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Thread: More light for FF?

  1. #1

    More light for FF?

    So I've heard that FF cameras have 1.3 stops more light but I can't wrap my brain around it for some reason. Because to me they're the ratios are the same just different sizes so wouldn't the lens and whatnot let in the same light? Does that mean that a medium format camera like a Hassleb let in even more light with the same FL and Ap?

    Also if the lenses today are in F-stop i'm sure they're like 90% true to the T-stop so why would the sensor size play a role in the amount of light? I can kind of see if your using an EF lens on a APS-C vs a EF-S lens on a APS-C being different....

    :|

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    Hansm's Avatar
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    Re: More light for FF?

    Hi Vincenzo,
    Not sure if I understand your question correct.
    A 2.8 / 100 mm lens will not be different on amount of light on Crop size camera op FF.
    The only difference is that the image is less cropped on FF compared to 1.3 cropbody.

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    Black Pearl's Avatar
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    Re: More light for FF?

    A f stop is a f stop regardless of what you put it in front of.

  4. #4

    Re: More light for FF?

    I'm not talking about the F-stop though more about the t-stop which would reach the sensor.

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    Re: More light for FF?

    Quote Originally Posted by Vincenzo View Post
    So I've heard that FF cameras have 1.3 stops more light but I can't wrap my brain around it for some reason.
    Of course the other possibility is that what you heard was wrong.

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    Black Pearl's Avatar
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    Re: More light for FF?

    My understanding of t stops is that they take into account the loss of brightness that occurs as the light passes through the glass in the lens. They are factory calibrated and therefor more accurate than a f stop which are derived by a simple ratio between the focal length and the physical size of the aperture - but - and its a big but, I'm struggling to see how they could be put into a practical stills photographic situation. Motion picture lenses use t stops which are an absolute so switching between them doesn't alter the exposure continuation of a given sequence. The difference is they are calibrated in t stops so you can control the exact exposure when switching, stills photography lenses aren't so even though there maybe a difference in the absolute light transmission between our lenses (not all are what they claim to be) there's not a lot we can do about it.

    Going back to my earlier post - surely a t stop is a t stop regardless of what you put it in front of.

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    Re: More light for FF?

    I'm sure I'm wrong here

    I suspect what Vicenzo has heard is something to do with 'the old chestnut' of the light per unit area falling on the sensor - let the thread go that way rather than discussing f and t stops, because I don't think they are relevant.

    My immediate thoughts regarding EF lenses on an APS-C body is that yes, it lets in more light, but that light is wasted, along with the angle of view it contained, because it falls outside the sensor. This is a commonly used lens+body choice (in many manufacturer's ranges) and allows one to use the sharpest bit of the lens's image

    Now; as to whether, at a given illumination, the level 255 comes out of the sensor at a different iso for the same aperture and shutter speed, I wouldn't know.

    Someone may like to look at the tutorials, as I vaguely recall reading something about this there.


    One could argue that an EF-S lens on a FF body lets in less light because it only illuminates a circle on the sensor and leaves the corners and sides in the dark. Of course, for practical purposes, this lens+body choice is daft, so the point about less light is irrelevant.

    Remember; I did say I could be wrong!

    Cheers,

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    Black Pearl's Avatar
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    Re: More light for FF?

    I suppose it boils down to how we are going to measure the amount of light.

    If we are talking about brightness at a given point on the sensor then the size of the sensor and the size of the image circle the lens produces are irrelevant. It is simple mathematics between the focal length and the actual size of the aperture that produce a f stop value and therefor a brightness level. This has to be the case as we can switch between any lenses and any body we like and get the same exposure.

    If we going to talk about the total amount of photons being converted into electricity at the sensor then yes a 24x36mm sensor will get and therefor convert more photons than a 15.8x23.6mm sensor.
    Crucially the exposure will still be the same.

  9. #9

    Re: More light for FF?

    Quote Originally Posted by Dave Humphries View Post
    I'm sure I'm wrong here

    'the old chestnut' of the light per unit area falling on the sensor
    yes that's exactly what i was talking about. I kept on hearing my friends and people saying that a FF because its bigger would let more light in than a aps-c but I couldn't understand how it would let more light in if both lenses are set to F/4 for example...

    and then I was thinking, maybe they thought that if you use an EF lens on a aps-c that ur 'loosing' some light around the edges and therefore the FF would have more light.....

    I mean yeah the amount of light would be more in a FF body like Dave said but that's parallel to the body size and it's still the same amount just spread over a bigger sensor...right?

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    Re: More light for FF?

    Quote Originally Posted by Vincenzo View Post
    yes that's exactly what i was talking about. I kept on hearing my friends and people saying that a FF because its bigger would let more light in than a aps-c but I couldn't understand how it would let more light in if both lenses are set to F/4 for example...
    It does, but it's from a wider area going in, and "over a wider area" coming out of the lens and heading towards the sensor (which "discards" a certain portion of it), so it evens out.

    Where it DOES get interesting though is that FF cameras have a tighter DoF (by about 1 stop over a 1.6x crop-factor camera) - so for a given DoF (which is dictated by Aperture), one may typically need up to TWICE the light when using a FF camera for the same DoF.

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    Re: More light for FF?

    Please define "FF" in this context. Thanks.

    I just found my answer....thanks....sorry for the "intrusion"...
    Last edited by jonjdoe; 17th October 2011 at 02:11 AM. Reason: Exercised a little patience....:)

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    Black Pearl's Avatar
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    Re: More light for FF?

    Where it DOES get interesting though is that FF cameras have a tighter DoF (by about 1 stop over a 1.6x crop-factor camera) - so for a given DoF (which is dictated by Aperture), one may typically need up to TWICE the light when using a FF camera for the same DoF.
    You might want to explain that one a bit more.

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    Re: More light for FF?

    Quote Originally Posted by black pearl View Post
    You might want to explain that one a bit more.
    Say two people are shooting in my studio. One has a FF camera with a 80mm lens, and the other has a CF camera with a 50mm lens (so save FOV). Sufficient DoF is a concern. The FF shooter may need F11, whereas the CF shooter may only need F8 for the same DoF. Therefore to get the same exposure (assuming, say, both are shooting at 1/125th @ ISO 100) then the F11 shooter will need the lights twice as bright.

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