Depth of Field for Enlarging Lens

[ASA1000]

You can calculate the optimum aperture for enlarging warped negatives, or curved field lenses, based on the focus spread (based on view camera focus equation of Hansma).

N = 20/(1+M) * square root of 'dv'

N = Aperture number
20 = user dependent constant (circle of confusion 0.15mm for me)
M = magnification
'dv' = millimeters of focal depth on the enlarger column.

Yes the optimum focal point is, for all practical purposes, in the middle point of the 'dv' or focal spread. Just like focusing a view camera. The 'dv' is obtained by moving the whole head up and down and recording the distance between sharp and blurry.

Thank you. This is good information for me!

[ASA1000]

Depth of field...now that's pretty straightforward. Dewpth of field, on the other hand...is a whole different kettle of fish!

LOL !! I was typing too fwst.....

[ASA1000]

Didn’t you mean depth of focus?

sigh...... field of focus :-)

[ic-racer]

One convention indicates the light rays go from the "Depth of Field" area to the "Depth of Focus" area.

So, based on that convention, in an enlarger the "Depth of Field" area is the negative. It [the enlarger] is a view camera set up for a high magnification macro. All the math is the same.

[Drew Wiley]

Slop? All depends. When everything else is sloppy, it will just add a little more to that look. With the relatively long focal length enlarging lenses used for large format printing, along with the lesser needed image magnification in print, there is distinctly more fine focus latitude than when working with MF negatives enlarged to the same size, where even a tiny amount of incorrect focal distance might have an impact unless the lens is stopped way further down than ideal.

[jnantz]

Like the title says; how much depth of field is there in an enlarging lens? Is there a way to know? Do shorter focal length enlarging lenses have more depth of field, like a wider angle lens on a camera?
Does everyone know about this, except me? !!
thx!

ASA,

it is like every other lens but you might run into people who say that it's use is optimized for 3 stops closed from wide open and how it is also optimized for flat field work. I've used a flat field lens for on-camera shooting for as long as I have had a LF camera (was sold on the camera as its taking lens when I bought it). ... after using it for IDK 25 years the internet told me everything from it won't take "good" pictures, to it won't cover the negative to it's a piece of junk ... maybe your lens/s will work too, one never knows ..

[Drew Wiley]

Some grain magnifiers have a built-in disc of cardboard down there. And that can be a bad thing, because it can shrink or swell depending on the humidity, and change thickness to a real degree.

[ASA1000]

ASA,

it is like every other lens but you might run into people who say that it's use is optimized for 3 stops closed from wide open and how it is also optimized for flat field work. I've used a flat field lens for on-camera shooting for as long as I have had a LF camera (was sold on the camera as its taking lens when I bought it). ... after using it for IDK 25 years the internet told me everything from it won't take "good" pictures, to it won't cover the negative to it's a piece of junk ... maybe your lens/s will work too, one never knows ..

Thanks! I'm working hard just to get back to making prints like I was 25 years ago. The images I made back then were more than adequate!
My depth of field question was related to enlarger alignment. While adjusting mine, in this new darkroom, I noticed that it takes a lot to influence the image but I didn't try different apertures...so I was wondering ...

[Mark J]

It's a good question and it's very relevant if you are aligning an enlarger column or setting the tilting lens stage ( in the case of eg. a Meopta ).
I'm not going to spend time arguing about which is 'field' or 'focus' for an enlarger !
The depth of focus at the film will be quite critical, as per a taking lens. If you have your enlarger lens set at f/8, then for critical sharpness, depth of focus, ie. the enlarger focus knob adjustment, will be around +/- 0.07mm ( the rule of thumb is ±2.lambda.F^2 for diffraction-limited, where lambda is the wavelength in mm and F is the f/number ).

If you are enlarging by X5, then the f/number at the paper side will be around f/40.
Using the above formula for the depth of ...focus... there , it will be much larger, around ±1.75mm.
You can also estimate the focus tolerance at the paper by multiplying the depth of focus at the film by the square of the enlarging magnification.

In actual fact the f/number won't be quite f/8 when you are using the lens at a finite mag ( ie. not infinity at one side ) and there is a more accurate formula for the f/numbers at each side, but it's not worth worrying about the differences for typical magnifications above about X4.

[ic-racer]

Based on the combined diffraction and Airy disk model and CoC size of 0.15mm in the print,

Enlarging lens at f8 and 5x enlargement allows 6mm of "Depth" at the baseboard side. (Equation is in post #4 above).

The equation is based on empiric data; anyone can see for themselves what 6mm does to the projected image.