Locus of Focus

[Manfred M]

Manfred, it just occurred to me that if one takes the maxim "fill the view with the painting", presumably to get the most out of the available pixels in the camera, then with a 100mm lens I would always be looking at having the picture occupy the full 24 degrees of the field of view. Moving back will get the rays from the object of interest closer to the axis of the lens, but only at the price of sacrificing the full use of the available pixels in the camera.

Unless your camera's sensor and the picture you are reproducing have the same aspect ratio, that is going to happen no matter what. Fill the frame is the way I would go, especially with the sensor you have on your camera.

What everything comes down to is really dependent on what you are planning to do with the images. If you are looking to display them on a website, the 8MP is going to be plenty, If you are planning to full-size inkjet prints, you are probably getting into the marginal range and if you are planning to produce twice life-size images, you won't be able to do it.

Paul has a valid concern, but that's something colour photographers have struggled with for year. Prints have to be corrected for the lighting conditions that they are going to be displayed under.

[Stasch]

Paul, I haven't had the courage yet to bring up colour matching ... you are indeed correct, that this is a huge hurdle. I am still recovering from the shock I had last fall when I took my first painting course at a community college in a nearby city (Hamilton, ON) and the instructor enlightened us about the phenomenon called "simultaneous contrast". I would prefer to call it "context dependent" since what it really means is that the colour that our brain tells us we see at a particular spot depends on the surrounding colours. There is an excellent example in Wikipedia under the heading "Brown", showing a chess board with a couple of checkers on it ... the squares they sit on appear to be of different colours, as do the checkers, but if you print it off and take a sheet of white paper and punch two holes in it, at a distance matching that of the checkers, then if you put the paper over the picture, covering up everything nearby except for the two checkers, then you see they are exactly the same colour. Some of my friends thought it was a "trick", an "optical illusion". But the truth is that it is a strong example of how our brain processes visual information and tells us what colour we are seeing. In particular our brain seems to turn up the contrast and separate colours ... this would likely have been quite useful to our ancestors living in the wild, to spot predators lurking nearby.

This was shocking because it meant that if I wanted to accurately paint a scene, I should not use the colours that my brain tells me that I see. If I am painting from a photograph, then I can use the white paper with a hole punched in it technique to see what local colours are "really" there ... If I paint what I see then the painting will likely end up being a painting that, compared to the real scene, has further colour shifts and increased contrast.

Before this bit of enlightenment I ran into problems getting the right colours on a poster for a mathematics department. The department wanted a certain antique white for the background of the text portions, and I had a colour sample to work with. But when I made a draft of the poster in Photoshop using this background colour and sent it to members of the department, they did not like the colour because it was too yellow. It could have been that their monitors were shifting colours, but now I would suspect "simultaneous contrast". Then there was the problem of getting the school's print-shop to turn out a poster with agreeable colours. On this matter we finally lucked out ... the technician set the wrong parameters for the paper type one day, and we loved the result.

So I am well aware that the road to getting good colours is going to involve compromises. One that I learned about recently was that the methods we have for reproduction of scenes, namely cameras, monitors and printing, all fall dramatically short of the dynamic range that our brain receives from nature (and paintings, etc). This has evidently led to an upswing in interest in HDR photography, which basically tries to trick our brain into believing that we are seeing a greater dynamic range than is really possible with the three above mentioned repro items. I don't know if you had HDR techniques at your disposal when you made the pictures of art in the gallery ... it might have helped (just a guess).

[pnodrog]

Stan you may want to have a read of this -http://gl.ict.usc.edu/Research/CMLR/...2003-print.pdf It is a bit theoretical but I liked the pictures. (yes I did learn to read from comics)

The technique of taking a couple of exposures was known but I did not come across the term HDR until later. The dynamic range was not really the problem. It was the fact that even taking extreme care in capturing the image you still only print a reasonable match that was dependant on the viewing conditions. The colour space of pigments in a painting certainly exceed any CMYK reproduction and act differently to lighting sources. Compromise is the word.

[Manfred M]

The problem with the poster for the math department is that you will never be able to get an on screen image to match a printed image; but if you use a colour managed process, they can be close. You are right, of course, because you have no idea as to how the monitors were set up to reproduce colours accurately.

