Shooting paintings and other flat, rectangular objects

RAH

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I volunteer at a small museum and sometimes I have to photograph paintings. I am probably going to get the new E-M5III to use for this, figuring High Res mode would be great. I have been using a Canon 80D mostly, mainly because it has more resolution than my 16mp E-M10II.

Anyway, my question is: are there any tricks for photographing flat objects like a painting to get them exactly square - i.e. exactly at a right-angle to the plane of the camera (so the left and right side are the same distance), the camera itself is square to the plane of the painting and also in the middle of the painting, etc.?

I have been using the technique of photographing straight down, with the camera mounted on a tripod with a lateral arm and a hot-shoe level in the camera's hot-shoe. This works very well, because you can then be fairly certain that the painting is exactly flat and so is the camera (adjusting with the hot-shoe level). So the only slight problem is getting the camera in the exact middle of the painting, but that's pretty simple and doesn't have to be exact anyway (you always have to crop some).

The big problem with this technique is that if the painting is large, it is difficult to get the camera high enough to get the entire painting in the shot, unless you use a very wide lens and that can give you distortion. And the lighting can get tricky shooting like this too. You can get shadows from the tripod legs unless you shoot straight down from inside the legs with a reversed center column, and then getting high enough is very hard.

So, it would be MUCH easier to just shoot horizontally - the painting on the wall or on an easel or the like, and just positioning yourself in front of it with the camera on a tripod, etc. Kind of obvious. But it seems to be MUCH harder under such conditions to get everything square, whereas it is a piece of cake getting things square shooting down.

So, anyone have suggestions on how to shoot square horizontally? Maybe a laser level or something tech like that? :)
 
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I do this for a business, reproducing paintings for artists who then sell giclee prints in their galleries. I'm using the hi-res mode on my EM-1 II which works great for this. I only shoot outdoors very early in the morning before the sun is above the horizon which gives me very flat even light, photographing from the front as some of the paintings I work with are very large. I visually try and get everything as square as I possibly can but it's never prefect. That's where photoshop comes in, using the skew tool and dropping a guide on each side it's very easy to correct for any lack of squareness. Been doing it like this for close to 20 years now and have never had a complaint.
 

RAH

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Thanks for the feedback, Highlander. I have used the skew tool in PaintShop Pro the same way and, as you say, it is very effective. But I do wish I could nail it without doing that. I just kind of wonder if that correction is changing the internal proportions in the painting some without detecting it. But, as you also say, no one ever complains, so I guess I am just worrying for nothing. It's good to hear that high-res mode works well for this. I can't wait to get the E-M5III! (I may be the only one saying this, it seems - ;-) )Thanks!
 

archaeopteryx

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I just kind of wonder if that correction is changing the internal proportions in the painting some without detecting it.
It's not, within the limitations of interpolating to the subject size of the pixels (and barring some bug in the transform, though it's not a difficult code to write or review the maths of). It's likely the accuracy with which you can define the transform and the residuals from lens distortion correction are larger sources of error.

So, anyone have suggestions on how to shoot square horizontally?
The cheap way is to level the painting, measure equal radii from its lower two corners with a tape or two, and set the camera up over the point where those two arcs intersect. Ideally, the lens's no parallax point would be over the intersection. It's a bit fussy to ensure the arcs are in the same plane but this method allows the walls and floor to be not perfectly flat or perpendicular to each other and for the painting to be not perfectly flat to the wall. The painting is probably not perfectly flat or square to itself so you may end up with a skew transform anyway, though.

You can extend this exercise to a triangulation to square the camera to the painting vertically as well as horizontally, since the painting is probably not perfectly vertical either.
 

RAH

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One final (?) question about shooting a painting horizontally - what's a good focal length to use? The thing is, if you shoot straight down, your fl is pretty much dictated by how large that painting is and how high above it you can get (i.e. how tall is your tripod). But shooting horizontally, you can use just about anything, it seems.

I mean, you could obviously use a wide lens and stand close, or a higher fl and move back some. I never really know which is better, assuming all the lenses allow you to conveniently position any lights you want to use. I'm thinking that this might be an even more important consideration if I get an E-M5III and use High-res mode.

I even bought a used PL 15mm f1.7 lens anticipating shooting straight down, but now I'm thinking that it might be better (and certainly easier) to shoot horizontally with say my Oly 60mm 2.8 macro (a macro lens is often sharpest, I think). Of course, I could still use the 15mm and just stand closer. A zoom lens would obviously be even easier, so perhaps use my PL 12-35 f2.8 or PL 12-60 f2.8-4 (at what distance??)?

