Large Format Focal Plane Meter - New Product

[Dave Gooding]

Hi All,

My interest is not so much photography as electronics. I am here both for a bit of advice and to shamelessly plug my product.

I developed a focal plane meter for my father's 4x5 Sinar several years ago, as my hobby was electronics and I am an engineer by trade. I have since supplied a couple more to other large format photographers, and then decided to productionise the design and to consider marketing it.

My question is whether people are interested in such a product at the price I can sensibly achieve (£120, roughly US$160 or Euro 130), and whether people have any suggestions or comments on the design.

It comprises a frame, which inserts into the camera in place of the usual film carrier, with a metering probe that can be seen through the ground glass and used to measure the brightness at any particular points of interest in the image. The general idea is that you can examine the brightness in highlights and shadows and adjust to an exposure such that any shadow or highlight detail of interest will be retained in the final print. My father, I think, was using Adams' zone system at the time I developed this and this type of meter is ideal for that purpose.

I believe that meters of this type have previously been produced e.g. by Sinar, very sophisticated but also very expensive. This was intended as a low cost, basic (but still accurate and reliable) alternative.

What do people think? Obviously a get rich quick system this isn't - the total world market would hardly keep someone in film for a year - but I thought it might be a useful product that people might be interested in.

See more details at http://www.dlgelectronics.com

Picture below - there is more at the link above. Please feel free to message me either here or via the link above.

I would also be interested if anyone has any other ideas for useful photographic/optical electronics products.
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Thanks!

Dave

[xkaes]

You are correct about other, similar products having been made. Gossen made a couple, but I am not knowledgeable about the details. They worked with Luna-Pro, Ultra-Pro, and perhaps other models. At least one was simliar to your design and reads in front of the ground glass. I think they made other attachments that read the rear of the GG. Minolta made the Auto-Booster and Auto-Booster II that came with four attachments -- one of which read on the back of the groundglass. The Minolta ones work with several of their meters, are useable for ambient and flash light (depending on the meter), and show up occationally on EBAY for around $50 or less. There may be other devises out there, as well from Sekonic and others. But perhaps comparing your design to these others could be useful in modifications.

[AtlantaTerry]

What about offering a version of your a probe that will attach to the photographer's already existing light meter? In the past I owned several Minolta AutoMeters that had that capability.

[Peter De Smidt]

Dave, probably the best use for this is for close-up photography, whether in the studio or otherwise. Not long ago I shot a small product in a light tent, and using a handheld meter, spot or otherwise, was a bit of a challenge.
https://u3595413.dl.dropboxusercontent.com/u/3595413/Sugar_Spoon.jpg

[Steve Goldstein]

Dave, that looks like a very interesting product, with a price and weight that are competitive with available spotmeters. The 8-stop brightness range could be limiting in some circumstances, but I don't think this would be a show-stopper. I've downloaded the user manual and will give it a careful read. I'm also an electrical engineer by day, at least for two more weeks.

Terry, I think trying to make it compatible with existing meters would be a lot of work because there are a lot of different meters out there. Dave would have to choose one or two, and then anyone who didn't own the one(s) he chose would be out of luck.

[Dave Gooding]

Thanks for your interest!

xkaes - I am interested in the feasibility of metering off the ground glass directly, and did have a concept using a pen-like sensor that you could put against the glass and meter a small spot. My idea was for a fibre-optic leading from the tip of the "pen" so that the tip could be made quite small allowing metering of an area of 1mm^2 or even less. However, when metering in this way don't you get ambient light entering the back of the camera through the glass or reflected within the glass and introducing errors into the metering? You must have to use a hood for this sort of application especially when you have the sun behind you. However, if people like metering off the glass, and find that stray light is either not a significant problem or don't mind using a hood then this is something I will try to develop further.

Terry - as Steve notes it would be difficult to make a probe compatible with existing meters as I rather suspect that they will interface differently both mechanically and electrically, so I would have to offer a wide range of them. Also, I am not sure if some of these meters would be calibrated to specific sensors.

Peter - that's an impressive picture. I am not a photographer myself but I can appreciate the technical difficulty of taking photographs like this. My meter is intended for situations like this where you want to meter not so much the overall scene but (in this example) the dark and light stripes specifically, to bring both the light and dark stripes in the range that the film and print can sensibly reproduce with saturation. Looking at your image, it would require metering quite specific small areas on the image. The sensor size on mine is approximately 5mm. Would this be small enough to get it wholly in the dark or wholly in the light stripes here? What format did you use? With a 4x5 or larger I would think it would work.

