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Thread: Focusing and other musings.

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    Focusing and other musings.

    Isn't it always the way.
    I finally get my MF gear together again after a long absence since I had to sell off my Arca and IQ180 at the start of the year, and the wet season has hit in force. I've had nothing but pouring rain and strong winds for the last week. Now there is a couple of cyclones brewing as well. Life is great in FNQ.

    Anyway I have been doing some reading about hyper focal focusing since this used to be my favourite way to do things. I had all the calculators and tried so many different settings but always felt a bit let down. I could never seem to really nail the right settings.

    A very helpful post by Wayne Fox in another thread (sorry no idea how to link to it at the moment) had me wanting to try another way. Basically just ditch the calculators and take many test shots at different apertures and note the setting where acceptable sharpness is lost. That then becomes your hyper focal distance.

    Brilliant. But then while trawling the Internet I came across this article. http://http://www.trenholm.org/hmmerk/DOFR.html This hypothesis is that at infinity it is easy to calculate what should and should not be resolved in your photo.

    This article makes a lot of sense to me and I am just wondering what others think of it. I understand it was written in the days of film but it should surely translate to digital.

    Now I just need the rain to stop so I can try it out.

    March is looking good
    Last edited by Dogs857; 8th January 2015 at 03:47. Reason: Fixing link
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    Re: Focusing and other musings.

    My own view on hyperfocal focusing and other aspects is at
    How to focus a landscape scene

    Anyway, it's related to the concept of depth of field which is always debated over and over again. Anyone knows that for large panoramic landscape prints which you can step close the standard recommendations falls apart as the viewer is virtually nosing the print. In that case you want to make the picture as sharp as needed where it counts.

    In many landscape scenes the inifinty has finer details than the foreground, so sharpness is more important there. On the other hand, sharpening software can make a good job so maybe you don't need maximum sharpness there and can give a bit more to the foreground.

    A tradeoff I like, which is actually resolution independent, is to set the CoC to the airy disk diameter, ie dependent on aperture, and aperture is usually f/11 or f/16, probably f/11 on IQ180. I think MF folks often shoot at a too large apertures which results in aliasing and thus false colors, I prefer to have a little bit more diffraction and less aliasing and get more depth of field as a bonus.

    Anyway the rationale with aperture-dependent CoC is that with smaller aperture diffraction limits resolution and thus the CoC should be related to that. With my CoC=Airy it will be sharper in the plane of focus, but the edge of this DoF will respond well to sharpening and in a print be extremely hard to differ from the focus plane, even when nosing.

    edit: 2x radius is 1x diameter... :-)
    Last edited by torger; 8th January 2015 at 04:45.
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    Re: Focusing and other musings.

    I never really gave much attention to hyper focal distances until I purchased a tech solution. I quickly determined the range for each wide I have and niw never go back to the Arca cards unless I have the 90 or 120mm out as they such a narrow DOF.

    To determine each approximate hyper focal I used a tape measure and a target that I could focus on.

    The process is tedious but well worth it as it saves a ton of time in the field.

    Paul
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    Re: Focusing and other musings.

    Thank you both.

    Torger that is a great article and I may need to read it a few more times to fully digest everything in it. I was hoping you would chime in. I love detail, and you provide that in spades.

    Paul

    I agree entirely, test test test. I have a Cambo now so the focusing is a little less precise but the theory would still be the same. I think I will give both methods a try and see which works better for me.
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    Re: Focusing and other musings.

    I'm a bit of a tech cam newbie, but a strategy that has been working well for me, is... not to focus :-)
    So at the beginning, I have calibrated my infinity focus (and on my Factum, it is pretty close to the zero setting for each lens). Now, it's basically set there, and if I need a bit more detail on a an area closer, I just close the aperture a bit more.
    I just need to do the same with ~1 degree of forward tilt, and I think I'll get even better results (but the optimal setting will not be at infinity anymore)

    I do have a Disto 5 to really focus on something else than infinity, but for the moment, I haven't used it. I guess my subject matters have been until now "grand landscapes", where the mountains in the background need the most detail.

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    Re: Focusing and other musings.

    That is what this article I found states.

    Keep the lens at infinity and stop down for your depth of field, then you can calculate what will and won't be in focus. He also gives a decent calculation for diffraction so you can judge how your infinity may resolve if you stop down too far.