So many lenses, so little time! Ultimately I suppose it doesn't matter too much, but with shooting a painting, you obviously want the edges to be sharp too, whereas with say landscape photography it isn't quite so important, IMHO (f4-5.6 seems to be the sharpest m43 aperture, from what I have read; guess I'd use f4). Any thoughts?
 
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I try and stick to 50mm (25mm in m4/3) for a focal length. I use my Oly 12-40 Pro set at 25mm and move the tripod to a spot where the painting fills the frame. I know some will say stick to a prime lens and that's sound advice but I can't quibble with my results from the 12-40 set at 25mm, everything is sharp right to the edges, I use aperture priority at f/5.6 and bracket the exposure. Sometimes I get lazy and adjust the zoom instead of moving the tripod but I don't stray too far from 25 mm, especially towards the wide end. I suppose the 45mm or 75mm would work just as well if you have enough room to get far enough back.
 

archaeopteryx

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(f4-5.6 seems to be the sharpest m43 aperture, from what I have read; guess I'd use f4)
Where possible, refer to tests of the lenses in question and allow for copy variation. Particularly with high resolution modes, you're looking for a level of optimization which calls for testing your lenses and the results will, in turn, be contingent on interactions between painting planarity, alignment, field flatness, depth of field, and extent of diffraction. I'm not aware of any m43 lenses which test sharpest outside of the range of f/2.8 to f/8 for magnifications below 1x using 12-20 MP.

Higher MP sensors and high resolution images are both likely to widen optimum apertures, though it depends on lens behaviour. I've mentioned about that somewhat in this thread, this one, and a few others.

Any thoughts?
Your proposed set of tests is reasonable given the questions you have so I don't see any reason not to proceed with collecting and examining the images. The data from doing so will provide more specific answers for your objectives than someone typing on a keyboard somewhere else. I can offer a few observations, though:
  • You'll find many threads here discussing image quality requirements as a function of image use. If you haven't already, I would suggest reviewing them with respect to whatever your plans are, particularly print size. This may help in efficiently allocating effort.
  • Roger Cicala's blog contains considerable detail on behavior of lenses with respect to best individual focus and field curvature and he's lately tested some macro lenses at finite conjugates as well. A look through what he calls the geek posts should provide a clearer sense of what to look for in test images and a baseline understanding of copy and focal length variation.
  • If your images include the paintings' frames, consider the frame shapes and level of perspective distortion that's acceptable.
  • Longer lenses are less sensitive to absolute x, y, and z positioning errors for alignment. Wider lenses are less sensitive to roll, pitch, and yaw error. Which set of errors is most influential will depend on your process, so the optimum tradeoff will as well.
  • In my experience of producing copy images of paintings, colour accuracy and control of white balance and lighting has been considerably more important than pixel peeping details of lens properties.

(a macro lens is often sharpest, I think)
Depends on the macro and the focus distance. Very loosely, non-macro lenses are typically optimized for infinity focus or something close to it. Macros are hopefully optimized for a closer focus but very little testing has been done to characterize the behavior. Most lens testing simply ignores that they're macros and proceeds with something near infinity focus. So you may wish to also evaluate your lenses across a range of painting sizes, especially if the museum's collection includes many small ones.

I can't speak to the Olympus 60 as it's not a lens I've used but, for example, my copy of the Panasonic 12-60 f/3.5-5.6 at 45 mm outresolves my copy of the Panasonic-Leica 45 f/2.8 macro if one's pixel peeping low magnification images. However, there is no functional difference whatsoever for basically any image use besides posting 100% crops to photography forums. That's not an activity I'd anticipate the museum curator or patrons to be heavily engaged in.
 
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RAH

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Interesting tips, guys. I think I'm leaning towards using my P20 f1.7; or perhaps my PL 12-60 (set around 25mm as suggested by Highlander).

Concerning your suggestion of 25mm, Highlander, I assume you are thinking of using a fl that is likely to give the least distortion, right (I mean, 50mm equiv being not wide and lot long)? Good idea, I think.

