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brighamr
16-Jun-2020, 02:02
Hi

I am doing some 10x8 macro transparencies ... which cost basically £40 a sheet film and process here in England
thankfully somebody else is paying but still every sheet counts
I could bracket +/- 1/3rd but that would probably push the budget over the edge and they would revert to digital which nobody wants

so what I would like to know is how do I calculate exactly

I have the math but where would I measure extension from
its from optical centre to film plane but how do i know where the optical centre is and then the film plane
after quite extreme movements is all over the place
4 corners of front and rear standards have probably +/- 10cm difference
obvious answer is to just average but is there a precise way

The transparencies will be displayed on light boxes so they need to be as near perfect as possible

best

robin

Dan Fromm
16-Jun-2020, 08:17
Not to be a complete idiot, but why calculate when you can measure? See https://www.pacificrimcamera.com/rl/02077/02077.pdf

Daniel Casper Lohenstein
16-Jun-2020, 08:53
Not to be a complete idiot, but why calculate when you can measure? See https://www.pacificrimcamera.com/rl/02077/02077.pdf

What about a camera / format independent Gossen meter with a measuring probe, https://gossen-photo.de/wp-content/uploads/DL/FOTO/AG/ba_messsonde_d.pdf to measure the light on the focusing screen (fresnel)?

They've also got the Profiselect device: https://gossen-photo.de/wp-content/uploads/DL/FOTO/AG/ba_profiselect_d_e_f.pdf

Doremus Scudder
16-Jun-2020, 10:55
Certainly, being able to meter at the film plane would be ideal, but not everyone has the resources to acquire the necessary equipment...

Figuring the bellows-extension factors by measuring the extension and calculating is a common and accurate method, so why don't we help the OP with what he asked for?

@OP,

I'll take a stab at it.

The basic formula for calculating the bellows extension factor is Ext² / Fl² = Exposure Factor
Where Ext = Bellows extension measured from film plane to lens nodal point (middle of the lens for most designs) and Fl = The focal length of the lens.

For the most accuracy, you need to know the exact focal length of the lens you are using and the position of the nodal point. If you have a newer lens that is listed in a manufacturer's specification sheet, then you can take the information from that. At any rate, for many lenses (i.e., not telephoto or retrofocus) the nodal point to measure from is where the aperture blades are.

Still, it's good to check. If you know the exact focal length of the lens, then you can simply measure from the film plane to a point on the lens shutter/barrel/etc. and mark yourself a reference point. If you don't, then it gets more complicated; you need to know the lens design and the location of the nodal point and then focus on infinity and measure from the nodal point back to the film plane.

Hopefully, you're using a lens that is easy to find info on, has a nodal point at the position of the aperture, etc. so the entire process is pretty straightforward. If not, you'll need to do your homework for the most accuracy. That said, if you have a lens that you don't have all the necessary info for, you could simply extrapolate what the focal length should be and where the nodal point of the lens most likely is and do a test or two.

It would help to know which lens or lenses you plan on using. Dan Fromm can certainly help you find whatever information there is about just about any lens that has ever been made; he's the resident expert.

And, if I'm wrong about anything in my recommendations above, I hope he'll step in with corrections.

Best,

Doremus

Daniel Casper Lohenstein
16-Jun-2020, 13:38
Figuring the bellows-extension factors by measuring the extension and calculating is a common and accurate method, so why don't we help the OP with what he asked for?

204776

Dan Fromm
16-Jun-2020, 14:11
Sounds good, Daniel, but the OP stated

where would I measure extension from its from optical centre to film plane but how do i know where the optical centre is then the film plane after quite extreme movements is all over the place 4 corners of front and rear standards have probably +/- 10cm difference

We usually calculate the correction for the center of the field and let what happens off-axis happen, but in this case without more information from the OP -- lens' focal length, how far is the center of the gate away from the lens' axis? -- we don't know what effect cos^4 will have. I'm sensitive to this because when I was writing my Horseman Optical Exposure Computer article I looked into cos^4's importance. If the OP is using a short lens, much decenterment and wants to be within 1/3 stop over the frame measuring directly is the way to go.

OP, I don't know what you mean by optical center. When doing closeup work and calculating exposure corrections, the distance that matters is from the lens' rear node to the film plane, measured on the lens' axis. Not as easy as most would like with much tilt/swing. For most LF lenses, the rear node is very close to the lens' diaphragm. The big exceptions (no pun untended) are telephoto lenses, whose rear nodes are considerably displaced from the diaphragm.

Measuring, given the gear, seems easier and allows better control when the subject isn't evenly illuminated.

cowanw
16-Jun-2020, 16:18
At 40 pounds a pop I would consider a 4x5 reducer and Horseman meter or exposing B&W and developing to check or some old Polaroid film any of which might cost 1-2 sheets.

