How to achieve critical focus?

Discussion in 'Medium Format' started by alex_bonham_carter, May 11, 2015.

  1. Hello all,
    I have recently been using a Mamiya RZ67 Professional with both a 90mm and 180mm lens and WLF. I bought it from eBay and I do not know the model of the ground glass it has a split screen dot in the middle with a second ring that does not act like the typical microprism. I would guess it is a Type C, but I could be mistaken. Now, I’m a child of the digital age and I am having a very difficult time achieving critical focus with this comparatively ancient camera. I know that there are issues with the focus/recompose method, but if I want my subject off-center, I must strain my eyes to imagine what the image would look like without the matte finish of the ground glass. Not a perfect solution. I bought a plastic 8x loupe, to help, but it really doesn’t fix my off center issue. I am beginning to think that the loupe is too strong and something in the 4-6x would be a better improvement. A loupe in the 4-6x range would allow me to see more of the image together.
    Even so, what is the best way to achieve focus on an off-center subject? If I lock the camera dead centered so that the focal point is on the subject, focus with the split screen, then recompose by rotating the ballhead on its fixed axis, would that be an accurate way of focusing? Though now that I say that, I would need to make sure that the ballhead base is also level to prevent any kind of tilt in the composition. I believe that the focus/recompose issue arises when the subject is focused on, then the camera is moved up or down, on an axis that is not fixed, such as hand holding. Most of my images are taken at night and I really enjoy still lifes, skylines, abandoned buildings, unusual cars, and the like. I would be more interested in using my 5DIII for people or anything that won’t put up with my slow fine-tuning. Obviously I could center the subject and just crop the final frame to get the image that I want, but then I would lose precious negative size. I’m wondering if I should just ignore the split screen and use a loupe to I’m a firm believer in tripod shooting and am not afraid to take the time to make a technically good photograph. I hope that the finished image is also aesthetically good, but sometimes I shoot a real dud despite accurate focus. ;)
    Any advice would be greatly appreciated!
     
  2. Dear Alex! First of all congratulations on getting The Mamiya RZ. Actually, this beast is still alive and shooting - many important photographers still use it and it has produced some of the world best images, so I am sure you won't be disappointed with it.
    If I may suggest, your post is a bit confusing in the sense that it's hard to grasp the exact question that you're asking.
    Try and post clear questions - the fewer the lines, the better - that way you will get more help !!
    Are your subjects in motion? Are you hand holding or shooting on a tripod?
    If you focus on a subject and then move the camera around, that subject might not be in focus anymore, depending on the distance from you and these subjects, what lens you are using and what aperture you are using. Assuming you are on a tripod and your subjects are not moving, to begin with, try and do this: do the composition as it were the exact final composition. Then put in focus the subject which is farther away. Then stop down the aperture (so f8, f11, f16, f22 etc.) while holding down the depth of field preview lever on the lens, which will make your ground glass go a bit darker, but will also show you which parts of your composition are in focus and which aren't. It might be hard to do this at night time, but I would start with this.
    Best,
     
  3. for FRS, you never rotate the camera to achieve the final composition because that rotates the plane of focus away from your subject. you always move the camera parallel to the desired plane of focus and subject.
    especially on 6x7, even f8 has a very shallow depth of field with portrait lenses and typical portrait distances. for architecture, environmental portraits it's a wider margin of error but not that much.
     
  4. Even when a camera is tripod mounted and centred at its lens' nodal point, when you focus on one object and then rotate the camera to a different direction, the off-centred object drifts inside focus. It's just geometry, illustrated for example here.
    To my knowledge, only one camera has ever genuinely solved "the focus/recompose issue", and that's the Hasselblad H series of 645 cameras, from the H4 model onwards. They have "Truefocus" technology, which uses knowledge of the lens focal length, distance to the object originally focused on, and amount by which the camera has been turned for recomposition, to generate and apply the geometrical focus offset.
    For any other camera with a central focusing area, one solution is to guesstimate an amount to "lean back" by after recomposition, to approximately correct the effect. The other is to just compose and focus with the object already off-centred; use a small movable loupe directly on a fine ground glass screen.
     
  5. Thank you for your feedback. The focus/recompose is as problematic as I suspected. Which loupe would you recommend for focusing on
    static off center subjects? I use a tripod all the time with this camera. Stopping down the lens to focus is a good trick, I'll be sure to try
    that!
     
  6. For most subjects I prefer a grid type screen. A split image screen would be more useful for copy work and maybe even for some architectural work. In dim light the halves of the split image circle can black out and even when they aren't blacked out, the two halves do not show very well whether you are in focus. In too many situations I find the split image focusing aid distracting.
     
