-
Welcome to the new forums! Please read this first. For known issues we are working to resolve, click here.
How will a sensor be able to achieve a very shallow dof...
- Thread starter Jothi
- Start date
Mystery member
Forum Pro
There's no connection between the amount of light and DoF. The total amount of light is the "exposure" and this is regulated by the combined effect of aperture and shutter speed.
Relatively simple optical and geometrical considerations determine the "region of acceptable sharpness", or DoF.
OzarkAggie
Senior Member
- Messages
- 2,153
- Solutions
- 3
- Reaction score
- 827
It's the lens that creates the dof. The smaller the sensor the more difficult it is because of the absolute or equivalent aperture.
For instance, if a sensor is four times smaller than a full frame (35mm standard) then you would add four stops to the relative aperture. If that's f/2 then the equivalent aperture is f/5.6 and that will determine dof. (multiply by 4)
Going the other way, if you wanted to achieve the equivalent of an f/2 then the lens would have to have an f/0.7 aperture.
So a larger sensor gives the advantage of using a lens that can actually be produced.
We use 35 mm as a standard for sensor size because it's so common.
Every crop sensor has a multiple. My G1X has a multiple of 1.8. Four-thirds a multiple of 2.0, and an APS-C is either 1.5 or 1.6 (Canon).
Here's a good place to get more information: Cambridge in Color
For instance, if a sensor is four times smaller than a full frame (35mm standard) then you would add four stops to the relative aperture. If that's f/2 then the equivalent aperture is f/5.6 and that will determine dof. (multiply by 4)
Going the other way, if you wanted to achieve the equivalent of an f/2 then the lens would have to have an f/0.7 aperture.
So a larger sensor gives the advantage of using a lens that can actually be produced.
We use 35 mm as a standard for sensor size because it's so common.
Every crop sensor has a multiple. My G1X has a multiple of 1.8. Four-thirds a multiple of 2.0, and an APS-C is either 1.5 or 1.6 (Canon).
Here's a good place to get more information: Cambridge in Color
OzarkAggie
Senior Member
- Messages
- 2,153
- Solutions
- 3
- Reaction score
- 827
The sensor is approximately 2/3rds (~16x25 mm) the size of a full frame 35 mm (24x36 mm)
Originally a film standard, hence the name.
Originally a film standard, hence the name.
Jothi
Well-known member
- Messages
- 176
- Reaction score
- 10
*the second para's third line
AND, in the second paragraph under the topic "Full Frame Sensors"
Plz explain me both...
Last edited:
David Wrench
Well-known member
- Messages
- 203
- Reaction score
- 114
The answers you have so far are correct, but I'm not sure that I fully understand the question. Perhaps you interpreted 'gathering more light' as meaning that a smaller aperture could be used, thus increasing the depth of focus? doesn't. A small-sensor compact and a full-frame camera should both, at the same ISO, require the same exposure for the same scene. The full-frame camera, however, would need a longer focal length lens to make the same image on its sensor, and would therefore give a shorter dof.
David
David
Gerry Winterbourne
Forum Pro
The terms are rooted in the history of photography. When cine was invented in the 19th century a standard width of 35mm was adopted for the film used. This film had two rows of holes down the sides to make sure each frame lined up with the rest. After allowing for the size of these holes and a margin around them there was 24mm left along the middle of the film for the actual picture area.
A standard was also adopted for the aspect ratio ( shape ) of film screens of 4:3. In cine the film runs vertically so for a width of 34mm this gave a height of 18mm for each picture. In the early 20th century camera makers decided to use 35mm film because it was already plentiful and cheap. However, with the film running horizontally instead of vertically it was the picture height that was 24mm.
The makers used the same holes for winding the film on so it was convenient to make the width of the picture a multiple of cine size: the multiple of two meant that shifting the film just needed twice as many sprocket holes to be used. This meant the width of the photo pictures was 2 x 18 = 36mm. Thus, 36 x 24mm became the standard size for cameras using 35mm film.
Towards the end of the 20th century camera makers came up with the idea of a smaller, more automated type of film camera. Because of its automatics it was called Advanced Photo Systems (APS). This system, being smaller, used smaller frame sizes, one of which was about 2/3 the size of the standard 35mm size; it was called APS-C.
The APS film system came too late to transform film photography because digital came along. Some early makers of digital cameras wanted to be able to use all the old film 35mm lenses by just putting a sensor in a film body, but there were technical difficulties and they needed to use smaller sensors. They decided to use the APS-C size that had been invented for the APS film system.
