Sharpest Aperture

[BobNewYork]

Some years back in either Photo Techniques or Camera and Darkroom there was an article in which he author gave a formula to determine the sharpest aperture to use under virtually every circumstance. In use, you set up, adjusted camera movements and then measured the difference, in mm, between bellows extension when focused on the near object and the far object that you wanted in focus. Bellows were then placed half way between these points and the taking aperture determined via the formula. I've used this successfully for a long time - but I can't find the damned article. I put it somewhere "safe" - and that was the kiss of death

I did use a spreadsheet programme to create a small chart which I laminated and remains in my bag, but I'd like to get hold of the formula again to do some refinements.

Anyone remember it? Anyone have the formula?..........Anyone?

Thanks

Bob H

 

[clay]

The article online here:

http://www.kenrockwell.com/tech/focus.htm

has the information you are looking for. Scroll toward the bottom to get to the meaty stuff.

 

[Ian Grant]

It's not quite as simple as Ken Rockwell states. It depends on the lens focal lenght type of design etc.

LF Tessar's, and similar designs like Xenar's of around 135mm to 150mm etc are generally bests at f22 and in fact give poor edge/corner sharpness until stopped down to f16.

However more modern 6 element designs are far better optimised and will give better results at wider apertures, this is where depth of field tables, calculators etc become more useful.

Ian

 

[archphoto]

I have been into 4x5 for the past 30 years.
My basic aperture has been f:22 for all lenses at 4x5 inch, exept for the Imagon ofcourse.
What I have understood is that the increase of DOF is 1/3 towards you and 2/3 away from you.
With a TC you have tilt to increase the DOF.
Using tilt (frontstandard only) should be done in small steps of 1 degree at the time, esp outside.
It is so easy to set the tilt at 15 degrees at once and than increase and increase and to find out that 3 degrees was what you needed.

Peter

GAS: RB, Sinar P2 4x5 and 8x10 inch

 

[clay]

Agreed that lens design has a practical effect as does the subject being imaged. I think the only thing that the Rockwell tables are trying to convey is the diffraction limited performance. The point being that even though a Tessar does get better corner to corner at f/22, that because of diffraction, some resolution is sacrificed in the center as a trade off for the improved corner resolution. No free lunch in other words. In practice, it just doesn't matter all that much for most LF shooting. It does matter for small formats, and shooting 35mm film at f/16 is just not a great idea if you plan on enlarging a lot.

It's not quite as simple as Ken Rockwell states. It depends on the lens focal lenght type of design etc.

LF Tessar's, and similar designs like Xenar's of around 135mm to 150mm etc are generally bests at f22 and in fact give poor edge/corner sharpness until stopped down to f16.

However more modern 6 element designs are far better optimised and will give better results at wider apertures, this is where depth of field tables, calculators etc become more useful.

Ian

 

[Ian Grant]

Have to agree Peter that f22 is usually the best aperture with most 5x4 lenses when you want optimum sharpness, and that's fine for tripod based work, which was all my LF work until recently.

The problems arise when you want good sharpness hand-held where you have to trade apertures against shutter speeds, I'm often shooting in locations now where tripods aren't permitted, so it becomes more important to know & understand how your lenses perform at wider apertures.

Ian

 

[Ian Grant]

Clay, the Tessar's were designed to give maximum sharpness & resolution at f22, somewhere I have the figures from Doctor Optic who were the last manufacturer of the CZ Jena designed Tessar's, resolution tailed of significantly after f32 due to diffraction for the135mm/150mm lenses.

Ian

 

[BobNewYork]

Thanks for the responses all. This wasn't the actual aritcle I was thinking of but it looks like it has what I needed - thanks. My LF lenses are all Symmar S at the moment so it works for me. (I say "all" - it's a 90mm and a 210mm!!)
Re the DoF and focusing 1/3 into it; I'm pretty sure that with the bellows set half way between the two "extremes" of focus you are indeed focused one third in, two thirds out - at least that's what I recall from the article I had in mind.

thanks very much again guys.

