Questions about focus and DOF technique and aperture

I have read the resources available on the LF home page about dof and optimal sh arpness techniques and have a question. I am new at this so please correct me i f i'm incorrect. I've learned to always set focus at the largest aperture possi ble ie., 5.6 for brightness reasons. But for example after using procedure 1 an d/or 2 (found on the LF home page) how is the aperture determined to keep dof. Since DOF and sharpness or a priority should the scene be metered relying on exp osure time only. Example if after taking appropriate readings you determine tha t you should expose for 1/30th at f8, but feel that you need more dof and need f 16 you could just adjust the exposure time right? How do you know the best aper ture for DOF, that is the real question.

Thank you very much!

Clark: If you're coming from another format, the basic rules for large format still hold true - you're depth of field increases as your aperture becomes smaller. And yes, you would need to adjust your exposure time if you change your aperture from F8 to F16. Simple examination of the groundglass will show your depth of field increase as you step down, you can also see it change as you apply movements. The "best" aperture for DOF depends on what you want from your chosen DOF. If by best you mean the most, it would be your minimum aperture, but diffraction would have a negative impact on image quality by then, so that's a tradeoff - that subject has been covered lots here. Merklinger has a web site with video clips showing how your DOF changes as you apply movements, and Leslie Strobel's " View Camera Technique" covers same. You may also want to look at any number of resources on basic exposure and camera setting techniques.

The easiest way, if the light is bright enough and the required aperture isn't too small, is to focus around the center of the scene and then just look at the ground glass while you gradually stop down. When everything needed to be sharp looks sharp, stop down one more stop to be safe and that's it. Unfortuantely, the light often isn't bright enough and/or the required aperture is so small that you can't see well enough on the ground glass to tell what is sharp and what isn't. In that case use the method that I thought Tuan described in his article on this site, which basically involves determining the distance that your lens travels between focusing on the nearest thing that needs to be in focus and focusing on the farthest thing that needs to be in focus. Tables exist, which I thought Tuan had in his article, that then tell you the required aperture based on the distance the lens travels. If they aren't in Tuan's article then let me know and I'll send them to you. In order to use this method, you'll need to tape a small milimeter ruler on your camera at some point where you can measure the distance the lens travels between focusing on the near and focusing on the far. This is easy to do with most cameras.

Everything is in the article Brian refers to, including all the relevant tables and scans of the two reference articles.

I have read and understand the articles on Tuan's site. However I am having difficulty getting some of the formulas ro work out right according to the tables given. I'll have to work on that! Sorry for the elementary questions I'm new to large format and am trying to learn all that I can!

Thanks for the responses!

Clark

If you don't already have one, I would suggest a focussing loupe. I use a Wista loupe (the lower power of the two) and I've found that it's much easier to check focus with it than just by looking at the GG. Just as an exercise, I would suggest that you take your camera out and practice trying to get everything in focus with different subjects/scenes. Play around with tilt/swing and aperture until you start to get the hang of it better. The theory behind it all can be a bit much at times, but for the most part it's easier in actual practice.