The funny thing in all of this is that the next painting I have to photograph is a pretty blurry painting - so the only thing that's sharp if you get up close to it is the actual grain of the canvas; the painted features are not sharp at all. And here I am worrying about which lens is sharpest and what aperture to use! Anyway, thanks folks!
 

archaeopteryx

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a fl that is likely to give the least distortion
Lens distortion is routinely evaluated in lens tests, so one can look at data instead of guessing (for example, at 25 mm the Panasonic-Leica 12-60 is one to three orders of magnitude above minimum evaluated distortion depending on processing path; if you want low lens distortion I'd also recommend considering measurements of the 20 f/1.7). Minimum perspective distortion occurs with telecentric lenses but I believe you'll find them impractical for most painting sizes. ;) A more practical approach to minimizing it is the longest focal length that's convenient given the painting size and working room. Again a matter of testing and selecting among the more useful tradeoffs. Personally I prefer short tele if it's an option but space constraints with larger paintings can mean normals or wides and I've occasionally needed ultrawides (though that was before stitching).
 
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RAH

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A more practical approach to minimizing it is the longest focal length that's convenient given the painting size and working room. Again a matter of testing and selecting among the more useful tradeoffs.
OK, it's a small painting and I have a big room - sooo, um, my Canon 80D with 400mm f5.6 lens! :dance2:
 

RAH

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Hey, now that's what I'm talking about! That seems like it would be perfect - I got a mirror out and tried it and well, of course, it works as you say - there's no way for it not to work! I do think it would sometimes be hard to implement. I mean, for example, a picture hanging on a wall. But it absolutely seems like it would be the preferred method to try to use first. You could even mark the center on the mirror, which I think would help. Thanks very much Hannety!
 

archaeopteryx

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I do think it would sometimes be hard to implement.
I would agree. If the curator of the museum you volunteer for is anything like the ones I've worked with you're not going to be allowed to risk damaging a painting by placing a mirror on it. In some cases you may not be allowed to move the painting, either. If it is permitted, I'd suggest carefully thinking through the logistics of associated with aligning to a mirror and ensuring the resulting registration matches that of the painting. Some of the things likely to be problematic are
  • ensuring the mirror's centre point is the same as the painting's centre point
  • ensuring the mirror's angle matches the painting in noncontact mounts or if mirror and painting are being exchanged
  • ensuring the painting isn't damaged in contact mounts
  • either keeping the mirror small enough its weight doesn't perturb the hanging angle or matching the mirror's weight and mounting to that of the painting
It may be useful to reconsider what alignment problem you're attempting to solve which isn't already addressed. For example, suppose you've a mirror of exactly the painting's size and are in a horizontal setup so that centre and mounting matching is easy. Is the resulting alignment going to be enough better than what you can get from aligning an EVF/LCD grid to an outline and centre taped on the floor and that it's worth getting mirrored acrylic or such and cutting it to match? With the painting out of the way you could also mark an offset centre and drop a plumb to it from the tripod for additional accuracy. (Which I suspect isn't something that's been considered, but it seems like less work. There's also no shortage of Arca options which could be used to avoid an offset mark.)

That said, I'm not sure why one wouldn't mark the centre of a mirror if using one. Centreing is intrinsic to every form of alignment target I can think of offhand, though for some targets the centre is imputed.
 

RAH

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Yes, I agree with the issues you mentioned, archaeopteryx. But I do think it is worthwhile in a lot of casees. The painting I mentioned that I have to photograph would work nicely, i think. It's fairly small, I can place it on a table tilted back somewhat and can probably place a mirror in front of it, touching its frame pretty accurately. It will be worth a try.
You mentioned "mirrored acrylic" - I was wondering if there are any VERY lightweight reflective surfaces (like cardboard) available. I guess I could look in a craft store. You don't need a perfect refelection - just good enough to make out your self and the front of the lens.

Edit: I just did a search and found some reflective "mirror" boards (8.5 x 11"), very cheap. Worth a look!
 

Hannety

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You mentioned "mirrored acrylic"
You can get cheap adhesive backed acrylic mirror tiles on Amazon. I have some about 6" square which weigh about 7g each. They are flexible so don't sit perfectly flat on their own.

If you trust the frames to be true and they hang flat you could put a mirror on a wall or easel for alignment then cover with the artwork?
 

archaeopteryx

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I was wondering if there are any VERY lightweight reflective surfaces (like cardboard) available.
You've just described a range of materials such as aluminized polyester and some forms mylar. So now you can figure out how to construct a tensile arrangement which provides the co-planarity necessary for good alignment. ;)
 

RAH

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