Users of these digital cameras had to get used to the idea that although they wee using the same lenses the smaller sensor didn't give the same field of view as on 35mm film. In effect they were getting a crop of the 35mm film frame, not the whole of it. Later, cameras with bigger sensors - the full 36 x 24mm of 35mm film - were introduced. Because those cameras deliver the full picture size of the 35mm film cameras they came to be called Full Frame cameras.
OzarkAggie
Senior Member
- Messages
- 2,153
- Solutions
- 3
- Reaction score
- 827
"Full Frame" is the same size as 35 mm film (24x36 mm). It's the standard because for 70 years it was the most popular film size, and the manufacturers made many lenses for that size.
Most of this stuff is explained in the link I provided. Here it is again:
Read this!
Most of this stuff is explained in the link I provided. Here it is again:
Read this!
Depth of field is for the most part derived from the distance to the object being photographed, focal length and aperture size. There is a secondary effect from the sensor size. The equation below describes the geometry.
The point where sensor size comes in is the circle of confusion (CoC). This describes the smallest feature on the sensor that needs to be sharp for the image to be considered sharp. In most applications this is set as 1/1500 the diameter of the sensor. So for APS-C sized sensor (Nikon or Sony) this is 28.3 mm / 1500 or 0.019 mm for a full frame sensor this would be 43.3 mm / 1500 or 0.029 mm. Since the c term appears in the lower portion of the fraction the larger it is the closer the near distance becomes and the farther the far distance, resulting in a larger DoF. So for the same lens and distance to the object the DoF of the larger sensor is smaller. In real life one would not use the same lens, same distance to take a picture of the same object. With the FF camera you would either use a longer focal length camera or move closer for the same picture. If is the changes in focal length, aperture and distance that results in a FF sensor having a small DoF in many situations.
There are many on-line and phone based calculators to figure out the DoF for a given situation. In most cases understanding what effects the DoF is most important as few are going to go to the trouble of using tape measures and calculators to take a picture. But if you understand the basics you can make adjustments while taking pictures to achieve the effect you want.
Not enough in focus, use a smaller aperture, a shorter lens or move back. Too much in focus, move closer, use a longer lens, or a larger aperture.
As far as gathering light goes, probably just a rough rule of thumb (to use an english idiom) Larger apertures give more light and produce shallower DoF, larger sensors gather more light and often result in images with shallower DoF.
Jacques Cornell
Forum Pro
Another way to view the matter is that the focal length affects DoF. f2 is f2 on MFT, APS or FF, and on each format it yields the same exposure. But, for the same Angle of View, the focal length is 1x on FF, 1.5x on APS and 2x on MFT. IOW, 12mm on MFT gives the same AoV as 18mm on APS and 24mm on FF. Shorter focal lengths naturally yield deeper focus. So, with smaller sensors you need a correspondingly larger aperture to get the same DoF.
Four stops down from f2 is f8. FWIW, MFT has 1/4 the sensor area of FF, and with the same AoV, f2 yields the same DoF as f4 on FF.
Two stops open from f2 is f1.
Voigtlander makes a range of f0.95 lenses for MFT.
--
The way to make a friend is to act like one.
www.jacquescornell.photography
Last edited:
Ace of Sevens
Forum Enthusiast
A large aperture lets in lots of light from all directions. Narrowing it limits the angles at which it can come in, making the image more defined.
Last edited:
Gerry Winterbourne
Forum Pro
This is pretty much nonsense. There's a tiny nugget of truth hidden in it, that I'll deal with first before explaining my first sentence.
These links explain lens flare https://en.wikipedia.org/wiki/Lens_flare http://www.cambridgeincolour.com/tutorials/lens-flare.htm
As you see, two things combine to cause flare: light coming from outside the optical path and internal reflections of that light inside the lens. The best way to avoid flare is to use a lens hood and keep sources of light away from the optical path. In macro the latter is usually easy because we want light falling on the subject from behind the camera.
However, there's always a certain amount of light coming from odd angles so there's always a tiny amount of flare even in the best-lit images. Flare reduces contrast, which in turn makes details harder to discern. So, yes, flare does reduce definition in an image. But if the scene is properly lit and the lens properly hooded the effect is very small. Internal reflections are slightly more likely to be visible if the aperture is wide, so the tiny nugget of truth is that in most cases a wide aperture makes a small difference to a tiny factor.
Now to why the generality of your statement is nonsense. All simple lenses suffer aberrations http://toothwalker.org/optics.html These aberrations are caused by the curvature of the glass, which is inevitably steeper as light is further from the axis of the lens. Designers use several elements whose aberrations cancel out but this cancelling is never perfect.
