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effective f-stop value
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PostPosted: Wed Dec 30, 2015 6:04 pm    Post subject: effective f-stop value Reply with quote
If one is using a medium-format lens (Mamiya 645 55mm F2.Cool with an adapter (Photodiox Pro) on a Nikon DSLR (D7200) ...

Do the F-stop values represent the same?
In other words, the Mamiya lens is much bigger. the F2.8 in medium format is a larger window of light than a native Nikon lens at F2.8.
So is an F-stop of 2.8 in medium format, effectively something akin to an F1.8 on Nikon??

Appreciate your thoughts.

Yossarian.


PostPosted: Wed Dec 30, 2015 6:59 pm    Post subject: Reply with quote
The f-stops are the same regardless of the format on wich the lens is used (lens-depend, not camere/format-depend).
f-stop = focal length/diameter of thr entrance pupil of the lens(= diameter of the image of the diaphragm as it can be seen through the front of the lens).


PostPosted: Wed Dec 30, 2015 7:02 pm    Post subject: Reply with quote
The F stop value will not be changed as it is the property of a lens. The main reason why medium format lens is biggest is to get a large are of illumination/image circle.


PostPosted: Wed Dec 30, 2015 7:21 pm    Post subject: effective f Reply with quote
Thats why I am asking about EFFECTIVE F-stop value... from the camera's sensor perspective.
Look at this picture. both are 55mm at F/4. obviously the one on the left is the medium format.
To me the medium F/4 is bigger, and therefore will get more light to the sensor.
Am I thinking about this wrong?
Mostly, just want to know if the F2.8 medium format lens would be as good in low-light as, say, a nikon F1.8.
I suppose shooting the two would be way to find out.


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PostPosted: Wed Dec 30, 2015 8:58 pm    Post subject: Reply with quote
The density of the light (number of photons per unit area) will be the same for the two lenses when operating with the same F-stop (but see note below). The medium format lens transmits more light in total but images it to a larger area. The light that is received by the DSLR sensor will be the same whichever lens is used. The exposure is determined by the light density not the total amount of light.

NB. Strictly speaking, this is true for lenses of the same T-stop value. Lenses with the same F-stop will not, in general, have the same transmission since the F-stop is a geometric value that does not take into account the light lost at each glass to air boundary in the lens (absorption losses are usually negligible). A complicated lens, even with state-of-the-art AR coatings will still have a T-stop somewhat different to the F-stop value.


PostPosted: Fri Jan 01, 2016 11:34 pm    Post subject: Re: effective f Reply with quote
Yossarian9 wrote:
Thats why I am asking about EFFECTIVE F-stop value... from the camera's sensor perspective.
Look at this picture. both are 55mm at F/4. obviously the one on the left is the medium format.
To me the medium F/4 is bigger, and therefore will get more light to the sensor.
Am I thinking about this wrong?
Mostly, just want to know if the F2.8 medium format lens would be as good in low-light as, say, a nikon F1.8.
I suppose shooting the two would be way to find out.




Notice you are seeing the exit pupils of the lenses. Yes, for the same F-number, the exit pupils may have different dimensions and be at different distances from the focal plane. However, if you look at the entrance pupils at the other side of the lenses, their diameters should be the same, no matter what type of lens.


PostPosted: Fri Jan 01, 2016 11:57 pm    Post subject: Reply with quote
Nikon 1.8 lens will be better in low light than Mamiya 2.8. Any 1.8 lens will be better in low light than any 2.8 lens, regardless of the format they are designed for.
Read carefully the wiki article about f-number:
https://en.wikipedia.org/wiki/F-number

Both SXR_Mark and Gerald are speaking about 2 lenses with the same f-number, not about comparing your Nikon f/1.8 with a medium format f/2.8 lens, as you asked.


PostPosted: Sat Jan 02, 2016 1:23 am    Post subject: Reply with quote
Lens made for medium format needs a bigger hole to let in the extra light needed to cover the area bigger than 24x36mm.

Learning about T-stop and F-stop difference might help here...

Gerald wrote:
... if you look at the entrance pupils at the other side of the lenses, their diameters should be the same, no matter what type of lens.


Huh? I don't think so...
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