Two23
16-Jun-2020, 16:19
This is what I use:

http://www.salzgeber.at/disc/

Kent in SD

Daniel Casper Lohenstein
17-Jun-2020, 01:53
Sounds good, Daniel, but the OP stated

OP, I don't know what you mean by optical center. When doing closeup work and calculating exposure corrections, the distance that matters is from the lens' rear node to the film plane, measured on the lens' axis. Not as easy as most would like with much tilt/swing. For most LF lenses, the rear node is very close to the lens' diaphragm. The big exceptions (no pun untended) are telephoto lenses, whose rear nodes are considerably displaced from the diaphragm.

Measuring, given the gear, seems easier and allows better control when the subject isn't evenly illuminated.

Perhaps the problem is neither to measure the distance of all four corners of the lens board to all four inverted corners of the screen, nor the use of a relatively inexpensive Profiselect attachment.

IMHO the problem - beside the "mother-of-all-problems" character of a 8x10 macro exhibition diapositive (!) - consists in forming a reasonable average value from all four measurements, given the costs of 40 £ for each try.

Anyway, I find it disadvantageous to spend 40 £ on developing an 8x10 macros of which I don't know if it is exposed exactly 1/3 EV or if its depth of field is large enough. - Remember that colours will bleach when transparencies are displayed on light boxes.

I would take the respective picture in 4x5 or even 6x7 (cheaper roll film, more dept of field than a 8x10 macro, bracketing) and then reproduce the best one using the 8x10 camera, without moving the lens or the film, just as a simple reproduction with a constantly increasing bellows factor.

How about digital test shots? You can attach a digital camera to a large format camera back. How about a digital image with many megapixels, which would be exposed on a transparent carrier?

Pere Casals
17-Jun-2020, 03:45
so what I would like to know is how do I calculate exactly

As you want exactitude... first you have to know how to get the exact numbers you use in the well known formula.

1) Know the Real Focal your lens has from datasheet, normally comercial denomination is quite close in practice to the real one, but for example Symmar-S 360mm has a true 350.5mm focal (http://www.mr-alvandi.com/downloads/large-format/schneider-symmar-s-data.pdf)

2) Know your Effective bellows draw. Real Belows draw depends on the Flange Focal Distance, so if you measure directly Belows draw you may get some error in the calculation, so to measure your Effective Bellows draw in the macro conditions first focus your camera at infinite and then measure what Relative Increment you need for close focus from the infinite focus position. Then add this Relative Increment to the Real Focal and youl'll have the Effective Bellows Draw bellows you have to use in the formula alongside with the Real Focal. This is exact.

3) Use belows formulas here: http://kennethleegallery.com/html/tech/bellows.php, but measure extension in the way I explain if wanting an exact compensation: Use Real Focal and caltulate the bellows extension by adding the Real Focal to the Relative Increment (close vs infinite), reiterating it.

Resources:

To determine the Real Focal, when unknown:

Tele and Retro lenses have particular Bellows compensation formulas for very close subjects, this is first class knowledge from a true master in optics, Mr Bigler:

___________

Beyond that, if wanting total consistency just attach a DSLR (or SLR) in the view camera back (without the dslr lens) to spot meter exactly, you'll project the image on the sensor. If you use a SLR in the back you may practice with bracketings, flash, etc with the real slide film you are to use.

If not using flash, use a shutter tester, LF shutters were sold new with a +/- 30% tolerance, so 1/30 could be 1/20 or 1/40, modern shutters were usually accurate than that 30%, but after many years best is checking the real speeds.

Daniel Casper Lohenstein
17-Jun-2020, 07:43
As you want exactitude ...

Know the Real Focal your lens ...

Know your Effective bellows draw ...

Tele and Retro lenses ...

If not using flash, use a shutter tester,...

I couldn't agree more! I would add that you should also develop your films yourself then. You should do some more tests first.

Bob Salomon
17-Jun-2020, 10:28
I couldn't agree more! I would add that you should also develop your films yourself then. You should do some more tests first.

Or use a Rodenstock pocket calculator that quickly and easily, as well as accurately, determines exposure correction based on the image ratio and doesn’t require bellows length or focal length.
It was also sold by Sinar and Linhof. Fits in a shirt pocket, no batteries, also computes Scheimpflug and DOF. Works for all camera formats from 35mm full frame to 810.

brighamr
17-Jun-2020, 16:38
Thanks for all the replies

Dan by optical centre I mean nodal point and I think I will mostly be using a 210mm macro sinaron

the image plane meter looks interesting I have the potential for that as my Sinar has a "metering back" but I don't have the probe or meter
dose any body have these ? dose it measure continuous light or just flash ?
I light with continuous light

Bob the Rodenstock calculator looks very interesting and it looks like Linhof and studio has them in stock

best

robin