Nikon FM2 Depth of Field Preview Help

Discussion in 'Beginner Questions' started by graceb1010, Jul 30, 2020.

  1. Hello!
    I recently purchased Nikon FM2n with a Nikkor 50mm 1:1.8 lens. I've been trying to figure out the DOF preview, but I can't seem to get it. When I press the DOF down on F1.8 the image is clear, but when I press it down on F22 the image is completely black. I thought the background would be darker on F1.8 since it has a shallow depth of field (Makes the backgrounds blurry?). I also can't tell what's in the depth of field vs not, on the the other F stops. Please help and correct me if I'm wrong.
    Thanks.
     
  2. What you're seeing is normal.

    Smaller apertures let in less light. Focusing screens tend to get dark at smaller apertures.

    If you hold down the DOF preview and turn the aperture ring step-by-step to smaller apertures. You may not see any appreciable difference between f/1.8 and f/2.8(it has to do with how the screen is ground). Somewhere around f/4 or so, you will see the split image area in the center start to black out, and it should be completely black f/8. The image will get progressively darker, but as you watch you should still be able to discern the image.
     
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  3. How do I tell what's in the DOF with the color changes?
     
  4. AJG

    AJG

    The easiest way is to look at the depth of field scale on the lens. With most manual focus 50 mm lenses you will see pairs of numbers representing the f/stops, usually starting with f/4. As Ben says, the screen gets progressively darker which makes evaluating depth of field difficult at best. The depth of field scale on your lens is likely to be much more useful.
     
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  5. That is true, but Nikon also does it a bit differently WRT to how the DOF scale is marked(or at least they did in the MF days).

    Some of the numbers on the aperture ring are painted a specific color. For the 50mm f/2 in my hand, you have green(f/4), orange(f/8), yellow(f/11) and blue(f/16).

    The barrel then has lines painted those corresponding colors, and the DOF scale works by matching the color up to the line. Not that it has any effect on usability(other than across-the-line consistency) but this was something Nikon did up to the AF era and contributed to the overall somewhat distinctive look of their lenses.

    IMG_1370.jpg
     
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  6. So if it's on F11 the DOF is between 30 and 7 feet?

    IMG_7878.jpg
     
  7. In your picture as shown, that's correct. There's some more technical stuff on how those numbers came about, but if you're printing 8x10 and smaller you can trust them.

    BTW, as the focus is set in your photo, at f/22 everything from ~6ft to infinity would be in focus(more or less). This works for any aperture, or at least one for which you have a depth of field marker-align the left DOF mark with infinity and you will have the lens set to be "hyperfocal."
     
    Last edited: Jul 30, 2020
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  8. If it's between 30 and 7 ft does that mean all of that is clear and everything after 30 is blurry?
     
  9. What's the case for the white numbers?
     
  10. It means that between 7 and 30 feet it is close enough, according to someone's definition, to in focus.

    Outside that, it starts to get less clear, but continuously, not a sharp cutoff.

    As to the original question, I might have used the depth-of-field preview a few times in the over 40 years since I bought my FM.
    Part of the reason for not using it, is that the image gets dark.
     
  11. It’s probably just as well to remember that there is only one plane of focus. When you focus your camera, let’s say at 5ft, it’s only those items that are 5ft away from the film plane that will be in correct focus. Beyond that to the front and back, everything will be out of focus, to some degree. You see this very easily when you shoot ‘wide open’ (at large apertures, or small f numbers, like f1.4 or f2).

    Now, smaller apertures give you increased depth of field (or depth of focus), but this focus is only really 'apparent' rather than actually in focus. If you look through your camera and start at a wide aperture, so you have a bright view, and focus on something that is receding (a wall running away from you etc.), hold down the DOF preview lever, and progressively close down to smaller apertures (bigger f numbers) you will see that what appears to be in focus increases to both the front and the back of the point you focussed on. It’s not really IN focus, it’s just less out of focus than with the wider, more open aperture you started with. The scale on your lens give you an idea of the ‘acceptable’ level of depth-of-field for ‘normal’ prints viewed at a 'normal' distance. If you blew up your shot to poster sized and looked at it from 4 inches away, you’d see that it wasn’t actually pin sharp in front of, or behind the plane of focus. However, from a few feet away, that poster shot would probably be acceptably sharp, and what you would ordinarily call, ‘in focus’.

