Tutorial #12: Understanding Metering Modes

Aside from Auto Focus, understanding Metering Modes is a prevalent issue for beginning photographers. Learning more about these settings will help you be prepared to shoot quickly so you don’t miss or screw up important shots. It’s nothing complicated, the only real issue is not knowing.


The best time to use these modes is when shooting with AV/Aperture Priority mode, TV/Shutter Priority mode or any of the other modes that don’t require full Manual or full Auto settings.

01. Center Weighted: This mode will average out the the exposure of the center of the image. This works well for portraits or close ups of objects that take up most of the image’s central area.

The reading is taken from a larger central area.

02. Spot or Partial: This mode will take a reading from a small 5% (spot) to 15% (partial) spot at the center of the frame. Some high end cameras will allow you to choose a point (usually an AF point) from which to get the exposure. Use this when you want to expose for small objects while ignoring most of the background. Good for back-lit images.

The reading is taken from a small spot in the center.

03. Matrix or Evaluative: This is often the default, but it can help you get some advanced auto metering. It allows you to use this in combination with auto focus points which I describe in detail in my last tutorial [Understanding Auto Focus]. It will take an average of the entire scene but place priority on the AF point selected.

Priority is placed on the selected focus area.

Button layout on a Canon 60D. 

The Exposure Mode screen on a Canon 60D.

Auto Exposure Lock [AE-L] Button:

This button allows you to lock the exposure taken when you half press the shutter button. The camera will not take another reading until you either attempt to change the AF point or actually take a shot. This is useful if combined with specific modes, such as spot/partial metering, so you are able to re-compose the shot after getting a reading from a small area.

The Auto Exposure lock typically looks like an asterisk.

For more info on exposure such as EV Values see my other tutorial: [Explaining Exposure]. Then you’ll be ready for my [HDR Photography] tutorial.

Tutorial #11: Understanding Auto Focus

The most often ignored or unknown settings for most beginners often deal with Auto Focus. I often see people accidentally change these settings then wonder why their camera “isn’t focusing properly anymore.” The following info should do away with all the frustration that results from just not knowing how your camera works.


The first most important thing to become familiar with are auto focus points and how to select them. The number of points and where they are situated depends on the make and model of the camera but there are a few typical configurations such as the 9 point system used on most entry to mid-range canon cameras. There is an option to manually select a point or to have the camera decide which point or combination of points to use.

You will typically see something like this in your viewfinder:

This is the dedicated Auto Focus Point Button on a Canon 60D. Although it may be elsewhere on other models it is labeled with “AF.”


The AF button will take you to the selection screen. The default setting is “Automatic”

You can then select any point you want with any of the scroll wheels or arrow buttons.


There are 2 types of focus sensors. They are distinguished by how they are able to focus. Point can focus by finding contrast with vertical lines in a scene or by finding contrast with horizontal lines. Larger max apertures on lenses will allow for more accurate focus. Most cameras will focus with any lens with a max aperture of 5.6 or larger. (there are hacks that will allow for smaller max apertures found on some third party lenses) The reason for larger apertures providing more accuracy is because the focus sensors require a lot of light to be more accurate at detecting contrast.

The vertical line sensor is the most common type. It detects horizontal lines. (once again photography names things so that you can get confused by it opposite meaning, lol.) The other, more accurate type, is the cross type sensor which detects both horizontal and vertical lines in a scene. This type can be typically found at the center point and a few surrounding points in higher end cameras.

The red crosses are cross type sensors, the blue lines are vertical line sensors.

Of course if you rotate the camera into portrait mode those vertical line sensors will now detect contrast with vertical lines instead of horizontal…


Autofocus deals with two settings and of course the Af/Mf switch on the lens. Some lenses let you adjust focus manually while set to auto others do not and attempting to can damage the lens.

01 Auto Focus Modes

The first thing to decide is what mode you should use. There is a setting for use on non moving objects and a setting for tracking moving objects.

Single shot Mode:

Canon calls it “One Shot” and Nikon calls it “AF-S” others might call it something else. When the shutter is half pressed, this setting focuses on a subject once then holds that focus.
You can put this to use by focusing and re-composing a shot without changing focus. It can be better put to use by combining it with the use of different focus points.

