Quiz: how many stops is this?

Discussion in 'Beyond the Basics' started by pcd, Jun 19, 2004.

  1. pcd

    pcd TPF Noob!

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    The sky is f8 (1/250) and the landscape is metering f4.5 (1/60). How many stops separate these exposure readings?

    Background info: I'm using the spot meter reading on a nikor zoom 35-135 mm with the following f stops listed on the ring: 3.5, 5.6, 8, 11, 16, 22.

    How do you figure out how many f stops separate the brightest and darkest areas of the picture you're trying to capture?

    Thanks,

    pcd
     
  2. photobug

    photobug TPF Noob!

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    Looks like 1.5 stops for the aperture. If f/4 to f/8 is two stops, then f/4.5 is half way between f/4 & f/5.6, thus that's 1/2 stop. That plus the full stop between f/5.6 and f/8 = 1.5 stops.

    F-stops progress in two numerical sequences, starting with 1.4 and 2.0. The progression doubles for each stop in each sequence.

    1.4, 2.0, 2.8, 4.0, 5.6, 8.0... every other number is doubled, and is one stop faster or slower than the one before or after it.

    So, your lens is 1/2 stop slower than a f/2.8 lens at its widest aperture. The difference between 2.8 and four is 1.2. Half of that is 0.6, added to 2.8=3.4, which is close enough.

    Shutter speeds work the same way. So, the difference in shutter speeds in your example is 2 stops: 250-125-60.

    So, how do you set your camera to get a good exposure? I'd split the difference and bracket a full stop in 1/2 stop increments up & down.

    Or you could switch to center-weighted metering and let the camera do it for you, and bracket anyway, though I'd probably only go a 1/2 stop each way.

    Hope that didn't confuse the crap out of you. :)
     
  3. drlynn

    drlynn TPF Noob!

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    I'm with the Bug on this one. I'd start in the middle, say f/5.6 at 1/125 and bracket at least 1 stop both ways in 1/2 stop increments.

    I might even go to +/-2 stops if I was shooting print film. It has so much latitude you might not be able to see much difference in the prints at less than 1.5-2 stops. You should be able to see differences in the negatives, tho.
     
  4. Bob_McBob

    Bob_McBob TPF Noob!

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    FWIW, the difference between f/4.5 and f/8 is about 1.66 stops ;)
     
  5. tr0gd0o0r

    tr0gd0o0r TPF Noob!

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    This may not be the "perfect" technical answer, but since the difference b/t f8 and f4.5 is almost 2 stops, then the two different exposures are almost the same, and since generally the landscape is the more important part of a photograph, i'd go with f 4.5 @ 1/60 (unless you have a tripod then i'd transfer that up to f8 @ 1/15 for DOF).
     
  6. Soulreaver

    Soulreaver TPF Noob!

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    Isnt the diff 4 stops, since he opened the diafragm from f8 to f4.5 , which is almost two stops, and increased the exposure time from 1/250 to 1/60, which are another two stops?
     
  7. photobug

    photobug TPF Noob!

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    No, not in auto mode. In auto mode if you change the aperture 2 stops the camera will adjust the shutter speed 2 stops in the other direction to keep the exposure the same.

    Go to this site for an example.

    Start at f/8 & 1/60. To keep the same exposure values if you change the shutter speed 2 stops to 1/250 you have to change the aperture to f/4 to keep the exposure correct.

    Shutter speed and aperture are married in every mode except manual. Move one, the other moves to compensate and keep the exposure "normal." Open the aperture 2 stops, the shutter speeds up 2 stops. The object for the camera (in auto mode) is to stay at "0", or perfect exposure.

    So, while there was an apparent total adjustment of 4 stops, because they are linked it's really only 2 stops. Unless you are in manual mode you ignore the value you aren't adjusting when counting stops of exposure.

    Clear as mud?
     
  8. tr0gd0o0r

    tr0gd0o0r TPF Noob!

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    and then just completely ignore my post because I'm pretty sure I have everything backwards.
     
  9. Ant

    Ant TPF Noob!

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    What a brilliant site...Thanks :)
     
  10. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    4 stops difference between f/8 @ 1/250th and f/4 @ 1/60th.

    f/8 @ 1/250th = f/5.6 @ 1/500 = f/4 @ 1/1000

    1/1000 to 1/500 to 1/250th to 1/125 to 1/60 = 4 stops

    It would be much easier to count the difference between the stops if you keep one variable (aperture or shutter) the same.

    When you say sky are you metering the blue sky or clouds?
    When you say landscape, do you mean green grass, dried brush, etc...?

    This is how I determine exposure for a typical landscape scene. Set the camera/meter for some decent DOF, say f/16. Meter the brightest part of the clouds that I would like to retain full detail (this is the key, you don't meter the brightest part, but the brightest part where you need to retain full detail); on a bright day with ISO 125 this would probably be about f/16 @ 1/1000th. Then I meter shadows where I would like to retain full detail; on the same sunny day with ISO 125 this might be about f/16 @ 1/8th. Then you can count the stops on the shutter: 8, 15, 30, 60, 125, 250, 500, and 1000 = 7 stops. 7 stops is very high contrast; I need to bring it back down to around 5 stops. To do this I would over expose middle gray 2 stops ( so expose at f/16 @ 1/30th ), and under develop at N-2 (normal dev time minus two). How long is "2"? It takes personal film speed tests to determine this amount precisely, but if you were using a traditional style BW film such as Pan-X or FP4+, it would roughly be about minus 40% from the normal dev time.

    If you cannot control you developing, meter off the darkest part (assuming that the darkest part is darker than middle grey) of the subject where you wish to retain full detail, and under expose 2 stops. You may end up with blocked highlights, but this is the lesser evil compared to loss of shadow detail. Overexposed highlights can be burned in or scanned and manipulated in PS. Underexposed shadows are clear on the negs, and contain no information to manipulate.
     
  11. photobug

    photobug TPF Noob!

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    I stand corrected, Soulreaver you were right. Everyone else please accept my apology for misleading anyone.

    Thanks Matt, for setting me straight. :)
     
  12. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Most of what you said is correct, but no matter what mode you are in the diff between f/8 @ 1/250th and f/4 @ 1/60th is 4 stops. I understood your reasoning about when metering in auto or priority modes.

    Or 3.5 stops using the exact settings in the initial post. My brain is too tired to deal with 1/2 stops today. It may be important to consider that the setting halfway between f/4 and f/5.6 isn't f/4.5, although my lightmeter refers to it as "f/4.0 and 1/2". f/4.5 would actually be a slightly wider aperture than half way between f/4 and f/5.6. But I understood what the poster was meaning :D
     

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