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.


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.

[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.

[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.

[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.