A computer screen is an additive RGB image that utilizes transmitted light. So far as I can tell, out eyes can see some 10 million distinct colours. The relatively basic sRGB colour space can represent some 16 million colours.

A print (or a painting) is a subtractive CMYK image. While printer manufacturers do not publicise this, so far as I can understand it, a print can only have a couple of hundred thousand distinct colours. Obviously mixing paint is an analogue process, so in theory, we should be able to produce a lot more colours than we will ever get out of a printer.

Really the bottom line is the only way that the posters should have been evaluated is as a hard copy in the lighting conditions that they were going to be displayed under.

HDRI is a much abused technique today. Conceptually it was a method of bringing back shadow detail and highlight detail that was beyond th dynamic range of the camera's sensor. In the case of reproducing a painting, this is not really going to make any difference, as I wouldn't expect either condition to occur. High contrast lighting is really where HDRI shines.

[Stasch]

Some months ago I purchased a couple of 250 watt 52K fluorescent bulbs (about $40 each), but I have not used them much so far. Here is a picture that I took (late one night) using them and the CoolPix 4500 ... it is a picture of my defunct remote control for the CP 4500:



There are several things to dislike about the CP 4500 produced in 2002, mainly due to a very dated, slow internal computer chip with quite limited memory; but for a compact camera it had what I consider to be amazingly advanced features. For example, on the White Balance menu one can choose between 3 different fluorescent settings. It also has EV bracketing of 3 or 5 pictures, in 1/3 EV steps ... that is not so easy to find on most cameras today, but very useful for HDR. The above macro picture looks pretty good to me ... but I am a real novice in such matters...

[Stasch]

I read on a forum about the Sigma 70mm macro lens that some were taking photographs of sections of an artwork and stitching them together in Photoshop. That sounds like an interesting way to really up the number of pixels (if it works) ... one would surely need a method to choose an exposure setting for the whole painting, and then apply an AE lock.

[Manfred M]

In that case what I would suggest that you do is try to pick up a colour swatch somewhere; if you have a connection with the university print shop, they are bound to have one. Borrow it one night and take a test shot to see how well the colours come out.

They look like this: http://xritephoto.com/ph_product_overview.aspx?ID=1192

I will take a test shot or two with mine whenever I an shooting where colour matters and will then use the test shot to set the colour correction for the rest of the shots I took under those lighting conditions.

If the colours match up nicely, you have a workable solution, if they don't you'll know and can work on an alternative solution.

[Stasch]

There is an interesting discussion of eyesight and HDR in Michael Freeman's Light and Lighting in Digital Photography, 2007, on pages 128 and following. He gives the following dynamic ranges (in EV stops):

High-contrast scene ......................................... 16-17
16 bit/channel TIFF file ................................... 16
Human eye .................................................. .. 15
16 bit/channel Raw from a 14-bit high end CCD ... 14

Active matrix monitor ...................................... 8+
8 bit/channel file (JPEG/TIFF)............................ 8
CRT monitor .................................................. . 7-8
Glossy paper ................................................. 7-8
Matte paper .................................................. 5

As of the date of this book (2007), although some cameras were close to capturing the full dynamic range using 16-bits per channel Raw, neither the monitors nor printout came anywhere near this range.

[Stasch]

Great suggestion, Manfred. I do not have the aforementioned connection, but I will figure out some way to get my hands on a color checker!

[jcuknz]

Thanks for the insights on photographing objects under glass. Sorry to hear about the shortcomings of my old CoolPix 4500 --- I had the idea they would be good for close up photos because of stories that they were very popular at one time for both photography with microscopes as well as with various long range scopes. (I also read that the addition of converter lenses was not so successful.)

