Langfords
Large apertures produce a shallow depth of field. They let in more light so require a shorter shutter speed.
Smaller apertures produce a larger depth of field.
Large apertures produce a shallow depth of field. They let in more light so require a shorter shutter speed.
Smaller apertures produce a larger depth of field.
The smaller the relative aperture, the higher the f number. f/1 is the widest aperture allowing the most light through, f/32 is the smallest, letting only a fraction of the light through.
Each change to the next highest number halves the amount of light passing through the lens. This is called one stop. There is the ability to adjust in half stops and one third stop increments.
The f-number system means that any lens set to the same number gives a standard image brightness, irrespective of the focal length or the camera size.
The actual f-numbers themselves denote the number of times the effective diameter of the aperture divides into the lens focal length. f/2 means the aperture diameter is one half the focal length. f/4 is on-quarter.
Each f-number takes into account two factors that control how bright an image is formed:
1) Distance between lens and image
For a distance subject (lens focused on infinity) the image is formed at one focal length from the lens. The inverse square law of light shows that doubling the distance of a surface from a light source quarters the light it receives. A lens of 100mm focal length forms an image only one-quarter as bright as a lens of 50mm.
2) Diameter of the light beam.
Doubling the diameter of a circle increases its area four times, meaning the image is four times as bright.
f number = lens focal length divided by effective aperture diameter
f-number of the lens's maximum aperture, together with its focal length are engraved on the lens rim.
Each change to the next highest number halves the amount of light passing through the lens. This is called one stop. There is the ability to adjust in half stops and one third stop increments.
The f-number system means that any lens set to the same number gives a standard image brightness, irrespective of the focal length or the camera size.
The actual f-numbers themselves denote the number of times the effective diameter of the aperture divides into the lens focal length. f/2 means the aperture diameter is one half the focal length. f/4 is on-quarter.
Each f-number takes into account two factors that control how bright an image is formed:
1) Distance between lens and image
For a distance subject (lens focused on infinity) the image is formed at one focal length from the lens. The inverse square law of light shows that doubling the distance of a surface from a light source quarters the light it receives. A lens of 100mm focal length forms an image only one-quarter as bright as a lens of 50mm.
2) Diameter of the light beam.
Doubling the diameter of a circle increases its area four times, meaning the image is four times as bright.
f number = lens focal length divided by effective aperture diameter
f-number of the lens's maximum aperture, together with its focal length are engraved on the lens rim.
Photography the new basics
Most digital SLR lenses will be at their sharpest when they are opened up from their smallest aperture setting by two stops
Most digital SLR lenses will be at their sharpest when they are opened up from their smallest aperture setting by two stops
Depth of field
pg 51 Langfords
Is the distance between the nearest and farthest parts of a scene that can be considered sharp or in focus.
Widest aperture (smallest f-number i.e. f/1.4) gives the least depth of field. Smallest aperture (highest f-number i.e. f/22) gives the greatest depth of field.
Two other significant effects:
1) Depth of field becomes less when focusing on something very close and greater when you are focusing on something far away;
2) The longer the focal length of the lens, the less the depth of field it gives even with the same aperture and subject distance.
Achieving significant depth of field when using telephoto lenses can be a great challenge.
Note to self: Experiment with the depth of field preview button
How depth of field works - pg 52
A lens focuses an image point at one distance only, depending on how far the lens is from the subject. Other parts of the subject nearer or farther away from the lens come to focus further away or nearer, forming disks instead of points of lights. These are known as circles of confusion.
Large circles of confusion that overlap give a blurred image. However, provided the circles are relatively small, they can appear sharp, since our eyes have limited resolving power.
You can rate an image acceptably sharp even when tiny disks are present instead of dots. The upper limit to what most people accept as sharp is taken to be 0.25mm diameter on the final print.
By accepting disks up to this size as sharp, subjects slightly nearer and father away than the subject actually in focus, start to look in focus too. If the lens aperture is made smaller, all the cones of light become narrower, so that images of subjects even nearer and farther are brought into the zone of acceptable sharp focus - depth of field has increased.
If you move farther back from the subject or change to a shorter focal length lens,, the positions of sharp focus for images of nearest and farthest subject parts bunch closer together. Their circles of confusion become smaller and depth of field has improved.
Greatest depth of field is produced when:
1) f-number is high
2) subject is distant
3) focal length is short
With subject beyond about ten focal lengths from the lens, depth of field extends farther behind the subject than toward the lens. Focus on part of the scene one third inside the depth of field required.
pg 51 Langfords
Is the distance between the nearest and farthest parts of a scene that can be considered sharp or in focus.
Widest aperture (smallest f-number i.e. f/1.4) gives the least depth of field. Smallest aperture (highest f-number i.e. f/22) gives the greatest depth of field.
Two other significant effects:
1) Depth of field becomes less when focusing on something very close and greater when you are focusing on something far away;
2) The longer the focal length of the lens, the less the depth of field it gives even with the same aperture and subject distance.
Achieving significant depth of field when using telephoto lenses can be a great challenge.
Note to self: Experiment with the depth of field preview button
How depth of field works - pg 52
A lens focuses an image point at one distance only, depending on how far the lens is from the subject. Other parts of the subject nearer or farther away from the lens come to focus further away or nearer, forming disks instead of points of lights. These are known as circles of confusion.
Large circles of confusion that overlap give a blurred image. However, provided the circles are relatively small, they can appear sharp, since our eyes have limited resolving power.
You can rate an image acceptably sharp even when tiny disks are present instead of dots. The upper limit to what most people accept as sharp is taken to be 0.25mm diameter on the final print.
By accepting disks up to this size as sharp, subjects slightly nearer and father away than the subject actually in focus, start to look in focus too. If the lens aperture is made smaller, all the cones of light become narrower, so that images of subjects even nearer and farther are brought into the zone of acceptable sharp focus - depth of field has increased.
If you move farther back from the subject or change to a shorter focal length lens,, the positions of sharp focus for images of nearest and farthest subject parts bunch closer together. Their circles of confusion become smaller and depth of field has improved.
Greatest depth of field is produced when:
1) f-number is high
2) subject is distant
3) focal length is short
With subject beyond about ten focal lengths from the lens, depth of field extends farther behind the subject than toward the lens. Focus on part of the scene one third inside the depth of field required.