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  1. #1
    light wait photophorous's Avatar
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    Depth of Field - DSLR advantage?

    If you read my "Minimum Shutter Speed" thread, you probably know where I'm going with this. I'm just thinking about some dorky technical stuff, and I'd like to hear what you all think about it. Here goes:

    Because of the crop factor with a DSLR, and most digital cameras, we use shorter focal length lenses to get an angle of view equal to 35mm film.

    Focal length affects depth of field. Longer lens = less depth of field (at the same aperture.)

    So, does that mean a small sensor digital camera gets more depth of field for an equal (to 35mm film) angle of view? Seems logical.

    Along the same lines, it's a generally accepted truth that most lenses don't perform optimally at a wide open aperture. So, to get the sharpest shots, we stop down a couple of stops. When we do this, does it improve the sharpness through out the frame, or only on the edges? I'm not sure.

    IF...a big IF...it's only the edge sharpness that is improved by stopping down, and you're using a lens that was designed for full-frame 35mm on a less than full-frame DSLR, you wouldn't need to stop down as much to get the same sharpness, due to the crop factor.

    I'm not sure all my assumptions are correct, but if we're able to preserve the lens sharpness using larger apertures on a DSLR, and at the same time we're using shorter focal lengths that give better depth of field, this equates to another low light advantage for digital.

    Am I crazy...or just wrong?

    Thanks for playing.

    Paul

    PS. I know this isn't going to make me a better photographer. I'm just curious.

  2. #2
    Senior Member Ronnoco's Avatar
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    Re: Depth of Field - DSLR advantage?

    Quote Originally Posted by photophorous
    Along the same lines, it's a generally accepted truth that most lenses don't perform optimally at a wide open aperture. So, to get the sharpest shots, we stop down a couple of stops. When we do this, does it improve the sharpness through out the frame, or only on the edges? I'm not sure.

    IF...a big IF...it's only the edge sharpness that is improved by stopping down, and you're using a lens that was designed for full-frame 35mm on a less than full-frame DSLR, you wouldn't need to stop down as much to get the same sharpness, due to the crop factor.
    .
    You ask some very good questions and an understanding of the technical aspects will indeed make anyone a better photographer.

    You are correct in that most lens don't perform optimally at a wide open aperture but the best f stop for overall sharpness, least distortion, least flare, and the lowest level of other lens problems varies with the lens maker and in some cases the particular lens. Minolta lenses for example were according to the tests better than Nikon and best at f 4. Longer lenses of all makers often tested out better at f 8 although I had a 200mm that was tack sharp at f 2.8. I say had, because I literally wore it out.

    Ronnoco

  3. #3
    Fluorite Toothpaste poker's Avatar
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    Re: Depth of Field - DSLR advantage?

    Quote Originally Posted by photophorous
    Along the same lines, it's a generally accepted truth that most lenses don't perform optimally at a wide open aperture. So, to get the sharpest shots, we stop down a couple of stops. When we do this, does it improve the sharpness through out the frame, or only on the edges? I'm not sure.

    IF...a big IF...it's only the edge sharpness that is improved by stopping down, and you're using a lens that was designed for full-frame 35mm on a less than full-frame DSLR, you wouldn't need to stop down as much to get the same sharpness, due to the crop factor.
    Yes. The edge softness would not be the same if the same exact lens settings were used on a 35mm and an APS-C DSLR. A TAMRON representative explained to me that there are such things as SWEET SPOTS for lenses, the center of a frame which is the most focused area. The DSLR would have less edge softness over the 35mm equivalent SINCE you are cropping out the rest of the outer frame, the true edge of the available image.

    The Tamron rep considered this a real PLUS for the DSLR/APS-C characteristic.

    How this applies to DSLR only lenses like Canon EF-S.....not sure.
    Canon 5D MKII & Canon 7D

  4. #4
    Be serious Franglais's Avatar
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    Re: Depth of Field - DSLR advantage?

    Quote Originally Posted by photophorous
    So, does that mean a small sensor digital camera gets more depth of field for an equal (to 35mm film) angle of view? Seems logical.
    Things don't go from being in focus to being out of focus in a flash. It's gradual. It also depends on how much you enlarge the image, what distance you view it at, etc. The more you enlarge, the earlier things look out-of-focus. As you are enlarging an APC-C image more, this tends to offset the advantage it has from using a shorter focal length lens.

    However digital technology introduces artificial sharpening of the image. This tends to make slightly out-of-focus parts of the image look in-focus. The switch to being out-of-focus becomes more brutal.