Stasch

I would guess that the popularity of the Nikon cameras came from their excellent optics and that the use of such short lenses resulted in much more depth of field than the SLRs that people had previously used. Personally I came on the scene a little later when I got my 5700 [ still available for use and respected as probably the best camera I have owned, though obviously limited Mp wise, I normally used it in 'studio' situations when later I had more than one camera ] The 5700 does have an excellent focusing system when in mid zoom and focuses in much closer than the SLRs I had used previously. There is a photo of a bug on another thread here at CiC. Results being similar to when a CU lens [500mm] is added at full zoom. So I have the option of being on top of the subject or at around 9 inches respectively.
The addition of WA converter lens to the 5000 was reported by knowledgable togs as excellent. I never bothered as I early on learnt about stitching and don't like the distortion of the short lens. The poor reports of adaptors comes from those who used the cheap ones designed for movie cameras and foisted on the unsuspecting digital user later. But those made for and by the original camera maker work well [ nikon, olympus etc]. But were expensive

In a copying situation with such small viewfinders [compared to copying with an LF camera with its ground glass screen ] I would use the longest lens the situation permitted to avoid the need for perspective adjustments later as much as possible.

As suggested elsewhere I wonder if you are searching for answers to non existing problems and should just get on with the job in hand.

[jcuknz]

I read on a forum about the Sigma 70mm macro lens that some were taking photographs of sections of an artwork and stitching them together in Photoshop. That sounds like an interesting way to really up the number of pixels (if it works) ... one would surely need a method to choose an exposure setting for the whole painting, and then apply an AE lock.

In a copying situation one normally arranges one's lighting so it is even over the whole subject though when editing with layers this is less important as one usually can adjust each layer to match the neighbours. But a lot of extra work because of laziness at the lighting stage. Different when shooting uncontrolled situations.

Yes it can work and was popular when we were using 3Mp cameras

[Stasch]

The suggestion for the longest lens possible is interesting, although I am not sure how the notion of perspective comes into play. I did get a start with my DSLR yesterday by buying the famous Canon nifty-50mm lens ...

[Stasch]

I found a website that looks promising for the kind of info that I was searching for, namely the Photozone in Germany. This site seems to be running on a small budget as it depends on supporters loaning them lenses to test; which is unfortunate because the tests are quite interesting. One that I particularly like is a distortion test based on photographing a rectangular grid about 120x80 cm and checking the resulting photo for the straightness of the grid-lines. (The other tests I haven't digested yet .)

The EF 50 mm and 100 mm Canon lenses, both normal and macro, are rated quite highly on this test, using a mixture of Full Frame and APS-C format camera bodies for the tests. This leads me to wonder: Is there any real advantage, beyond making more room for the lighting equipment, to getting more than the cheap nifty-50mm f1.8 II lens (which I bought yesterday) for photographing artwork?

[Manfred M]

I found a website that looks promising for the kind of info that I was searching for, namely the Photozone in Germany. This site seems to be running on a small budget as it depends on supporters loaning them lenses to test; which is unfortunate because the tests are quite interesting. One that I particularly like is a distortion test based on photographing a rectangular grid about 120x80 cm and checking the resulting photo for the straightness of the grid-lines. (The other tests I haven't digested yet .)

The EF 50 mm and 100 mm Canon lenses, both normal and macro, are rated quite highly on this test, using a mixture of Full Frame and APS-C format camera bodies for the tests. This leads me to wonder: Is there any real advantage, beyond making more room for the lighting equipment, to getting more than the cheap nifty-50mm f1.8 II lens (which I bought yesterday) for photographing artwork?

Try it and see how things work out. In my case, the 50mm lens would be too short, with my full-frame camera as I would get to close to the picture being photographed and this will start impacting the light, You certainly won't lose anything trying it..

[Stasch]

Manfred, thanks for all the helpful advice (likewise to the others who responded). In particular the examples of photography of artwork that you posted were impressive and persuasive ... they have been coming back to mind over and over these past few days. The advice that was most difficult to come to terms with was the value/necessity of stopping the lens down to get depth of field --- the idea of having a lovely fast lens that one cannot use as a fast lens when photographing artwork just didn't sit well with me. But reality eventually won out, and the need for careful DOF calculations faded away. From what I read at the Photozone site, a few stops down from wide-open can reduce the distortion as well as increase the DOF. Maybe I should learn to think of my 50 mm f/1.8 lens as "really a f/5.6 lens"....

