Formula for using a front cell only

[Jonathan Brewer]

Assuming that by doing this, I get a usable image, is there a rule of thumb/formula for using the front cell only of a LF lens in terms of a different focal length from the lenses original configuration? Does it depend on the construction of the lens? If I take the rear cell off the shutter, then focus w/the front cell to infinity, will that give me a true indicator of whatever the change in the focal length happens to be?

[Leonard Evens]

I believe that the sum or the reciprocals of the focal lengths of the two components adds up to the reciprocal of the focal length of the combination. (But I could be wrong.) If so, if you knew the focal length of the rear compoenent and the focal length of the combination, you could calculate the focal length of the front component. But if you only know the focal length of the combination, I don't think there is any formula which could help you.

If you use the front element to focus at infinity, you can measure the distance from the lensboard to the infinity position, and for many lenses, but not all, that might be pretty close to the focal length. Also, if you knew enough details about how the lens was designed you could find the exact position from which to measure.

[Jonathan Brewer]

It's a 229mm lens, the specs refer to its construction as........... '4 elements in 2 air-spaced doublets'..................when I remove the rear cell, I get nothing with the lens on my 4x5, on an adaptor mounted on my 8x10, it has enough bellows draw to present me with an image, it appears as a lens with a much greater focal length since I have to crank the bellows out quite a bit.

[Donald Qualls]

What the powers of front and rear groups might be depends very strongly on the design of the lens. For instance (just to pick a bad example), if I take the front group off a Cooke triplet, I'll get a remaining (simple meniscus or planoconvex) lens with a *shorter* focal length than the whole lens; this is becuase the concave middle element is stronger than either of the convex elements.

Also worth nothing that with some lens designs (again, the Cooke triplet is a prime bad example) the remaining glass is poorly corrected for color, aberrations, and/or distortion (the rear singlet of the Cooke has a lot of spheric aberration, chromic aberration, and IIRC a significant amount of barrel distortion).

Conversely, with my Componon, I find there's little difference in image quality between using the rear group alone as compared to using the whole lens.

I might mention that most convertible lenses that aren't listed as "triple" are more commonly used with the rear group alone than with the front group alone -- I don't know if this is because of better correction, or because it's less of a PITA to remove the front group (which hangs right out there in front) compared to pulling the lens/shutter/board off the camera to remove the rear group.

[Ernest Purdum]

Some LF lenses are highly asymmetrical and one cell alone will probably not give a usable image. Symmetrical lenses such as yours are apt to provide an image,but the quality of the image would probably vary considerably from one lens to another.

The reason for taking off the front cell when using one cell alone has to do with distortion (as well as the PITA factor).. If the diaphragm is behind the lens, pin-cushion distortion results. In front, the distortion is the barrel type, generally thought of as the lesser evil.

Focusing at infinity and measuring the distance to the groundglass will give you a rough estimate of the focal length, good enough for most purposes. Accurate measurement needs a lot of bellows. You focus at infinity, mard the amount of extension, then refocus until the image is exactly the same size as the subject, (you need two good steel rules for this) then measure the amount of extension from the mark at infinity to the new position.

[Jonathan Brewer]

I'm getting a usable image for what I plan on using this config for, when I mount the front cell of this lens on my 8x10, the front cell is sitting in front of my Ilex #5, with both cells on, the lens opens to F4.5(the width of the iris diaphragm on the Ilex is close to 2.5 inches perhaps a little smaller.

If I get a rough estimate of the focal length of the front cell only, focused at infinity on my 8x10, can I divide that number by 2.5 to come up with an F number, or is this being simplistic?

[Jonathan Brewer]

Actually I mis-spoke, the lens is in a Copal 3, so the question would be, can I divide the length I come up w/the lens focused at infinity by the width of the iris diaphragm to come up with an F number, or am I missing something?

[C. D. Keth]

Actually I mis-spoke, the lens is in a Copal 3, so the question would be, can I divide the length I come up w/the lens focused at infinity by the width of the iris diaphragm to come up with an F number, or am I missing something?

Almost. The single cell focused at infinity will tell you the focal length in close-enough detail.

Since the iris is behind the cell, just measuring the iris opening isn't quite good enough. What you want to do is focus the cell at infinity. Then replace the groundglass with a piece of card with a small hole in it, perhaps an eighth of an inch. then place a lightbulb behind the new "pinhole back". Now place a white piece of paper in front of the lens. Now you can measure the circle of light projected onto the paper at each stop mark to find the entrance pupil diameter. This is the diameter you want to use in your calculation above.

You can either measure the diameters at existing iris marks to get adjusted f-stops for the single cell, or you could calculate the diameters for regular standard f-stops and then make new marks where those fall on the iris scale.

[Jonathan Brewer]

Christopher...............let me see if I have this straight, replace the groundglass w/the front cell focused at infinity, with a piece of card(cardboard?) with a hole approx. 1/8 of an inch, place a lightbulb in front of the card(how far away?), then place a white card(how close or does it matter?) in front of the front cell, and measure the diameters at each higher/lower setting(Fstops marked on the shutter) on the diaphragm?

Going from wide open to closed down on the iris diaphagm stop by stop as you measure the sucessively smaller diameters, is that it?

[C. D. Keth]

Christopher...............let me see if I have this straight, replace the groundglass w/the front cell focused at infinity, with a piece of card(cardboard?) with a hole approx. 1/8 of an inch, place a lightbulb in front of the card(how far away?), then place a white card(how close or does it matter?) in front of the front cell, and measure the diameters at each higher/lower setting(Fstops marked on the shutter) on the diaphragm?

Going from wide open to closed down on the iris diaphagm stop by stop as you measure the sucessively smaller diameters, is that it?

The distance of the lightbulb from the black card doesn't matter much, a few inches should be fine. The white card should be close to the lens, a centimeter should do fine. All this is doing is turning the camera into a projector and measuring the diameter of the unfocused image at its smallest point: near the lens.

[Jonathan Brewer]

Thank you Christopher, I appreciate it, I'll measure it this way and see what I get.

[Ernest Purdum]

Will the front cell screw on in place of the rear cell? This arrangement is usually considered preferable for distortion reasons. Also, with the lens in this position, the iris opening divided into the focal length gives you the f stop accdurately.

[Ted Harris]

Jonathon, FWIW while I can't give you an exact formula I can tell you that I have shot with the front cell only of an Apo Symmar 210 ... did it by accident while shooting formal shots for a wedding ... too long a story for here and yo can find it in the archives. Bottom line is that it appeared to be somewhere in the 300-360 range or a bit longer and that the images came out fine.