I haven't quite got my head around Jae's math and how I would use the Disto 5 should I decide on purchasing one vs the E-Module though. I think further explanation for my aging grey matter is required (hint hint)
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In my two previous posts, I discussed methods I developed to maximize the usability of a Tech Camera, in my case Rm3di. It is very important to remember that I am talking about techniques of using a 'machine' accurately, not about creating artistic masterpieces. Some may argue that my methods of focusing is an overkill and there are ways to get 'good enough' instead of 'absolute.' Again, I am being a mechanic who 'fine tune' a Formula 1 car for a driver so he can take advantage of it and show its maximum performance on the track and may win the race (for me, as a driver, I am taking advantage of the camera to it's maximum potential but haven't won any race yet. But just puttering around with a finely tuned machine is lots of fun too).
The main attractions of a Tech (and View) Camera to a photographer are two folds (my personal opinion); first, the best possible optical lenses available; second, mechanical 'movements' - shift, tilt, swing and accurate focus ring. The main challenges of a Tech Camera to a photographer who purchased it is its difficulty to use, mainly how to focus accurately with or without Tilt/Swing.
CONTROL based on visual feedback is very powerful and useful. See how we steer cars, we don't measure the angle of turn we need to make before making a precise turn of steering wheel (in other hand, we usually end up in accidents if we try keep on driving with badly fogged up windshield), see how Apollo 13 crews steered the crippled module to a successful reentry based on visual feedback of seeing the earth through the module window (but they could do it only because they are NOT ordinary weekend Piper flyers).
A Tech Camera with MFDB lacks an 'Excellent' visual feedback to focus an 'Excellent' lens via an 'Excellent' camera movement. In control theory, the weakest link determines the final result. The combined accuracy of a Tech Camera is 60% if the accuracy of the visual feedback using a GG (or my dated P45 display) is 60% even with 100% accurate lens and camera movement. The same analogies apply to the Lens component of Tech Camera (reduced lens performance by stopping down the aperture) and Focusing component (i.e., relying on HFD or Infinity focus setting).
How do we measure distance (to focus accurately)?
- It is not likely a Tech Camera will have an Auto Focus such as Nikon and Canon.
- LiveView may solve the problem in 2-3 years (IQ4xx).
- A tethered shoot has its place.
- eModule from A-S? First of all, it is only for RM camera from A-S. I only know how it work and what it does based on what I have read from this Forum. My present impression is that it is an excellent concept but not-so excellent method of measuring distance. It measures (let me know if I am wrong) the distance the way we manually focus a SLR camera, rotate a focus ring until the image is sharp, then translate the angular rotation into focus distance. I don't know yet how many rotations it requires to focus from Infinity to let say 1 meter. With A-S Focus Ring, SK35 rotates slightly over 360 degree (1 rotation), SK47 rotates 720 degrees (2 rotations), and SK90 rotates 1,620 degrees (4.5 rotations) to focus from infinity to 1 meter. The same control theory applies here, the weakest link determines the final result. The number of rotation of eModule focus ring determines the effectiveness of Rm3di's helical focus ring. If the focus ring of eModule rotates 360 degree (Canon 5DII rotates approximately 90 degree), its accuracy with SK35 will not diminish, with SK47 it will diminish to 50%, and with SK 90 it will diminish to 25%. (I apologize in advance if eModule reads the distance by other means, not relying on the angular rotation of focus ring.)
- I gave up on GG for my own reason and do not have opinion on it anymore.
- Laser Rangefinder provide the most accurate means at a reasonable price and is portable. It would have been a breakthrough product if eModule was based on a laser rangefinder.
How to use a Laser Rangefinder (LR)?
A LR is one tool any Tech Camera users should have. With HPF ring(s) or with printed card(s) for A-S, accurate focus can be made with a one-click distance measurement. Keep the picture of the cards in iPhone, and you will always have it with you. You may get a simple App which provides HFD and Near and Far Focus distance with a measured distance or make a quick spreadsheet. Without any intention of insulting our forum members, it is very important to read manual and go through all functions. Spend an hour with it. I have met too many who have Disto but didn't know it could be in either metric or imperial mode, different decimal points, the location of sensor, front or back assignment of the reference point, etc. I have Disto manual in both iPhone and iPad in PDF format to refresh my memory how to measure the horizontal and vertical distance of a inclined object point.
How to use LensTilt?
SIMPLE LENS TILT
If you are limiting the lens tilt with the 'Plane of Sharp Focus' that is parallel to horizontal ground, and not to tilt the camera, it is easy to find both lens tilt angle and the required focus distance. You can have a simple spreadsheet with only two user input; f and J, and use it in an iPhone. The equations are as follow:
alpha = arcsin (f/J), where alpha is lens tilt angle, f is lens focal length in mm, and J is the vertical distance between lens axis and your Plane of Sharp Focus in mm.
Fd = J / sin(alpha), where Fd is the calculated focus distance in mm.
If you are focusing the ground plane, J is the camera height from ground; if you are focusing on items on horizontal table top, J is the vertical distance between the lens axis and table top; if you are focusing on the street below from rooftop, J is the vertical distance between lens axis and the street, etc.
J is always located vertically along the lens nodal point.
It should be noted that the conventional concepts of DOF or HFD do not apply once a lens tilt is made. DOF is not two parallel planes from the lens anymore but a wedge (above and below the Plane of Sharp Focus).
HFD also lose its relevancy and I will explain it using examples:
- SK35 at 8.0 aperture and CoC setting of 3 pp of P45 (0.02mm)
- Ground to be the plane of Sharp Focus
- If Camera Height is 1 meter, Tilt angle is 2.1, Focus distance is 29.6 meter and HFD is 8.1 meter
- If Camera Height is 1.5 meter, Tilt angle is 1.4, Focus distance is 68.8 meter and HFD is 8.1 meter
TILTED PLANE OF SHARP FOCUS
If you want to have the Plane of Sharp Focus in angle to the ground (to have small cactus flowers in foreground and tall cactus tree in the back in focus) without camera tilt, the calculation become more complex. You have to have several more user input for tilted Plane of Sharp Focus; vertical and horizontal location of both Near and Far Object relative to the Lens Nodal Point. And J is not anymore the vertical distance between the lens axis and the ground and will be calculated based on the locations of Near and Far Object.
TILTING CAMERA
Tilting the camera in addition to the lens tilt makes geometry even more complex.
What is 'Lens Equation'?
A "lens equation' will translate the measured object distance into the distance between the lens nodal point and the sensor plane for a specific lens. In place of 'lens equation', ALPA provides HPF rings and A-S provides the helical focus ring and lookup tables and both work 'good enough'. It is an industry convention that the object distance is between the sensor plane (not the nodal point) and the object. The equation can be developed with the data provided by manufacturers (ALPA and A-S) or with your own calibration data.
It streamlines the workflow using 'calculation' on an iPhone, more accurate without 'mental' interpolation, and provide a means to enhance the usability of HPF ring with a simple modification.
Jae M