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Depth of field

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  • #503
    Claff
    Participant

    This came from one of my shoots over the weekend:

    Camera says the photo was done at 1/640 shutter, f2.8 aperture and that’s the limit of my knowledge of photo terms.

    Other info:
    Focal length: 55mm
    Metering: Matrix

    Camera is a Nikon D100 with a Sigma 28-70mm D “Aspherical” lens, picture was shot in full “P” mode with the camera deciding all settings.

    The problem is that I cannot seem to get an entire picture, or at least the subject of the entire picture, all in focus. From reading up on things, depth of field is primarily a function of aperture, and I know I can’t go any lower than f2.8, so there’s nowhere to go but up. What will I have to compensate for if I go with a different aperture – faster shutter? steadier hands?

    #4515
    Klahanie
    Participant

    First of all, I like the photo just fine the way it is. The focus is right on his/her beautiful eyes and face.

    I’ll explain this as best I can to the best of my knowledge. That’s not saying much though since I mostly run willy nilly and rely on intuition or luck. 🙂 Here goes: The smaller the number of your f stop, the larger the aperture will open. This will allow the most light in, but it will also shorten your depth of field. If you increase your f-stop to say, f8 or f11 more of what you see in the viewfinder will be in focus. Just remember that the aperture will not open as large so you will need to slow down your shutter speed to allow for more light. If the lighting is good you should be able to shoot it without a tri-pod. I don’t know what the lighting situation was for the photo, but I would imagine you wouldn’t need to go to a shutter speed any slower than 200 if you stick to an f-stop of f8, which I think would work for what you wanted. Good luck, and just shoot, shoot, shoot!

    #4516
    sleeping
    Participant

    There’s an old rule of thumb that the safe speed for handholding = 1/focal length, so at 55mm that would be 1/50 or thereabouts. It really depends on your tolerance for sharpness and how steady your hands are, though.

    As you get to that point you can use a camera support or increase the ISO (use faster film, or go into the menus and change the setting on digital) which will allow you to use smaller apertures and/or faster shutter speeds, at the cost of increased noise/grain.

    I don’t know if the d100 is the same as the d70, but on the D70 in P mode you can change the combined aperture and shutter speed (keeping the same exposure) using the rear command dial – this makes experimenting with the differences really easy.

    #4517
    Claff
    Participant

    Thanks for the replies, and the compliment. The subject is Kurt, one of our three Pugs. It was taken at the dog park under overcast late afternoon skies so available light was starting to become a problem.

    Just poking around with the D100 I see it does have that feature where I can adjust the aperture/shutter while in P mode, so now I have something else to fiddle with. One of these days I’ll crack open that tome that came with the camera and really figure out what all those buttons and switches do.

    #4518
    Diggin
    Participant

    I think its best if you get out of AP or SP modes completely and stay in manual. Even outdoors in daylight I might use the flash to make up for the loss of light because I can still get incredible DOF. Luckily my daughter felt like posing for this example. (Flashed off of ceiling)

    Excuse the tint. I didn’t adjust or calibrate the colors


    #4519
    SaintDiluted
    Participant

    You said that you have trouble getting the whole subject in focus… Generally, in a case like a portrait or a situation where you have something in the foreground and some extraneous detail in the background, you are better off using a shallower DOF, like happened in your dog photo. Your dog has a great face, and the fact that the hindquarters are out of focus moves attention to the face.

    It can be a challenge getting the focal plane where you want it, for sure, but it generally makes for a more technically pleasing photo. Extremely shallow DOF can give you what the Japanese (and mostly everyone else) call ‘bokeh,’ where you get circles in the background that give artistic flair to the photo.

    There’s an example of extremely shallow DOF… I actually took this at F/1.8 but with the lens on the camera backwards (for macro goodness) and had to focus manually by moving the camera away and toward the subject.[/img]

    #4520
    schnee
    Participant

    Some term defns. The tome that came with your camera will likely refer to the aperture in f-stops. Things like “f/2.8” and “f/22”. f/2.8 is a larger aperture than f/22. When you “stop down” you move from a large aperture to a smaller one – f/8 to f/11 for example. It gets confusing b/c the numbers increase as the aperture decreases.

    By selecting smaller apertures, your depth-of-field increases – f/22 has lots of DOF, f/2.8, not so much.

    #4521
    staplermofo
    Participant

    Lucky you, the D100 has a depth of field preview button! I think it’s on the front of your camera, right by the lens.

    I’m not sure about the D100, but on my rebelxt I can hold the dof preview button down, then fiddle with the aperture until I see what I want. Try it out.

    Of course, now that this thread is 7 months old and you’ve probably already found a great solution you’ve been using for months.

    #4522
    Claff
    Participant

    Of course, now that this thread is 7 months old and you’ve probably already found a great solution you’ve been using for months.

    You’d think that, but you’d be wrong.

    I do things the easy way these days: shoot everything five or six times with various settings assuming at least one of them will be usable. With the pugs it’s not always that easy since they have limited patience for sitting still in the face of click (fiddle) click (fiddle) click (fiddle) etc etc.

    #4523
    Curious
    Participant

    if your camera has the depth of field preview button then you’re golden. for one thing you can simply look through the viewfinder while trying the different settings. with a SLR, or in this case a DSLR, what you see is what you get. at least as far as depth of field goes. you may only be seeing 90 – 95% of what will be in the final picture but that’s for another time.

    and i’m assuming there is an info display that shows the increasing shutter open time as the aperture closes down. great DOF isn’t much good at 1/15. unless you have Diggin’s tripod. note his shutter at 1/8. it will be a long time before you can hand hold at that speed.

    since you are shooting digital and the only cost is in your time try this. take a 3/4 inch or 1 inch tape measure and extend it out a few feet on a table top or something similar. then put a small item at 2 or 3 feet as a focus and reference point. then take the several sots you mentioned above using the full range of f stops. that should graphically show you how f stop affects DOF since you can see the numbers blur both front and back. you will notice that the front DOF is smaller than the rear DOF. no, i don’t know why. it just is.

    that said there are two other things to keep in mind. the longer the lens the shallower the DOF all things being equal. and the closer you are the shallower the DOF. i’d bet there are tables for different lens and distances if one was to google. film lens used to have a scale for DOF but my digital don’t.

    good luck. and take lots of shots. even ones that don’t strike you at first can often be saved with a judicious crop job.

    #4524
    ch4r7ie
    Participant

    here’s a website devoted to dof

    http://www.thebokeh.com/-home/

    #4525
    renko
    Participant

    Some digital SLRs also have depth of field calculation built in, as a special function. My 30D has it. You focus lock on point 1, then on point 2, and then on your final framing choice. The camera computes the appropriate aperture so that both point 1 and 2 are in focus, given your final composition choice.

    Depth of field is a function of two things, magnification (not focal length) and aperture. There is a wiki article that gives the appropriate equations. Most simply, the greatest your magnification (the closer the focus point is) and the larger the aperture, the shorter the depth of field.

    It’s not a bad idea to commit some depth of field numbers to memory. For example, depth of field for your 50mm lens, at 10 feet, 20 feet and infinity, for a small range of f-stops. If you have a few numbers in your head, you can get a better idea of your results, even for different lenses (magnifications) and f-stops.

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