Help on COF/DOF for P67 lenses

Hi all,

 

I'm a newbie to this excellent forum. As someone who's recently

moved into medium format and a few months back invested in a P67-II

(having had a used RB67 for a short while) I would describe myself

as an enthusiastic beginner! Landscape photography is my main

reason for moving to MF and the P67.

 

My questions concerns DOF and Hyperfocal Distance focusing. Both

concepts I understand, and like several posts on this forum I miss

DOF scales from recent lenses - my 55-100mm zoom in particular. In

order that I can make use of DOF tables or Palm/PocketPC

applications that can calculate DOF in the field, I presume it's

necessary to know the Circle of Confusion (COF) of the lenses I

use. Is there somewhere I can find the COF for each lens I own or

do I just use a blanket COF value, (i.e. I know that all Canon EF

lenses have a COF of 0.035mm), for Pentax Lenses or Medium Format in

general?

 

I also get confused when I read elsewhere that a COF value should be

used depending upon the final intended use of the print and how much

it is expected to be enlarged. Then I get just plain confused....

If someone could point me in the right direction I'd appreciate it.

 

I've always found this forum to be an incredible place to find

information, and have based many purchasing decisions on information

researched here. Thanks to everyone taking part.

 

Regards

Hi Martin,

with my 55/4 I consider a circle of confusion of 0.045 mm . This is my personal taste of course and if I relied upon the DOF marks on the barrel I would be completely out of track.

My choice does not relate in any way to considerations of enlargements and/or viewing distance from the print but simply what I consider acceptable and pleasing under my 8x loupe .

 

Cheers

 

Roberto

There is a problem with using DOF tables for the 55-100 because the barrel does not delineate exactly enough where the distances are. So, even when you know a hyperfocal distance, it is difficult to set the focus to that distance. I have put hyperfocal marks on mine(f/32 and f/11 only) based on my other lenses in that focal length range. This required some trial and error to get them exactly right however. My f/32 hyperfocal marks for my 55-100 are; for 100mm, half way between the 5 and 8 meter marks, for 75mm, 12 feet, for 55mm, 6.5 feet.

Hi Martyn,

 

There is some Confusion about the COF

Here are mine thoughts, I'm not claiming to know the truth.

 

The Circle of Confusion is a measure of accepted unsharpness on the negative. Imagination a very small light spot, if it is in focus it will occupy close to no negative area, when it becomes out of focus it will be come bigger and bigger. When it reaches the diameter of 0.035 mm (Note that this is a one dimension notation)in terms of 35mm we still experience this as sharp. A small calculation shows that 40 line pairs/mm resolution gives a "feature-size" of 1/(2*40) = 0.0125 mm so only 3 times smaller in diameter as the Circle of Confusion.

 

In the end we are not interested in the negative but in the final print / slide!

 

The experience of sharpness is limited by the resolution of your Eyes!

Since the number and arrangement of sensitive elements in a human eye limit are fixed at 8 line-pairs /mm at a normal viewing distance 30 cm!?!.

 

This leads to a max print size of 40/8 * 36 mm 18 cm wide for 35 mm

and 40/8 * 69 = 34.5 cm wide for 6*7

 

With increasing print size also the viewing distance increases.

So this helps

To go short: The circle of confusion in the final print of a 35 mm film at 18 cm wide is (180 mm / 36mm ) * 0.035 = 0.175 mm

If we keep every thing the same but use 6*7 instead

we get 0.175 mm /(180/70) = 0.068mm

 

So the circle of confusion should be linear to the length scaling of the negative. ( because diameter is the one dimension field)

 

Basically I would expect that what was linear to the (negative) area so it would be 0.035mm * 4 = 0.14mm that would be true if the CoF was expressed in area (two dimensions)

 

If I'm right in MF 0.2mm CoF is used.

 

My guess is that the MF Cof was defined earlier and that it is more relaxed than for 35mm.

 

Kind regards

 

Gert Jan

Martyn, I do not know if this is of any help for you, but comparing the results of my little DOF spreadsheet application with the figures published by Pentax for certain lenses, I found out that Pentax assumes 0.05mm COF for their medium format lenses. Thus, if you feel comfortable with the DOF scales on your fixed focal length Pentax lenses (assuming you have one), you might use this number. If you think the scales are too optimistic, you might decrease this number (say to 0.045mm in a first attempt). Best regards

Hi all,

 

Thanks to all of you who've contributed answers, it's all been very useful. I just this evening stumbled across a free windows application called DOFMaster ( http://dfleming.ameranet.com/custom.html ) Not sure if any of you guys have used it - it seems like it could be useful for producing DOF scales, even for zoom lenses!!!.

 

Having played with it a bit and tweaking the COF values in the program whilst comparing the results to the DOF scale on my 45mm lens - I can confirm that my 45mm lens appears to have a COF of around the 0.045mm to 0.05mm (which was also mentioned in a couple if your replies).

 

So I guess I should base any calculations I do in future around that value and experiment a bit and see if I like the results.

 

Many Thanks

 

Regards