I just learned about f-stop equivalency...

Discussion in 'Photography Equipment & Products' started by nerwin, Feb 26, 2016.

  1. nerwin

    nerwin Been spending a lot of time on here!

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    I was watching a photography video this morning and learned about this. I had no idea there was such thing as f-stop equivalency.

    So a Sony RX100 M3 24-70 or Nikon DL 24-85 have 35mm equivalency of focal length but not aperture. Their 35mm aperture equivalent is actually f/4.9-7.6.

    Samething applies to a 50 1.8 on a Nikon DX body. While the lens is a 50mm with a maximum aperture of f/1.8 on full frame but when its on a 1.5x DX crop body, its equivalent of a 75mm at f/2.8 on full frame. However, would this apply to dedicated DX lenses?

    I could be completely wrong. But it kind of makes sense because not all the light coming through the lens will hit the smaller sensor. But because the 35mm full sensor uses the whole image circle of the lens, it collects most of the right? I'm slightly confused haha.


     
  2. Ysarex

    Ysarex Been spending a lot of time on here!

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    Slightly confused. The f/stop equivalence that you're talking about pertains to DOF not exposure. Exposure with an f/2.8 f-stop on a crop sensor is the same exposure as with an f/2.8 f-stop on a full frame camera. As far as exposure f/2.8 is f/2.8 regardless of the camera sensor or film size.

    The issue comes up when trying to compare DOF between cameras of different sensor size. The smaller the recording media whether film or sensor the greater the DOF in general. You get more DOF from small sensors and small film and less DOF from large sensors and large film. So the f/stop equivalence factors you're seeing are used by people who want to compare DOF between different sensor size cameras.

    Joe
     
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  3. Solarflare

    Solarflare No longer a newbie, moving up!

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    Why yes, for depth of field that true.


    Exposure though is light per sensor area. Thus the size of the sensor doesnt change it. And its depends upon T-stops, not f-stops.

    T standing for Translucence - how much light is actually channeled through the lens

    While f is standing for focal length. Thats because the aperture is defined as k = f / D. K being the aperture, f being the focal length, and D being the diameter of the pupil. So if you have for example a 50mm f1.0 lens, it has a pupil of 50mm diameter. If you stop it down, aperture blades are forming an increasingly smaller hole, so at f2 it would be for example only 25mm now, and only allow 1/4 of the light through that it allowed before.

    F-stops are only related to T-stops in that the f-stop is are always lower than the T-stop. The only way to make them equal would be to have an optics without any light loss at all, and well thats not possible.


    And as always, optical features like aperture are part of the lens, not the sensor. The lens doesnt care what kind of sensor you use with it. It will always operate at the focal length, f-stop and T-stop thats typical for the lens, and it will also have the depth of field that corresponds to that focal length and f-stop.

    Thus if you use a 50mm f1.8 lens on an APS-C sensor, its focal length stays 50mm, its f-stop stays 1.8, and its T-stop stays whereever the heck it is at f1.8, hopefully something close like T1.9 or T2.0.

    The only thing thats changed is the viewing angle. And yes, if you recompute the depth of field, you would have to have a different dof - aperture. Its much easier to remember though that the 75mm equivalent you use on the APS-C sensor will have the exact same depth of field as a 50mm f1.8 on a small format sensor. Because depth of field only depends upon optics, not the sensor.



    P.s.: Ooops, of course exposure isnt specified in T-stops ... it only depends upon the T-stop of the lens plus the current luminence of the scene.
     
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  4. JustJazzie

    JustJazzie Been spending a lot of time on here!

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    I've gotta agree with the comments above. When I bought my hand held light meter, the didn't offer a FF version.

    I too, think they were referring to the DOF rendering between the two.
     
  5. Ysarex

    Ysarex Been spending a lot of time on here!

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    This is incorrect and a common misconception. The size of the sensor (film) is a DOF determinant. DOF changes with the size of the recording medium. To calculate DOF requires a value for the variable CoC (circle of confusion) and that value is determined by sensor size: Online Depth of Field Calculator

    Joe
     
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  6. Braineack

    Braineack Been spending a lot of time on here!

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    in regards to DOF, not exposure.
     
  7. Solarflare

    Solarflare No longer a newbie, moving up!

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    Uh-hu.

    Well, you are obviously wrong with that assumption, and your link doesnt prove it either.

    DoF is exclusively a function of the optics. Which is logically extremely obvious. What can the sensor do about the light falling on it ? Absolutely nothing, except recording it. Thus DoF happends in the optics, nowhere else.

    The only reason larger sensors offer more DoF is because for a larger sensor, longer focal lengths are required. Thus a MFT sensor uses a 25mm, an APS-C sensor a 33mm and a FF sensor a 50mm optics for the same field of view. Which means the FF sensor gets a lot less DoF. But not because its larger. If you use the FF sensor with a 25mm optics, it sees the same DoF as the MFT sensor with it, except it gets a whole different viewing angle.

    The only influence of the sensor is the pixel size, which indeed changes the DoF a bit. Because mathematically no matter which focal length and which aperture - only the focus plane is in focus.

    But this actually fights the DoF "advantage" of the larger sensor, since the larger sensor has larger pixels and thus is more tolerant. This influence however is nonlinear - meaning to get twice the DoF, you would need 4 times as large pixels. Also recently larger sensors tend to have more and more the same pixel size as smaller sensors, so I prefer to just ignore this issue altogether.



    P.s.: The best way to quickly disprove your claim is to go to the link you provided yourself and scroll to the end of the "camera" list. Instead of any camera, there is a list of CoC values. How could there be CoC values without any sensor size ? Thats because sensor size isnt part of any of the formulas to compute DoF. Only focal length, aperture, distance and CoC are.
     
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  8. gsgary

    gsgary Been spending a lot of time on here!

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    As they say a little knowledge is very dangerous
     
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  9. KmH

    KmH In memoriam Supporting Member

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  10. Braineack

    Braineack Been spending a lot of time on here!

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    That website uses this formula:

    Zeiss formula - Wikipedia, the free encyclopedia

     
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  11. Ysarex

    Ysarex Been spending a lot of time on here!

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    DOF_sensor.jpg

    Joe
     
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  12. table1349

    table1349 Been spending a lot of time on here!

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    [​IMG]
     
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