice Hopster is much better exposed with only .6 seconds of a shutter speed, but we’re losing Lamoin’s hooves and the chair is getting dark. Still, I like this better than the first. Spot Metering time.

ISO 100, f/8, .3 Seconds

As the spot is only on Hopster’s white ‘fur’, the camera is attempting to exposure properly for it. The camera did a decent job, but you can notice what happened to the rest of the scene. Dark, dark, dark. What happens, though, if we move the spot to the darkest part of the scene, to the area just below Lamoin?

ISO 100, f/8, 10 Seconds

Zoiks! While that one patch of bison fur is exposed well, the 10 second shutter speed makes everything else overexposed. Can we mitigate this a bit?

ISO 100, f/8, 3.2 Seconds

That’s a bit better as far as Lamoin is concerned. Hopster is still blown out and not the least bit happy.

It is important to note DLSRs are often equipped to favor metering at times. Such as in Matrix Mode. Often the focus points that are locked on a subject will get a slight bump in their overall importance for light metering as the camera knows this is likely the main subject. Others take it a step forward and use face detection software to know where a face is and expose for that, instead of an overall evaluation of the scene.

What About My Phone?

Your phone uses Matrix or Evaluative metering most of the time. It looks at the whole scene and tries to balance the exposure.

Using an iPhone as an example (although most phones work the same), a large rectangle appears to show where the metering and focus are locked, when locked. In this example, the box is closer to center-weighted than true evaluative.

But that makes the leafs a little over exposed. All you have to do to change the area where your phone meters and focus is to tap that spot on the screen. In this case, I want the leafs better exposed, thus I tap them.

That’s better. What happens if I tap the ground to make it better exposed?

Whoa now! The ground is brighter but now the leafs are totally blown out and my Shutter Speed is so slow that things got blurry.

You might also notice how your phone will put a focus and exposure box around someone’s face when it finds one in the scene? That’s because the camera assumes you want the face exposed correctly and 99 times out of 100 that is true.

The point here is you can pick your exposure area in a phone just like you can in a DSLR or mirrorless camera. This “tap on the screen” method is also true for most drones.

The Right Mode for the Right Scene

So which mode is right for which situations? Without being by your side for every shoot you go on, I can make some generalizations:

Matrix/Evaluative Mode
- Good general walk around setting
- The beach
- Midday sun
- Cityscapes
- Lightning
- Bison
- Mountain ranges
- Grand vistas
Center Weighted
- Concert/Band
- Sunsets/Sunrise (to meter and then recompose)
- Group shots
- Waterfalls
- Portraits
- Birds
Spot
- The Moon
- Sports action from afar (where the subject is pretty much filling the center of the lens)
- Back-lit subjects
- Yeti
Wrapping Up Metering Modes

Play around with the modes and get to know them better. You will find times when the main subject is still too bright no mater how much you adjust the exposure compensation (more on that next week).

That is likely because your camera is in Matrix Mode and trying to balance everything. Switch to Center Weighted and chances are you will find things starting to come around. Get more precise with Spot and the control becomes even more finite.

To access the different metering modes, look for a dial or knob with the icons above on it. On some Canons it is a button press then a wheel turn. On Nikons it can be a rotatory dial on the side of the eyepiece.

Next Up: Camera Modes(and how to stop using the green rectangle!!)

Questions? Pop ’em like Pez in the comments section below. or email me at peter@peterwestcarey.com.

Photography Basics – A 43 Day Adventure, and its companion 40 Photography Experiments, are series written by professional photographer Peter West Carey. The series are designed to unravel the mysteries of photography, helping you can take better pictures. Subscribe here to receive all the updates and bonus material. Your comments are always welcome.

If you enjoy the series, consider learning photography first-hand on a professionally led international photo tour in Nepal or Bhutan. More information can be found at Far Horizon Photo Tours.
October 11, 2018