focus

Focus, Depth of Field and Perspective

Focus, Depth of Field and Perspective

Digital PhotographyDigital Photography

Sharp focus attracts the eye.Sharp focus attracts the eye.

Sharp focus attracts the eye.Sharp focus attracts the eye.

Think of the “plane of critical focus” as a pane Think of the “plane of critical focus” as a pane of glass stretched from one side of the scene of glass stretched from one side of the scene

to the other where everything is in focus.to the other where everything is in focus.

Notice that the depth of field extends about Notice that the depth of field extends about one-third in front of the plane of critical focus one-third in front of the plane of critical focus

and two-thirds behind it.and two-thirds behind it.

1/31/3 2/32/3

The photographer directs The photographer directs your eye to the point of your eye to the point of interest by using focus.interest by using focus.

Micha Bar-AmMicha Bar-Am““Yad Vashem Holocaust Memorial,” Jerusalem, 1981Yad Vashem Holocaust Memorial,” Jerusalem, 1981

The “plane of critical focus”The “plane of critical focus”is also calledis also called

the “image plane.”the “image plane.”

What exactly is sharpness,What exactly is sharpness,and how much can it be controlled?and how much can it be controlled?

In theory, a lens can onlyIn theory, a lens can onlyfocus on a flat plane atfocus on a flat plane at

one single distance at a timeone single distance at a time(the plane of focus)(the plane of focus)

and objects at all otherand objects at all otherdistances will be less sharp.distances will be less sharp.

But, in most But, in most cases, part of the cases, part of the

scene will be scene will be acceptably sharp acceptably sharp

both in front of both in front of and behind the and behind the

most sharply most sharply focused plane.focused plane.

Light from a single point on the subject reaches the lens in the shape of Light from a single point on the subject reaches the lens in the shape of a cone. Behind the lens, that same light converges again in the shape of a cone. Behind the lens, that same light converges again in the shape of

a cone, to a corresponding image point.a cone, to a corresponding image point.

If your film or sensor is located at the tip of that cone of light, that image If your film or sensor is located at the tip of that cone of light, that image point will be exactly in focus.point will be exactly in focus.

If the recording surface is not at the tip of the cone and instead If the recording surface is not at the tip of the cone and instead slices through the cone in front of or behind the focus point, it will slices through the cone in front of or behind the focus point, it will

record a small circle instead of a point.record a small circle instead of a point.

A circle can be just small enough that you can’t see that it’s a circle A circle can be just small enough that you can’t see that it’s a circle instead of a point. This size is called a “circle of confusion.”instead of a point. This size is called a “circle of confusion.”

Any points in the recorded image that make a circle this size or Any points in the recorded image that make a circle this size or smaller will appear to be in focus. smaller will appear to be in focus.

Any points within this range are inside the depth of field.Any points within this range are inside the depth of field.

Reducing the size of the lens aperture makes the base of each Reducing the size of the lens aperture makes the base of each cone of light smaller and its angle narrower.cone of light smaller and its angle narrower.

The same size circle of confusion can be recorded farther from The same size circle of confusion can be recorded farther from the focus point.the focus point.

So even though a smaller aperture doesn’t change the actual So even though a smaller aperture doesn’t change the actual location of the plane of focus, it widens the range of points that location of the plane of focus, it widens the range of points that

appear to be in focus, thus increasing the depth of field.appear to be in focus, thus increasing the depth of field.

There are actually 3 different ways to control the depth of field in a

photograph:

There are actually 3 different ways to control the depth of field in a

photograph:

change the aperture

change the focal length

change the distance to subject

change the aperture

change the focal length

change the distance to subject

We have already learned how toWe have already learned how toadjust the aperture to change theadjust the aperture to change the

depth of field.depth of field.

In actuality, the apertureIn actuality, the apertureworks works along withalong with the focal the focallength of the lens to definelength of the lens to define

depth of field.depth of field.

The longer the focal length, the less light reaches the sensor or film, The longer the focal length, the less light reaches the sensor or film, therefore a long lens will form a dimmer image than a short lens therefore a long lens will form a dimmer image than a short lens

unless more light is admitted by the aperture.unless more light is admitted by the aperture.

Apertures of the Apertures of the same number are notsame number are not

all the exact same size!all the exact same size!

The sizes of The sizes of apertures areapertures aredetermined so that determined so that at a given f-stop at a given f-stop number the same number the same amount of light amount of light reaches the film, reaches the film, no matter what no matter what the focal length of the focal length of the lens.the lens.

If the focalIf the focallength of the lens is length of the lens is 100mm, you need a 100mm, you need a lens opening of lens opening of 25mm to produce an 25mm to produce an f/4 aperture. f/4 aperture.

