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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 29 Apr 2015 (Wednesday) 14:01
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Confusing lens field-of-view / depth question

 
burb1972
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May 01, 2015 17:04 as a reply to  @ post 17537504 |  #16

sensor density does effect dof, especially at low fstop 1.2 etc, pixel pitch will change the fstop of a lens, like 7d will lose almost a stop of light on an f1.2 lens.
http://www.dxomark.com​/Reviews/F-stop-blues (external link)


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gjl711
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May 01, 2015 17:29 |  #17

burb1972 wrote in post #17540000 (external link)
sensor density does effect dof, especially at low fstop 1.2 etc, pixel pitch will change the fstop of a lens, like 7d will lose almost a stop of light on an f1.2 lens.
http://www.dxomark.com​/Reviews/F-stop-blues (external link)

Don't confuse DoF, f-stop, and t-stop. All different things. If you read the article it says nothing about the f-stop being changed but the quantity of light hitting the sensor is not as efficient at larger apertures. The f-stop is still the same and the optical properties of the lens as well but the amount of light hitting each pixel is less effective. DoF stays the same, f-stop staues the same, the amount of light is lessened.


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burb1972
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May 01, 2015 19:29 as a reply to  @ gjl711's post |  #18

isnt fstop a measure of the amount of light?and a decrease in the amount of light would increase the fstop?


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gjl711
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May 01, 2015 19:52 |  #19

burb1972 wrote in post #17540117 (external link)
isnt fstop a measure of the amount of light?and a decrease in the amount of light would increase the fstop?

Not really. f-stop is the ratio of the aperture to the focal length. It has nothing to do with the amount of light coming through the glass. Think of it this way, I have a f/1.4 50mm lens. I set it to f/1.4 and there is a certain amount of light coming through the lens. I now stick on a 10 stop neutral density filter thus reducing the the amount of light by 10 stops. If f-stop would be a measure of the amount of light, I have just increased my f-stop to f/45. But it's not. The lens is still f/1.4, DoF is still as it is at f/1.4 but the amount of light is if were f/45.


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BigAl007
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May 01, 2015 20:15 |  #20

burb1972 wrote in post #17540117 (external link)
isnt fstop a measure of the amount of light?and a decrease in the amount of light would increase the fstop?

No f Stop is simply a ratio of the effective aperture diameter (in simple lenses the actual diameter) of the aperture and the focal length. In theory if you change the ratio so that the diameter is increased or decreased by a factor of the square root of two, you will double or halve the area of the aperture so doubling or halving the amount of light passing through. That is equivalent to changing the aperture by one stop either way. It is this value that is important in DoF calculations, as the Absolute (Effective) Aperture is one of only three variables that are absolutely necessary to know to calculate the DoF. The others are the Absolute Reproduction Ratio, and the visual acuity of the observer.

All of the above is fine, but when it comes to the actual amount of light reaching the sensor it falls down. It assumes that there is zero light absorbed/reflected while passing through the lens. Modern lenses with their multi coatings are actually very good, and transmission rates are usually around 98% to 99%. If it is absolutely essential that you get the correct exposure as it is in cinema for example, where the editor will have to cut the film from multiple cameras together then they use T or Transmission stops. These are carefully calculated to take into consideration the light losses from the lens. If you look at many of the new manual focus lenses that are being offered for cine on DSLR work, they are marked with T-stops.

All of the above applies to both film and digital sensors. Digital sensors also have issues with light coming from very oblique angles. The light is at such an angle that it fails to actually reach the sensor well. This is true even for sensors with advanced gapless micro lenses in front of them. This will affect the edges of the sensor more then the center. However if you have a very wide aperture, on a relatively short focal length even sensels in the center of the sensor will be seeing light rays from quite oblique angles. Hence the fact that you lose some level of exposure from very wide aperture lenses on digital compared to using film. Although you lose some of the low light advantage that a wide aperture should give you, you still get the DoF control. So at f/1.2 your lens may have a T stop of 1.8 on digital, while by the time it's at f5.6 it may have an equivalent T stop of 6.3. In this situation the T stop value is actually dependent on the fact that you are using digital and not film, if we consider the additional light lost through sensor constraints.

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burb1972
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May 01, 2015 20:22 |  #21

wow, thanks for the detailed explanation.


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May 01, 2015 21:05 |  #22

Here is another great site.

http://www.dofmaster.c​om/dofjs.html (external link)


Here is a 'bokeh' and 'blur' calculator for fun as well.

http://howmuchblur.com ….8-on-a-0.9m-wide-subject (external link)


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vengence
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May 02, 2015 21:00 |  #23

Something that's not immediately obvious, but is if you delve into the math of DOF, is one of the major factors of DOF is simply subject size vs sensor size. If you're taking a picture of an object that's the same size as your sensor, the DoF is going to be razor thin. If you're taking a picture of something that's hundreds of times the size of your sensor, i.e. a mountain, the DoF is going to be enormous.




  
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Post edited over 4 years ago by xpfloyd. (2 edits in all)
     
May 03, 2015 01:21 |  #24

vengence wrote in post #17541121 (external link)
Something that's not immediately obvious, but is if you delve into the math of DOF, is one of the major factors of DOF is simply subject size vs sensor size. If you're taking a picture of an object that's the same size as your sensor, the DoF is going to be razor thin. If you're taking a picture of something that's hundreds of times the size of your sensor, i.e. a mountain, the DoF is going to be enormous.