    I find all this quite interesting as I have always had SLR / DLSR's up until this point. They were all autofocus and I never really gave much thought to how things work. For Landscapes I used to do the focus into 1/3 of the scene thing and was happy with results.

    It was only after I started taking things seriously that I started paying much closer attention to how things were actually being taken, and how they looked when you printed them out.

    This is why I loved having a tech camera, it made me think about my photography so much more than I was.
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    Re: Focusing and other musings.

    Two pdf books "The INs and OUTs of FOCUS" and "FOCUSING the VIEW CAMERA" authored by Harold M. Merklinger, also the author of article that 'Dogs' referenced can be downloaded here:

    Download The INs and OUTs of FOCUS

    I've read The INs and OUTs of FOCUS and found it to be informative. I need to read it again though to really absorb the material.
    Last edited by Jamgolf; 8th January 2015 at 18:46.

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    Re: Focusing and other musings.

    This is probably a bit against the grain, and also not always the case, but I think it is far more important to focus on the most important part of the image...and I mean that both literally and metaphorically. Not every landscape image needs perfect infinity to foreground sharpness, and understanding to what you are trying to draw attention is more important. Hyperfocal focusing can allow one to ignore this choice, which is not good! With medium format digital and good lenses, you are only going to have one area in perfect focus, and it should be the primary subject of the photo.

    Of course, there are going to be situations where it is better to focus slightly off the main subject so that you can extend the area of acceptable sharpness, but personally I think it is detrimental to be too formulaic about it... I say this from both an artistic and a technical standpoint. I am a printer and a landscape photographer. I use the S series and print large, and the areas of maximum sharpness are clearly visible in large prints, even when using wider lenses at medium to small apertures...when the system is good enough and the prints large enough, you will see the transition, and it can be distracting to have the main conceptual focal point be different from the main optical focal point.
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    Re: Focusing and other musings.

    Quote Originally Posted by Stuart Richardson View Post
    With medium format digital and good lenses, you are only going to have one area in perfect focus, and it should be the primary subject of the photo.
    I think this is quite interesting. So take the example of a (smallish) barn in front of a grand mountainous landscape. The barn is not big in the frame - the mountains are.
    Without the barn, the picture would look dull - it's just a mountain after all.
    So in my mind, the "subject" is the barn, it makes the picture. One would have the tendency to focus on that. It is the subject after all.
    But then most of the frame is out of focus - since the barn occupies only a small area.

    So I would focus on the mountain (at infinity), and try to keep the barn "pretty close to focus", but not in focus.

    It's a tough choice. I don't want to start focus stacking. Tilt could be a solution to have "everything" in focus...
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    Re: Focusing and other musings.

    Quote Originally Posted by Stuart Richardson View Post
    Not every landscape image needs perfect infinity to foreground sharpness, and understanding to what you are trying to draw attention is more important.
    So true.... and has been my preferred way of shooting for a long time.

    Victor

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    Re: Focusing and other musings.

    throwback to the group f/64 vs the pictorialists
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    Re: Focusing and other musings.

    Miska -- in this case, I would choose the barn. That is the little detail which will draw the eye, and more likely to be scrutinized. The mountains themselves will not likely have a ton of detail, or at least not that would not be adequately served by photographing at a medium aperture...their outline will likely still be crisp and natural looking. Keep in mind that our eye is also a lens and does not have infinite depth of field...when you are focusing on something the objects in the periphery are not fully in focus! It is not a pinhole...

    I think it is actually kind of rare to have a composition where if you focus on the main subject, the rest of the composition is not well served just by using the right lens and aperture...
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    Re: Focusing and other musings.

    It depends a bit on style, I'm more of the f/64 type I guess. I use tilt a lot, and sometimes even swing, but not as much as 8x10" folks do.

    f/64 can sound like a really small aperture but if the reference was 8x10" (?) , that's only about f/11 on a medium format digital camera. The extended and optimized DoF with tilt and swing.

    I agree though that it is quite rare that you actually need hyperfocal, focusing on a main subject works well most of the time, and that's good for me because I focus on ground glass and have no hyperfocal presets. With tilt in the arsenal, hyperfocal becomes even more rare (although you can have some convenient presets involving tilt and hyperfocal if you happen to have a high precision focusing ring).

