PDA

View Full Version : Minimum focus question.

kev curry
13-Nov-2008, 10:25
I wouldn't know where to start with this one so I'm asking you good folks of higher consciousness:) ...how close will a 300mm Nikkor M / Flange focal distance of 290mm focus with a maximum of 395mm of bellows?

kev

13-Nov-2008, 11:34
From a spreadsheet I have ( I posted a copy in an earlier thread)

Focal Length (mm) 290
Distance to Subject (meters) 2.7
Distance to Subject (feet) 8.9325
Bellow draw(mm) 324.8962656
Bellows draw(inches) 12.79119156

Magnification 0.12033195
Compensation 1.255143679
Compensation in stops 0.327852522

JGB

13-Nov-2008, 11:37
D'oh I misread the bellows length.

Focal Length (mm) 290
Distance to Subject (meters) 1.1
Distance to Subject (feet) 3.639166667
Bellow draw(mm) 393.8271605
Bellows draw(inches) 15.50500632

Magnification 0.358024691
Compensation 1.844231062
Compensation in stops 0.883019421

JGB

kev curry
13-Nov-2008, 12:21
Thats excellent Jim.

Many thanks
kev

drew.saunders
13-Nov-2008, 13:57
1/(1/290 - 1/395) = 1090mm

It's 1/fl - 1/bellows = 1/distance

cowanw
13-Nov-2008, 19:25
Which leads me to the stunning and counter intuitive revelation that, for a given bellows draw, the shorter the focal length, the bigger the magnification.
Regards
Bill

Leonard Evens
14-Nov-2008, 08:46
Which leads me to the stunning and counter intuitive revelation that, for a given bellows draw, the shorter the focal length, the bigger the magnification.
Regards
Bill

What is misleading your intuition is the fact that for the same subject distance, if you decrease the focal length, you decrease the magnification. You become aware of this in practice because objects appear smaller on the gg when you use a smaller focal length lens. So you begin to identify smaller magnification with smaller focal length.

But if you fix the image distance, the subject distance gets smaller when you decrease the focal length, hence the ratio of image distance to subject distance, i.e., the magnification, gets larger.

The problem is that it is hard to think of relationships involving more than two variables, and for better or worse, many such relationships in large format photography involve multiple variables. If you find something counter-intuitive, the problem is usually that your intuition is overly simplified. Even a professional mathematician like myself sometimes has difficulties of this kind.

cowanw
14-Nov-2008, 09:48
Precisely so
Kindest Regards
Bill

What is misleading your intuition is the fact that for the same subject distance, if you decrease the focal length, you decrease the magnification. You become aware of this in practice because objects appear smaller on the gg when you use a smaller focal length lens. So you begin to identify smaller magnification with smaller focal length.

But if you fix the image distance, the subject distance gets smaller when you decrease the focal length, hence the ratio of image distance to subject distance, i.e., the magnification, gets larger.

The problem is that it is hard to think of relationships involving more than two variables, and for better or worse, many such relationships in large format photography involve multiple variables. If you find something counter-intuitive, the problem is usually that your intuition is overly simplified. Even a professional mathematician like myself sometimes has difficulties of this kind.