Subject size is irrelevant. Its focal length, focus distance and aperture. Sure if you frame a whole mountain in your shot the dof would be different than if a persons face filled the frame but that's simply to do with the altered focus distance (assuming the same sensor size in both shots, same focal length and same aperture)


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May 03, 2015 04:29 |  #25

Many years ago I read an article in Popular Photography by Herbert Keppler that taught me one of the main factors in determining DOF is magnification. Magnification is the result of focal length and focus distance.

More magnification means less DOF and less magnification means more DOF.

So in reality it is just magnification and aperture that determine DOF.


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May 03, 2015 09:20 |  #26

msowsun wrote in post #17541380 (external link)
Many years ago I read an article in Popular Photography by Herbert Keppler that taught me one of the main factors in determining DOF is magnification. Magnification is the result of focal length and focus distance.

More magnification means less DOF and less magnification means more DOF.

So in reality it is just magnification and aperture that determine DOF.


The important thing to remember is that the Magnification amount in question is the ratio between the actual size of the subject in real life, and the size of the subject in the final image. That and the actual diameter of the aperture, not the f/number, are all you need for DoF; apart from the visual acuity of the observer so that you know at what size they stop seeing a blur disk as a spot. Actually both Magnification Ratio and Aperture are both inversely proportional to the DoF. This is of course really only of academic interest, as you need to define more of the variables to make much use of the basic proportionalities. Fortunately we are usually coming from the direction of knowing the variables that are needed to allow us to calculate the DoF from the equipment that we are using, things such as film/sensor size, focal length, focus distance, f ratio (from which it is simple to determine that important absolute aperture size) and so on. But to do so we still have to specify the final print size and viewing conditions, as part of the calculation.

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May 03, 2015 18:29 |  #27

msowsun wrote in post #17541380 (external link)
Many years ago I read an article in Popular Photography by Herbert Keppler that taught me one of the main factors in determining DOF is magnification. Magnification is the result of focal length and focus distance.

More magnification means less DOF and less magnification means more DOF.

So in reality it is just magnification and aperture that determine DOF.

The above truism applies in the context of a single format size, like if you only shoot 135/FF. Compare two format sizes, and the truism falls apart due to the change in FL to achieve the same AOV at the same camera-to-subject distance. But, ignoring the format size consideration, it is a truism I use in discussions, too!


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Wilt
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May 03, 2015 19:13 |  #28

BigAl007 wrote in post #17541528 (external link)
The important thing to remember is that the Magnification amount in question is the ratio between the actual size of the subject in real life, and the size of the subject in the final image. That and the actual diameter of the aperture, not the f/number, are all you need for DoF; apart from the visual acuity of the observer so that you know at what size they stop seeing a blur disk as a spot. Actually both Magnification Ratio and Aperture are both inversely proportional to the DoF. This is of course really only of academic interest, as you need to define more of the variables to make much use of the basic proportionalities. Fortunately we are usually coming from the direction of knowing the variables that are needed to allow us to calculate the DoF from the equipment that we are using, things such as film/sensor size, focal length, focus distance, f ratio (from which it is simple to determine that important absolute aperture size) and so on. But to do so we still have to specify the final print size and viewing conditions, as part of the calculation.

Assuming 20/20 visual acuity while using a DOF calculator, I compared:


  1. 35mm format using 50mm lens, f/4 at 23'/7m to capture subject area which is 16.4'/5m wide
  2. 35mm format using 200mm lens, f/16 at 92'/28m to capture subject area which is 16.4'/5m wide
  3. Fictitious format 48x72mm (double 135 format) using 100mm lens, f/4 at to capture subject area which is 16.4'/5m wide
  4. 35mm format using 200mm lens, f/4 at 92'/28m to capture subject area which is 16.4'/5m wide


DOF calculations for the four above situations:

  1. 1.68 meter deep total DOF zone
  2. 6.73 meter deep total DOF zone
  3. 0.72 meter deep total DOF zone
  4. 1.66 meter deep total DOF zone


Far field blur plots for the four above situations:

IMAGE: http://i69.photobucket.com/albums/i63/wiltonw/POTN%202013%20Post%20Mar1/DOF%20blur2_zpswqgdqetf.jpg

So what have we learned here? (All four situations frame the same area.)
  • At the same Aperture Diameter, situation 1 and 2 have very different DOF zones.
  • At the same Aperture Diameter, situation 2 and 3 have IDENTICAL far field backgroud blur -- in spite of very different format sizes!
  • At the same f/stop, situation 1 and 4 both provide SAME DOF zone when the subject is identically sized in the frame -- in spite of very different FL lenses.

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May 04, 2015 02:05 |  #29

And if I followed you correctly here, you just prooved that it's exactly magnification and aperture that determines depth of field.
Remember that magnification is not only dependent on how you shoot the image, but also how big you print it and at which distance you view the printed image.
Thus you can easily modify the depth of field after taking the picture. Just go closer or far away, and it changes.


Anders

  
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May 04, 2015 11:55 |  #30

apersson850 wrote in post #17542575 (external link)
And if I followed you correctly here, you just prooved that it's exactly magnification and aperture that determines depth of field.
Remember that magnification is not only dependent on how you shoot the image, but also how big you print it and at which distance you view the printed image.
Thus you can easily modify the depth of field after taking the picture. Just go closer or far away, and it changes.


^ in the context of a single format size. APS-C is not the 'same format' as FF (as illustrated in #1 vs. #3)


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Confusing lens field-of-view / depth question
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