Asking experts: light stops and stuff (true or false)

Discussion in 'Open Discussion' started by F1L1P, May 20, 2010.

  1. F1L1P

    F1L1P Mu-43 Veteran

    Jan 2, 2010
    I can’t figure out what (light) stop would be. Here is what I assume- true or false:

    Light stop is equivalent to 1 EV

    If you double light stops value you double the light intensity entering the sensor

    In order to double the amount of light you need to exponentially increase number of light source. Eg. 2 light bulbs will yield 1 light stop, 4 light bulbs will yield 2 light stops more intensive image, 8 bulbs- 3 stops and so on. Each time you increase amount of light by one stop you (need to) double previous amount of light source?

    1 light source is 1 light stop? 2 light sources is still 1 light stop?

    1 light source is 0 EV, 4 light sources is +1EV?

    I understand aperture affects exposure. So e.g. Panasonic 20mm f1,7 will have XY value of light stops on m43 or 43 body and the same amount on full frame body, but the difference will be visible in DOF which is, let say, side effect of sensor size. Fast lens is fast lens no matter which body it is mounted on, right?

    ISO value has also an effect on the exposure. Doubling the ISO amount, we double the sensor sensitivity- ISO 400 is 4x more sensitive/ will produce 4 times more light intensive image than ISO 100? ISO 800 is 8x more, ISO 1600; 16x...?

    What is the ratio between aperture and light stops? I understand there is chart which explains this:
    1.0 1.4 2 2.8 4 5.6 8 11 16 22 -> Each of this aperture is by 1 light stop more intensive to light than the one before? Or If I need faster shutter speed I would be able to use 3 times faster shutter speed (if aperture changes from f4 to f2 or from f16 to f8).

    Thanks for help! :thumbup:
  2. kevinparis

    kevinparis Cantankerous Scotsman

    Feb 12, 2010
    Gent, Belgium
    its really very simple

    there are three variables that control exposure

    aperture of lens (f stop eg 2.8)
    shutter speed (eg 1/30 sec)
    sensor/film sensitivity ( eg ISO 400)

    doubling or halving any one of these will be the equivalent of a change of 1 EV either way.

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  3. Djarum

    Djarum Super Moderator

    Dec 15, 2009
    Huntsville, AL, USA
    Real Name:
    Just looking at aperature, an f-stop doubles the area of the light coming into the image plane. So, f1.4 vs f1.0 is double the amount of light. 2 is double the light of 1.4, 2.8 double that of 2..etc. Each of these "doublings" is going up by one stop.

    When it comes to ISO's in the digital world, I think much of using the doubling and having rules of thumb go out the window. Historically, 100, 200, 400, 800, 1600 were considered some form of "f stops" or light stops. Lets say I I was taking a picture at f4 and at ISO 800. Conventional wisdom would say that if I go to f2.8, I should be able to take that same shot at ISO 400. I've never found this to be the case in my digital cameras..ever. I don't know if manufactures design sensor sensitivity and designated ISO's around equivalent fstops as in the example I mentioned.

    For reasonable shutter speeds, I've found doubling and halving to be pretty accurate to doing the opposite at the aperature. But when long shutter speeds are required, I've found that the rule also gets thrown out the window.
  4. Djarum

    Djarum Super Moderator

    Dec 15, 2009
    Huntsville, AL, USA
    Real Name:
    For the most part, yes. In my own experiences, though, there are times where this doesn't always follow suite. I think much of this has to do with how the manufactures set up their ISO ratings.
  5. Streetshooter

    Streetshooter Administrator Emeritus

    Dec 15, 2009
    Phila, Pa USA
    I'll try to make this easy to digest....
    There exist in photography a "half/double" relationship that is based on the Inverse Square Law.
    What this means is something like this....
    Less is more and more is less...applied as so:

    If you close the aperture to f16, a very small hole, it's because there is more light...
    If you open to f2, a very large hole, it's because there is less light.

    If you go to a fast shutter speed, say 1/500 you are letting light in for a very short time and...
    if you go to say, 1/30 you are letting the light in for a longer time.

    The combination that makes exposure is Intensity xs Time. Intensity is the Fstop and time is the shutter speed.

    A way to keep a certain combination of fstop/speed is to use the EV...meaning Exposure Value.
    By changing the EV, you are keeping the exposure equation and changing the ISO to your given choice.

    What you are calling Light Stop is really a term that came about many, many years ago. The old cameras had an aperture ring or needle that just went around and didn't stop with a click. Then I believe it was Ilex but not 100% sure, invented a stop at the given fstop mark. So, the term fstop was invented because the aperture actually stopped at a given fstop.

    So, if you are at f2.0 and go to f2.8 you have halved the light volume thru the lens.
    If you go from f2.8 to f2.0 you have doubled the volume of light thru the lens.
    This is the half/double inverse square law at work.

    Here's a tricky spot....ISO is tricky....
    if you are at ISO 100 and go to ISO 200 you have doubled the speed of the sensitivity.
    if you go from ISO 200 to ISO 400 you have doubled the speed.
    if you go from ISO 400 to ISO 800 you have doubled the speed again but....!

    from 100-200 is 1stop...from 100-400 is 2stops and from 100-800 is 3 stops.
    You stated that in your ISO question but your calculations are wrong due to the half double inverse square law.

    You absolutely must grasp the concept of the Inverse Square applies to photography and all things in life by the way.

    Set your camera on a table or tripod.
    Take a light reading in consistent light and move the aperture 1 stop at a time, watch the shutter speed change, Then do the same with the shutter speed and watch the aperture change.