Bob H

 

[edtbjon]

Apart from what the title of the thread says, i.e. in practical terms stop down halfway down the scale (where you usually find f/16-f/22 for 4x5" lenses), the question seeem to be about DoF. If you do a search for e.g. "large format" and "DoF scale", I recon you will find lots of information on the subject (apart from Ken Rockwell's excellent article). I do personally use this DoF tech. all the time when I shoot LF and have done so since the 80'ies. It's built into every Sinar camera since the F and P series were introduced and really makes life under the dark cloth much easier. All you have to be aware of is that these scales often are designed with a final print of 8x10" in mind. (The DoF scales of most 35mm and MF lenses also have the same target, i.e. a 8x10" print.) If you want to print bigger, you have to stop down more and then it's good to know the formula with the acceptable Circle of Confusion etc. (Which should be in the Ken Rockwell article as well as on other places.)
Oh yes, about the 1/3 - 2/3 vs. ½-½ thingy. When you have set the focus, it should be halfway between the far focus plane and the close focus plane. That is what happens on the film plane side of the lens. On the other side of the lens (i.e. the real world which isn't upside down ) the close focus plane is at about e.g. 1 meter (OK then, 1 yard for you americans), the actual focus plane is at 2 yards and the far focus plane is at about 4 yards. So it's both 50-50 and 33-67, depending on which side of the lens you are.

What I don't understand in some of the above answers is why you talk about different lens designs. Every lens of a specific focal length does have the same DoF at any given aperture opening. This regardless of it being a simple meniscus lens or a complicated zoom lens.

//Björn

 

[Sirius Glass]

The rule of thumb [or any other appendage one might choose to use] is:
35mm f/ 8
120 f/11 - 16
4x5 f/22
5x7 f/??
8x10 f/??

Please fill in the blanks.

Steve

 

[clay]

I agree that f/22 optimizes the sharpness across the field, but it does so at the cost of absolute sharpness in the center. Check out this mtf graph of a tele-xenar (tessar design) from Schneider:

http://www.schneideroptics.com/pdfs/photo/datasheets/tele-arton/tele-arton_56_250 2.pdf

and look at the last two graphs showing mtf from f/11 and f/22. The f/22 is more uniformly sharp across the field, but absolutely less sharp in the center than at f/11. The difference is so small that it is of no practical consequence, but the fact is that if you want the sharpest possible image in a subject in the middle of your frame, you would be better off shooting at f/11 than at f/22, even with a tessar. But again, in practical terms, it is of no consequence, unless you are shooting with tech pan on your 4x5.

Clay, the Tessar's were designed to give maximum sharpness & resolution at f22, somewhere I have the figures from Doctor Optic who were the last manufacturer of the CZ Jena designed Tessar's, resolution tailed of significantly after f32 due to diffraction for the135mm/150mm lenses.

Ian

 

[BobNewYork]

Well it's not entirely about DoF. The idea is to find the aperture that will give you the sharpest image for the depth of field required. It's a trade-off between a small enough aperture to create the DoF you want and the diffraction caused by stopping down too far. Since I saw this my taking technique has been to find the bellows extension difference between near and far and then consult the chart for the "best" aperture. My shutter speed falls out of all that. Kinda like Large Format Aperture Priority!!!!

 

[keithwms]

Bob, for LF my strategy is this. Starting with a wide aperture I focus and then attempt to gain sufficient coverage and sufficient effective depth of field via tilts and shifts. Failing that I stop down. Then... rinse, repeat... no formulas, no math In LF you are almost always working on issues other than resolution.

f/22 is certainly already incurring diffraction softening. Most LF lenses are giving maximum resolution values across the frame at around f/16 or even f/11. Indeed f/22 is the aperture at which the lens typically reaches highest coverage, not the highest resolution across the image circle.

In MF, lacking the tilts, you wind up stopping down more than would be necessary if you did have tilts. C'est la vie.

 

[j_landecker]

This article on the LF photography site covers what you're looking for, and there are links to the Photo Techniques article under "References" at the bottom of the page:

http://www.largeformatphotography.info/fstop.html

Jim