  7. The key to critical focus is assuring that your eyepiece or loupe is focused on the focal plane of the screen before trying to focus the lens. Grid lines are useful, if they are embossed on the ground portion of the screen. Lacking that, the faint lines of a Fresnel lens in the screen can be a target for your eye. Keep that in focus as you adjust the lens.
    Modern screens are frosted lightly in order to enhance brightness. As a result, you can see through the screen so that you focus the lens using a virtual image behind the true focal plane. Locking your eye on to the screen itself is key to accurate focusing of the lens.
    I use a "chimney" finder for critical focusing with my Hasselblad. It has a compound eyepiece which is adjustable, and the body of the finder blocks all extraneous light. If there's something like that for the Mamiya, it is an inexpensive and lightweight (if bulky) solution. Prism finders with adjustable eyepieces are good too, but expensive and heavy.
    A loupe for viewing film on a light table works too, if it has an opaque skirt. It doesn't need to cover the entire screen, but that's the best way to exclude ambient light. Magnification in the 4x to 6x range is adequate. More magnification makes it hard to see the image for the screen structure itself. The best high power magnifier is an Hastings triplet, available (typically) in 7x, 10x and 20x versions.
    A triplet magnifier is held close to your eye, not at arm's length like a geriatric reader. It works by letting you hold your eye close to the object under examination and is virtually distortion free. That distance is very short - about 1" at 7x and 1/4" at 20x.
    A good loupe will cost $200 to $300. A Baush & Lomb triplet will cost about $50. For the money, you get a flat field without chromatic aberrations or coma. For $20 you get both at no extra charge ;)
     
  8. Thanks! For a "good" loupe, I was thinking either a Wista 5x or a Cabin 3.5x. Obviously I could spend more, but I'd prefer
    to cap it at about $150. What do you guys think about them apples? I think the plastic 8x is not giving me enough (any)
    adjustments to make sure that my focal point is dead on the ground glass texture. I lose power, but gain fine tuning ability.
     
  9. Doesn't your WLF already have a loupe you can flip open to view through?
     
  10. You're overthinking this. Just focus with the split image rangefinder and recompose. That's how I did it on Nikons with similar viewing screens for 30 years before AF came along and so did most if not all of my friends. Yes, in certain combinatons of angles that can be off by a hair but unless you're shooting wide open with a long lens it will be well within your depth of field. As Alan says, your WLF already had a loupe, so no need to buy another one.
     
  11. Craig, that's fair. I'm not trying complicate the process too much, but I just prefer to be as exact as possible. I'm making
    13x19 sized prints and I can see images coming out of the printer that I hurried a little, and they suffer. I don't have an
    extensive history with film so I have to check and recheck to make sure the image is going to come out as I want it to.
    Just trying to get a leg up on this old tech. Very often I shoot stopped down to f/8-11, so it's absolutely not a deal breaker
    to use the pop up loupe. Thanks for everyone's feedback!
     
  12. Haven't used a big Mamiya SLR but with other MF types (Hassy, Rollei TLR/SLR, Mamiya C, etc.) when necessary, I focus with the ground glass when the camera is positioned for the desired composition at short range. Just because there is a centre split does not mean you have to use it for every shot. If the screen fitted to your particular RZ is not conducive to this approach and you want accurate focus at close range with the lens open, I suggest you procure a screen that makes it easier for you to use the ground glass, because, broadly, I disagree with Craig's comments above re: DOF covering any camera adjustment. Never knew Nikon made medium format cameras, either...
     
  13. hmm ... I have a type A focusing screen on my RZ and I have no problem focusing on the off center part of the screen. anyway for critical focus, I use the loupe and the center spot, then I sidestep parallel to the desired plane of focus to change composition. given the usual subject (my kids) and handheld RZ, the results are good :)
    I learned this with a Lubitel TLR, it did not have a fresnel type focusing screen, so you had to look THROUGH it on the subject. if you focused your eyes on the focusing screen, you could not focus since you saw norhing ...
     
  14. Ok so what is the difference by shifting the camera after focusing? Who's the math wiz here?
    Let's say I'm shooting a 180mm lens on a 6x7 camera like an RB67. I focus on the subject's eye at 15 feet at let's say f/11. I then re-aim the camera so the person face is at the right edge of the viewfinder.
    So how many inches did the focal point and DOF shift?
     
  15. Let's say I'm shooting a 180mm lens on a 6x7 camera like an RB67.​
    OK, a 180mm lens on 6x7 format (let's assume it's in landscape orientation) gives a horizontal field of view of 22 degrees...11 degrees from centre to edge.
    I focus on the subject's eye at 15 feet at let's say f/11. I then re-aim the camera so the person face is at the right edge of the viewfinder.​
    That would mean a shift of about 10 degrees. The focal plane, old (centred) distance/direction to face and new (off-centred) distance/direction to face form a right-angled triangle, with:
    cosine (10 degrees) = 15 feet / X, where X is the new distance to the person's face.
    X = 15 feet / cosine (10) = 15 / 0.9848 = 15.23 feet
    So how many inches did the focal point and DOF shift?​
    The face is now (15.23 - 15) = 0.23 feet (~ 3 inches) closer to the camera than the focal plane.
    I'll leave the DOF shift calculation to others!
     
  16. May I suggest that you call Really Right Stuff and discuss the use of a Nodel Slide & Clamp set-up.
    This consists of a slider bar and release clamp that allows you to easily slide the camera from side-to-side allowing use of the camera's standard center focus for off-center compositions.
    It is usually used to shoot panoramic images to be stitched together in post, but also allows you to slide to one side, focus, and then slide back to shoot. I use one of these when shooting table-top in studio where critical focus is heightened due to shallow DOF when working close up with off-center compositions.
    Looks like this: (they make different bar lengths). Can't hurt to call them and ask.
    http://www.reallyrightstuff.com/Shop/Nodal-Slide-Options/192-Duo-Package-MPR-192-Duo-Clamp.html
    - Marc
     

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