I have been trying to photograph some of my oil paintings, so that I can have the 5x4 transparencies scanned onto digital. I thought that, assuming everything was square and level, if I focused sharply on one part of the image, the rest would also be in sharp focus.? Evidently, this is not the case, - I'm getting scans back where the centre of the image is perfect but other areas, especially out towards the edges, are very blurred.
Any advice please, on how to cure this.?
Thanks.
Sounds like you are doing it correctly: front and rear standards parallel and parallel to the artwork.
What lens and aperture are you using? It seems you are not using a small enough aperture.
If you are certain that your camera and subject are perfectly square and lined up, it sounds like your lens doesn't have enough coverage for the size of your subject.
Try closing your lens down to at least f22 and using a slower shutter speed. You will probably need to use more light. If that's not possible, use a faster speed film.
Longer process lenses such as APO Nikkor and APO Artar stopped down might give you better center to edge sharpness for your type of application.
Zheng L.
Istead of focusing in the middle of the painting, which is clodsest to the lens, you could take more advantage of DOF if you focus at the 1/3 mark or so on the long axis. This will be mid way distance ( from your lens) between the centre of the painting and the corner. If you figure the distances you can choose the required aperture and DOF.
Regards
Bill
I agree, it sounds like your lens does not have a flat field at the distance you are working. To try and make it work better try focusing on the edge, then the center and split the distance on your focusing standard. Then stop down and see if it is any better.
Possible problems, ranked in order of what I think might be most likely (YMMV).Originally Posted by Jim Conaghan
... I thought that, assuming everything was square and level, if I focused sharply on one part of the image, the rest would also be in sharp focus.? Evidently, this is not the case, - I'm getting scans back where the centre of the image is perfect but other areas, especially out towards the edges, are very blurred.
Any advice please, on how to cure this.?
Thanks.
1) You're really not orthogonal to surface you are photographing (you mention other areas but don't confirm that the focusing fall-off is uniform in all directions). Tolerance is pretty tight in close focusing. That's why document copiers for smaller format cameras used a column like an enlarger has...to keep the centerline through the lens.
2) You haven't stopped down enough to get the depth of field needed across the image (see 4)
3) You have a lens that has some shutter damage that isn't otherwise noticeable (this does occur in 0 shutters but not often in copal 1 or 3). I've had this happen on a 0 lens before.... Frankly if you are using a 0 lens, you're probably using a fairly short focal length and this lens will have problems with #4. 0 shutter size has always been mechanically weak for hold alignment in copals....
4) Not all lenses are created equal for copying flat fields at close distances. If in doubt, borrow a process lens with shutter. A good APO Artar (cheap!) or Red Dot (even if optimzed for 1:10) might help you solve it if this is your problem. Borrow from a friend if possible.
5) One more effect if you are using a longish exposure (3 sec or longer). Film "pops" in long exposures if you pull the dark slide and immediately try to expose without waiting for it to equalize with the temperature and humidity inside the bellows. John Sexton pointed this out on longish exposures with landscape images.... the effect is random areas with blurs as the film in that smallish region pops in/out of focus during the exposure. Its more prevalent with 8x10 or ulf than 4x5 but John had a couple of good 4x5 examples at a workshop back in 1991.
....John recommended before long exposures to pull the darkslide, wait a minute, then lightly tap the holder in the camera back to give an impulse to allow the film to normalize. Then make the longer exposure. Frankly I was skeptical about this until I saw it on a couple of my aspen exposures up in Castle Creek....then I became a convert.
...and there is an expensive alternative if you can find it -- Sinar made 4x5 and 8x10 holders that had a sticky material on the inner septum; your film was lightly "glued" to the back to ensure it didn't move or pop during long exposures. I looked into these and they were very expensive when still available....needless to say the Sexton tapping method won on price-performance merit.
Note that the effect of 5 is random out of focus regions in the image, with a random distribution and random shapes.
Stop down too far and you will be in diffraction and still have a problem. Use a lens designed for the job. A process lens for flat field art reproduction on 4x5 is optimized for use at f22 only.
I'm betting your lens has a non flat field. Pretty much any modern 150 LF lens from Rodenstock, Schneider or Nikon will give you surprisingly good results. Modern meaning within the last 15 years or so. A dedicated process or macro lens might boost the quality a bit, but the garden variety current major maker 150 is amazingly good. I am not sure I could see the difference in anything but a big enlargement.
I'm an artist, and shoot my work using a 4x5 with a rollfilm back. I use a Caltar/Rodenstock 150 / 5.6 and am totally pleased with the results. I level the camera with a bubble level every shoot. I stop down to 11 or smaller. I use a magnifier to make sure I'm well focussed. I use an spotlight to brighten a place on my painting so I can see well enough to focus precisely. The spot gets turned off for the actual exposure.
I found that it took a while of shooting and making incremental adjustments to my process to get consistently good results. Shooting artwork demands routine and complete attention to detail.
Best,
C
Many thanks to all who took the time to reply to my query, your advice is very much appreciated - there's certainly a few aspects here that I will have to look into.
For those who asked, the lens used was a Nikon Nikkor-W 210mm 5.6, - Copal #1, which was recommended for this type of use.
Thanks again,
JC.
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