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Apeture, Depth Of Field And A Question...
My question not so much about how aperture affects Depth Of Field. What I woudl like to undertsand is WHY fast shutter speeds a large apetures give shallow depth of field...

Comments (17)

Scoot4nat wrote:.

My question not so much about how aperture affects Depth Of Field.What I woudl like to undertsand is WHY fast shutter speeds a largeapetures give shallow depth of field..

The shutter speed has no effect on depth of field, but the aperture does. There is an explanation here:.

Http://www.cs.mtu.edu/.../~shene/DigiCam/User-Guide/950/depth-of-field.html.

Best wishesMike..

Comment #1

Thank you for the link Mike, Ill see if I can find an answer there..

To Graystar - Thank you for your in-depth explanation! How very helpful! I accpe t that shutter speed does not affect DOF but I'm pretty sure apeture does...

Comment #2

Just read that article. It gives a very simple explanation of why apeture affectes DOF. Thank you again...

Comment #3

Scoot4nat wrote:.

Thank you for the link Mike, Ill see if I can find an answer there..

To Graystar - Thank you for your in-depth explanation! How veryhelpful! I accpe t that shutter speed does not affect DOF but I'mpretty sure apeture does..

The "they" refers to the plural "shutter speeds", as that was your usage. You had already said that your question was not about aperture (or at least that's what it sounded like when you said "my question is not so much about how aperture affects Depth Of Field"...little did I know...).

How much explanation do you expect for something that doesn't exist?..

Comment #4

I will accept that your use of the word "they" referred specifically to shutter speeds. However, I had asked why large apetures affect DOF also (the mispelling of the word "and" [a] may have not made this 100% clear)...

Comment #5

Mike703 wrote:.

There is an explanation here:.

Http://www.cs.mtu.edu/.../~shene/DigiCam/User-Guide/950/depth-of-field.html.

That site says: "Since circles of confusion are formed by light rays passing through the lens tube, the size of a circle of confusion is proportional to the amount of light that can pass through the lens tube. This means smaller (resp., larger) circles of confusion will be formed if less (resp., more) light can pass through. Restricting how much light can pass through the lens is the function of the diaphragm in the lens tube that sets the aperture values. Therefore, a smaller aperture means a smaller diaphragm opening, which, in turn, means allowing less light to strike the film/CCD plane. Thus, we have smaller circles of confusion and, as a result, a sharper image!".

And that is wrong! Obviously: if it was right dof would be greater in dim light!.

There is a diagram on p49 of Ansel Adams' book The Camera that explains (correctly) why smaller apertures give deeper dof..

'Some of the money I spent on booze, women and fast cars, but the rest I squandered' - George Best..

Comment #6

From what I read, I took it to mean not how much light got through the diaphram, but the proportion of space on the sensor which was exposed to light; that is to say that the larger the apeture, the larger the area on the sensor exposed to light, hence the larger circle of confusion and the shallower DOF..

???.

Can anyone shed any light on this? (No pun intended)..

Comment #7

I can show you images at F3.5 where everything is in focus and images at F22 where there is a very narrow depth of field..

Depth of field is dictated by aperture, focal length, and distance to subject.

Shooting a 100mm macro lens at F22 from 6" away will yield a narrow field, while shooting a landscape at F3.5 at 10mm will yield a fairly deep DOF..

Here's an example. Both shots taken with the same lens. Unfortunately they are at different f stops - first one F5.6, the second F8, but for the sake of demonstration they are close enough..

First shot I was at the minimum focus distance for the 400 F5.6L, which I think is about 11 feet..

The second shot, the rock formations in the foreground were approx 2-3 miles away, and the horizon was approx 15-20 miles away. At F8 the DOF is measured in miles, whereas the first picture it's measured in inches. To be exact, using the depth of field calculator http://www.dofmaster.com/dofjs.html - the bunny photo has a DOF of 0.04 feet or approx 1/2". At F8 it would be 0.06 feet (.72"). For the second shot using a subject distance of 2 miles (10560 ft) yields a near focus distance of 2602 feet (1/2 mile) and an infinite DOF beyond that..

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Comment #8

IMac, therefore iAm wrote:.

Depth of field is dictated by aperture, focal length, and distance tosubject.

True, but& from a practical point of view we're almost always looking to take a particular picture. Except at macro distances, focal length and distance to subject pretty much cancel out DoF-wise for a given composition. You get about the same DoF whether you zoom with your lens or zoom with your feet. So the only significant variable at the photographer's disposal for controlling DoF is aperture..

It might be noted that while DoF remains fairly constant at a given composition, a greater distance and a longer focal length put more of the DoF in front of the focus point (better balance front and back) and will blur the background more...

Comment #9

Because of geometric laws..

A small aperture gives you a small angle and the tolerance to focus is higher (it take longer distances that the point become a large enough disc to observe). At large apertures the distance you have to travel is smaller.As an example..

A small aperture will give you 1.5 deg while a large one 10 deg. If the threshold of the disc radius you see as a disc not as a point is 0.03 mm you have a1*tan(1.5)=a2*tan(10)=0.03.

So a1/a2=tan(10)/tan(1.5)=6.7. So if you have a1=1.15 mm you have a2=0.17 mm. So at the same distance the DOF will be 6.7 times deeper for the aperture that gives you 1.5 deg (according to the Thales law).VictorBucuresti, Romaniahttp://s106.photobucket.com/albums/m268/victor_petcu/http://picasaweb.google.com/teodor.nitica/..

Comment #10

Scoot4nat wrote:.

I will accept that your use of the word "they" referred specificallyto shutter speeds. However, I had asked why large apetures affectDOF also (the mispelling of the word "and" [a] may have not made this100% clear)..

