f-stops

Discussion in 'Photography Beginners' Forum' started by Thru_These_Eyes, Jun 12, 2007.

  1. Thru_These_Eyes

    Thru_These_Eyes TPF Noob!

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    Does anyone have their own simplified version of their understanding of F-Stops? I get aperture, but the f-stops are really complicating me in my photography course. Help!!:(
     
  2. Seefutlung

    Seefutlung TPF Noob!

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    Aperture and F/Stop are the same thing.

    Traditional F/Stops are:
    1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 44 and 64

    The number represents a ratio between the size of the aperture and the focal length of the lens. So it is actually 1:5.6 or 1:22 (which is why the greater the F/Stop number the less light hits the film/sensor.)

    A 100mm lens with an F/Stop of 4 mean that the length of the lens is 4 times longer than the aperture diameter, so 4 into 100mm is 25mm. So the diameter of the aperture at F/4 on a 100mm lems is 25mm (which is probably more info then most people care to understand ... but there it is.)

    F/Stops work similarly as shutter speed and ASA/ISO in controlling the amount of light hitting the film/sensor. Each full F/Stop increment either doubles or halves the amount of light,

    With a starting F/Stop of ... say ... 5.6 and you select F/8 ... this full F/Stop increment has just halved the amount of light hitting the sensor from F/5.6.

    Once again starting at 5.6 and you move a full increment in the other direction to F/4 ... you have double the amount of light hitting the sensor from 5.6.

    So every full F/Stop increment either doubles or halves the amount of light hitting the film/sensor from your starting aperture.

    So when a person state to open it up a stop ... that means to double the amount of light hitting the film/sensor. Three ways to open up a stop are:

    1) by lowering the shutter speed one full increment (i.e. from 1/250 to 1/125 is a full stop - a doubling of the amount of light from 1/250.);

    2) by opening up the aperture one full increment (i.e. from F/11 to F/8 will double the amount of light hitting the film/sensor); and

    3) raise the ASA/ISO one full increment (i.e. starting at ASA/ISO 200 a full increment would be to double the ASA/ISO to 400 ... this doubling of the sensitivity of the film/sensor is equal to a full stop. Sorry, if #3 is confusing ... if it is just ignore it and I'll provide more info/help on this topic.

    Gary
     
  3. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    F-stop is just the scale that we use to represent the aperture. It's a ratio of the focal length and the diameter of the aperture.

    So if you have a 50mm lens and it's set to F2...then the actual diameter of the aperture is 25mm. At F4, it's 12.5mm.

    This means that lower F numbers represent bigger apertures. F2 is larger than F4.

    The scale is not linear in terms of 'stops'. Each 'stop' is higher by a factor of (the square root of 2). So if you made a scale of full stops it would look like this...each step is one whole stop smaller.
    F1.0 -- F1.4 -- F2 -- F2.8 -- F4 -- F5.6 -- F8 -- F11 -- F16 -- F22 -- F32 -- F45 -- F64
     
  4. deanimator

    deanimator TPF Noob!

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    Err, yeah...

    It´s the size of the hole in the lens that the light goes through. :p

    Get an old manual lens from an SLR, and turn the aperture/f-stop ring and have a look.

    It´s important to understand why it is useful...for example, Depth-of-Field*






    *eek...what´s that???
    We will tell you if you want to know...:wink:
     
  5. agwhite

    agwhite TPF Noob!

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    Ok im new to the forums and photography so i have a question or two.

    I think i understand that math and how light works relative to aperture and lens size. Now is the ideal ratio between the two different relative to the subject matter or is there a rule to follow pretty much always?

    Also the body and lenses i have all have automatic functions, so im guessing it sets the right ratio and all that automatically correct? But do i loose something by not doing it myself? Since electronics only undestand math instead of art... i dont know just shooting off things ive been wondering.
     
  6. deanimator

    deanimator TPF Noob!

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    Time to find out about DEPTH OF FIELD :p
     
  7. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Modern cameras have meters that automatically adjust the shutter speed and aperture to get the exposure....but you are right, the camera doesn't know what it's shooting or what you, the photographer are trying to do.

    We use the aperture to control the Depth of Field. Wider apertures mean a more shallow DOF...smaller apertures get you more DOF.

    We use shutter speed to control how motion appears in the photo. A faster shutter speed freezes motion (both by the camera and the subject) and a slower shutter speed will cause movement to become a blur or not to be recorded at all.

    It's up to the artist to decide what is most important for a photo and choose settings accordingly. Often, it's a compromise based on the light you have to work with...but there is also the option to add your own light, which changes things.
     