Steve, the eight stop brightness range can be changed - I could even mod my existing stock very easily - I would have to substitute a different scale in the indicator and change one resistor value (if you look at the circuit diagram in the manual you will see how straightforward this would be . The reason for a plus/minus four stop range was that I had understood that was sufficient to accommodate the intensity range that could be reproduced and that anything beyond that would be full black or full white. Bear in mind the intended usage of the meter was to set the meter for your intended whole-image exposure (you dial the exposure in on a knob on the meter) then poke around in the image with the metering probe to check that shadows and highlights fall within a +/- 3 or 4 stop range or whatever of that, then fine tune the exposure a stop or two as necessary to ensure you get the shadow or highlight detail you want to be reproduced in the photograph. However, as a non-photographer myself I am open to advice on whether a +/- 4 stop range around the nominal exposure is sufficient to capture the full reproducible range of the film.

Thanks again, Dave.

[xkaes]

Here's what info I've got FWIW:

First, is the Minolta Booster II with the GG reader (attached) and the flexible incident reader attachment (great for tight spots). These will work with most Minolta meters as plug-ins.

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Second, is the Minolta Booster II with the GG reader (attached) and connected to a Minolta Flash Meter II.

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The next pictures are close-ups of the GG reader itself. It is 38mm long with an 18mm thread to attach to the Booster or Booster II. The front, that is placed on the GG, is a clear, 2mm thick, plastic plate with a diameter of 15mm. In the center of this plate is a 5mm section that reads the light. The metal tube is maybe 0.25mm away from the GG. The interior of the tube is aluminum, just like the outside -- not black.

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I suppose some light could "get around" the spot that is read, but a dark cloth should take care of most of that. For me, the bigger deal is the GG, and the lens fall-off. The instructions basically say to take a regular reading of a gray card in the scene. Then take a reading on the ground glass of the same gray card. Basically, the "difference" between the two readings is used to adjust future readings on the ground glass. There is an "adjustment" dial on the Booster which is turned to achieve the same exposure obtained from the the non-GG reading.

But, IMHO, the readings taken on the edges of the GG need to be adjusted for any light fall-off -- which varies with the f-stop -- as well asthe GG itself, which gets less light at the edges -- varies with the GG /fresnel used, even if there is no light fall-off.

[Dave Gooding]

Thanks xkaecs, that's actually very informative and tells me it is at least feasible to meter off the ground glass. I am not sure why readings should be lower at the edges and corners of the image unless there's any vignetting by the camera optics and in any case you should be metering the light that the film will actually see, unless there's some behaviour of the ground glass scattering for off-axis illumination. I have a physics degree and I can also get some support on this from someone with doctorate in optical scattering so between us we should be able to work out how to do it.

I've got another product development just starting now so I'm a bit busy but I will get the optical scattering Ph.D lady started on looking into this one.

[xkaes]

I'll give you any info I can, but it will be based on my current GG setup. It would have to be adjusted for each GG/Fresnel screen avaiable -- and there are a lot of them,

[Bob Salomon]

Thanks xkaecs, that's actually very informative and tells me it is at least feasible to meter off the ground glass. I am not sure why readings should be lower at the edges and corners of the image unless there's any vignetting by the camera optics and in any case you should be metering the light that the film will actually see, unless there's some behaviour of the ground glass scattering for off-axis illumination. I have a physics degree and I can also get some support on this from someone with doctorate in optical scattering so between us we should be able to work out how to do it.

I've got another product development just starting now so I'm a bit busy but I will get the optical scattering Ph.D lady started on looking into this one.

Maybe you should look at what Linhof did. They made a focusing metering bellows that attached directly to the ground glass back and had an eyepiece with a rubber collar and, originally a 2x and currently with a 4x loupe. The 4x unscrews and becomes a fixed 2x and a removable 2x pair of loupes. The rubber collar was made to fit the microscope adapter of a Gossen LunaPro meter. The bellows will compress enough so a meter reading the diameter of a quarter could be read at any point of the gg or it could be pulled back all the way to meter the entire gg or any portion of the gg.

