Illumination requirements for RTI image capture of gravestones

[jonpat]

Reading weathered gravestones is a difficult task. Recording images using RTi seems the appropriate method to interpret worn and faded text. However it opens up a number of questions about how to properly take the images:

1. What lumen should the light source have given picture are taken in daylight? Can the lumens be too high?

2. What plane should the lightsource spheres be place in given the stone of vertical? Sit on top of the stone/attached to the face of the stone?

3. Modern torches can have an adjustable field of focus - should the light source always illuminate the whole picture frame or should it be focused narrowly and move across the lettering.

Jon O'grady

Melbourne, Australia

[Dave Martin]

Hi Jon,
Yes, RTI can be a great help on weathered gravestone inscriptions:

Q1. What lumen should the light source have given picture are taken in daylight? Can the lumens be too high?
A:   You need the directional light to be many many times brighter than the ambient illumination. A continuous light (be it LED or halogen etc.) is unlikely to be adequate/appropriate. A powerful flash is usually the answer outdoors; but to reduce the ambient impact you need a very strong ND (neutral density) filter on your lens.

Q2. What plane should the lightsource spheres be place in given the stone of vertical? Sit on top of the stone/attached to the face of the stone?
A:   The spheres need to be in the same plane, for an upright slab I would typically have two, either side of the slab, so that the plane of the slab cuts the middle of the sphere. I have several spheres that I have drilled and tapped with 3/8" thread as used for camera kit and then mount the spheres on lightstand arms.

Q3. Modern torches can have an adjustable field of focus - should the light source always illuminate the whole picture frame or should it be focused narrowly and move across the lettering.
A:   The illumination needs to be point source but not constrained light like a narrow torch beam; also the intensity of a torch beam will fall-off as you get off-axis, which likely lead to sub-optimal results.

Dave

[jonpat]

Hi Dave, thanks very much for your answers - most helpful. Hopefully I can ask your assistance in subsequent questions. I need a highly portable configuration of equipment  as I expect to be covering 2-4 graveyards in a day. Initially I will work in Sussex rural graveyards.

1. What did you use for spheres. I can't imagine you drilled holes through Xmas decorations.

2. Based on the need to have much higher lumens that sunlight it seems a i need a lightsource of 16k to 32k lumens - yes?

3. I note your specification for a neutral filter.

[Dave Martin]

Hi Jon,

More to follow on equipment, but firstly, I’m a little curious as to your plans to RTI capture 2-4 graveyards in a day - unless they’re just a small number of monuments in each, then I think that might be optimistic. Bear in mind that for each slab you have to (1) Carefully position the sphere(s) on their mounting appropriately; (2) Setup your camera on its tripod, frame the shot so camera is looking at the centre of the slab perpendicularly and includes all of the slab and the sphere(s), and focus (always manual, never AF); (3) Take the photographs (you’re probably looking at a minimum of 25-30); (4) Depending on how you plan on separating the myriad of photographs into one folder per monument, either at the start or end of the batch (or both) for each monument you may at a minimum want to shoot an image in the set of a blank card or a ‘slate’ with monument plot number or similar.

Then, periodically, you’ll need to stop and quality-check the images you’re acquiring; and of course pre-preemptively change camera batteries (if it runs-out part way through a set, you need to discard the set and start again because the movement associated with changing battery will mean not all images have exactly the same camera position). You'll also have to change flash batteries / battery packs; and you may have to let the flash'rest' / cool down after a few hundred shots (or have two flash units that you alternate between).

I would suggest throughput is more likely to be measured in tens of monuments per day; a slick operation, with an assistant, might just crack a hundred in a long day?

Dave

[Dave Martin]

Camera tripod

Unless it is as perfectly laid out and tended as a Commonwealth War Graves cemetery, getting the camera in the correct position to frame the different monuments can be tedious. Plots can be different heights, different surfaces, kerbed or otherwise, …

Adjusting a tripod can take a surprising amount of time. You might think a pan/tilt head would be useful – and it would IF the tripod is level, but it won’t be and it is not worth the faff to level the tripod for each monument plot. A ball head helps, but then you’re usually having to use two hands – one to move and prevent the camera toppling, and the other to lock/unlock the ball head. What I found great for this purpose was a Squeeze-grip Manfrotto 222 head – you can do it all with one hand – squeeze the grip and adjust, let go and it locks, and you can adjust the drag.

A cheap bull’s-eye level on top of the camera (or otherwise suitably affixed) is a useful aid to setting the camera.

You still have the problem of accommodating monuments which significantly differing height. You may have no alternative but to adjust the tripod leg lengths. One thing I found useful though was having a tripod with a centre column that can be moved up & down by just using a thumb on a spring lock lever. I have much better tripods, but for this feature alone I used to use an old Cullman 2903.

For monuments, to make best use of every pixel you’ll probably be shooting mostly in Portrait, so unless your camera is in a cage you’ll need an L-bracket or similar (don’t reply on hanging it off to one side using the ball head as even the slightest droop will spoil the RTI set).

To help with setting the camera for each monument, and focusing, it is extremely useful if you have a monitor which you can view from above. Whilst my cameras (Sony Alpha) have modest rear screens with some degree of articulation, I found it was worth mounting a remote HDMI monitor (mine is 7-inch diagonal) to one side of the camera, via a small ‘magic arm’. The monitor’s weight was counterbalanced on the other side of the tripod by the USB ‘power bank’ I used to power the camera via a dummy battery / battery eliminator.