    The depth of preview lever gives you a chance to ‘preview’ the scene before you take the picture, to help you assess what will be in ‘acceptable’ focus. There is a downside of course, in that the view gets dimmer and dimmer as you close down the aperture. This is probably why, as others have pointed out, it’s a feature that nearly all high end cameras have, but few people use very often. It’s useful from the point of that when you don’t use it, you get shown the depth of field you’ll get with the lens wide open.
     
  12. AJG

    AJG

    Depth of field scales are determined by defining a "circle of confusion"--when an out of focus point is defined by a small enough disc on the film or digital sensor that the eye "confuses" it for a point. As Ben mentioned above, DOF scales on lenses are usually adequate for 8x10 prints depending on how critical you are. As Glen mentioned, there is not a sudden fall off just beyond the markings on a DOF scale, but a gradual loss of sharpness. If you want to see a good diagram go to Ansel Adams' The Camera.
     
  13. James G. Dainis

    James G. Dainis Moderator

    For cameras that did not have a DOF button, one could press the lens release and turn the lens slightly so the aperture lever no longer made contact. That would allow the aperture to close down to the aperture setting and one could view the DOF. To show people what the DOF button did, I would have them frame a shot through a window at the landscape.. Then they could see that the frame of the window would get sharper as they pressed the DOF button with the lens set at a small aperture.

    I don't know how many times I have heard, "I press this button and the only thing that happens is the viewfinder gets darker." That is when I tell them to head over to the window.
     
  14. On normal to long lenses, the DOF markings can get too close to be easily readable. The 105mm f/4 Micro I have is close to this point. The Nikon method of using colored lines makes this somewhat less of a problem, but it can still get cramped. Also, there comes a point where you run out of easily distinguished colors :) . If you look on the 20mm lens I showed above, it has lines for every aperture except f/5.6. I'd have to look at the 20mm f/4 I use a lot more often than the one above(which is a somewhat rare and valuable lens that's not great optically and also is large compared to the f/4 version) but I think it does have a line for every aperture.

    If you're using DOF markings to estimate for an aperture that isn't marked, you can eyeball it as being somewhere between the closest two that are marked.

    Depth of field is kind of a fuzzy concept anyway(pun intended) and those markings are at best an estimate. They worked great for reporters who would probably not print big(unless it was a big headline story), and where halftone newspaper printing would hide a lot of other stuff. Only a fraction of photos, especially from 35mm, will ever get printed over 8x10 anyway, which again is about what the markings assume(if I'm planning on printing a film image larger than 8x10, I do it on 6cmx6cm 120 film at a minimum, and if possible either 6x7cm or 4x5" sheet film).

    I suspect that even lower resolution DSLRs and the ability to see at 100% magnification with just a mouse click have made photographers even more aware of how limited depth of field really is. By the time you get to 24mp full frame or so, you can easily see that there really is only one plane actually in focus and all the aperture changes is how quickly focus falls off from that. At typical web resolutions or even moderate print sizes, the old rules still hold, but "pixel peepers" easily see how true this is.
     
  15. Without diffraction, there would be one distance with everything in sharp focus.

    With diffraction, there is no point at which everything is in perfect focus, but some that are closer than others.
    With a perfect lens, and the focal distance exactly right, you still get a circle in the focal plane.
    One could, then, define depth of field based on the size of the actual circle to the size of the diffraction limited circle.
    (I don't know that anyone does that.)