    Charles

  5. #5
    Viewfinder and Off-Topic Co-Mod walterick's Avatar
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    Re: Depth of Field - DSLR advantage?

    Hey Paul

    You sure are asking the tough questions recently That's cool, I dig the "dorky technical stuff" too.

    These I feel I can answer a little better for ya.

    First off, you've got several different things going on here at the same time. Let's look first at your initial question:

    "Focal length affects depth of field. Longer lens = less depth of field (at the same aperture.)"

    Paul, this is a very common misconception. Focal length does not affect depth of field! Here are the reasons we are so easily tricked into thinking it does: 1) magnification - a telephoto lens compresses an image and draws it's background forward, making the background larger and harder to discern when out-of-focus, and 2) we are used to seeing pictures from telephoto lenses with blurry backgrounds and we are used to seeing wide-angle photos with everything in focus.

    Try this as as an experiment: take a close-focusing wide-angle lens and put something in your viewfinder that has an easily-recognizable background. Get as close as you can to that subject and note the size of the image in your viewfinder. Now switch to a telephoto lens and put the subject in the frame again at the same size it was in the wide-angle lens. If each lens has the same aperature and the subject is magnified to the same size in the viewfinder, the depth of field should be the same. Remembering of course, you are seeing more of the background in your wide-angle lens than you are in your telephoto lens. Remember also, good telephoto lenses are designed to have "smooth" out-of-focus areas (bokeh) and wide-angle lenses generally are not. This quality of good telephoto lenses might make telephoto lens' background look "blurrier" or smoother.

    Therefore, aperature aside, depth of field is affected by the size of your subject in your frame and not the focal length of the lens.

    "So, does that mean a small sensor digital camera gets more depth of field for an equal (to 35mm film) angle of view?"

    Nope! ;) Depends on how big your subject is in the frame!

    "Along the same lines, it's a generally accepted truth that most lenses don't perform optimally at a wide open aperture. So, to get the sharpest shots, we stop down a couple of stops. When we do this, does it improve the sharpness through out the frame, or only on the edges? I'm not sure."

    You've got 2 different issues here One is the aperature at which a lens is sharpest. You are correct in saying that a lens is usually sharpest stopped down a couple of stops (usually 2.) Therefore you're correct in saying that your lens is normally sharpest stopped down. But you've got another issue here: the kind of image a lens creates.

    What is the shape of the image your lens creates?

    You take a photograph and get back an image that is rectangular (or in some instances, square.) But your lens is round, right?

    That means your lens is creating a circular image on the back of your camera. Your film is rectangular, so it will capture whatever light falls onto it, giving you a rectangular section of that round image. Since the center part of that circular image is sharpest, your camera manufacturer designed that film to sit right in the middle of that circle! Thereby giving you the sharpest image possible. Take a look at the first three pictures below:

    1) The circular image your lens actually produces. Remember, the image is sharpest in the center and softess at the outside.
    2) The portion of that image your film actually sees. Notice that the corners of the image are sticking out into that "soft" portion of the circle - hence, the corners of your frame are going to be softer than the middle. Now, the advantage your DSLR gives you is that it creates an even smaller rectangle, thereby pulling its corners further into the sharper part of the circle and giving you a closer crop than you would get with your 35mm camera.
    3) Final image you would get back from Walmart; except yours should have a better composition

    Stopping your lens down will improve corner sharpness as well as center sharpness. But because your corners are so much softer than your center, they will tend to sharpen up faster than your center will. Which is in part why landscape photogs often stop down more than portraitists do. Who cares if the corners of your out-of-focus background are blurry in a portrait? But the corners of your landscapes had better be sharp or your calendar shots won't sell!

    I hope this serves to help answer some of your questions for you. It's kind of a verbose answer to your questions. And it was kinda early in the morning here when I started typing this but I think everything's correct ;)

    BTW, gaining more knowledge of the workings of photography and your camera will always make you a better photographer I think
    Attached Thumbnails Attached Thumbnails Depth of Field - DSLR advantage?-circle-small.jpg   Depth of Field - DSLR advantage?-circle-small-rect.jpg   Depth of Field - DSLR advantage?-final.jpg  
    Walter Rick Long
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  6. #6
    has-been... another view's Avatar
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    Re: Depth of Field - DSLR advantage?

    Let me get some asprin first. Good questions, gotta think on this a little bit.