  8. agwhite

    agwhite TPF Noob!

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    Ok...
    Aperture controls the amount of light that hits the sensor...
    Wider aperture=more light and a shallow DOF
    Narrow aperture=less light deep DOF

    now im with Deanimator and have to know more about the Depth of Field.
    ive heard the term and seen it relative to pictures and understand it has something to do with the sharpness of the subject relative the background or something right?
    From a pro though, how would you explain DOF?
     
  9. agwhite

    agwhite TPF Noob!

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    Just thought of a question while looking at my last roll

    In terms of aperture... for shooting inside in fairly low light... do you want a wide aperture to let light in to compensate? Is that common sense or is there more to it than that.
     
  10. sabbath999

    sabbath999 No longer a newbie, moving up!

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    Agwhite, I hate to give you this kind of answer, but it is the truth...

    It depends.

    The aperture you choose depends on what you are trying to do with the shot. While, on the surface, your thoughts make a lot of sense, when you take it a bit deeper you need to look at what you are trying to do with your photograph to determine whether you should shoot with your lens open wide (lower numbers like 2.8 or 3.5 or wherever your lens goes), or if you need to use much longer shutter speeds... or if you need to increase your ISO either by changing it on your digital camera, or using "faster" high ISO film.

    As the others have stated, depth of field is critical, but if your subject is moving and you have to increase your shutter speed to keep it from blurring, you may have to make compromises.

    If you are confused, then bracket your exposures (which basically means try it a bunch of different ways). If you are shooting digital, you can later go back and see which settings gave you which effect, and you can use that as a learning tool.

    Whether you choose to make aperture your priority or shutter speed your priority depends upon the picture you are trying to take. Back in the dark days of the first automatic film cameras, most cameras that had automatic exposure systems that either allowed you to choose aperture or shutter speed priority, but not both. Then the "program" cameras came out, which allowed you to switch between the two or use the "program" mode which you still see on most DSLR's today. Times have changed for the better.

    Of course, the curmudgeons out there will say that the only REAL way to take a picture is in full manual mode... but I am not one of that crowd.
     
  11. Seefutlung

    Seefutlung TPF Noob!

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    Generally speaking, playing with the aperture is usually a good initial response.

    In low light situations most experienced/skilled photogs will set their shutter speed to the lowest they can use (either because the lens is being handheld or the subject is moving or both of these critical elements need to be accomodated.) After setting one's shutter speed the next critical element would be DOF ... if DOF isn't all that important or critical than usually a proper exposure can be attain through aperature adjustment. If DOF is important then one has to attain a proper exposure through ASA/ISO adjustment.

    Trade-Offs in Low Light:
    1. Low Shutter speed trade-off is blurry everything if handheld, blurry action if camera is on a tripod or the action exceeds the shutter speed (i.e. rule of thumb states that you can handhold a 28mm at 1/30 of a second and still get crisp images. But shooting a bicycle wizzing by or even someone running fast and pretty close to the lens, the 1/30 will not stop the action cleanly.)

    2. Wide Aperture trade-off is shallow DOF and an overall less sharp image. Although the image may appear sharp at say 2.8 ... if the same shot was taken at F/5.6 or 8 ... the image would even be sharper. Additionally, shooting at lower numbers (larger apertures) increases the criticalness of focusing. Shooting a normal person face with a 100mm lens, (not Cyrano de Bergerac), at say 1.4 or 2 and you focus on the nose ... the eyes will not be in sharp focus. (Also DOF in a P&S is just crazy because of the rear element to sensor plane is so shallow, as opposed to a SLR, that laws of optics just heaps tons and tons of DOF on a P&S.)

    3. Higher ASA/ISO trade-off is image quality. As you increase the ISO to compensate for low light your grain/noise increases and the image becomes more contrasty. Different films can handle elevated ASAs better than others and certain cameras will perform much better at ISOs of 800, 1600 and 3200 than others. Most dSLR hold their own with minimal IQ loss up to 400 ... at 800+ one can start seeing IQ differences between cameras.

    Gary
     
  12. Thru_These_Eyes

    Thru_These_Eyes TPF Noob!

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    Thank you guys so much for the very helpful advice. I am still a little confused, but I think what I may just do is go out, take pictures of the same thing with different apertures/f-stops. I've never been good at math and the ratios KILL my ability to understand when its best to use what aperture. Depth of field I understand. Isn't that when you change the setting on your camera to the 'infinity' symbol? I am taking courses thru NYIP (its an "at home") course, and I am learning everything myself thru books and DVD's. I'll be taking a hands on class this fall b/c the at home course is not working for me anymore.
     

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