Dof

[Thompsonr]

There is a lot of very technical info on this forum on DOF which looks really interesting. However I'm rather under the gun. I just photographed some oil paintings (60"x48") with my 4x5 and a 150 mm lens and studio flashes. The picture was about 12 ft from the camera. The result was beautiful in the centre but very soft around the edges. I assume this is a DOF problem and not a lens problem. We're talking 3-4 in difference in the range of the centre and the top and bottom of the picture.
- One solution is to reduce the aperture and open the shutter and pop the flash multiple times. Prefer not to do this.
- I could use my 210 mm lens but not sure how much difference this will make to DOF

Is there a source of DOF easy to comprehend equations or a table for a 4x5 DOF?

I can't think of any other solution.

Does anyone have ideas please.

Many thanks
Raph

[Dan Fromm]

Not DoF, because you're trying to image a plane (the painting) on to a plane (the film). Coverage.

What lens? What shooting aperture? Film plane parallel to the painting?

Changing focal length won't change anything.

[Thompsonr]

Shot at f11 with a circular polarizer to eliminate the reflections (very glossy and textured). Studio lights full blast.
The lens is a Caltar 11-E, 1:6.3 150mm.
I also have a Caltar ii - N, 1:5.6, 210mm
B&W circurar polarizer (could this be the problem? I have a linear polarizer, cokin unfortunately)
Yes the film plane and the picture were parallel and the camera dead centre.

[Brian Ellis]

I don't understand what you mean by "we're talking 3-4 in difference in the range of the centre and the top and bottom of the picture."


Apart from that, leaving everything else the same but going from a 150mm lens to a 210mm wouldn't increase depth of field, it would reduce depth of field (the longer the focal length lens the shallower the depth of field, everything else remaining equal) though with a painting depth of field isn't usually a problem.

[Sheldon N]

At f/11, you are likely seeing the effects of resolution falloff towards the edges of the image circle. Stopping down to f/22 may improve this, assuming you have enough flash power.

Here's some testing info from Chris Perez's website. Each column shows the measured lp/mm resolution from the center, midpoint edge and extreme edge.

Rodenstock Geronar 150mm
f/11 60 38 24
f/16 76 48 34
f/22 48 54 48

[Peter K]

Circular polarizers are specially for SLR-cameras with TTL-measurement, but there is no difference with a LF camera if you use a linear or a circulare polarizer. Exept the filter is dirty, scratched or so.

But to avoid reflections from the texture you have to illuminate the painting with polarized light and with the polarizer on your lens you can choose the amount on texture to reach the film.

As Dan mentioned before, Dof is no isue with photographing paintings.

But to avoid

[John O'Connell]

- I could use my 210 mm lens but not sure how much difference this will make to DOF


Try the 210mm lens---since you're shooting an object of fixed size to a fixed reproduction ratio on the film, DOF should be identical with both lenses.

[Thompsonr]

Tks for all your thoughts.
Sheldo, I checked Chris web site, very interesting. If I interpret your extract correctly,
(Rodenstock Geronar 150mm
f/11 60 38 24 ) I'd be getting less than half the resolution at the edges? Thats not so good.
I've posted the image at
http://www.pbase.com/photoimagerie/image/95782714/original
Any further thoughts would be appreciated

Many thanks for all your help

[jmcd]

I think a G-claron with a flat field would be perfect for this sort of thing.

You can of course use multiple pops with your flash so you can stop down to f22 or so for more improved use of your image circle.

[Peter K]

Try it with your 210mm lens. 150mm is much too short for 4x5" and this kind of work, specialy with a Geronar.

[Sheldon N]

Tks for all your thoughts.
Sheldo, I checked Chris web site, very interesting. If I interpret your extract correctly,
(Rodenstock Geronar 150mm
f/11 60 38 24 ) I'd be getting less than half the resolution at the edges? Thats not so good.
I've posted the image at
http://www.pbase.com/photoimagerie/image/95782714/original
Any further thoughts would be appreciated

Many thanks for all your help

Yes, that's correct - half the resolution at the edges when used at f/11.

However, looking at your image I think I see another issue at play. You have pretty good left right sharpness at the middle half of the picture, but your top/bottom sharpness is lacking. It looks like you may have some inadvertent tilt in your image. If this were just an image circle issue you would see unsharpness all the way around the image.

I would use a couple sticky post-it notes marked with a big black "X" on the edges of the artwork to assist in focusing. Use a good loupe and then tilt/swing until all four edges/corners are equally sharp on the ground glass. Then pull off the sticky notes to make the final exposure.

More careful focusing, plus stopping down to f/22 with multiple flash pops should make a dramatic difference.

[Bill_1856]

Looks to me like a crappy lens or scanner. Also, did you recheck the corner focus carefully after stopping down?