02 Continuous Mode:

Canon calls it AI Servo, Nikon calls it AF-C mode. Both allow constant focusing on selected focus points. This mode is best for moving objects because it continuously tracks an object to re-adjust focus should it’s position change before the shot is taken.

03 Automatic

Sometimes there will be a third setting that will automatically detect which mode to use. This typically doesn’t work very well and it’s better to choose a mode yourself. Canon calls it AI Focus, Nikon calls it AF-A.


Now that you know how to use auto focus and a bit about how it works it can really help improve your compositions and how quickly you can prepare yourself to take a shot.

Selecting ONE SHOT mode and combining it with using the center point then quickly recomposing the shot is one way to quickly autofocus on a particular part of a scene. Otherwise you can try to quickly select the point you think will fall over the part of the scene you want in focus. If you want to control of where to focus and avoid out of focus photos it’s best to manually choose your focus point.

Here are some samples of compositions and the points I selected during shooting. With enough practice, you will learn where the sensor will be able to achieve focus as well as which focus point to select for optimal sharp focus.

At times the focus points have actually influenced my compositions to a certain degree. Depending on which part of a scene i want in focus, I will tilt the camera so that a selected point falls on the subject. It’s made for some pretty interesting compositions particularly when shooting events .

[life in edit] Bokeh Photoshop Brushes Download

I made brushes for Photoshop! They simulate different types of apertures like 9, 8, 7, 6, 5 blade apertures and those “fun” shapes like hearts stars! I may update with more shapes later…

I even made one with my logo like anyone would actually use that. :D  

You can download it here: http://lifeinedit.deviantart.com/#/d50tsms

Tutorial #10: Motion Blur & Panning

These colder months mean shorter days with less light and more opportunities to use slower shutter speeds without needing ND filters, so this is a good time to go over this topic…

Motion Blur and Panning both illustrate movement in a still image. Not unlike Futurist painter Giacomo Balla’s Dynamism of a Dog on a Leash seeing the image trail shows the position of an object elapsed over a period of time. In the case of a photo that time frame would be dependant on a slow shutter speed.

For some reason anytime I think of motion blur this painting always comes to mind.
Dynasim of a Dog on a Leash 
The faster an object is moving the faster of a shutter speed you can get away with using while still capturing motion blur. Anything 1/30 and slower is a good place to start. Something like 1/10 is ideal – especially if you aren’t using a tripod. Just remember there’s a difference between camera shake and motion blur. At least one thing in the photo must be in sharp focus otherwise the blur in your photo is just undesirable camera shake.
If your lens has some sort of image stabilization and you’re using a wide angle you can get away with using pretty slow shutter speeds and no tripod. The image below I shot handheld with a shutter speed of 1/4.

17th & Beaumont
If a scene is fairly well lit it’s possible to show down the shutter speed just enough to capture motion by closing up the aperture as much as possible. I shot this carnival ride in late afternoon light at a shutter of 1/15 with the aperture stopped down to f/22.

Carnival Ride
In some cases you can use a flash set with a second curtain shutter to get a trailing effect and a sharp well lit image. What a second curtain shutter does is make the flash go off right before the exposure ends rather than right after the shutter first opens. Doing this makes the trails appear to drag behind the subject in movement rather than through it.
The other way to depict movement in an image is a technique called Panning which involves the camera following the object in movement so the background is blurred rather than the subject. It can be tricky to get the speed in which u swipe the camera to keep up with the moving subject, and the only way to really master it is practice and a lot of trial and error.
I panned the image below at 1/5 on a tripod by just swiveling the tripod to follow the ambulance as it passed by.
Ambulance Pan

This pan I shot handheld with a shutter of 1/40. The shutter was a bit fast so the background blur is very subtle but the subject still pops in comparison.
Cat Walk

There’s a lot to be explored with motion blur whether it’s adding a sense of movement to something that doesn’t move or using it to isolate a the subject just as bokeh does. Just be prepared to take a lot of shots without expecting them all to even be usable.

Tutorial #9: White Balance

White Balance is getting neutral colors such as whites and grays to appear neutral in a photo by compensating for the temperature or tint of available light. Often the color temperature chosen is based on an artistic decision. It’s important to know how to set it so you have control over whatever it is you want. Shooting in RAW will allow you to adjust the white balance in ways that aren’t possible with jpgs. In the days of film, you had to use film specifically designed for certain temperatures of light or filters to correct white balance.
The hotter the light the bluer it appears, but according to color theory, artistically choosing for it to appear golden or red gives a feeling of warmth whereas making it blue makes it appear cold. This color association is why typically food will be shot in warmer temperatures – it’s more appetizing – and things like ice will look best cooler.