If the focalIf the focallength of the lens is length of the lens is 200mm, you need a 200mm, you need a lens opening of lens opening of 50mm to produce an 50mm to produce an f/4 aperture. f/4 aperture.

What does all this mean to you?What does all this mean to you?

When adjusting your camera for depth of field, JUST REMEMBER:When adjusting your camera for depth of field, JUST REMEMBER:

The smaller the aperture, the greater the depth of field

The shorter the focal length of the lens, the greater the depth of field

The greater the distance from the subject the greater the depth of field

The smaller the aperture, the greater the depth of field

The shorter the focal length of the lens, the greater the depth of field

The greater the distance from the subject the greater the depth of field

The smaller the aperture, the greater the depth of field

The shorter the focal length of the lens, the greater the depth of field

The greater the distance from the subject the greater the depth of field

Two more techniques let you control depth of field:

Two more techniques let you control depth of field:

Zone focusing

Focusing on the hyperfocal distance

Zone focusing

Focusing on the hyperfocal distance

For both of these techniques, you need a lens with a

depth-of-field scale.

For both of these techniques, you need a lens with a

depth-of-field scale.

Zone Focusing

Zone Focusing

Zone focusing is useful Zone focusing is useful when you want to shoot when you want to shoot rapidly without rapidly without refocusing, and can refocusing, and can predict approximately predict approximately where, if not exactly where, if not exactly when, action will take when, action will take place.place.

To zone focus, use a lens’s depth-of-field scale to find the f-stop To zone focus, use a lens’s depth-of-field scale to find the f-stop settings that will give you adequate depth of field.settings that will give you adequate depth of field.

Suppose the nearest focus point you want sharp is 7 feet away and the Suppose the nearest focus point you want sharp is 7 feet away and the farthest is 13 feet away. Turn the focusing ring until those distances on the farthest is 13 feet away. Turn the focusing ring until those distances on the distance scale fall opposite a matched pair of f-stops on the depth-of-field distance scale fall opposite a matched pair of f-stops on the depth-of-field scale. If you set your lens aperture to that f-stop, objects between the two scale. If you set your lens aperture to that f-stop, objects between the two

distances will be in focus.distances will be in focus.

Suppose the nearest focus point you want sharp is 7 feet away and the Suppose the nearest focus point you want sharp is 7 feet away and the farthest is 13 feet away. Turn the focusing ring until those distances on the farthest is 13 feet away. Turn the focusing ring until those distances on the distance scale fall opposite a matched pair of f-stops on the depth-of-field distance scale fall opposite a matched pair of f-stops on the depth-of-field scale. If you set your lens aperture to that f-stop, objects between the two scale. If you set your lens aperture to that f-stop, objects between the two

distances will be in focus.distances will be in focus.

This is a typical This is a typical digital lens with a digital lens with a DOF scale.DOF scale.

Here the aperture is Here the aperture is set to f/11.set to f/11.

The depth-of-field The depth-of-field scale shows that scale shows that objects from 1 to 2 objects from 1 to 2 meters will be in meters will be in focus at this focus at this aperture.aperture.

Hyperfocal DistanceHyperfocal Distance

Instead of focusing on infinity, (as shown above), set Instead of focusing on infinity, (as shown above), set the distance scale so that the infinity mark lines up the distance scale so that the infinity mark lines up

opposite your chosen f-stop on opposite your chosen f-stop on the depth-of-field scale.the depth-of-field scale.

As shown above, the infinity symbol As shown above, the infinity symbol falls just within depth-of-field scale for falls just within depth-of-field scale for

the chosen f-stop: f/8.the chosen f-stop: f/8.

Perspective:Perspective:How a photograph shows depthHow a photograph shows depth

Perspective is Perspective is the way the the way the brain judges brain judges depth.depth.

The brain judges depth in a photograph mostly by The brain judges depth in a photograph mostly by comparing objects in the foreground with those in the comparing objects in the foreground with those in the

background. The greater the size differences perceived, the background. The greater the size differences perceived, the greater the impression of depth.greater the impression of depth.

Compare the scale of the bird to the Compare the scale of the bird to the cage in these 3 views.cage in these 3 views.

When changing the focal lengths When changing the focal lengths only, the scale between the bird and only, the scale between the bird and

the cage remains consistent.the cage remains consistent.

Expanded perspectiveExpanded perspective seems to result from seems to result from a very wide lens, but using any lens close to a subject a very wide lens, but using any lens close to a subject stretches distances because it magnifies objects near stretches distances because it magnifies objects near the lens in relation to those that are far from the lens.the lens in relation to those that are far from the lens.

Compressed perspectiveCompressed perspective is usually associated with a is usually associated with a long focal length lens. It is because the lens is relatively far from long focal length lens. It is because the lens is relatively far from both foreground and background that size differences between both foreground and background that size differences between

near and far parts of the scene are minimized.near and far parts of the scene are minimized.