To meter properly the back had to have a Fresnel installed and, of course, the user had to determine how much light was absorbed and lost by the gg/Fresnel.

[interneg]

This is just me thinking aloud, but how much does that control box do that a smartphone running an appropriate app & plugged into the the probe couldn't? If this was feasible, I could see it being possible to integrate your metering with relevant documentation - ie how much + or - development etc - & even automate that process to a certain extent, perhaps incorporating tested film curves in the app to enable you to tailor exposure to specific films in a convenient manner. The ultimate stage would be to integrate this with an electronic in-lens shutter, with the smartphone as the controller, but that's perhaps getting away from your design goals...

[Dave Gooding]

This is just me thinking aloud, but how much does that control box do that a smartphone running an appropriate app & plugged into the the probe couldn't? If this was feasible, I could see it being possible to integrate your metering with relevant documentation - ie how much + or - development etc - & even automate that process to a certain extent, perhaps incorporating tested film curves in the app to enable you to tailor exposure to specific films in a convenient manner. The ultimate stage would be to integrate this with an electronic in-lens shutter, with the smartphone as the controller, but that's perhaps getting away from your design goals...

Interesting. I don't know a lot about smart phone apps - I'll have to find someone that does.
The only bit in my box that might be problematic in a smart phone is the front end stage. The signal range is huge to cope with exposures from hours to 1/2000s or so; putting that directly into the smartphone would inply an input sensitivity in the sub microvolt range and effective accurate analogue to digital conversion of 24 bits resolution. Therefore I think I'd have to put the front-end analogue logarithmic amplifier outside the phone. However this could be made tiny and integrated with the sensor if I can get a bit of power out of the phone to drive it.
Another interesting job to put on the increasing list of things to work on!

[xkaes]

I am not sure why readings should be lower at the edges and corners of the image unless there's any vignetting by the camera optics and in any case you should be metering the light that the film will actually see, unless there's some behaviour of the ground glass scattering for off-axis illumination.

There is SEVERE light FALL-OFF with some lenses -- more with wider lenses, and more with lenses used opened up. But even if that is not an issue, the light that hits the edges of the film plane (GG) will be less that what hits the middle.

[faberryman]

It is an interesting project, but wouldn't a traditional spotmeter accomplish the same thing, be easier to use, and be a lot more versatile?

[xkaes]

It is an interesting project, but wouldn't a traditional spotmeter accomplish the same thing, be easier to use, and be a lot more versatile?



Do you mean a spot meter of the scene or a spot meter of the ground glass?

[Dave Gooding]

There is SEVERE light FALL-OFF with some lenses -- more with wider lenses, and more with lenses used opened up. But even if that is not an issue, the light that hits the edges of the film plane (GG) will be less that what hits the middle.

But isn't it the light actually hitting the film plane that you want to measure, since that is what the film records? I mean the whole point of metering in this way is to measure the actual amount of light at that point that the film will be exposed to, surely?

I can see that the ground glass would give a different scatter, reflection and transmission away from the lens axis and that would have to be accounted for, and we are going to plan some tests for this to see how much difference it makes and how well we can capture both the directly transmitted and forward-scattered light. Backscatter and speculation reflection is of course light that is just lost and cannot be captured when metering off the viewing side of the screen.

What sort of accuracy do you need in the metering? Is one-third or one-quarter of a stop sufficient? Doing better than this over a full exposure range of, say, 1/1000s to an hour will be challenging.

[xkaes]

But isn't it the light actually hitting the film plane that you want to measure, since that is what the film records? I mean the whole point of metering in this way is to measure the actual amount of light at that point that the film will be exposed to, surely?

You are absolutely correct, and it is absolutely a challenge.

And using a spot meter of the scene (or any meter of the scene) does not account/adjust for light fall off due to the aperture and the lens design. At least metering off the the GG can attempt to do that -- but the photographer needs to decide where in the scene to expose for (or use other methods such as central ND filters, dodging/burning, etc.).

[faberryman]

Do you mean a spot meter of the scene or a spot meter of the ground glass?
Spot meter of the scene. This seems like a solution in search of a problem.

[xkaes]

Spot meter of the scene. This seems like a solution in search of a problem.