  7. #7
    light wait photophorous's Avatar
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    Re: Depth of Field - DSLR advantage?

    Quote Originally Posted by another view
    Let me get some asprin first.
    Good advice.

  8. #8
    light wait photophorous's Avatar
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    Re: Depth of Field - DSLR advantage?

    Hi Rick,

    Thanks for the excellent explanation. I'm sorry I'm so slow replying. I had a very busy weekend, sitting on an inner tube floating down the Guadelupe River trying not to spill my beer. I hope you had a good Memorial Day weekend too.

    What your saying about focal length and DOF makes sense. Longer focal length does not mean less DOF. You actually have the same DOF, but it appears compressed due to magnification. So the f-stop you need to keep a Wildebeest in focus from snout to tail is the same regardless of whether or not you can smell his breath. Gotcha. But, I'm still trying to process how the crop factor affects the compression in an image.

    For example, if you take two different images, one with a 30mm lens and one with a 90mm lens, with your subject filling up the same amount of each image, you'll have more peripheral objects in the image from the 30mm than with the 90mm. (Obviously camera location relative to subject will change.) Also, foreground and background elements in the image from the 90mm lens will appear closer to the subject than in the image taken with the 30mm lens. This is the compression that causes the DOF to appear shorter. So, I see two basic differences between a wide angle and a telephoto: 1) angle of view - amount of peripheral objects in the frame 2) distance compression - perceived distance between FG, subject & BG.

    Now comparing digital to film. [I realize digital vs. film is really irrelevant in this discussion. It's just the lens length and the size of the image plane that creates these differences, but it's easier to discuss in the terms that we're most familiar with.]

    If you take two different images, one with a 75mm lens on a (35mm) film camera, and one with a 50mm lens on a DSLR with a 1.5x crop factor, both will have the same "angle of view." Am I correct in thinking that means the amount of peripheral objects in the two images will be the same? What about the perceived compression? Will objects in the BG (or FG) appear to be at equal distances from the subject in both images, or will the BG & FG elements still appear closer in the image taken with the longer lens, regardless of the crop factor? My thinking is that the angle of view changes according to the crop factor, but the distance compression would not. The amount of compression would appear the same regardless of how the image circle is cropped.

    So, it seems like this is a difference between 35mm film and a 1.5x DSLR sensor, that can not be equalized or countered. No matter what you do, you're either going to have a difference in the angle of view or in the perceived distance compression. The only way to make them the same would be to crop the image from a 35mm film camera, which from a optical standpoint, would make the two identical.

    So with my theorized DOF advantage removed from the picture, it still seems that the crop factor gives you a small advantage in lens sharpness, because the softer areas of the image circle are cropped by the sensor. That's cool. I'll take what I can get.

    This will all make more sense to me when I get home and have a chance to play with my lenses.

    Thanks for reading and replying.

    Paul

  9. #9
    Poster Formerly Known as Michael Fanelli mwfanelli's Avatar
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    Hmm...

    Quote Originally Posted by walterick
    Paul, this is a very common misconception. Focal length does not affect depth of field! Here are the reasons we are so easily tricked into thinking it does...
    Well... I think it's more a matter of how you think of depth of field. I seldom use different lenses and keep the image size the same. That's great for illustration purposes but is rarely the way people think about using a lens.

    For example, my 45mm lens on a Pentax 67 is wide angle. But the DOF, with that lens at that FOV, is much smaller than the DOF of the "equivalent FOV" 22.5 mm lens on a 35mm camera. I really think that people think about a lens' FOV much more than getting the same image size. We choose "wide angle", "normal", "telephoto" accepting (and expecting) in advance that the image size will be different. We don't try to make everything the same image size.

    In this situation, at least from a practical point of view, I think that the FOV does indeed depend upon focal length.
    "Sometimes I wonder whether the world is being run by smart people who are putting us on, or by imbeciles who really mean it." --Mark Twain

  10. #10
    has-been... another view's Avatar
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    Re: Depth of Field - DSLR advantage?

    Actual situations when you're shooting and the science behind it isn't always the same thing, I agree about that. Taken to the extreme, my Coolpix has a sensor that's not quite the size of a fingernail. The lens is about 7-21mm which gives it about a 28-80 equivalent to a 35mm film camera.

    Because the 7mm focal length gives an angle of view of 28mm in terms that we're used to working with, we're shooting that 7mm lens a few feet away from a subject (just an example here). Loads of DOF, even wide open. It may or may not be an advantage; depends on what you're trying to do.

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