Visual evaluation of depth of field

[leonard_evens]

I generally use the "focus spread" method to estimate the needed

f-stop for desired depth of field. That involves measuring the spread

and either doing some arithmetic or using a table. But I've noticed

that as I stop down, the image appears to come into adequate focus

long before I reach the calculated f-stop. I've also double checked

by measuring subject distances and using the usual formulas to

determine the hyperfocal distance (and hence the needed f-stop). As

expected these prove consistent with the focus spread method.

 

So I am wondering just what is going on. In my case, I have been

highly myopic since I was about 10. When my cataracts were removed

and interocular lenses inserted, I found I saw very well in good light

but I had problems in dim light. My opthamologist told me she often

sees this in patients with a history of moderate to extreme myopia.

There is some minor degeneration in the retina which is masked by the

cataracts and becomes apparent when they are removed. For most

activities, this is a minor annoyance, but it certainly limits what I

can see on the gg when the lens is stopped down. I have a very bright

Maxwell screen, but I can't see much above f/16-22. I can do somewhat

better with a loupe, but then of course it is hard to evaluate the

entire scene at once.

 

I wonder if it may be that in effect my visual acuity drops as the

light dims and hence my effective coc for viewing purposes gets

larger. I normally view with some +5 diopter glasses I had made for

the purpose, which allow me to get about 6-7 inches from the screen.

I figure my viewing coc is about 0.135 mm, and it could double or more

in dimmer light.

 

How does this work for those of you with good visual acuity, even in

dim light? Can you generally rely on the gg for estimating the depth

of field you are getting as you stop down? I've been told that many

people can do this well.

 

Are there any useful tricks for helping evaluate dof visually, even if

one can't see too well when stopped way down?

[michael_briggs2]

You might estimate the "correct" aperture by the several techniques, then take photographs at these apertures, make prints of the size you would normally make, and then examine them to see which have the dof that you were aiming for. Until you do that, you don't know which aperture determination technqiue is correct. There is good theory behind the equations behind the focus-spread technique, but you don't know that the coc value that you are using is the correct value for your needs until you have done some experimental tests.

 

When I use the ground glass to evaluate focus, I use a X4 or X5 loupe or magnifier, then to take the photo I stop down one additional stop past the aperture that I judge to be correct by viewing the ground glass with the magnifier. I have found this procedure to produce prints that agree with what I expected from viewing daylight scences on the ground glass with a magnifier. If the light is dim, I might use more than one stop extra. My explanation for the extra stop is that it compensates for being able to judge detail better in the print than on the ground glass.

 

In very dim light, I use varying combinations of the focus spread method, viewing the ground glass, and experience. Carefully arranging your dark cloth to block all ambient light will make a big difference -- a small amount of sunlight will prevent your eyes from adapting to the ground glass illumination at f32.

 

In studio situations, one can bring in additional light, perhaps using a desklamp to illuminate the critical object for dof. In some situations, even outdoors, you can place a small flashlight pointed at the camera at the critical dof spot or spots in the scence.

[michael_briggs2]

Also check that your dark cloth is truly opaque -- frequently the goals of minimizing bulk and weight lead to darkcloths that transmit some light. The transmitted light will reduce your ability to judge dof at small apertures.

[brian_ellis3]

When you did your arithmetic you were assuming a certain size circle of confusion at a certain size enlargement I assume. When you're looking at the ground glass you're not looking at the enlarged version you used to figure your circle of confusion, you're looking at a considerably smaller version. That would be my guess as to why it looks sharp on the ground glass before you've reached your calculated aperture.

 

I don't think your difficulty in seeing well when the aperture gets below F16 or F22 is that unusual. I too have trouble much beyond that range except in bright sunny conditions and I don't photograph in bright sunlight too often.

[bill_proud]

Leonard,

 

I supposedly have good vision but can't see much beyond f/16 on the ground glass even under perfect conditions. I've tried the optimum f stop theory but find it a bunch of hooey. F/32 is better than f/22 and f/45 is better than f/32. Stop down to f/45.

 

good luck

[philip_sweeney]

Like Bill I'll agree depending on the ambient light its hard to see when one stops down. For myself with 6 inch lenses and above, at adequate distances (I know thats one hell of a relative statement but I hope you will get the point), outside, I never use anything wider than f22. Closer in - smaller.

 

In low light with a 90mm lens I have a prefocus pencil mark for 12.5 feet at f22 (CoC = 0.1mm). No swing, no tilt. And manytimes stop down to f32

[paul_coppin]

Some of your concerns expressed relative to the visual acuity of your eyes in dim light is valid. Your eyes behave like a lens with a diaphragm and a variable plane of focus - as the light dims, your pupils enlarge (=open up) to let more light in, and you have the attendant change in depth of field/focus, pretty much the same as a lens. Bright lights, our pupils stop down - more DOF, less accomodation required. As we get older, our eyes begin to lose the ability to be squeezed and stretched to optimize the plane of focus on the retina. Dim GGs get to be a pain literally and figuratively, and thats why many of us need a higher powered loupe - grain focusing!, which of course needs more light, and so the circle evolves. All those autofocusing 35mm marvels weren't designed for our kids you know!

[pvp]

DOF is, to a large degree, subjective -- it is dependent on a user-specified circle of confusion -- which is in turn dependent on the eventual amount of enlargement. The enlargement issue, IMO, makes it hard to adequately determine DOF on the ground glass; what is sharp to your eyes there might not be sharp enough at 30x40. I'm still very much in the learning stages of large format photography, and I often use the focus spread method of determining the best f/stop. When in doubt, or if I forgot to take my paper brain with me, I stop down until it looks good on the GG, then stop down another stop. Or two.