So metering a scene using a 50mm lens with 300mm of extension with a spot meter and running a bunch of calculations is somehow easier than just reading the light on the groundglass? Maybe in your world.

[faberryman]

So metering a scene using a 50mm lens with 300mm of extension with a spot meter and running a bunch of calculations is somehow easier than just reading the light on the groundglass? Maybe in your world.
I do not know of any 50mm lens for large format, certainly none that would focus with 300mm of bellows extension. But that is just my world.

[xkaes]

I do not know of any 50mm lens for large format, certainly none that would focus with 300mm of bellows extension. But that is just my world.

There are plenty. I'll just mention a few that I have. Here is my Mamiya 37mm f4.5 fisheye -- makes absolutely, unbelievable close-up shots:

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Here's still more, among them a Minolta 12.5mm f2.5, a Minolta 25mm f2.0, and a Minolta 50mm f3.5. I've used all of them with more than 300mm of extension:

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Then there's my Schneider 47mm XL f5.6 and Fujinon SWD 65mm f5.6 that I've used with long extensions at times, but I think I've made my point. Happy spot metering -- when it's appropriate, which isn't too often, I use a Minolta Autometer II with 5 degree spot attachment.

[faberryman]

Of the six lenses you mentioned only one is a 50mm lens - the Minolta 50mm f3.5. Is that a large format lens or an adapted lens? What is its image circle? What is the focus distance with 300mm of bellows extension?

[Bob Salomon]

I do not know of any 50mm lens for large format, certainly none that would focus with 300mm of bellows extension. But that is just my world.

45mm and 55mm Apo Grandagon.

[xkaes]

Of the six lenses you mentioned only one is a 50mm lens - the Minolta 50mm f3.5. Is that a large format lens or an adapted lens? What is its image circle? What is the focus distance with 300mm of bellows extension?

Yes, only one is 50mm. You've got me there! I just chose that, at random, as an example, but FOUR of the lenses I mentioned have focal lengths SHORTER than 50mm. FYI, microscopic lenses are used on large format cameras all the time. Only two of mine are microscopic lenses, but at high magnifications -- which these lenses are designed for -- large formats (in my case, 4x5) are completely covered. And by the way, the results are amazing -- and metering with flash is easy when metered of off the ground glass. No need to try to make a spot meter reading of a moth's wing.

[faberryman]

Yes, only one is 50mm. You've got me there! I just chose that, at random, as an example, but FOUR of the lenses I mentioned have focal lengths SHORTER than 50mm. FYI, microscopic lenses are used on large format cameras all the time. Only two of mine are microscopic lenses, but at high magnifications -- which these lenses are designed for -- large formats (in my case, 4x5) are completely covered. And by the way, the results are amazing -- and metering with flash is easy when metered of off the ground glass. No need to try to make a spot meter reading of a moth's wing.
What is the distance between the lens and the moth's wing?

[Oren Grad]

However, as a non-photographer myself I am open to advice on whether a +/- 4 stop range around the nominal exposure is sufficient to capture the full reproducible range of the film.

That is not sufficient - it's far short of the range that many negative films can record. Whether the output medium is so limited is irrelevant, as photographers often face the task of assessing subject brightness ranges that far exceed that and figuring out how best to record them on film for later interpretation in the darkroom or image editor and translation to a chosen output medium. Unless there is a compelling engineering reason to do so, you should not arbitrarily truncate the range - provide as much as the components will allow, and let the user decide how to apply that capability to the metering tasks at hand.

[Dave Gooding]

That is not sufficient - it's far short of the range that many negative films can record. Whether the output medium is so limited is irrelevant, as photographers often face the task of assessing subject brightness ranges that far exceed that and figuring out how best to record them on film for later interpretation in the darkroom or image editor and translation to a chosen output medium. Unless there is a compelling engineering reason to do so, you should not arbitrarily truncate the range - provide as much as the components will allow, and let the user decide how to use it.
Thanks. what sort of range can a film sensibly cover? I did run a series of 20 or 30 test exposures on Ilford HP5 as part of a calibration exercise (to calibrate a sensor against which each unit is checked and adjusted). I measured the negative intensity and plotted it against exposure. It follows an S-curve with a roughly linear region of +/- 4-ish steps in the middle but there are measurable intensity differences between consecutive exposures even at 10 stops away from the nominal, although to the eye these extreme ones have no discernible difference. Would you really operate the film at these extremes?