Making “Close-Ups”Making “Close-Ups”

Close-up PhotographyClose-up Photography

The best way to take close-up The best way to take close-up

photographs is with a photographs is with a macro lensmacro lens..

There are several types of There are several types of macro lenses and they are macro lenses and they are often distinguished by often distinguished by their focal lengths.their focal lengths.

A 50-60 mm lens is typically usedA 50-60 mm lens is typically usedfor small objects and product shots.for small objects and product shots.

A 90 - 105 mm range macro lens A 90 - 105 mm range macro lens is versatile and can be usedis versatile and can be used

for anything from insects to smallfor anything from insects to smallobjects.objects.

A 120 - 200 mm range lens givesA 120 - 200 mm range lens givesyou more working distance and you more working distance and

is commonly used for insectsis commonly used for insectsand other small animals.and other small animals.

If a macro lens is not If a macro lens is not in your budget, a in your budget, a reasonably priced reasonably priced alternative is a alternative is a

close-up lensclose-up lens..

A close-up lens attaches A close-up lens attaches to the front of your to the front of your camera lens. They come camera lens. They come in different strengths in different strengths (measured in diopters). (measured in diopters). The higher the diopter The higher the diopter number, the closer you number, the closer you can focus.can focus.

Close-up lenses are relatively Close-up lenses are relatively inexpensive and small, but image quality inexpensive and small, but image quality will not be as good as with other close-up will not be as good as with other close-up

methods.methods.

Still another option for close-up photography is Still another option for close-up photography is

a a bellowsbellows which fits between the lens and which fits between the lens and the camera to increase the distance from the the camera to increase the distance from the

lens to the sensor.lens to the sensor.

Extension tubes Extension tubes work essentially the samework essentially the sameway as a bellows. The object of both is toway as a bellows. The object of both is toincrease the distance from the lens to theincrease the distance from the lens to the

sensor.sensor.

Extension tube attachedExtension tube attachedbetween the camera bodybetween the camera body

and lens.and lens.

How big is a “close-up?”How big is a “close-up?”

A photograph is considered aA photograph is considered a

close-upclose-up if it is 1/10 life size (1:10)if it is 1/10 life size (1:10)to life size (1:1).to life size (1:1).

Macro-photography Macro-photography generally refers to generally refers to an image that is anywhere from an image that is anywhere from life size (1:1) to as big as 10 times life size life size (1:1) to as big as 10 times life size (10:1).(10:1).

Micro-photography Micro-photography is achieved by is achieved by photographing through a microscope to photographing through a microscope to get an image larger than 10 times life get an image larger than 10 times life size.size.

Other terms commonly used are:Photomacrography and Photomicography.

Other terms commonly used are:Photomacrography and Photomicography.

Depth of field is Depth of field is shallow in close-ups shallow in close-ups so focusing so focusing becomes critical.becomes critical.

A 50 mm lens at a A 50 mm lens at a distance of about 12 distance of about 12

inches from the inches from the subject has a DOF of subject has a DOF of

1/16 inch when the 1/16 inch when the aperture is set to f/4.aperture is set to f/4.

At f/11 the DOF increases -At f/11 the DOF increases -but only to 1/2 inch!but only to 1/2 inch!

Here you can see how increasing the distance Here you can see how increasing the distance between the lens and the sensor increases the between the lens and the sensor increases the

size of the image.size of the image.

Regardless of the method used to take a close-up, you must Regardless of the method used to take a close-up, you must move the lens farther from the sensor to focus closer to your move the lens farther from the sensor to focus closer to your subject.subject.

And the farther the And the farther the lens is extended, the lens is extended, the dimmer the light that dimmer the light that reaches the light-reaches the light-sensitive surface.sensitive surface.

So you must increase theSo you must increase theexposure time so the photo exposure time so the photo will not be underexposed.will not be underexposed.

Use a tripod to Use a tripod to take close-ups to take close-ups to

prevent motion prevent motion blur.blur.

Use a cable release if you have one.Use a cable release if you have one.

You can also use a flash toYou can also use a flash to““freeze” the motion.freeze” the motion.

These photos These photos were made using were made using a macro lens and a macro lens and a flash.a flash.

FiltersFilters

A polarizing filter A polarizing filter removes removes

reflections from reflections from surfaces such as surfaces such as

glassglass

This works best at aThis works best at a30 - 40 degree angle.30 - 40 degree angle.

A polarizing filter also makes A polarizing filter also makes distant objects clearer and the sky distant objects clearer and the sky

darker.darker.

The effect works best when you are shooting at a 90 The effect works best when you are shooting at a 90 degree angle from the sun.degree angle from the sun.

Other popular types Other popular types of filters are soft of filters are soft

focus and star (cross focus and star (cross screen) filters.screen) filters.