You can always shoot using one of the camera presets. In most situations these or Auto White Balance [AWB] will work fine, but custom white balance is always the most accurate. Even if you want to adjust it later when editing it’s best to get it right when shooting.
The typical presets are: Auto, Daylight, Shade, Cloudy, Tungsten, White Fluorescent Light, and Flash. I’ve made this chart with the Kelvin measurement for some typical light situations:

If you want more accuracy use the “Custom White Balance” setting. You will need something white like a sheet of paper to photograph a sample from. Some higher end cameras will let you choose the actual temperature number like in the chart above. Here’s is what you would do on a Canon. I used a Rebel T1i in the example.

First, you will need to take a photo of something completely white under the lighting conditions you will be shooting in.

Press the menu button, go to the second screen and select Custom WB.
Then you’ll need to select the photo you took of something white.
Then when you press the “Set” button to select OK it’s set.
It will remind you to set the White Balance to Custom.
You can do that in the White Balance Menu. On the Canon Rebels it’s the up arrow labeled WB

Post Processing:

There are 2 variables to adjust: Temperature and Tint. Temperature determines how blue [cool] or red [warm] the whites appear. Tint is the balance of green and magenta – similar to color balance settings on a tv. Different types of light sources produce different tints and temperatures of light. Most fluorescent lights produce a green tint, daylight has a cool temperature, and incandescent bulbs produce very warm light.
To edit in Camera Raw you adjust the the sliders for Temperature and Tint as desired. It’s a pretty intuitive interface.

Although the best way to edit white balance is to shoot and edit in RAW, you can make adjustments in Photoshop by using “Hue/Saturation” to adjust tint if the photo is too green. Sliding “Hue” to the left will add more magenta hues and sliding to the right will make it greener.

Image > Adjustments > Hue/Saturation
shortcuts: ctrl+alt+U or cmd+option+U
For non-destructive editing use:
Layer > New Adjustment Layer > Hue/Saturation…
To adjust the temperature the easiest thing to do is use “Photo Filter.” Select the desired filter and adjust the density until you get the result you want, or even use a custom color to use as a filter.

Image > Adjustments > Photo Filter…
For non-destructive editng:Layer > New Adjustment Layer > Photo Filter…

The Results:
  The images are in order of warm, neutral, cool.

Tutorial #8: Calculating Hyperfocal Distance

Shooting at your camera’s Hyperfocal Distance will allow you to capturing the maximum depth of field possible for any given aperture. This is most useful for shooting landscapes or any scene where you want as much of the foreground and background in focus as possible in one shot. For every camera film or sensor size and aperture combination, there is a distance at which you must focus your lens on to achieve this maximum depth of field. It is the closest distance at which you can focus while still keeping objects at infinity appearing to be in acceptable focus.
:( The Math Stuff:
Of course there are all sorts of calculators and apps that will do the math for you, but in case you’re ever stuck in a position where that kind of tech isn’t available it’s always good to know how to do it yourself – even for someone who hates math as much as I do…
The formula is fairly simple:

Let’s use the example of shooting at a focal length of 17mm with an aperture/f-stop of f/8 on a Canon with an APS-C sized sensor. Take the focal length and multiply it by itself [square it] then divide it by the product of the f-stop number and the circle of confusion number and add the number of the focal length. The circle of confusion depends of the cameras sensor size The chart above contains the most common sensor types, but with a bit of research you can find the circle of confusion for even point and shoots and phone cameras. The result that you get will be in millimeters so you will need to convert that to something that is usable such as meters or feet. Just move the decimal 3 spaces to the left and round it off. You will get about 2 meters as your hyperfocal distance.That’s about 6 1/2 ft.

((17 x 17) /( 8 x 0.018)) + 17 = 2023.94mm = 2.02m = 6.63ft

Shooting Tip 01:
Some lenses have markers for showing focus distance (many Sigmas lenses). However, these markings can become unreliable under different temperature conditions that can cause the parts of the lens to expand and contract. A quick trick you can use is to either measure (if you want to be very precise…) or make an educated guess of the hyperfocal distance you’ve calculated and find an object to auto focus on at that same distance. Once focused, switch the lens to manual focus to lock it.