The scale is a trade off between allowing accurate estimation of fractional-stop variations and giving enough range. Going to +/- 5 or 6 would be no problem but at +/-10 you'd be peering at a finely-spaced scale and struggling to see quarter-stop differences.

A +/-4 range implies you are covering a range of 256x in the brightness between deepest shadow and brightest highlight - and getting the detail in these regions - in the same print. +/-6 implies a factor of 4000 across the image. Is this really true? Obviously if it is the case then I need to expand the range since it is exactly these scenarios this meter is intended for!

I'll find my film exposure test data and post it here.

[Dave Gooding]

Here are the results from my calibration test exposures.
I inserted a meter sensor in the back of the camera and measured its output at a given aperture when aimed at a grey test card in near-uniform illumination. I then ran a series of exposures at different exposure times without changing the aperture or the set up. I then measured the opacity of the developed negatives, plotted them out and thus determined the calibration data for the sensor I used during the test run. This particular sensor was then retained and is used for the check and calibration of each meter produced.
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[Dave Gooding]

Actually looking at this again I think I must agree that +/-4 is no enough and I should at least go to +/-6, more if other films give a better range.
The advice I had when I designed the meter was that +/-3 was enough but I think that was based on the print range rather than the film.
Any other opinions before I amend the design? I'll retrofit any mod into my stock as it is a very simple thing to do.

[xkaes]

What is the distance between the lens and the moth's wing?

I can tell you've never used a microscope, and apparently have done little macro work either. In MANY microscope situations, the subject and the lens are so close that oil is used to connect then together. I could photograph a butterfly's wings with a distance of 1 inch to 1/10 of a millimeter between the two. If you've got a spot meter that can somehow fit in there, PLEASE tell the photographic world!

[Oren Grad]

Thanks. what sort of range can a film sensibly cover? I did run a series of 20 or 30 test exposures on Ilford HP5 as part of a calibration exercise (to calibrate a sensor against which each unit is checked and adjusted). I measured the negative intensity and plotted it against exposure. It follows an S-curve with a roughly linear region of +/- 4-ish steps in the middle but there are measurable intensity differences between consecutive exposures even at 10 stops away from the nominal, although to the eye these extreme ones have no discernible difference. Would you really operate the film at these extremes?

If you used a single development time for your negatives, you haven't yet actually tested how long a subject brightness range (SBR) the film can usefully record. There's a whole body of technique around how to manipulate exposure and development, in light of a given film's sensitometric properties, to optimally capture any given SBR.

There are special film/developer combinations, such as T-Max 100/DI-13, and techniques, such as David Kachel's SLIMTs, that are designed to tame SBRs well beyond 10 stops to make them printable on enlarging paper. If you're going to scan rather than print directly to silver, high-DMax/long-straight-line films like T-Max 400 can capture ridiculous SBRs that, with considerable effort in digital manipulation, can be reduced to something that can go on paper.

No need to dive into the details of all that if it's not your primary interest. The take-home remains that yes, as a photographer I can use as long a brightness measurement range as you can give me.

[europanorama]

If you remember vivitar 130/LX130 160/LX160 exposure-meter in the 80ties(obsolete since bad electronics used): they offered a spot(aufsatz) dont know english term. with normal dome and flat calotte for GG-metering which i never used. but i still have it. maybe i should try with gossen variosix. maybe not working. production was stopped due to the mentioned problem. LX had motorized wheel.

[Bernice Loui]

Sinar Booster-1 and Probe FCM with Sinar metering back.
http://www.galerie-photo.com/manuels/sinar-shutter-system.pdf

What does this film plane metering system offer that is different?

Film plane metering can work in a studio setting OK, out doors these systems can bring on a host of difficulties due to light leakage to the metering probe and portability of the system, dust, dirt and more can easily cause problems.

As for microscope image capture, majority of that need today is done using a digital camera or digital video system that light meters and compensates as needed.
There was a time when film based microscope photographic systems were a significant item.

Early example like the Zeiss Photomicroscope & Ultraphot system, Wild Heerbrugg/Leica-Leitz MPS-11 system, Wild Heerbrugg M400 and others. These film photographic systems all had some sort of film plane metering system and uber low vibration shutter systems.


Bernice