Stopping down may mean that I will need to acquire some more lighting if I want to enjoy the softer lighting via something like a soft-box. I find it strange that camera stores do not seem to sell the white soft-box fabric so one can build one's own version of a soft-box ... I am pretty handy with a hammer, saw and nails, and can imagine putting together a workable in-house contraption for lighting, like building a box frame around the place where the art is to be mounted for photographing, and covering it with said fabric. Since my photography is just a personal hobby, I don't need elegant, light-weight soft-boxes that are designed for mobility, and to make a good impression on ones clients ... I wouldn't mind having them, but for my purposes it seems the $$ could be better spent on more exciting aspects of photography (like saving for that special lens, and perhaps even a dream camera body with more megapixels, etc.).

[Manfred M]

Well, your f/1.8 50mm is still a reasonably fast lens and there is nothing to prevent you from shooting wide open for other things. I primarily choose the DoF that I want and then adjust shutter speed and possibly ISO to get what I want. For things like portraiture, I tend to shoot darn close to wide open so as to ensure an in-focus face, but a nicely blurred background.

Photo stores sell softboxes, not supplies that let you make your own. If you want make a diffuser; especially if you are looking at a cool light source, consider using a white vinyl shower curtain liner. Nice and cheap at your local dollar store....

[Glenn NK]

The suggestion for the longest lens possible is interesting, although I am not sure how the notion of perspective comes into play. I did get a start with my DSLR yesterday by buying the famous Canon nifty-50mm lens ...

Perspective is not affected by the focal length of the lens - it is only affected by the distance from the object(s) to the image.

For a flat two dimensional surface, it doesn't matter what the distance is - perspective is not a concern because there is none.

As for "flat field" lenses, some tests do check this out:

http://www.photozone.de/canon_eos_ff...0f28ff?start=1

In addition to its distortion at both ends, this popular Canon zoom lens does not have a flat field of focus (Field Curvature).

Glenn

PS For what it's worth, I'm in complete agreement with Manfred's comments.

[Stasch]

Glenn, thanks for the link to the field curvature example ... indeed my fears would have been confirmed in this case, that the "locus of focus", i.e., the "field of focus" has a "field curvature". What I got wrong was that I expected the curvature to be in the other direction, towards the camera.

This field curvature does not seem to be significant with the prime lenses mentioned for photographing 2D artwork, at least at the single focal distance used for photographing the 120 x 80 cm grid in the Photozone reviews (unfortunately they only did this for the one distance, so there may still be concerns ... will the lenses work well closer up?)

I wish reviews of lenses would publish diagrams like the one in Photozone, but go further and shade in a cross section of all the region that would be "in focus", what I like to call the "locus of focus", instead of just a curved line, since there will be a depth of focus all along the curved line. That way one could tell if focusing on the corners (as suggested in the Photozone article) would put the center, and hence the entire scene, into the field of focus. In the hypothetical example pictured here (based on the diagram from Photozone)



one has such a shallow DOF along the field that focusing at the corners will definitely put the center out of focus ... it seems that there is no way, with the given camera settings (and hence with the given field of focus), to put everything in the plane into focus.

Stan

[Manfred M]

Im that case Stan, you'll have to explain to me why there are so many excellent reproductions of works of art available all over the place. They were taken with real cameras using setup very like the ones that have been posted here previously.

In reality, the issue is this is a minor issue; getting your camera lined up properly so that the optical axis is perpendicular to the plane of the artwork is a real world problem that will have a much greater impact on a sharp image than this. Try calculating how much of the image will be out of focus if your camera alignment is off by a few degrees. Thankfully DoF is going to compensate there too. Of course, we are also assuming that the artwork is perfectly flat too; again that is not a realistic assumtion either; as any form of mounting is going lead to some degree of deformation; either sagging or bowing.

[pnodrog]

As I said earlier taking the photograph was not the problem. It is about 12 years since I was doing it but I am reasonably certain I was using a Nikkor 24-120mm at about 80mm to 100mm depending on size of artwork at f5.6 or f8. Full frame 35mm with velvia 100 film. From memory the camera was on a tripod between 2 and 4 metres away. Getting the artwork fully in focus was never a problem however if I went beyond 100mm there was a bit of distortion (from memory I think it was barrel). It was all before the camera settings were saved on file. I never totally filled the frame with the image as I wanted to avoid any softness or vignetting.

I cannot remember if you have indicated in what form or size you are going to output the images you are going to so fastidiously take. Sorry but I think you are not grappling with the what will be your main problem.