    I'm an amateur and don't shoot a huge volume of images, I just scanned to find a hyperfocal example image from the last year and well I did not find a single one, but I understand if you have a preset on the camera you can use it more often. It does happen though that a scene of mine don't really have a main subject, it's more about composition and atmosphere than pointing at some specific. In intimate landscape scenes it quite often happens that I don't focus on anything particular, but on the other hand in those cases infinity is rarely visible so it's not hyperfocal anyway.

    Attached one such example. I guess you could argue that the fallen tree closest in the foreground is the main subject, but my intention of the scene was more global. I don't really remember where I placed focus but I think it's between the fallen tree and the closest standing tree in center, and letting the background become slightly soft, shot at f/16.
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    Re: Focusing and other musings.

    Trees are hard, it's true! Even then, I tend to just pick a main point and then stick to it, but I know what you mean. For this image for example, I just used my judgement and chose the second closest tree and shot at f16. It was the 70mm S lens, so depth of field is not huge even at f/16. This came out very well, but the closest tree is slightly out of focus when you look at the huge print. But in this case, the main point of interest for me was the path flowing in the woods, so I wanted the area in the middle in sharper focus. The background and foreground are both less sharp than the middle, but in this image that is fine. You could make the point that I am mentally focusing hyperfocally here, but that is one reason to include it! There are always exceptions to every rule!
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    Re: Focusing and other musings.

    always makes the logical mind chuckle:
    "There are always exceptions to every rule"
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    Re: Focusing and other musings.

    What can I say, I am a nihilist.
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    Re: Focusing and other musings.

    Quote Originally Posted by torger View Post
    f/64 can sound like a really small aperture but if the reference was 8x10" (?) , that's only about f/11 on a medium format digital camera. The extended and optimized DoF with tilt and swing.
    Can you please expand on this for me?? I have heard this said before but I always thought aperture was the same regardless of format size.
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    Re: Focusing and other musings.


    For a fixed f-ratio, DoF decreases as you make the system bigger. You wouldn't think so, because the bigger sensor/film needs less enlargement, and that allows a larger CoC, but that effect scales only linearly with size.

    What is surprising is that the size of an out of focus point image grows *quadratically* with system size. You get one factor from the bigger lens opening - rays passing through opposite sides have a bigger angular separation, and another factor from the increased aperture to sensor distance - the rays have more time to diverge, making a bigger blob on the sensor.

    So in the final print or viewed image, OOF point sources make disks that scale linearly with camera size. To get the same DoF from the bigger camera, you need a smaller aperture. 8x10 has a MUCH bigger sensor, so you need a really small aperture to get things in focus, hence f/64.

    I love this calculation (yes, I am a Mathematician), but it is heresy to allude to it in certain fora. People are strange.

    Matt
    Last edited by MGrayson; 11th January 2015 at 05:41.

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    Re: Focusing and other musings.

    Although I am a Mathematician, I don't like formulas. Or, at least, I find pictures more convincing than formulas, and don't really believe a result until I can see it. I have always wanted to write up this explanation that, given a distance to the subject, it is the physical size of the lens aperture alone that determines the size of an OOF disk relative to other elements in the picture - the size, if not the quality, of the bokeh.

    Here's the setup as seen from above:



    The image with infinite DoF looks like this:



    Now suppose we want that OOF light to make a disk of a certain size:



    Let's see what happens if we move the light a bit behind the subject



    so that the OOF disk is just barely visible:



    But to be visible at all, the rays from the light must still enter the lens



    If the lens is smaller than that, no light gets in, so the OOF disk can't be visible. We didn't use anything about the focal length, sensor size, film vs. digital - all that matters is the size of the opening.

    Yes, if I move the lens further from the subject, the required lens opening grows. So if you change focal lengths but keep the subject size the same, you have to change the opening as well, and THAT leads to a constant f-ratio. A 200mm at f/4 has the same OOF look (to first order) as a 100mm at f/4 if the 100mm lens is half the distance to the subject.

    Phew - I feel better now.

    --Matt
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    Re: Focusing and other musings.

    the actual opening measures to be the focal length divided by the f-number.

    so f/64 is a smaller opening than f/32, assuming f is the same.

    as marc points out it is the actual opening size that is relevant to DOF, however, the reason f/64 is required for 8x10 is because typically the focal lengths used are longer as the format gets larger.

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    Re: Focusing and other musings.