Shooting Tip 02:
Keep in mind that shorter focal lengths and smaller apertures increase depth of field so using these can get you shots that appear in focus from close in the foreground all the way to the farthest parts of the background.

The Results:

Notice how the stuff up close is just as sharp as the stuff furthest in the back.

…mainly religious imagery not intentional lol…I just found these to be the best examples because the statues are close in the foreground…and I’ve shot quite a lot of local landmarks for work…i guess there are not really many non-religious statues around here now that I think of it…

Tutorial #7: Macro Photography for Under $10

Macro lenses are expensive, so good thing there are cheaper ways to do macro photography. I’ll go over Reverse Lens Mounting and a bit on using extension tubes.
Supplies: First, I’ll go over the things you will need and some you might want to have…
01. A tripod (optional)

02. An SLR camera with a kit lens or any lens 50mm or wider.

03. A reverse mount ring (optional but recommended)

04. AND/OR An extension tube

Shooting Tip 01: Aperture Size & Depth of Field
The results will have a very shallow depth of field, so you will need a small aperture to increase the depth of field enough to have a decent amount of the image in focus rather than just a couple of millimeters. You will want to set the aperture to f/11 or f/16. You might not be able to see much when composing the shot if there isn’t a lot of ambient light, but it can mean the difference between this:

and this:

Shooting Tip 02: Manual Aperture Hack
If you’re shooting Canon and the lens you’re using doesn’t allow manual aperture control there is a “depth of field preview” button right underneath the lens release button. This button temporarily sets the aperture so that you can preview the depth of field by closing up the aperture to the setting of your choosing. Normally, the aperture is set to the max aperture [wide open]. I don’t really see the difference in its intended use, but it has a much more useful purpose.

Just set the aperture in the camera’s settings and press the DOF button while removing the lens. It will hold the aperture setting once removed.
Shooting Tip 03: Lighting
The lack of light from using a very small aperture will likely cause you to need more light and even a higher ISO to avoid camera shake. A flash can come in handy. Sometimes, you can get away with using a built in flash if you can figure out how to bounce the light just right, but you will probably need an external flash or some other light source. It could even be something you already have lying around like a desk lamp…
Shooting Tip 04: Focal Length & Magnification 
The wider the angle (the shorter the focal length) the more magnification you will get. My favorite focal length to use when shooting bugs is 35mm. Using an ultra wide lens will produce some extreme macro shots like the ones i posted below taken with the 10-22mm lens.
The Camel Logo on a Cigarette
Eye Shadow

 Shooting Tip 05: Focusing 
You will need to manual focus each shot. It helps to physically move the lens [and yourself] back and forth to focus once you have the focal length and the focus set. Once you try it out you’ll get a feel for the limitation of how it the lens is able to focus. You will need to be so close you are nearly touching the subject – especially with shorter focal lengths [18mm] 


Method 01: Free-lensing
Reverse Free-lensing is probably the cheapest method. It’s possible to do it with just the kit lens, and no extra attachments need to be purchased. In this method all you do is hold your kit lens reversed in front of your lens-less camera. You will also get a tilt-shift effect depending on how you hold the lens. There are a few drawback such as, dust getting into the camera and lens and light leaks. Here is a photo I took with a 10-22mm wide angle lens of my hair. You can clearly see my terrible split ends. :-/

Method 02: Using a Reverse Mount Ring
A Reverse Mount Ring can be purchases anywhere online for anywhere between $4 & $8. It’s probably best to purchase a common size like a 58mm ring and just get step up or step down rings for any other lens sizes. To use it, just attach the ring to the filter end of the lens and then attach to the camera. I find this to be the best way to get good macro shots. I don’t have to worry about light leaks or dropping the lens. Dust (and other things) can still enter the open end of the lens. I once had a jumping spider jump right into the lens! D:

I eventually got the spider out… This guy, actually:

Method 03: Extension Tube
Using a 50mm lens and an extension tube can give you similar results to reverse mounting, but extension tubes can cost $10 or more. It simply attaches between the lens and the camera to increase the focal length, but it doesn’t do much to increase working distance. You still need to get extremely close to focus on a subject.

