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  1. #11
    Senior Member
    spb_stan's Avatar

    Re: F- stops for different lenses

    Lots of confusion in this thread as to the meaning of Aperture and Transmission. Texas has it right, the aperture just tells of a ratio of focal length to effective aperture diameter. That is all it tells you, and does not infer anything about light transmission. a complex lens with 23 elements will almost always have less light transmitted through it than a simple 3 element lens made of the same material. But lens elements are not of the same material, they are selected for their ability to compensate for path disturbances by color frequency, scattering, collimating etc. The Transmission factor is measured as a T number. F stops are calculated by simple measuring the dimensions of the lens and T-stops are actual light transmission efficiency. T-Stops are almost always lower in transmission than F-stops suggest since F apertures assume dry air as the medium of transmission and T-Stops ratings are actual lens optical path attenuation measurements.
    DSLRs using through the lens metering and VF compensate so exposure is pretty close to T-stop but that has limits when wide open when no more aperture vane movement is possible, you can see that camera gain has to make up for the lower transmission than ideal. So a f/1.4 lens with a T-stop rating of 1.8 might give a lot less light than a lens with identical physical dimension f/1.4 but T-stop t-1.5. The result in low light will be a lot lower noise and better DR for the latter lens.
    T-stops are not so important for most general photography because there is nothing to compare within individual images, each is a stand-alone visual experience. Where it becomes very important is when images have to match when changing apertures and lighting. A case there is astrophysics and cinema where scene exposure has to be carefully calibrated to match another scene or angle.
    General photography can be sloppy in light gathering attention to detail but movies are another story and astrophysics calibration of the light transmission is critical for optical measurements to mean anything other than just pretty pictures. Actual light intensity falling on a pixel can make the difference between just a pin of light and evidence of an entirely undiscovered galaxy.
    Why don't lens makers list T-stop? It is expensive to measure and to assure a certain specification and not demanded by still photographers so why bother. Cinematographers, however, have the need and budget for it so demand that spec.


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  2. #12
    Staff
    Super Mod
    hark's Avatar

    Re: F- stops for different lenses

    Quote Originally Posted by bandit993 View Post
    Take for example a $16,000 Nikon 600mm F/4 lens set at F8 and a $1250 Sigma 150-600mm C (@600mm) at F8. Will the camera need different amounts of light or set the ISO at the same level?
    Providing you don't zoom with the Sigma, the readings should be the same. BUT if you move the zoom to 550mm or some shorter focal length than 600mm, then the field of view changes possibly causing a different meter reading.
    Cindy
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  3. #13
    Senior Member
    Texas's Avatar

    Re: F- stops for different lenses

    What irks me is the F stop numbers are on all lenses and I assume them to be mostly believable.

    Even zooms that have an F stop range that changes with a particular zoomed focal length print the range on the lens.

    But macro lenses, that's another story. Their printed and spec'ed F stop is measured at infinity focus and is way off for any close focusing. Well probably not off at all based on how the max advertised F stop is calculated. But you do need more iso or more light to get the same exposure when you close focus.

    Many zooms have a much different than advertised focal length when focused closely.

    I'm supposing that a fixed focal length macro lens keeps its advertised focal length when close focusing, but could be wrong.

    Of course this is nit-picking, they all have excellent photo capability.
    Last edited by Texas; 05-25-2018 at 05:48 PM.
    D750, D100, D200, Nikon 1 J5
    (Once owned: EL, F2AS, D50, D90, D300s, and D7100)

  4. #14
    Senior Member
    pforsell's Avatar

    Re: F- stops for different lenses

    Quote Originally Posted by Texas View Post
    But macro lenses, that's another story. Their printed and spec'ed F stop is measured at infinity focus and is way off for any close focusing. Well probably not off at all based on how the max advertised F stop is calculated. But you do need more iso or more light to get the same exposure when you close focus.
    Bellows effect is a well known fact, unavoidable, and widely documented. It is not cheating or "fine print" or hidden knowledge. I haven't older books available right now, but at least Ansel Adams' Camera and Lens from 1948 has the formula to calculate the bellows effect. I assume the lens manufacturers assume that macro shooters are interested in their stuff more than the average joe, and know all this.