    Then when you grasp this, take a reading and adjust the EV and watch what changes.....

    If ya get stuck, call and I'll send my telly number...

    ps...I forgot....
    There is a thing that happens at slow, long shutter speeds. It's called reciprocity. The issue is more intense with film but digital has it also.
    The digital cameras solved it by exposing longer than the marked time...say 8 seconds will really be around 10 seconds.
    Reciprocity is the failure of the recording device with low light levels.....
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  6. squeegee

    squeegee Mu-43 Veteran

    Jan 26, 2010
    I have an f-stop related question...

    An F-stop is the same regardless of the sensor size right? (with respect to the amount of light let in, I realise the DOF has a conversion factor)

    i.e. an F1.4 lens for a full frame camera should have the same exposure as an F1.4 lens for a 4/3rds sensor right?
  7. Streetshooter

    Streetshooter Administrator Emeritus

    Dec 15, 2009
    Phila, Pa USA
    Correct. Fstop is the same with all lenses just as shutter speeds are the same.
    Funny how that works huh....there really is a few constants in the photography universe....
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  8. fks

    fks New to Mu-43

    May 16, 2010
    SF Bay Area
    not entirely true. the elements in a lens absorb a little bit of light, so you may not get the same exposure at the same ISO and aperture settings with lenses made by different companies (the movie industry uses the t-stop, which measures the actual light transmitted). there's also the issue of f-stops being approximations (eg f/2.8 is actually f/2.86).

    in practice, i've noticed as much as a 1/2EV of difference between lenses at the same focal length and aperture but of different brands. zoom lenses are more prone to this, most likely because they have more elements that move around relative to each other.

  9. Streetshooter

    Streetshooter Administrator Emeritus

    Dec 15, 2009
    Phila, Pa USA
    not entirely false either. This is an accepted standard that most camera/lens makers use. We are stuck with the subtle differences but for the most part, f 5.6 will be f 5.6 for the majority of shooters out there....
  10. cosinaphile

    cosinaphile Mu-43 All-Pro

    Dec 26, 2009
    new york city
    what couild i possibly add ?

    double or half .... iso or..... f stop or...... shutter speed

    play with the camera on manual and review to kill a few minutes here and there and it will become second nature in no time
  11. Amin Sabet

    Amin Sabet Administrator

    Apr 10, 2009
    Boston, MA (USA)
    Exposure is based on the amount of light gathered per unit of sensor area, not total light gathered for the sensor.

    Doubling the exposure time (eg, 1/50s exposure instead of 1/100s) doubles the amount of light gathered per area, which is equivalent to +1EV.

    Increasing the aperture area by 2-fold results in a doubling of light intensity and is equivalent to +1EV. Each doubling of aperture area reduces the f-number by a factor of sqrt(2).

    Doubling the ISO technically does not change the exposure (since the light gathered per unit of sensor area is unchanged), and by the same token does not technically increase the number of light stops, but increases the apparent exposure two-fold and so is commonly referred to in terms of stops of light.

    So, if you want to increase apparent exposure by a factor of 2 (+1EV or "1 stop" of light), your options are:
    • Change from 1/(2X) shutter speed to 1/X shutter speed
    • Lower your f-number from f/X to f/(X/(sqrt(2)))
    • Increase ISO from X to 2X
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  12. mauve

    mauve Mu-43 Top Veteran

    Mar 9, 2010
    Paris, France
    Just to follow up on Amin, because he said something very right here : ISO bumping doesn't change exposure. It changes *Gain* in the digital world, and like any amplifier, it amplifies everything, not only what you'd wish : noise first and foremost, but also *contrast*. So in the end, the image is noisier, and less detailed because there's less "steps" available for shades. ISO is really the last control you wish to tweak because there's so many trade off to face.
  13. flash

    flash Mu-43 Hall of Famer

    Apr 29, 2010
    1 hour from Sydney Australia.
    Real Name:
    So much for simple :)

    1. The amount of light in a scene is measured in EV's for the purpose of still photography. Often you'll see manufacturers show a specification for the autofocus in EV's. This is the minimum amount of ight that the AF will work in. EV is not the same as exposure. But a 1 unit change in EV does mean a 1 stop change in exposure.
    2. Exposure represents the "brightness" of the final recorded image and is measured in stops. Stops are descried as a "change". ie: I'll open up the aperture by three stops". They are not an absolute measurement of light level like EV.
    2. You can affect exposure in three ways. Shutter speed, aperture, sensitivity (ISO).
    3. A stop is a doubling or halving of the prevoius value. ie: you double the shutter speed to get a 1 stop increase.
    4. Shutter speed stops are made by doubling or halving the timje the shutter is open.
    5. Aperture speed stops are controlled by opening and closing a diaphram inside the lens (usually). You add or subtract a stop by doubling or halving the AREA of the opening. The f-stop number (1, 1.4, 2, 4, 8, etc) represents approximately the diameter of the diaphram relative to the focal length of the lens. ie: If you have a 100mm lens and you want f1 you need the width of the lens to be 100mm across. If you want f2 then it would be 50mm across. This is why a "slower" lens has a smaller diameter than a "fast" lens.
    6. ISO describes the sensitivity of the sensor/film to light. ISO 200 is twice as sensitive as ISO100. There are trade offs for going above and below the native sensitivity of a sensor. Above is usually noise and below is usually dynamic range.
    7. If you change one measure of exposure and then another by the same amount in the opposite direction exposure will be the same.
    8. Correct exposure is the amount of light recorded that you want recorded, not what a light meter necissarily says it is.

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