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Thread: F-stop measurement across different brands

  1. #1
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    Default F-stop measurement across different brands

    Hi all, I am trying to understand this f-stop thing and getting no where. The question I have is when x brand produced a lens which says f2.8, L brand also has a lens that says f2.8, can we safely assume that both lenses have the same maximum aperture of f2.8 based on a certain measurement, barring the quality of the glass, practically will allow the same amount of light enters its iris? What about consumer camera that claims f2.8, such as the Panasonic FZ-series Leica f2.8 (420mm!!)?

  2. #2

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    eh...lets see...most prosumers at f2.8 will give u almost the same thing as on a lens...

    correct me if i am wrong on this (anyone) .....the 2.8 on most digicams are a relative number to the camera..... so u wun get the same dof at 2.8.... when i open my lens to 2,8, its prob bigger than the lens head of a Digicam le... (this is wat i tink...need to confirm)

    cheers..

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    Quote Originally Posted by L.Lee
    Hi all, I am trying to understand this f-stop thing and getting no where. The question I have is when x brand produced a lens which says f2.8, L brand also has a lens that says f2.8, can we safely assume that both lenses have the same maximum aperture of f2.8 based on a certain measurement, barring the quality of the glass, practically will allow the same amount of light enters its iris? What about consumer camera that claims f2.8, such as the Panasonic FZ-series Leica f2.8 (420mm!!)?
    If I can remember somewhere in the back of my head, F-stop measurement is derived from an equation or something... So I think its safe to say that at f2.8 or any other aperture most glass would be the same....
    The equipment can only bring you so far - the rest of the photographic journey is done by you.

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    The panasonic fz10 f2.8 lens is 36 to 432mm (35mm equivalent), it is actually only 6 to 72mm with a much smaller sensor than the DSLR sensor. So the aperture is actually much smaller than the 35mm f2.8 lenses.
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  5. #5

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    The F-number (e.g.: 2.8), is derived from focal length divide by diameter of the hole (aperature)

    200mm / 2.8 = 71mm
    or
    200mm / 71mm = 2.8

    focal length 200mm, diameter of aperature 71mm, thus U'll get f/2.8.

    Though the FZ10 is 35 to 432mm (35mm equivalent), however, the actual focal length is only 6 to 72mm (due to smaller sensor, so focal length needed will be shorter also).

    therefore, 72mm / 2.8 = 25mm
    The hole only needs to have diameter 25mm, thus a smaller lens.
    Last edited by AReality; 14th November 2004 at 09:06 PM.

  6. #6

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    Yeah, areality has got the technicals nailed down. It's just a ratio between the objective lens diameter and aperture diameter. The same thing goes for telescopes and middle/large format lenses. All are relative...

    You have to think in terms of relativity. A p&s digicam sensor is so much smaller, so a 2.8 lens does not need to be that big. If we fit a 35mm 70-200/2.8 lens to say a 5mp digicam, it'd be WOW! Of course there are minus points of small sensors too.

    And anyway, a f2.8 on a zoom would be "darker" than a f2.8 on a prime. Usually it ranges from 1/2 to 2/3 stops depends on how many elements the zoom has. That's to say if you manually set 1/100s f2.8 on a 50mm prime, it should look brighter than a 1/100s 2.8 picture done on a 28-70/2.8 lens.
    Last edited by 2100; 14th November 2004 at 09:41 PM.

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    I see, I see...ths is getting very educational now. In the case of 200mm f2.8 with an aperture size of 71mm, I presume you are refering to radius size right? Else the lens barrel is going to be huge and this is unlikely judging from the filter size of a 200mm 2.8L which is in the region of 70-80mm.

    I believe we can also conclude that the prosumer lens is handicapped only by the smaller size of its sensor right? (beside the quality of the glass).

    Can we also conclude that for a given f-number, the amount of light the camera can receive through the lens is also subject to the quality of the glass and the number of glass elements used?

    If the above question is true, than the f-measure is purely on apeture size (as dicussed earlier) and the brightness/speed of the lens can not be determined by just the f-stop right?

  8. #8

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    Quote Originally Posted by 2100
    Yeah, areality has got the technicals nailed down. It's just a ratio between the objective lens diameter and aperture diameter. The same thing goes for telescopes and middle/large format lenses. All are relative...
    I think you are mixing things up. f-stop is simply the ratio of focal length to size of aperture for camera lenses.

    Objective lens diameter is essentially the aperture for scopes and binoculars.