Separate message to follow later re spheres and light sources.

Dave

[jonpat]

Thanks Dave, That is incredible experience. The one aspect I didn't explain is that I am not doing a whole graveyard but just a few monuments in each graveyard. I'm not expecting to do more than 6 monuments a day.

 

[jonpat]

Hi Dave, I have been tinker ing about the camera settings for the photographs.

It seems to me the aim is to set the f-stop first as low as possible, the ISO highest level and then the highest shutter speed. Would that be correct?

thanks

Jon

[Dave Martin]

Spheres

Jon, you mentioned baubles (in jest) but they are unlikely to be up to RTI! Apart from mounting issues, they are unlikely to be sufficiently spherical.

Requirements are for a sphere (or, more accurately, a hemisphere) which is geometrically spherical, and has a black (or dark) uniform smooth/polished/glossy surface.

For objects of the size of gravestones, black snooker balls are ideal, and they’re cheap enough to be replaced if they get scratched.

Mounting the Spheres

It needs mounting so that, ideally, the centre of the sphere or the equator of the hemisphere is in the plane of the surface you’re imaging. If you’re imaging something where you can have the spheres to the side then that’s relatively easy, it’s a bit harder of you have to position a sphere in the middle of an artefact.

If the monument is recumbent but proud of the ground, I will just place a snooker ball either side of the stone, and pack it up to a suitable height. If it needs help to stop it rolling away, I use either a brass ‘cup washer’ that it used in furniture (from e.g. B&Q) or the cardboard cylinder centre from a roll of insulating tape. Don’t be tempted to use ‘BluTac’ as the residue it leaves on the surface, if on the upper surface next time, can spoil the results.

I have also put a flat spot on a snooker ball  by hand-held sanding against linisher belt (don’t be tempted to put it in a vice without very good protection!).

To mount them on ‘arms’ I drilled and tapped the snooker balls. I started by tapping standard ¼” as used for basic photographic kit, but I found that didn’t fare well (short, fragile thread in the ball material). I then tried 3/8” – worked better, but thread started to wear. I thought about several alternatives such as gluing-in a length of 3/8” studding so I could use a coupler, but what I ended up doing was gluing a standard photographic 3/8” to ¼” reducer which worked almost the same way as a Helicoil insert I would use in Acetal or aluminium.

Supporting the Spheres

Photographic lightstands are ideal for use with vertical gravestones.

You’re ideally looking for an un-interrupted hemisphere facing the same way as the face of the slab, so ideally site the lightstands behind either side of the monument and then bring the arm forward until the centre of the sphere is in the same plane as the face of the monument.

To prevent distracting highlights from the lightstand arm, I sleeve the end nearest the sphere in matt black – in fact I literally use old black socks, held in place with black shoelaces.

Whatever you do, don’t be tempted to use flexible goosenecks – they will droop!

Multiple spheres

Even though most processing still only uses one sphere, I will always use two if I can. Partly future-proofing in case dual-sphere processing in future, but mainly as a precaution in case one of the spheres moves – e.g. nudge when moving the flash around, or brush with clothes, or lightstand slowly sinking on grass. Wind can cause things to move, and whilst you can suspend a small ballast (sand) bag from the centre of the lightstand tripod, you need to beware of the wind causing movement of the sphere on the end of the lightstand arm.

[Dave Martin]

Illumination

For successful RTI, you need a sole (or dominant) isotropic point light source with constant intensity (it only needs to be isotropic within the cone that will hit the subject/monument so maybe a 90 degree cone is enough).

Indoor, with no/low ambient light, you can use a studio lamp of some sort.

Outdoor, in daylight, you have very limited options other than a powerful flash.

You have mentioned modern (LED) torches but I really don’t think they will work.

Firstly, it needs to be bright enough to ‘overpower’ the ambient light, and I don’t believe there are many (any?) LED sources that meet the needs of RTI and are price-competitive with a suitable flash.

Most powerful LED lamps adopt measures which render them unsuitable for RTI – they either have multiple LED chips/cobs and/or use reflectors. Either of those measures mean that it is no longer a point source, and also that it is no longer isotropic. Multiple LEDs = multiple sources. Reflector means source broader – e.g. a torch with a 100mm reflector effectively means a 100mm dia source - again, no longer a point source. A reflector also means no longer isotropic as light is stronger in the ‘beam’.

[Dave Martin]

Camera setup

Re:

On 11/20/2024 at 4:11 AM, jonpat said:

.... camera settings for the photographs. It seems to me the aim is to set the f-stop first as low as possible, the ISO highest level and then the highest shutter speed. Would that be correct?  ....

Steps:

1. Remove ND Filter.
2. Frame the shot.
3. Focus (using centre of image).
4. Adjust F-stop so you have sufficient DOF to get the whole monument face in focus. Do not stop the lens down any further than you have to in order to get everything in focus.
5. Replace ND.
6. Set shutter speed as appropriate for your flash.
7. Adjust ISO if necessary to get sufficient capture, don't use ISO any higher than you have to to avoid introducing any more noise than you have to.

Don't use auto-focus / auto-exposure / auto-WB. Conceivably you may get away with auto-ISO.

[jonpat]

Dave, thanks for all the information. I now have  to complete my setup and go out and practice. I will get back to you once I've competed some experiments in the New year.