Questions about...

Discussion in 'Open Discussion' started by swampduck, Apr 16, 2013.

  1. swampduck

    swampduck Mu-43 Veteran

    335
    Mar 29, 2013
    Taneytown , MD
    Dan
    What is the relationship between f-stop and ISO? Two lenses like the Oly 17mm 2.8 and 1.8. What ISO on the 2.8 version would get me approximate "speed" of the 1.8? At ISO 200 on the 1.8, would the resulting ISO on the 2.8 need to be 400, 800? With sensors handling low light so well now, is it feasible to have a "poor man's" fast lens in the Oly 2.8 and still be able to get the necessary speed? I know that other factors come into play here. 1.8 DOF > 2.8 DOF. But what else do I get in the 1.8 related to IQ?
     
  2. slothead

    slothead Mu-43 All-Pro

    Aug 14, 2012
    Frederick, MD
    Dan,
    Two times the ISO is the equivalent of one major f stop. The major f stops are 1.4, 2, 2.8, 4, 5.6, 8, etc. The f stop 1.8 is in intermediate stop a little larger than 2 a little smaller than 1.4 so iso200 using 1.8 would be close to ISO400 at 2.8 (assuming the same shutter speed). There has been a tendency as of late by Nikon and Canon to make and sell f/2.8 lenses as fast lenses, especially if they are zooms (that need more glass to work anyway). Oh, and the advantage of 1.8 over 2.8 (other than available light) is a decreased depth of field, and hence bokeh. Does that help?
     
  3. RobWatson

    RobWatson Mu-43 Hall of Famer

    F 1.8 dof < f2.8 dof
     
  4. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    F-stops is how wide your aperture is. The smaller the F-stop the wider the aperture. A wider aperture lets in more light allowing a faster shutter speed, but also reduces your Depth of Field and puts more of the photo "out of focus". Of course, a shallow DOF (from a small F-stop, or wide aperture) can be used both to your advantage or disadvantage. For instance, taking a portrait of a single person you might want a shallower DOF to make your subject pop out of the image by de-focusing the background, or for a group shot, lifestyle shot, landscape, or action shot you may need a deeper DOF (small aperture, higher F-stop) to get more in focus.

    ISO is your light sensitivity. In the film days it was known as film speed, as you remember. The higher the ISO the faster the shutter speed you can use but the more noise you introduce into the image, degrading the quality. You generally want to use the lowest ISO you can get away with while still maintaining a sustainable shutter speed.

    Naturally, you require a fast enough shutter speed to counter camera shake if shooting hand-held, and also to stop motion if shooting action.

    So as you can see, Aperture, ISO, and Shutter speed are all interconnected. Each of them are measured in "stops", and one stop is the equivalent of one Exposure Value (EV) on your light meter. So if your camera is reading 0 EV for instance (what it considers to be neutral exposure), then if you stop down your aperture by one stop you will need to either slow down your shutter by one stop or bump your ISO by one stop in order to maintain that 0 EV exposure.

    One stop does not equal one number, by the way... For instance, one f-number does not equal one f-stop. These are the values which equal a full stop...

    Full EV Stops (brightest to darkest):
    Aperture: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32
    Shutter: 1s, 1/2s, 1/4s, 1/8s, 1/15s, 1/30s, 1/60s, 1/125s, 1/250s, 1/500s, 1/1000s, 1/2000s, 1/4000s
    ISO: 12800, 6400, 3200, 1600, 800, 400, 200, 100, 50

    So for instance... one scene may produce a 0EV meter reading at f/2, 1/30s shutter, and ISO400. You would get the same 0EV meter reading at f/4, 1/15s, and ISO200 (two stops down in aperture, one stop up in both shutter and ISO).

    0EV on the meter does not necessarily equate to "perfect exposure". The ideal exposure for each scene varies from scene to scene. 0EV is what the meter reads as neutral but that may not be the exposure you want for your image.

    For instance, if shooting a low-key scene (like at night, or in a dark atmosphere), then the camera will want to boost the exposure in order to make the dynamic range more neutral. On the other hand, you want your image to be dark because that's what the scene actually looks like. Therefore, you will probably need to go into a negative EV number to get the proper exposure for the scene.

    If shooting a high key scene (ie, a white-background studio shot, shooting in white snow, shooting in bright sunlight, etc.) then the camera will want to drop the exposure in order to prevent the white from being too white and blowing out. Again, that's not what you want for a high-key scene, so you may need to shoot with a positive EV number on the meter.

    This is what Exposure Compensation is for, if you're using Aperture or Shutter priority modes. If shooting in Manual mode, then you may want to adjust your settings past 0EV.

    The big advantage of a Live View camera like our Non-Reflex digital system cameras (ie, Micro Four-Thirds), is that you see on the screen what the exposure on the image will look like, not just what the meter says. You also see the color balance like it will be saved. That is a huge, under-appreciated advancement in photographic technology.

    It is very important to be able to choose how your camera exposes, as the photographer. This is also why it's usually best to use a more specific metering pattern rather than letting the camera grab readings from the entire frame. I choose to use Center Weighted metering most of the time, which gives me a more deliberate subject to meter off of without getting thrown off as easily as spot meter by landing on a shadow or highlight. Of course your subject may not always be in the center of the frame, but if you use either Manual Focus or Back Button Autofocus, then it is very easy to lock in your meter reading (with AEL, or Auto Exposure Lock) then recompose the image.
     
  5. RobWatson

    RobWatson Mu-43 Hall of Famer

    Technically F stop, ISO and shutter speed are not related in the slightest. In "M" mode I can choose each one as I see fit and there is no constraint! If I now impose a requirement of a certain exposure level (or EV as mentioned above) now there is very much a 'connectedness' and the "exposure triangle" is created.