Ah, I thought that was a misspelled at, as in in combination with (fast shutter speeds at large apertures.).

A magnifying glass can burn something by focusing the sun's rays. You focus the rays by moving the glass up and down until the circle is a point. That circle is the circle of confusion. The smaller the circle, the better the focus..

On the web site below there is a good graphic which demonstrates why the circle of confusion is smaller for out-of-focus objects when the aperture is smaller. A small aperture keeps the circles small..

Http://www.aesthetic-endeavors.com/photo/dof/.

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Comment #11

Here is a video on DOF:.

Http://snodart.com/tutorials.php.

Here is how aperture cannot be separated from shutter speed for the same amount of light:.

Http://www.dryreading.com/camera/index.html.

And here is DOF article:.

Http://www.photozone.de/depth-of-field.

Scoot4nat wrote:.

I will accept that your use of the word "they" referred specificallyto shutter speeds. However, I had asked why large apetures affectDOF also (the mispelling of the word "and" [a] may have not made this100% clear)..

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Anandahttp://anandasim.blogspot.com/http://olympuse510.wikispaces.com/http://picasaweb.google.com/AnandaSim/http://www.flickr.com/photos/32554587@N00/..

Comment #12

I'm not sure I have every heard anyone put it the way you describe. If the field of view at/near the focus point is kept constant, the DoF is relatively insensitive to changes in the remaining geometry.Leonhttp://homepage.mac.com/leonwittwer/landscapes.htm..

Comment #13

DOF is:.

(1) Directly proportional to f ratio: if you double f ratio, from f/8 to f/16, eg, the dof doubles..

(2) Directly proportional to the square of the distance from the subject to the camera: if you move from 100m to 200m away from the subject the dof increases by a factor of four..

(3) Inversely proportional to the square of focal length: if you change focal length from 100mm to 200mm the dof is reduced by a factor of four..

Note that these calculations apply whether there is film, a 36 x 24 sensor, an APS-C sensor, a sheet of paper, or nothing behind the lens; crop sensors DO NOT affect the dof of the image formed by the lens. Note also that points 2 and 3 are why some people (eg, the Cambridge in Colour tutorials) say, wrongly, that focal length does not affect dof: if you change to a longer focal length but then walk further away to keep the image the same the dof will be affected in opposite directions and the final result will be about the same..

When you use film, there is an image formed on the negative, and if that image is enlarged to make a print, so the circles of confusion are enlarged as well. If the film is 35mm film the negative image has to be enlarged a lot to make (say) an 8 x 10 print, while if the negative is 8 x 10 it does not have to be enlarged at all, so smaller film formats have shallower dof..

In the same way, people often talk about the image on a small sensor having to be "enlarged" more than the image on a bigger sensor to make a print, but that is wrong, of course: there is no "image" coming out of a digital camera, just a string of numbers. What does happen is that the circle of confusion in the lens image is the same for all sensors, but (say) a 0.01mm circle is a smaller proportion of a 36 x 24 sensor than of a 6 x 4 sensor. What matters is how many pixels are covered by rhe circle of confusion, because that will determine whether the circle of confusion is or is not bigger than 0.1mm on the final print (roughly, the size it needs to be for the image to look really sharp)..

'Some of the money I spent on booze, women and fast cars, but the rest I squandered' - George Best..

Comment #14

Les Olson wrote:.

That site says: "Since circles of confusion are formed by light rayspassing through the lens tube, the size of a circle of confusion isproportional to the amount of light that can pass through the lenstube. This means smaller (resp., larger) circles of confusion will beformed if less (resp., more) light can pass through. Restricting howmuch light can pass through the lens is the function of the diaphragmin the lens tube that sets the aperture values. Therefore, a smalleraperture means a smaller diaphragm opening, which, in turn, meansallowing less light to strike the film/CCD plane. Thus, we havesmaller circles of confusion and, as a result, a sharper image!".

And that is wrong! Obviously: if it was right dof would be greaterin dim light!.

I can't see a problem with this explanation. It says that if the aperture is narrow, the circle of confusion is smaller, so there is more tolerance until it appears to be out of focus (i.e. it can deviate more from the focus plane and the d-o-f is greater). The term 'less light' may be what is confusing here: it doesn't mean 'dim light' (and hence a wide aperture), it means a smaller spot on the sensor as the result of a narrow aperture..

Best wishesMike..

Comment #15

Nicely put.......

DoF is mainly effected by f/stop and Focused Distance. Shorter Focal length Have at any given F/stop and Focus point MORE DoF than Longer Lens's...

I only have a 30mm with a Dof Scale on it....

But Compare the DoF scale on differant FL lens's..to see. That is...F/8 on a 30mm at the 6' focus will have 3.9'-15' in focus.....on a 50mm focused at 6', it will less....like 5-12 feet.

Examples from a DoF scale on an OLD Pentacon 30mm f/3.5 in M42 mount.

F/stop......Focused Point......DoF4................3 feet..........2.5-3.5 feet4................6 feet..........5-11 feet4................12 feet........10-inf feet.

8................3 feet..........2.2-4.3 feet8................6 feet..........3.9-15 feet8................12 feet........5.5 - Inf feet.

16...............3 feet..........1.9 - 11 feet16...............6 feet..........2.7 - Inf feet16...............12 feet.........3.5 - Inf feet.

So as the lens is closed down, the DoF gets a bit deeper at each f/stop using the same focus points as guide for instruction..

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What I shoot with - in Profile.Arbib..

Comment #16

Even though I thought I've understood enough aboutdepth of field, I always learn something new readingthe threads.Thanks...

Comment #17

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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