Choosing an F stop for sharpness

How do you decide which f-stop is best for your compositions?

Twice now I have been asked why I chose f64 in my two recent posts in the ESS Gallery. My response is there; it has to do with the fact that I cannot see the ground glass with my glasses. For the rough focusing I need to look over my glasses instead. Perhaps I need a special prescription for Large Format photography. For fine focusing I use my glasses and a 50mm lens for a loupe.

So I chose f64 just to be sure of the sharpness. Although I am satisfied the image is sharp through the loupe.

But why would I chose a wider aperture over a smaller aperture when I want sharpness throughout?

Thank you for your help. I really need it.

Will
Edmonton, Alberta
Canada

Hi Will,
small apertures reduces the sharpness due to diffraction. When a wave pass through an hole that has a width similar to the wave's length, it changes its angle of propagation (this phenomenon is called diffraction).
Since the light is a wave, and the aperture is an hole, the lenses are affected by diffraction. The amount of diffraction depends by the diameter of the aperture. With large apertures the diffraction is negligible; with small apertures that diffraction becomes a serious problem.

Juza is right about diffraction. I would suggest printing a small hyperfocal chart for the lenses you use most. Focus distance is ahrd to measure with most LF cameras that don't use special cams for each lens but the hyperfocal chart will still give you ballpark numbers for near/far sharpness at a given aperture.
Remember that the hyperfocal distance represents near/far points in front and behind the focus plane, which with LF is not necessarily parallel to the camera. If you use tilt or swing you will need to take it into account.

Guy

Have you been to largeformat info web page? I found a simple way to add DOF calculator to my Arca Swiss, but I guess that it's as simple to put one on another model of camera. I guess that you are using 4x5 camera. With this format I'm routinely using f32, and if I find out that I need smaller f-stop still, I would not think twice to use it. Usually with film as large as 4x5 diffraction is less of a problem than unsharp photograph.

Marko

To my thinking, diffraction is a bit less of a concern with a LF lens than it is with the typical 35mm lens. For one thing, an f-stop is actually a ratio rather than an absolute size. So, f32 on my Schneider 90mm lens is physically larger diameter than is f32 on my Nikon 80-400mm lens. There is a very big reason why I often end up choosing a wider aperature for sharpness though--wind. It constantly blows where I live and a faster shutter speed helps to reduce blur due to wind.


Kent in SD

Hi Will,

I agree with Guy's idea of having a DOF chart with you in the field. I never go out to due landscape photography without one.

The principal reason, as Kent mentioned, to use a larger larger aperture is because of the wind. In partcular, I think it is difficult to judge the effect of wind on trees that may be in the middle ground distance of an image. Where there is vegetation in the foreground that can be easily seen it's easy to look at and decide whether to use a large aperture or not even shoot because of the wind.

I started keepng a chart with me after realizing, when making large prints, that in many of my images that had trees that were in the midddle ground view they were blurry from the effects of wind. I think in many cases if I could have shot at one or two stops larger I wouldn't have had a problem.

Perhaps if you get a chart you could work on stopping down one stop more than what should be sharp according to the chart, in order to be on the safe side. A good idea would be to go out in the field somewhere close by where you can shoot some film in practice and then cloesly review the film, rather tha waiting until you are out on the road or shooting something important. Randy

f/64 in 8"X10" format is roughly equivalent in DOF as f/8 in 35mm format when printed at the same final image size. If you are shooting slow film, you might be tempting "reciprocity failure" which can shift color images.

Will, all the above re diffraction etc. is absolutely correct. However, I am going to respond to your query re special glasses for LF. I am an optometrist and work with patients all the time that need critical intermediate and near distance focus. Bifocals work at reading distances of 14-18 inches but you have to lift your head a lot to get into the zone. Trifocals will focus at 18-26 or so inches which is probably too far for your needs. Depending on whether or not you have significant astigmatism in your correction a good pair of over the counter reading glasses may suit your needs. Another option is a jewlers loupe of about +5 (not 5 X mag, but + 5 diopters) that can be hand held or clipped to your glasses and swings down when needed.

Another option is a special pair of glasses made for your specific needs. Perhaps glasses that have a distance Rx on top and bifocal placed rather high focused for the distance you need. If it is placed high you won't have to hold you head back so much to use it.

Ask your optometrist for his/her help. When you go to the office be prepared to discuss the working distance you need when judging focus on your camera.

Regards,
Ed

Will,

You might also consider a loupe that has adjustable focus. I have an older Schneider 4x that does this. Here's how to focus the loupe, if you decide to go this route...

Set up your camera with movements set to neutral, but do not install a lens.

Remove the back and attach a 'sticky note' to the 'film-side' of the ground glass. Reinstall the back.

Get under-the-hood and adjust the focus of the loupe until the edge of the sticky note is tack sharp.

Essentially, what you have done is correct the focus for your eye and also where it need to be: in the plane of the film.

-PB

Thank you. I have been looking for hyperfocal distance charts and depth of field charts but so far found only those relating to digital slr photography. I searched through the Large Format Page website too. I may opt for a library book on LF photography that I remember. It may have a section on hyperfocal distance. The internet is not the only and final source of info

I was already concerned with diffraction when I chose f64. However, under what circumstances is f64 safe to use? The fact that it is available on the lens indicates that it can be used in some cases. Is f64 for compositions with near-far elements?

