Stopping Down Too Much?

Is it possible to stop down a lens too much so that it actually

starts degrading the image, I ask as I'm going to shoot some macro at

1:1 on 10x8 at f64 to get the required DOF, but cant see the image on

gg because of light loss from extension + small aperture

if anyone can help I'll be greatful

Dave

As the lens is stopped down defraction begins to be a consideration. Depending on what you will do with the image, (enlarge or contact print), will depend on the impact of defraction on your final image. If you were to contact print your 10X8 negatives, I would doubt that you would notice the effects of defraction. Certainly F64 is not excessive with that format.

As you stop down the lens, diffraction becomes more of a problem. Diffraction is a consequence of the wave nature of light. Because of it, the image of a point is actually roughly a disc shaped blur, the size of which depends on the f-number. The smaller the aperture, the larger this disc. This would happen even for an ideal lens without aberrations. But the larger the format, the less diffraction is a problem because the negative will be enlarged less, so an aperture which might be a problem for 4 x 5 may not be so much a problem for 8 x 10.

 

There are various theories about how to choose the proper f-stop to balance depth of field against diffraction. See

 

www.largeformatphotography.info

 

for an extensive discussion of this matter and also other view camera basics anyone trying to do what you want to do should know.

Yes.

 

While, on the one hand, you obtain greater depth of field with a decrease in aperture (larger f-stop) and decreasing abberations, there's a point at which diffraction begins to erode sharpness. I'm no expert, but most lenses are diffraction limited at about f22. Thereafter, diffraction becomes significant enough to cause a net degradation of the image.

 

The effect isn't as great in L.F., because the negative isn't enlarged as much. For example, those who contact print their negatives can live with significant diffraction, because it's not as noticeable.

Dave, Don't forget that at 1:1 if your shutter is telling you f64 you are likely at f128. Everything is compounded. At some point you may as well take the lens/ shutter off and put a pin hole on.

I recently did a shot at about 1:1, on 8x10, of a box, the inside of

the box, and the wall behind it. No movements would help focus.

I stopped down to f90, which I have done frequently in the field

with very sharp results on contact prints. But at the close

distance, the distortion was magnified, and the image was soft,

not out of focus exactly, but mushy.

David, you certainly know how to pick difficult work! Shooting 8x10 at 1:1 really pushes the limits. If you contact print, and end up with a 1:1 life size image, have you tried shooting the same image with 4x5 or 2 1/4? You could shoot 4x5 at 1:2 and achieve the same size image (by enlarging to 8x10) with much more depth of field and fewer headaches. You are the final judge. However, I think a comparison test would prove interesting.

Everything that has been said so far is accurate but you left an important

element out of your original post. What lens are you using? If you take Ken's

post a step farther this is one of the important points.

 

It may be counterintuitive but for 1:1 and other photography of small objects in

the studio think wider rather than longer lenses (at least that is my technique).

If you have 4x5 available and are no tthinkin gof contact printing the 8x10

then 4x5 IS a better choice but all teh same principles apply in 8x10. You will

find that you can achieve the composition you want with more ease with a

shorter as opposed to a longer lens and still have more depth of field and less

bellows extension to worry about.

 

For example, for food and other table-top sorts of things on 4x5 I generally

use 180 mm.

 

While I am working in closer than I would with 240 or 360 I still ahve enough

room to arrange the objects and I am not working with an unmanageable

bellows extension, usually not even with one that requires any exposure

increase (which is not true with the longer lenses). Depth of field, as you are

finding out, is still a problem but I usually use f45 or f64 and that will solve

most of the problem not solved with movements. At these distance you may

have to settle for some small part of teh scene being slightly out of focus or

rearrange your composition to avoid same. Another part of the equation is

light, when you are working in close and want to use a small f stop it also

helps if you can use enough light, placed in such a way as to give good

definition to the edges of your subject and also to allow the use of a faster

shutter speed to avoid any wiggle or shake, etc.

 

Can't say much more without knowing what lens you are using and I seldom

shoot 8x10 for this sort of work. My guess is though that if I were to do so I

would start with a 240 lens as opposed to anything longer.

Diffraction can degrade an image. Insufficient depth of field can degrade an image. As between the two, most people would tolerate the fairly minimal loss of image quality that diffraction causes as compared with the major, and usually obvious, loss of quality from insufficient depth of field. With macro work, especially 8x10 macro work (I have to ask - why 8x10 for this work?), depth of field is usually the principal concern, diffraction would be considered only if you have a choice of several apertures that will all produce the necessary depth of field. So if you need F64 to gain the necessary depth of field, I think that's what you should use and not worry about diffraction.

David,

 

Are you using a digital darkroom. In such a case, please consider that sharpening in Photoshop may "clean up" some of the diffraction effects, but most probably none of lacking DOF. For that reason, I would not worry about a small lens aperture, even if it leads to diffraction.

"While I am working in closer than I would with 240 or 360 I still ahve enough room to arrange the objects and I am not working with an unmanageable bellows extension, usually not even with one that requires any exposure increase (which is not true with the longer lenses). "

 

This can't be right? Exposure increase is a function of magnification, not physical bellows extension!

Sorry for the confusion, that is what I get from writing too early in the morning

or too late at night. I was thinking of the total combination of distance, focal

length and light. I mentioned lighting afterwards but meant to put it in here in

terms of keeping your light levels such that you could work with shorter

exposures as well as reduce shake and eliminte reciprocity failure with very

small f stops and long exposure times.