Method 04: Combination
You can go on to combine the extension tube and a reverse mount ring to get even more magnification, but it still wont increase working distance and the depth of field may become too shallow. But of course, you can experiment with that if you like…
The Results:
I find that my best shots were taken with the reverse mount ring method.

I even shot some stereoscopic 3D macro shots, so you can get pretty creative with this stuff. [Cross your eyes until you see the 3D effect]

and to give you a better idea of the scale of some of these things…

If you want to see more examples, check out the Flickr Set with all my Reverse Mount Macro Shots.

Sure, a real macro lens will give you some good working distance, but the point here is really to have fun doing macro photography for under $10. You get what you pay for, sort of…

Tutorial #6: HDR Photography

High Dynamic Range Photography is fairly easy to do with a tripod and some HDR software and an image editing program. I’ll go over several ways to do it.

Dynamic range simply means the range of values between lightest and darkest visible tones. The idea behind HDR Photography is similar to the Zone System. I think of it as an extension to the Zone System and one of the modern applications of it in digital photography. It’s not just about making cool almost surreal photos, the goal is to capture tones in every zone. For more info on zones and tonal values see my previous post: [Tutorial #5: The Zone System]
For this tutorial, I’ll be using both Photomatix to quickly create HDR images and Photoshop to create them manually. First I’ll go over Photomatix. It’s reasonably priced at $99 for the pro version and $39 for the light version. http://www.hdrsoft.com/ Anyone can use the trial version which never expires but does watermark images made with some of the advanced features.
Photoshop CS5’s new feature is not very good…it really REQUIRES raw files for actual HDR images…and just gave me an error message about my images not being the same size…so converting to JPG or shooting in JPG will result in a terrible image.

no, not ok, adobe. :<
It once even rendered a completely black image. Not very HIGH in dynamic range at all. :-/

Step 01: Shooting
If you want to do HDR the right way, you will need to Bracket. This means taking anywhere between 3 and 5 different exposure values of the same image. One correct exposure and then over and under exposed by 1 and 2 stops. I would strongly recommend using a tripod and shooting in RAW. If you take your photography seriously you should always shoot in RAW – even if right now you aren’t using or editing those files.
When shooting try to work quickly and shoot with similar lighting which may be difficult with landscapes if it suddenly becomes overcast or sunny. Clouds and plants also move pretty quickly on windy days. When putting the images together later it will cause ghosting from the different objects not being in the same place in each photo. That’s why the tripod is important. There may be similar difficulties with motion blur and ghosting on longer exposures. You want each of the 5 images to be as identical as possible so they line up properly.
Step 02: Pre-Processing
It’s definitely a bit tedious if the image is made from multiple shots, but it is possible to create them from one correctly exposed RAW file so you don’t have to worry about ghosting from any moving objects. You wont get all the dynamic range you can get from shooting multiple images but you can still get a similar effect.
When processing the RAW file export an image at the EV values of -2, -1, 0, +1, +2. For more on this see my previous post [Explaning Exposure]

EV -2EV -1EV 0EV +1EV +2
For demonstration purposes i’ll be using this one exposure method.
Step 03a: Processing in Photomatix
The first thing you do is load your bracketed photos [3-5 photos].

If the different photos were made from one file the meta info will have the same exposure value recorded for how it was shot. You will need to manually fill in the exposure value and the increments at which they were adjusted to. I did them in whole stop increments from -2 to +2.

When you press “ok” it will give you a few options for automatically dealing with several things, such as alignment and ghosting. I didn’t need to check any of these since I did mine off the same image.

It will then composite a preview image that you can adjust. The screen will look like this:

[click image to enlarge]
There are several detailed settings on the left and the filmstrip along the bottom contains various presets that you can apply and then fine tune on the left panel. You can use the loupe to magnify details. The top bar has all the zooming/viewing options, and there is a histogram toolbar as well.
The settings you decide on will depend on your taste. Some people love the “painterly” look and some hate it because it starts to look more like a painting than a photograph. Reducing the “smoothing” setting will increase the effect while increasing it will reduce it. The rest of the settings are best explored depending on the photo and what look you’re going for.
Step 03b Processing in Photoshop:

This step is more advanced. You need some familiarity with layers & blending modes and layer masks. First, open the correctly exposed image [EV 0]. Then “Place…” the underexposed images over the the original layer and place the over exposed images on top of those. If you need to adjust the alignment you can do so as you place each image by reducing the opacity of the placed layer and moving the layer until it matches the one under. ghosting elimination will come later.