    Also, for an ideal lens of focal length f, there is a relationship between the distance to an object
    (closer than infinity) and distance to the image plane. This is less true for the real lenses that
    we buy, though for usual cases, not all that far off. For most lenses, though, the actual image
    of a point source will be a circle larger than the diffraction limit circle. The increase in the
    size of the circle for objects of different distances will, then, be less than considering
    only diffraction.

    As someone else noted, there is a size that one does not notice for an 8x10 print at normal viewing distance.

    Prints larger than 8x10 are normally viewed from a large distance, so that about the same
    circle of confusion applies to those prints. Or, for prints from digital images, one does not need
    proportionally more resolution for larger and larger prints.
     
  16. Use a digital camera with LiveView.
    You can then see exactly what you'll get in the final image.

    SLR screens don't show depth-of-field accurately anyway. The screen is optimised for viewing brightness, and depth-of-field is seen as deeper than it will actually show in the finished print or on screen.
     
    Last edited: Aug 1, 2020
  17. My 2 cents: with an SLR you are seeing what the film sees. When you stop down the lens to higher and higher f-stops you are closing the diaphragm in the lens in order to give the film less light. This has the added effect of expanding the region that is in focus. As I said you are looking through the lens and seeing exactly what the film sees. But, to mitigate the view darkening as you increase the f-stop (which it is doing since increasing the f-stop means decreasing the brightness impacting the film) modern lenses don't close down the diaphragm until the moment the shutter release is pressed so that your eye can continue to see the scene in full brightness, i.e. at the minimal f-stop of the lens. The DoF button closes the diaphragm to the selected f-stop so of course the scene will darken to whatever f-stop is selected. This can't be helped but it allows the viewer to get some idea of exactly what the film will see on exposure and this includes just what is in focus, the so called depth of field (DoF). As said eloquently by others this is not a precise region but falls off gradually from the exact focal distance selected in both directions, near and far. All the discussion about the lens values given for DoF at various f-stops is in the eye of the beholder. Surely you are not going to get a tape measure and if the lens says the DoF is for 7 to 16 feet measure out the limits and expect everything there to be crystal sharp and everything outside that to be blurry. It's a continuously varying process.

    If the far limit includes infinity then you've got what's called a hyperfocal distance of precise focus selected and everything from the selected distance out to infinity should be acceptable.
     
  18. Aside from that, I've always considered distance markings on lenses to be only approximate anyway.

    MF lenses of moderate focal lengths do generally have a hard infinity stop that unless the lens has been messed with should be dead on. Long lenses often indicate infinity as a range(and even MF ones will usually focus beyond infinity to allow for expansion of the glass in the sun), and AF lenses rarely have a hard stop at infinity.
     
  19. Yes.

    But it seems that the times that I am most interested in depth-of-field preview are when the scene is already not so bright

    Well okay, there are two times when you want to use DoF preview. One when you want to be sure you have enough
    depth of field, and the other when you want not to have too much. That is, when you want the subject to be in focus,
    and when you want the background not in focus.

    But really, I haven't found that it works all that well.

    It now occurs to me, though, that with mirrorless (MILC), that the electronic viewfinder could generate
    an appropriately bright view as the aperture varies. Could even do electronic preview, computing the
    appropriate blurring or not blurring. (The latter being deconvolution, fairly well understood by now.)

    So far I haven't been so interested in MILC, especially at the current prices.
     
  20. Not really. You're looking at an image projected by the lens onto a piece of semi-translucent plastic, via a Fresnel condenser lens and maybe a glass condenser as well. What you're seeing has an 'aerial image' component to make it brighter than would be seen on a simple ground-glass screen.

    This distorts the depth-of-field shown; making it appear greater than what will be seen on film, or on a digital sensor.

    The optical arrangement of a Nikon viewfinder also prevents any aperture greater than f/1.8 being accurately represented. Therefore an f/1.4 lens will look no brighter through the viewfinder than an f/1.8 lens, and appear to give exactly the same depth of field.

    Strange, but true. Try it!
     

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