Ed, I appreciate your help in suggesting a special pair of glasses so I can see the ground glass. That is the most logical and practical answer to my situation. I could use this along with the jewelers loupe you mentioned. I may have to talk to a jewelers to find out where to get one. Especially when it can clip onto my glasses and leave my hands free. That seems very ideal for my circumstances.

Preston, I also really like your suggestion of the adjustable focus loupe. I will see if I can find one.

Thanks again, all, for your response.

Will
Edmonton, Alberta
Canada

I suggest strong reading glasses from the drug store. Find ones that allow you to focus 8-12 inches away with a magnified view. I use this method instead of a loop. If you can get everything in focus with movements, stop down to the sharpest setting for the lens. Otherwise, stop down until all is sharp.
Knowing hyperfocal is good, but use conservative numbers if you want to print big. I probably stop down to f64 once every 200 exposures. On most lenses f64 is well into meaningful diffraction with 4x5.
Generally a shot that requires f64 is a near/far relationship that can't use movements. Not a great setup for large format. Most of us probably don't set up those kind of shots.

I've had bifocals since before I started shooting LF and I've never found it a problem. You might have to bend your head more that you'ld like but it's worth it for a sharp photo.

I'll stop down to f/64 if the image has a lot of depth in it that I can accomodate for with tilts and swings. With my wide lenses up to 150mm, I usually shoot at f/22-32 and with the longer lenses I'll shoot at f/32-64. My 300mm is pretty good at f/64.

- Dan.

I have come up with a temporary solution. Until I can get glasses with the prescription I need. I found that combining my glasses which I use for near distances, such as viewing this screen, and reading glasses from a drugstore works very well. It means wearing one pair atop the other but under the dark cloth who cares?

I did not notice any signs of diffraction in the transparencies I took at f64 so far. Could I tell by examining the transparencies? Even under magnification they look alright.

Dan, I don't know why I have this problem. But I do. Likely I just need reading glasses from the optomitrist.

Thank you all for your help.

Will
Edmonton, Alberta
Canada

4x5 film and processing is so expensive but if you want to see if there's a difference in a well focused image shot at f/22 and f/64, shoot both images and compare them. Personally I don't want to spend that kind of money. I just know from experience that my 150mm is much sharper at f/32 than f/45 and my 300mm is good at f/32-64.

- Dan.

Thank you Dan. I think I will pass on the comparison test. Your knowledge and experience is all I need.

Will
Edmonton, Alberta
Canada

The nicest benefit of reading glasses is having two hands free.

Perfect focus with the view camera is a breeze. If not already present, put a millimeter scale on the base of your view camera so you can determine the locations where the front standard needs to be in order for bring the near and far objects into focus, in other words, the two extremes of the focal range of your image. Measure this distance and set the front standard exactly halfway between these two points. Now your front standard is set at the perfect location to achieve the maximal focus. To determine the f-stop, you'll have to go back to the distance between these two focus extremes. I used to have a chart that showed this, and don't recall the numbers exactly, but I think it was something like 4 mm = f/64, 3 mm = f/45, 2 mm = f/32, and 1 mm = f/22 where the distances are those the front standard traveled between the near and far focal points after you had done the appropriate tilts and shifts.

Thanks very much, Tony. Until now I had ignored the scale on the base of my camera. This is exactly the response that I needed.

As for the glasses issue, you all have guided me to the solution. Thank you.

Will
Edmonton, Alberta
Canada

I wasn't sure of my numbers. This has all the equations you need:

http://www.math.northwestern.edu/~len/photos/pages/dof_essay.pdf

That is something one ought to be judging for each composition depending on how much the different areas of the image are in focus and how large one might be enlarging an image. There are many compositions that are impossible to bring all areas of a frame into sharp focus. For instance a tree trunk top to bottom in a frame's near foreground with a level plain receding into the distance. No experienced photographer with an interest in bringing all elements into focus would waste a moment considering such a scene. For credible compositions that are increasingly less marginal, one can stop down less and less towards the len's optimal optic center f-stop settings. As a landscape photographer that tends to frame scenes with fore, midddle, and back grounds, I rarely find scenes with level even plains where I might get all of a frame's areas into optimal sharp focus. So for usual mixed subject distances tend to do as you do, stopping down towards the smaller f-stops. For my type of informal work, there is little need to involve lens math and depth of field charts etc. With my 150mm lens, I avoid using the F45 to F64 region unless the various subject distances are going to be marginal. Likewise with my 90mm will prefer the f22 to f32 stops rather than f32 to f45 if it will work. Even stopping down to maximum aperatures, I will still at times end up wasting sheets because I gamble I might be able to pull in enough sharpness only to find out later that the resulting focus was not large format worthy. Personally I dislike any out of focus components to my landscape images so tend to err on the stopped down side. I know from experience that with the film I use, drum scanning, then Lightjet printing, that I can stop down to the near maximum stops on my lenses and still produce great highly detailed prints at the 30x37.5 inch size I max out at. Other photographers interested in being able to print to say 40x50 and larger might have to consider stopping down less and forget about trying to compose more marginal compositions.

...David

Other photographers interested in being able to print to say 40x50 and larger might have to consider stopping down less and forget about trying to compose more marginal compositions

David, I gather from this that compositions can influence the size of the print. And that not all 4x5 trannies will make good 40x50's or bigger.

Thanks for your reply. Choosing the right lens is easy for me. I have only one; 150mm.

Will
Edmonton, Alberta
Canada