You can play around with different layer orders and opacities – even blending modes – but i recommend setting any underexposed images to “Mulitiply” and overexposed images to “Screen.” Mulitply blends only the dark values into the final image composited and Screen only blends in the light values. this way your correctly exposed base image is being enhanced by the values in other images.
You can get even more tones values by using other filters such as “Diffuse Glow” and a Layer Mask to hide and reveal dark and light parts of each layer.
The Diffuse Glow settings I used are below, but yours may vary. You just want it to have a a soft glowing effect on the highlights. The final touch for this layer is to set it to screen mode and adjust the opacity as desired.

I added a bit of a vignette by using a copy of EV -1 layer in multiply mode over the screened layers and then using the layer mask to hide the center of the layer with a large brush in black.


The Results.
[click images to enlarge]
 The images that are done manually aren’t as detailed and simply have an HDR effect, but it is possible and actually more time consuming in comparison… After trying Photomatix i highly recommend it. I know a lot of others do too.

I’ll do a proper HDR image later on to show how color can be exaggerated, and how the results from multiple images differ and are ultimately better.

Angle of View & Focal Length

Use this chart to determine what focal length you should use depending on the horizontal angle of view you need to capture. The longer the focal length/narrower angle of view, the more flattening that occurs in the image, meaning that objects that are at a distance from one another appear to be in much closer proximity and within a flatter space than on a wide angle/short focal length. A wide angle lens will increase perceived spacial depth and at the same time increase distortion.

*The following is for full-frame/35mm cameras. You will need to do some conversions if you shoot with a different sensor size such as a Canon with a X1.6 crop factor or a Nikon with a x1.5 crop factor you will need to multiply the focal length indicated on the lens by the crop factor before referring to the chart [50mm x 1.6 = 80mm]. There are ways to convert for other image sensor sizes. Another popular one is for the Four Thirds System. The Four Thirds website lists the 35mm equivalents of all their lenses here: http://www.four-thirds.org/en/microft/lens_chart.html

Tutorial #5: The Zone System

The Zone System is a method used to take a scene as it appears to the eye and create an image to reflect the artist’s vision of it by controlling the exposure of lightness and darkness values. It’s a great method for achieving the highest dynamic range possible in one exposure, or to determine the proper way to composite an High Dynamic Range (HDR) image from multiple exposures. This will allow you to achieve correct, ideal exposures. It can be applied to film or digital in black & white or color.  
Use the following scale for guidance in differentiating between the different values. Each zone is an increment of one stop.

In order to apply the Zone System, you must look carefully at your scene and decide what objects fit into the middle value, Zone V. Figure out what Zone you are metering on. To your camera, anything you meter on will be considered Zone V and the meter will give you settings for that. The point is to match what you visualize in the scene as belonging in Zone V with what the camera will capture as Zone V, or middle gray. You want the objects in the scene to fall into their designated zones as follows:

Zone 0:  Darkest Shadows, night sky

Zone I:  Very dark shadows.

Zone II:  Pure black/dark surfaces, first instance of detail.

Zone III:  Dark shadows, dark surfaces with textures: cloth, plastic, paper.

Zone IV:  Blue sky, dark green foliage, shadows on dark skin or dark surfaces.

Zone V:  Gray card, dark skin, green foliage, shadows on light skin & surfaces.

Zone VI:  Average skin, yellow foliage, light stone.

Zone VII:  Very light skin, soft shadows on white surfaces.

Zone VIII: White surfaces with textures: Paper, Sand, Snow, Cloth.

Zone IX:  Pure white surfaces in direct light. On the digital scale this zone is synonymous with Zone X and appears pure white.

Zone X:  Reflection highlights, direct sources of light.

For example, if you meter on a dark object that falls into Zone III, then you will have to compensate the exposure by stopping down by two [-2] stops. The reason being that the camera will give settings that will shift the Zone III objects into Zone V and cause over-exposure. To correct this, and move the Zone III objects back into Zone III you must adjust your settings to be 2 stops darker [Zone III - Zone V = -2 stops]. If your camera were exposing for Zone VII tones then you would need to adjust the exposure by two stops [Zone VII - Zone V = +2] to avoid under-exposure.