    As a photographer/artist/technical specialist it is up to me to decide what is the most important element - for high speed action freeze shutter speed to freeze the motion and EV=0 with my widest aperture now I am forced to use a specific ISO. If the F stop does not yield sufficient DOF then I must stop down to a higher F stop and must increase ISO to compensate.

    One can also use F stop, focal length and subject distance to manage DOF ... first things first.

    IQ is a massive can of worms.

    (F2.8/F1.8)^2 = 2.5 so use 2.5 times effective EV based only on aperture. One can use either ISO or shutter speed (or a bit of both) to compensate for changes in F stop.

    If you just compare F1.8 @ ISO 100 to F2.8 with out also stating shutter speeds then one cannot answer the question. If shutter speed and EV are the same then F2.8 ISO 250 will net the same EV as F1.8 @ ISO 100.

    This assumes both lenses have similar T numbers ... that is another story.
     
  6. Rinaldo

    Rinaldo Mu-43 Regular

    92
    Aug 9, 2012
    Sao Paulo, Brazil
    Ming Thein did a nice review about the 17/1.8, also comparing it to the 2.8 version and with the 20 as well... maybe it can help...

    Comparative lens review: The Olympus M. Zuiko Digital 17/1.8 – Ming Thein | Photographer

    Althought I'm switching from 14/17/20 to the 17/1.8, I think you're right on thinking this way. I don't see all that diference between them, according to MT's tests. The 17/2.8 is a nice lens IMO. I'm switching mostly because I do a lot of video and love the snapshot ring feature on the 12. And also I want to think simpler... let's see how it goes... :redface:
     
  7. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    All three are directly related to exposure. :)
     
  8. slothead

    slothead Mu-43 All-Pro

    Aug 14, 2012
    Frederick, MD
    What are T numbers by the way? I had ever seen them mention until these SLR Magic lenses appeared.
     
  9. speedandstyle

    speedandstyle Mu-43 Hall of Famer

    t-stops were created for cinema lenses. When you film with two different lenses the light needs to match exactly{or at least it did back in the old days - easy fix now in post}. Anyway the "t" stands for "transmission" as in how much light is transmitted. Each lens is set up and tested to measure how much light is actually transmitted at a given f-stop and then marked. The numbering system is the same as f-stops however starting at 1 then 1.4 then 2 then 2.8 etc... Cinema lenses also use a clickless aperture so you can change it while shooting, the focus ring is geared for control with a follow focus and the markings are on the side rather than the top of the lens.

    [​IMG]
     
  10. GaryAyala

    GaryAyala Mu-43 Legend

    Jan 2, 2011
    SoCal
    F/Stop = aperture = an adjustment diaphragm used to control the amount of light hitting the sensor.

    The aperture is graduated into f/stops. An f/stop is a ratio of the aperture diameter to the focal length of the lens. So an f/2 aperture opening (f/stop) indicates that the aperture is set to a diameter which is half the focal length of the lens. If you were shooting with a 17mm lens, the aperture diameter at f/2 would be 8.5mm.

    Every full stop increment will either double the amount of light hitting the sensor or halve the amount of light hitting the sensor, dependent upon if one opens up the aperture or closes the aperture.

    Full Stops are as follows:
    f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128


    ISO = a light sensitivity rating for the sensor. The higher the rating (ISO number) the greater the sensor's sensitivity, the less light is required for a proper exposure. The lower the ISO number/rating the sensor is less sensitivity to light, therefore more light is required for a proper exposure.

    Doubling the ISO rating/number also doubles the sensor's sensitivity to light, which in turn halves the light requirements of the sensor for a proper exposure. Halving the ISO rating/number will double the sensor's light requirements for a proper exposure.

    Opening the aperture from f/11 to f/8 = 1 full stop, this is a doubling of the light hitting the sensor. (and vice versa)

    Increasing the ISO from 200 to 400, doubles the sensitivity to light of the sensor, this in turn means that half light is required for a proper exposure. (and vice versa)

    Each doubling or halving of the ISO rating/number is equal to one full stop.

    Shutter Speed works similarly, every doubling of the shutter speed halves the amount of light hitting the sensor. A shutter speed of 1/60th allows half as much light as 1/30th. Each halving or doubling of the shutter speed equals one full stop.

    Remember each full stop, is a stop of light regardless if it is aperture, ISO or shutter speed.

    So if you have a proper exposure and ... say ... you open up the aperture one stop for less DOF (allowing more light to hit the sensor), you have to either double the shutter speed or half the ISO in order to maintain the proper exposure.

    If your settings for a proper exposure are:

    1/250 - f/5.6 - ISO 400

    open up the aperture one stop to:

    1/250 - f/4 - ISO 400

    you need to compensate for the extra stop of aperture light by dialing in:

    1/500 - f/4 - ISO 400
    or
    1/250 - f/4 - ISO 200

    After attaining a proper exposure setting, any adjustment to aperture must be equally compensate by an adjustment to either shutter speed or ISO.

    After attaining a proper exposure setting, any adjustment to shutter speed must be equally compensated by an adjustment in aperture or ISO.

    After attaining a proper exposure setting, any adjustment to ISO must be equally compensated by an adjustment in shutter speed or aperture.

    There are partial stops and you can compensate by combining settings, but that stuff starts to get unwieldy, (I tried to keep it simple).

    Good Luck,
    Gary
     
  11. slothead

    slothead Mu-43 All-Pro

    Aug 14, 2012
    Frederick, MD
    Thanks Speed.