    On the comparison of f/64 8x10" and MFD f/11 I made;

    If we assume infinite resolution on the sensor and infinite resolving power of the lens, ie diffraction is the only factor limiting resolution, there's a zero sum game between formats in terms of resolving power and DoF.

    If you make the format larger, you need a longer focal length to match field of view (FoV) and then you need a smaller aperture to match DoF, and then you get punished more by diffraction so you end up not gaining any resolving power despite the larger format.

    If you go the opposite direction from larger to smaller you need a shorter focal length to match field of view, but to resolve as much as the larger format you need to reduce diffraction and thus open up the aperture, ending up on a zero-sum.

    Conveniently enough, the you can use the crop factor both on focal length and f-number to find the corresponding match when going between formats.

    8x10" diagonal is about 320mm, 48x36mm MFD is about 60mm, that's a 320/60=5.33 crop factor. 64/5.33=12, that is f/12. For full-frame 645 it becomes ~f/13, so f/11 was a bit of exaggeration on my part, you need to get down to 44x33mm for that...

    That is if you want to resolve as much as 8x10" at f/64, you can't shoot at a smaller aperture than f/11 with your 44x33mm sensor (and you need like 600 megapixels on that sensor of course...). The point was showing an example that f/64 is not that deep DoF as it might sound in modern ears.

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    Re: Focusing and other musings.

    Quote Originally Posted by vjbelle View Post
    So true.... and has been my preferred way of shooting for a long time.
    Mine too.

    Airlight reduces the benefit of hyperfocal distance focussing, often significantly, ocassionally hugely.

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    Re: Focusing and other musings.

    Sorry to go on about mathematical abstractions when you would all rather consider Art, but I find the corollaries to the above analysis very interesting.

    For instance, want to know how big the disks of distant OOF lights will be in your image? Imagine your lens opening sitting at the plane of focus. If you are shooting a 50mm at f/1.0, then the opening is 5 cm, or 2". So imagine a 2" diameter disk however far out you are focused, and that's what the distant lights will look like. Focus closer, disks get bigger. Here's an example:



    That's probably a 50 f/1.4 wide open, so those disks should be 1.4" = 50mm/1.4 across at the plane of focus, which is the window grill. That middle section is 10" across, so that looks right. In another shot, where I'm closer to the window, the disks look bigger:



    but they still look about 1.4" across at the plane of focus, which is now closer.

    For specular lights not so far away, it's a bit more complicated. I'll spare you.

    We now return you to your photography forum...

    --Matt
    Last edited by MGrayson; 12th January 2015 at 16:22.

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    Re: Focusing and other musings.

    Thanks Matt, Torger and JLM

    I get it now, for some reason it was just escaping me but I wasn't taking into account the size of 8x10 and the length of the lenses. It's quite simple once you stop and think.
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    Re: Focusing and other musings.

    There is a lot of good points being raised here.

    I have finally gotten a break in the weather so will get out and try some different techniques and see which I feel works for me best.

    I have a GG with my Cambo but only a 3x loupe so was thinking I will use that more for framing than critical focusing. My eyesight is still pretty good so I may be able to get things pretty close.

    Time for some testing.
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    Re: Focusing and other musings.

    Quote Originally Posted by MGrayson View Post
    Sorry to go on about mathematical abstractions when you would all rather consider Art, but I find the corollaries to the above analysis very interesting.

    For instance, want to know how big the disks of distant OOF lights will be in your image? Imagine your lens opening sitting at the plane of focus. If you are shooting a 50mm at f/1.0, then the opening is 5 cm, or 2". So imagine a 2" diameter disk however far out you are focused, and that's what the distant lights will look like. Focus closer, disks get bigger. Here's an example:



    That's probably a 50 f/1.4 wide open, so those disks should be 1.4" = 50mm/1.4 across at the plane of focus, which is the window grill. That middle section is 10" across, so that looks right. In another shot, where I'm closer to the window, the disks look bigger:



    but they still look about 1.4" across at the plane of focus, which is now closer.

    For specular lights not so far away, it's a bit more complicated. I'll spare you.

    We now return you to your photography forum...

    --Matt
    Those disks are an image of the aperture. You notice the disks are becoming oval as they move away from the optical axis (and perpendicular to the radial axis). Those show the mechanical vignetting in your optics. The difference in area between the circular aperture image and the vignetted aperture are proportional to the light falloff.
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    Re: Focusing and other musings.