    Quote Originally Posted by Texas View Post
    I'm supposing that a fixed focal length macro lens keeps its advertised focal length when close focusing, but could be wrong.
    Some may, but none that use unit focusing do. All of my macro lenses shorten when focused to closer than infinity.

    It might be interesting to some if manufacturers published a graph or a table that shows the true focal lengths at all focusing distances, but I don't think it would mean much to the general public, or much to anyone at all. For example, if I found out that my 400/2.8VR is only 390mm at minimum focusing distance, would I replace it with some other 400/2.8? Which one? And why?

    Another issue is the strong focus breathing of some lenses, like the 70-200/2.8VR II but that problem has been fixed with the replacement FL version, which is absolutely stunning in every way. I would buy that one if I was a zoom shooter, but I prefer primes.
    Last edited by pforsell; 05-25-2018 at 06:35 PM.
    9 Nikon single-digit pro bodies from D1H to D5.
    12 Nikon three-digit consumer bodies from D100 and up.
    56 Nikkor prime lenses from AIS 8/2.8 to AFS 400/2.8VR
    4 Nikkor zoom lenses: 14-24/2.8, 17-35/2.8, 28-70/2.8, 70-200/2.8VR
    My fastest lens is f/1.2 (x3) and slowest f/2.8


  5. #15
    Senior Member
    Woodyg3's Avatar

    Re: F- stops for different lenses

    Everyone should understand the difference between f-stops and t-stops. Here is a vid that does a decent job of explaining.

    https://petapixel.com/2016/12/30/f-s...plain-english/
    Thanks/Like Texas, ryan20fun Thanks/liked this post
     
    Woody Green

    Clear Eyes, Full Hearts, Can't Lose

    D500, D7200, D7100, D70

  6. #16
    Senior Member
    Texas's Avatar

    Re: F- stops for different lenses

    Back to bandit's initial concerns, setting the F stops to the same value on two different lenses, everything else being equal, will give the same exposure on both.

    That is, unless one lens of the two is a macro/micro and you are close focusing with it.

    In that case, relying on matching the F stops on both lenses to get the same exposure will be folly since the F stop shown on/by the macro lens will no longer match that of the other lens as to light transmitted to the sensor/film.

    It may not be cheating to print a single F stop spec on a macro lens. But it is nice that zoom lenses of a certain design do note a change of F stop vs. focal length.

    I'd like to see macro lenses do something similar based on focus distance but I fear the extreme F stop range would scare off buyers. That bellows effect is a big one.
    Last edited by Texas; 05-27-2018 at 05:16 AM.
    D750, D100, D200, Nikon 1 J5
    (Once owned: EL, F2AS, D50, D90, D300s, and D7100)

  7. #17
    Senior Member
    spb_stan's Avatar

    Re: F- stops for different lenses

    Two lenses with different T stop ratings but same aperture will have different exposure settings, they have to, their sensors will be supplied different light intensity. I am not sure why everyone is saying there is no difference. Would they say a f/1.8 lens and a f/1.4 lens would have the same exposure? They might get a similar exposure once the Triad is adjusted to produce the same exposure but they will require different settings to do it.
    Thanks/Like Kevin H Thanks/liked this post
     

  8. #18
    Senior Member
    Texas's Avatar

    Re: F- stops for different lenses

    I think they will be mostly, not absolutely, the same exposure - say within a third or half stop. Close enough.
    D750, D100, D200, Nikon 1 J5
    (Once owned: EL, F2AS, D50, D90, D300s, and D7100)

  9. #19
    Senior Member
    480sparky's Avatar

    Re: F- stops for different lenses

    Quote Originally Posted by Woodyg3 View Post
    Everyone should understand the difference between f-stops and t-stops...........
    Why?
    Don't mind me... I'm out roaming around somewhere between Zone III and VII.


    Go forth and actuate!


    My Website.

  10. #20
    Senior Member
    Woodyg3's Avatar

    Re: F- stops for different lenses

    Quote Originally Posted by 480sparky View Post
    Why?
    Because knowledge is a good thing.

    Photographers should understand that there can be differences in exposure between two lenses set at the same f-stop and shutter speed due to differences in t-stop, that's all. The difference may never make a practical difference for most, but I thinks it's still good to understand.
    Thanks/Like ryan20fun, Texas Thanks/liked this post
     
    Woody Green

    Clear Eyes, Full Hearts, Can't Lose

    D500, D7200, D7100, D70





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