Focusing question

[Leonard Evens]

Gentlemen, I am greatful for your generous advice. So much mystery at this point in my LF adventure. Leonard, I will certainly look at the near/far technique, which was already on my to do list. I did check out that article from Northwestern, and broke out into a cold sweat over the math. I suspect it will all seem a bit less ominous after a few readings. Again, obliged to you all. David

I think you can skip the math. The part of the article I was referring to was where I explained how I made a scale to put on my focusing knob. Basically all you have to do is measure the distance the standard moves on the rail or camera bed when you turn the know one full turn, as well as the circumference of the knob. The ratio of the second distance to the first tells you how much distances on the rail are magnified on the scale. Using that information, you can make marks on the scale in mm and, we hope, tenths of a mm, corresponding to such movements on the rail.

Looking at what I just wrote, I can see it is not terribly clear, but I hope when you actually do it, it will become a lot clearer.

[John Berry]

Donald, thanks for yours and fair points. My step 4, fine tuning, would include front tilt to maximize focus. I have not quite figured out rear tilt. Care to educate me? Obliged. David

The best education on that is next time out, try it both ways. First use front tilt to get the image you want. Then square up the front and use back tilt to get what you had with front tilt. You might find they have different character. Rear tilt will change perspective. I use it a lot on purpose.

[Nathan Potter]

Leonards knob scale idea is one I've used on some occasions, especially for macro work. The idea is similar to that used on vernier calipers and micrometers to increase the accuracy of linear measurements. In the case of rail measurements on view cameras the improvement in rail position accuracy is derived from the ratio of the knob circumference to the geared circumference (the gear that moves the standard along the rail for focusing). This can be very significant as Leonard points out. For example, with the lens wide open (so the subject material is very visible) you can focus on the near and far subjects that you want to be in focus and note the delta in the film plane position from the accurate reading on the focusing knob. If you have generated a chart of DOF for a particular COC and f/no. and the lens you are using then you can set an approximate center of focus using a rail position intermediate to the near and far readings. It's all a PITA so I seldom go to all this trouble.

Nate Potter

[Leonard Evens]

Leonards knob scale idea is one I've used on some occasions, especially for macro work. The idea is similar to that used on vernier calipers and micrometers to increase the accuracy of linear measurements. In the case of rail measurements on view cameras the improvement in rail position accuracy is derived from the ratio of the knob circumference to the geared circumference (the gear that moves the standard along the rail for focusing). This can be very significant as Leonard points out. For example, with the lens wide open (so the subject material is very visible) you can focus on the near and far subjects that you want to be in focus and note the delta in the film plane position from the accurate reading on the focusing knob. If you have generated a chart of DOF for a particular COC and f/no. and the lens you are using then you can set an approximate center of focus using a rail position intermediate to the near and far readings. It's all a PITA so I seldom go to all this trouble.

Nate Potter

I don't see what you find so difficult about it. For focusing, you just set the standard halfway between the positions for the near and far points. To determine the proper f-number to use, you use the focus spread, or distance in mm between the near point and far point. If you then follow the approach recommended on the lfphoto.info web page, yo appeal to Paul Hansma's table, which you carry with you, from which you read the f-number from the focus spread. This method supposedly balances diffraction against defocus. But, ignoring diffraction, you can get a quick estimate by multiplying the focus spread by 10 and dividing the result by 2. (That assumes a coc of 0.1.) For example, suppose the focus spread were 4.6 mm. The calculation gives 10 x 4.6/2 = 46/2 =23. So you would stop down to f/22, or if you wanted to hedge your bets, a hlf stop to a stop further.

I do this all the time, and the whole process, including focusing on the near and far points, takes less than a minute.

[Doremus Scudder]

At the risk of shameless self-promotion...

I wrote an article for View Camera Magazine about using basic movements with the field camera that might be of interest to you. It purposely avoids mathematics and uses a strictly visual approach.

You can find it on the free articles page of the View Camera web site. Here is the link:

http://www.viewcamera.com/documents/...aMovements.pdf

You can download the pdf. document.

One note: Leonard Evens pointed out to me after the article was published that the illustrations of the depth of field "wedge" are not representative of the true situation. I showed them as flaring out when in reality they are simple wedges with straight edges. The illustrations are still useful for helping visualize how depth of field works when movements are applied.

I also use the focus-spread method for finding the optimum f-stop. I would second the above suggestions to dig through Mr. Hansma's article on the LF home page. I have made stickers for my cameras with the f-stop tables as well as adding millimeter scales to a couple.

The drill is easy: Apply all the movements with the lens open, then pick your near and far points, note the distance in millimeters between the near and far points, position the focus exactly at the midpoint, read the appropriate f-stop from the table, stop down, meter, and shoot.

Hope this helps.

Best

Doremus Scudder

[Nathan Potter]

Leonard, I didn't explain the PITA very well. Over the years in getting away from industrial type photography where I was used to employing a lot of technical stuff I've come to focus more on fine art photography. So now when I'm in the field my mind is totally consumed by the subject matter and I find any consideration of technique seems to be subservient to finding and framing the scene. I can't explain it well but in my mind there seems to be a conflict between the technical and the art that I can't handle simultaneously. I realize that at times this approach is detrimental to a well framed image and maybe it's just a phase in my photography.

Nate Potter

[timparkin]

Leonard, I didn't explain the PITA very well. Over the years in getting away from industrial type photography where I was used to employing a lot of technical stuff I've come to focus more on fine art photography. So now when I'm in the field my mind is totally consumed by the subject matter and I find any consideration of technique seems to be subservient to finding and framing the scene. I can't explain it well but in my mind there seems to be a conflict between the technical and the art that I can't handle simultaneously. I realize that at times this approach is detrimental to a well framed image and maybe it's just a phase in my photography.

Nate Potter

Heh... by your definition, the whole of the 'craft' of large format use is a PITA for me (having only started last October). Hopefully, all aspects of it will become second nature sometime in the next decade. I've distilled the focus knob scale rules into a simple Ebony rule of thumb... half a turn is f/45, quarter of a turn is f/32, 1/8 turn is f/22, and a wiggle is f16 :-)

Tim

[robert amsden]

The BtZS* software for a pda will figure it based on f stop and subject distance.

[ignatiusjk]

Dear deliberate1, what size lens you have on the camera also matters the angle of view the lens has will determine if you need to use the movements. The old rule was focus 1/3 of the way in a stop down to f32. With LF it's important not to complicate things.Sometimes the more we think the worse it gets.

[Bill_1856]

http://bobatkins.com/photography/technical/dofcalc.html
The most important thing to remember is that DOF is about 1/3 in front of the principal object, and 2/3 behind it.