    Yep, you can see the aperture vignetting clearly. I just never appreciated that they were images at 1:1 scale at the plane of focus. It's also a credit to the optics that the disks are so clean.

    --Matt

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    Re: Focusing and other musings.

    since diffraction was mentioned...it is my understanding that diffraction starts to show when the actual opening size gets relatively small. it is related to the wavelength of the light and the opening size, not the f number directly. meaning f/64 on a 200mm lens will show less diffraction than f64 on a 50mm lens. another way to think about it is that an edge will diffract light and with smaller physical openings, the surrounding "edges" start to affect each other more.

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    Re: Focusing and other musings.

    Diffraction is proportional to the f-number. It is the angular size of the exit pupil that is important--an f/64 light cone has the same angular size regardless of focal length. The impact of diffraction changes with the format size.

    The entrance pupil is related to resolution at the object plane, unlike the exit pupil which is resolving power at the image plane. A larger entrance pupil gives greater resolution at the object plane, but the diffraction, the spot size, at the image plane is a product of the f-number. Note, magnification changes with focal length. At a given f-number and FoV, the larger format has greater resolution at the object plane, the same spot size/diffraction at the image plane, less depth of field, and more depth of focus. Crazy world...

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    Re: Focusing and other musings.

    Quote Originally Posted by Dogs857 View Post
    Keep the lens at infinity and stop down for your depth of field, then you can calculate what will and won't be in focus.
    Not really. DoF and CoC simply models our perception of sharpness in an image--if you don't like what your DoF scale tells you, find a better CoC and recalculate the values for DoF. Changing the value of the CoC does not change the image, it is just adjusting the model to reflect how you perceive an image. Two people can see the same image and think different values for the CoC are correct and neither would be wrong.

    Only the image plane is in focus. But as DoF shows, you can go away from the plane of focus and still get what appears to be a sharp image. "In focus" has nothing to do with sharpness, it is simply an optical property--you can have an in focus image that does not appear sharp.

    Because DoF is simply a perceptual model, pixel resolution has no effect on it. Take two images with two 35mm sensors, one with 24MP and another with 37MP and make two 40" prints. The DoF in each print will be identical. The only difference is the 37MP image will have higher frequency detail.
    Will

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    Re: Focusing and other musings.


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    Re: Focusing and other musings.

    Quote Originally Posted by jlm View Post
    Jim, do you need help? It is a long entry. From your Wikipedia article:

    The ability of an imaging system to resolve detail is ultimately limited by diffraction. This is because a plane wave incident on a circular lens or mirror is diffracted as described above. The light is not focused to a point but forms an Airy disk having a central spot in the focal plane with radius to first null of

    d = 1.22λN
    where λ is the wavelength of the light and N is the f-number (focal length divided by diameter) of the imaging optics. In object space, the corresponding angular resolution is

    sin theta = 1.22(λ/D)
    where D is the diameter of the entrance pupil of the imaging lens (e.g., of a telescope's main mirror).
    As you can see, diffraction at the image plane is d = 1.22λN where N is the f-number of the system. Entrance pupil is not a factor, but simply f-number. This determines resolving power at the image plane from diffraction.

    In object space, the angular resolution is dependent on the diameter of the entrance pupil: sin theta = 1.22(λ/D). That is the resolution in the object space in front of the camera. But this is unrelated to f-number and does not change diffraction at the image plane. BTW, that is based on an object at infinity. When objects are closer to the camera, then the angular size of the entrance pupil defines resolution at the object plane (just like the angular size of the exit pupil in image space). This is also known as the numeric aperture--commonly used with microscopes.

    That is what I actually wrote in my post above.

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    Re: Focusing and other musings.

    I think Will is right. It's true that diffraction scales inversely as the width of the opening, but the light travels further to the sensor in the larger camera, and that cancels out the scale. Or think of the view from a spot on the sensor looking out. You can't tell how far away the aperture is, i.e., the focal length of the lens. You can only see a disk whose angular size is determined by the f-ratio.

    It sounds paradoxical that depth of focus would be bigger on the larger camera, as the optical system doesn't care which is the sensor snd which is the subject. But when we scale up the camera, we don't change the subject distance, while we do change the sensor to exit pupil distance. That's the source of the asymmetry.

    Crazy world, indeed.

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