    And anyway, a f2.8 on a zoom would be "darker" than a f2.8 on a prime. Usually it ranges from 1/2 to 2/3 stops depends on how many elements the zoom has. That's to say if you manually set 1/100s f2.8 on a 50mm prime, it should look brighter than a 1/100s 2.8 picture done on a 28-70/2.8 lens.
    Be careful of how you are comparing here. Setting f2.8 on a 50mm prime and comparing it to a f2.8 zoom is not valid unless the prime only has f2.8 as its largest aperture. What you see through the lens at any time is the open aperture. So you might be comparing 1.8 or 1.4 to f2.8 here.

  9. #9

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    Quote Originally Posted by L.Lee
    I see, I see...ths is getting very educational now. In the case of 200mm f2.8 with an aperture size of 71mm, I presume you are refering to radius size right? Else the lens barrel is going to be huge and this is unlikely judging from the filter size of a 200mm 2.8L which is in the region of 70-80mm.
    Diameter. not radius. But with new technology, this may not hold true anymore.

    Quote Originally Posted by L.Lee
    Can we also conclude that for a given f-number, the amount of light the camera can receive through the lens is also subject to the quality of the glass and the number of glass elements used?

    If the above question is true, than the f-measure is purely on apeture size (as dicussed earlier) and the brightness/speed of the lens can not be determined by just the f-stop right?
    Dunno wat you're trying to say.
    But for a given F-number for any focal length, the amount of light falling onto the sensor/film per unit time is the same.

    E.g.:
    Wider angle has wider field of view.
    Wider angle has shorter focal length.
    Shorter focal length == smaller aperature hole (for same F-number)
    Thus, the wider the field of view, the smaller the hole.

    So why is the amount of light falling onto the sensor/film per unit time is the same?

    A wider field of view has more rays going into the lens than a smaller field of view. Thus the smaller hole. The calculation will be quite mathematical, I don't bother to explain, coz it'll take ages.


    .

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    Quote Originally Posted by L.Lee
    ....I presume you are refering to radius size right? Else the lens barrel is going to be huge and this is unlikely judging from the filter size of a 200mm 2.8L which is in the region of 70-80mm.
    Oops, I meant perimeter.

    Still on the same subject, but this time allow me to given an example:
    Is it true to say that I can always maintain the same shutter speed &/ ISO for a given camera body and scene when I change from a f2.8 lens to another f2.8 lens of different make/model, while achieving the same exposure?

  11. #11

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    You are right Zerstorer, thanks for correcting me. Have been thinking of digiscoping, where focal length / objective diameter gives you the max f-stop. For lenses, it's the same eg 400/5.6 would mean 400/5.6 = 71mm min obj lens diameter (ie 77). For my sigma 50-500 with 86mm diameter, it's 500/86=5.81, round it up and it's f6.3. Guess things are not absolute and they don't use the extreme edges of the glass.


    Quote Originally Posted by L.Lee

    Still on the same subject, but this time allow me to given an example:
    Is it true to say that I can always maintain the same shutter speed &/ ISO for a given camera body and scene when I change from a f2.8 lens to another f2.8 lens of different make/model, while achieving the same exposure?
    Yes. Say you take 300mm on a FZ20 and set at 1/100 f2.8 ISO 200. You use a D70 with a 70-200/2.8 lens and set at 200mm 1/100 f2.8 ISO 200 (200mm becomes 300mm effectively after 1.5X cropping). They will look more or less the same for exposure. More or less, coz there will be variances in terms of sensor sensitivity, different light attenuation through different lenses (ie more glass elements and the lousier they are the more attenation) etc... If you use the metering modes like Shutter, Aperture priority, different bodies will give different readings anyway so that might not be accurate even though subject/focal length are the same.

  12. #12

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    You mentioned the Panasonic consumer cameras and its claim of 432mm at 2.8. Yes, a FZ20/10 has true 400+ mm at f2.8 capability. And its Mega O.I.S works. We have a shooter in the World of Nature using this cam and it's good.

    The problem is the sensor. I don't care about 4 or 5 megapixels between FZ10 and 20, very little difference. Compared to digital SLRs, they have 1/2.5 sensors. That's even smaller than the 1/1.8" ones found in the C5050 or 2/3" one in the F717. Problem with small sensors is that they are quite susceptible to noise, though latest advances with processing and sensor manufacturing is keeping this in check. But no matter what, sensor size is still bound by physics laws and that is why if you check out the samples of the FZ20, ISO 100 on it looks worse than ISO 100 on a Sony F717. That "worse" part i am talking about is noise, and that destroys detail to quite a huge certain extent when above a certain threshold.

  13. #13

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    Quote Originally Posted by AReality
    Diameter. not radius. But with new technology, this may not hold true anymore.



    Dunno wat you're trying to say.
    But for a given F-number for any focal length, the amount of light falling onto the sensor/film per unit time is the same.