[Ken Lee]

Is everything level ? Level the subject, then level the camera. Make sure the lens is in the center of the subject. You shouldn't have to use any movements.

Another approach is to step further back, so that the subject occupies only the center of the groundglass image. Now, you will be using the best portion of the lens. If the film is fine enough, and the scanner is good enough, you will get more even results.

[Ole Tjugen]

I think sheldon nailed it: Front, back and image plane were NOT parallell. That's the only way to get this kind of unsharpness.

[Murray]

I usually have a problem with where I have the artwork...either a wire/strap wall hanging system or an easel usually gives me some tilt. I prefer the easel because it doesn't move in wind currents. I measure the angle with one of those construction inclinometers then match the camera tripod to start.

As others pointed out the ground glass focussing should take care of the fine tuning beyond that.

[vonstauren]

What do you mean by DOF? The depth of field as Brian understood it or the depth of focus? Me thinks you had a film wrongly put in your film holder or simply not plain.

[Mark Woods]

Brian, the DOF is the same for any lens as long as the image size is the same. This is such a misnomer that it drives me crazy. I would look at the coverage area and see if the area is on the out side limits of the lens. If so, it could be soft. I'm about to purchase a magnifying glass and determine the F/stop and shot some shots. I have a hunch that they are going to be very pictorialst.

[Sheldon N]

Brian, the DOF is the same for any lens as long as the image size is the same. This is such a misnomer that it drives me crazy.

Yes practically speaking, but not quite exactly. It's functionally true for lenses that are longer, but falls apart to some degree when dealing with wide angle lenses. It has to do with second order effects.

However for the purposes of this shooting situation, yes - switching from a 150mm to a 210mm lens won't really change DOF much.

If you like reading argumentative and tiring threads regarding depth of field, this one is a good one:

http://luminous-landscape.com/forum/index.php?showtopic=24509&st=0

:)

[Thompsonr]

Thanks, I set the whole thing up again. I think Sheldon and others hit the nail on the head. The painting was probably twisted requiring a small amount of front lens swivel. As you point out, on closer re-examination, there is a line of focus down the centre and its just the left and right edges that are out of focus. The painting was oriented vertically to get the shadow below the skirts. The colour strip was on the RHS.

Thanks very much for the helpful discussion, most appreciated.
A great forum
Ralph

[Leonard Evens]

I'm chiming in late. Let me say that I agree that at least one of your problems is some degree of tilt. You can make sure the standards are parallel as follows. Turn the camera so that it points straight down. Use a bubble level to adjust the rear standard tilt and swing so the ground glass is level in al directions. Then use a torpedo level across the front lens barrel and adjust the front standard so that it is also level in all directions. Then when you set up the camera, make sure it is exactly square on the scene. You can do this by using postits as suggested above.

The depth of field in this case, since the subject is relatively close, is dependent just on the reproduction ratio and not on the focal length, so it shouldn't matter what focal length lens you use. The DOF at f/11 at 12 feet should be just be at least a foot in front and in back of the plane of focus. (The exact amount would depend on your criterion for sharpness.

It is also possible that curvature of field may play a role, but given the DOF available, it shouldn't make enough difference to produce the results you show in your example. It is much more likely that the plane of focus is passing through the subject rather than lying right along it.

Let me note though in passing, that the DOF does in general depend on the focal length if the subject plane is sufficiently far away.

[Brian Ellis]

"Brian, the DOF is the same for any lens as long as the image size is the same. This is such a misnomer that it drives me crazy."

Mark - You need to read what I said. I said that "leaving everything else the same," the longer lens will produce less depth of field. The only way to change focal lengths but keep the image size the same is to change the camera position, in which case everything else hasn't remained the same.

[Drew Bedo]

Hello Brien;

I have only read a few of the responses to your question/problem, and there is a lot of good stuff here. I'd like to throw in a concept that
I learned from Steve Simmons at a LF workshop in the late 90s...."The Ground-Glass is truth". What you see there is what the film sees. If it looks sharp it is sharp.If it isn't sharp, don't take the shot. Best wishes in this, let us know how it all works out.

[Ole Tjugen]

..."The Ground-Glass is truth". ...
That's correct. And DOF is an illusion, but a useful illusion - since we humans are easily fooled by illusions. ;)

[Thompsonr]

Thanks to everyone again, all your feedback is most enlightening for me as a novice to 4x5. I've reshot the art work being much more careful with angles and positions of both the camera and the painting. I also used the stickies in the corners, very useful suggestion. I like the "whats on the ground glass is true" point but its very hard to see the corners, especially when you stop down to say f11. (I do have a fresnel lens).

I'm now making several changes;
- looking for a much better loupie
- measure and mark precise locations on the floor to position the paintings and camera in exactly the right place
- make some reusable corner pieces marked with crosses to easily locate at the corners of the paintings

I've printed all you responses for future mind jogging, Thank you all again,
Ralph