The reason wide apertures give soft images is that aberrations are worse when a bigger area of the surface is open to light. It has nothing to do with the incident angle of the light. Narrowing aperture improves sharpness by cutting out the areas of the lens with worse aberrations. There is, however, a limit to this, because of diffraction http://www.bbc.co.uk/schools/gcsebi...y/home_energy/introduction_to_wavesrev6.shtml
The general effect is that most lenses start out relatively soft at their widest aperture, improve to a peak of resolution and then get progressively softer again. But, to reiterate, none of this has anything to do with the angle of light reaching the lens.
Chris R-UK
Forum Pro
Ace of Sevens may also be getting confused with one effect of using non-telecentric lenses with digital sensors.
Older pre-digital lens designs didn't worry too much about ensuring that all light rays leaving the lens hit on the film/sensor surface vertically. These lenses are described as non-telecontric. Modern telecentric digital lenses are designed to minimise non vertical light rays.
This wasn't too much of an issue with film but digital sensors with microlenses on the top effectively collect their light at the bottom of shallow wells and some non-vertical light rays therefore do not reach the sensor. This effect is most pronounced at the sensor edges and causes vignetting, loss of resolution and loss of contrast in the image corners.
With DSLR lenses which have a long distance between the back end of the lens and the sensor this effect is normally minimal and can be dealt with by the microlens design around the edge of the sensor. However, it can become very pronounced when using certain older rangefinder lenses on a mirrorless camera where the rear of the lens can be very close to the sensor. The effect is worst on FF cameras like the Sony A7 because there is no cropping of the image circle. I have seen a test of a 25 year old Leica lens mounted on a FF, APS-C and M4/3 bodies and the FF image was virtually unusable because of problems in the corners. The effect was nearly gone on APS-C and totally gone on M4/3 because of the cropping of the image circle.
Note that this is caused by the non-telecentricity of light leaving the back of the lens, not light falling on the front of the lens.
clack
Forum Enthusiast
- Messages
- 288
- Solutions
- 3
- Reaction score
- 124
Well, it is a literal interpretation of illustrations like this one:
Taken from the DPReview 'Equivalence' article
--
Every thing has already been photographed - but not yet by everyone. (Karl Valentin)
Ace of Sevens
Forum Enthusiast
That's what I was going for. Out of focus is caused by light from a given object spread out on the sensor. If it's not in the focal plane, light coming off at different angles gets directed to different parts of the sensor. Closing the aperture blocks the more extreme angles, making the circle of confusion smaller.
clack
Forum Enthusiast
- Messages
- 288
- Solutions
- 3
- Reaction score
- 124
Here are some illustrations which include a photographed object:
Camera Apparatus A - Light from Venus's belly button* goes towards Apparatus A, within a light cone of angle a. Inside Apparatus A the light coming from that cone is projected onto an imaging surface. Light coming from a point on Venus's right shoulder is behind the focal plane. Its image on the imaging surface inside the apparatus will be blurry.
* If so inclined, replace Venus of Milo with Michelangelo's David. With a fig leaf, of course.
Apparatus B is half as large as camera A. Light from Venus's belly button approaches within angle b which measures half the number of degrees of angle a. Light coming from a point on Venus's right shoulder is behind the focal plane but will be imaged less blurry. The narrower angle of the cone of light creates a deeper depth of field. The front of Apparatus B is exposed to a light cone a quarter the size of that of Apparatus A. Because its image surface also is a quarter of the size of that of A, B can still achieve the same photographic exposure on its inside.
Apparatus C, is like A but stopped down two stops. The cone of light coming from the outside world is the same as in B.** We have the same depth of field. It also means that the front of Apparatus C is exposed to the same amount of light as Apparatus B. On its inside however Apparatus C stretches the cone of light. We would have to adjust shutter speed to achieve same photographic exposure***.
** That means image B and C are created from the same set of real-world photons. Therefore we call situation B and C photographically equivalent.
*** Or we could amplify the incoming signal, e.g. by electronic means.
--
Every thing has already been photographed - but not yet by everyone. (Karl Valentin)
Last edited:
That is basically the only way, if you change the aperture only and keep everything else the same. Open more, you get more light and a shallower DOF because the only way to get more light from a point is from rays that do not hit the center of the pupil so directly and that shrinks the DOF.
EDIT: I just saw that Ace of Sevens gave more or less the same answer.
Last edited:
Mystery member
Forum Pro
I'm with Gerry on this one; these back-woods explications of optical phenomena aren't helpful, and simply enlarge the circle of confusion.
Last edited:
Keyboard shortcuts
- f
- Forum