    E.g.:
    Wider angle has wider field of view.
    Wider angle has shorter focal length.
    Shorter focal length == smaller aperature hole (for same F-number)
    Thus, the wider the field of view, the smaller the hole.

    So why is the amount of light falling onto the sensor/film per unit time is the same?

    A wider field of view has more rays going into the lens than a smaller field of view. Thus the smaller hole. The calculation will be quite mathematical, I don't bother to explain, coz it'll take ages.


    .
    you are explaining why two different prime lens, a wide angle and a telephoto lens would differ in their aperture diameter, for the same F-number, eg f/2.8.

    you're saying the wide angle (say 35mm) would have a smaller aperture diameter (12.5mm) at f/2.8.

    compared to telephoto (say 200mm) would have a larger diameter (71.4mm) at f/2.8.

    that's fine and dandy. what about zoom lens? the aperture diaphragm is the same one. how does it work for those variable aperture (eg. f/4-5.6) or even constant aperture (f/2.8) zoom lens when you zoom from wide to telephoto? does the aperture diaphragm dilate from 12.5mm to 71.4mm internally when you zoom from 35mm to 200mm at f/2.8?

  14. #14

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    Quote Originally Posted by AReality

    Quote Originally Posted by L.Lee
    Can we also conclude that for a given f-number, the amount of light the camera can receive through the lens is also subject to the quality of the glass and the number of glass elements used?

    If the above question is true, than the f-measure is purely on apeture size (as dicussed earlier) and the brightness/speed of the lens can not be determined by just the f-stop right?
    Dunno wat you're trying to say.
    But for a given F-number for any focal length, the amount of light falling onto the sensor/film per unit time is the same.

    .
    what L.Lee is trying to ask is, if the light falling onto the film/sensor is the same for two different lens, as the aperture formula is a purely physical dimensions one (ie only focal length and aperture diameter), but why doesn't the quality of the glass, the number of lens elements, and the resulting difference in the transmission of light play a part? so with the aperture being just focal length/aperture diameter, why then would there be a difference between cheap glass with low light transmission, and your super L lens with high transmission and low light loss?

    ok, maybe that's not what L.Lee is literally asking, but along the same lines of questioning.

  15. #15
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    If you want to talk about actual light tranmission, you need to talk about T-stops and not F-stops. T-stops are used in cinematography if I am not wrong. F-stops only approximate T-stops but are close enough for most photographers...

  16. #16

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    Quote Originally Posted by eudoofus
    what about zoom lens? the aperture diaphragm is the same one. how does it work for those variable aperture (eg. f/4-5.6) or even constant aperture (f/2.8) zoom lens when you zoom from wide to telephoto? does the aperture diaphragm dilate from 12.5mm to 71.4mm internally when you zoom from 35mm to 200mm at f/2.8?
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  17. #17

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    since u all start talking about aperture.... i would like to know did anyone compare a prosumer camera with a dslr lens.... in terms of same aperture same distance..... does the dslr lens be better in any way?

    Those carl zieus on sonyf828 n L lens on canon pro1.... how good r they compared to those lens used on dslr? are they the lowest grade in dslr world?

    me not slr user.... pardon me if my logic is weird....

    Thank you

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    Quote Originally Posted by kegler
    since u all start talking about aperture.... i would like to know did anyone compare a prosumer camera with a dslr lens.... in terms of same aperture same distance..... does the dslr lens be better in any way?
    depends on what you consider, "better". a prosumer PnS would combined telezoom, standard AND macro all in one. SLR lens? add a closeup filter if it doesn't have a macro option that produces closeups of things that well.

  19. #19

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    Quote Originally Posted by sehsuan
    depends on what you consider, "better". a prosumer PnS would combined telezoom, standard AND macro all in one. SLR lens? add a closeup filter if it doesn't have a macro option that produces closeups of things that well.
    ok...i didn't state my question clear enough...heehee....

    example:
    compare as in f828 at 28mm f2.0 and with 300D kit lens 18mm(1.6x) which is 28.8mm with a 3.5f? i think do u think they r on par? or how different?
    maybe on par in terms of amount of light goes into the cam?

  20. #20

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    Quote Originally Posted by kegler
    ok...i didn't state my question clear enough...heehee....

    example:
    compare as in f828 at 28mm f2.0 and with 300D kit lens 18mm(1.6x) which is 28.8mm with a 3.5f? i think do u think they r on par? or how different?
    maybe on par in terms of amount of light goes into the cam?
    Can't compare apples to oranges.
    However,

    The amount of light falling onto the sensor/film per unit area, for the same F-number, is the same for all focal lengths. Regarless of the sensor size.

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