Figuring focal length

[jwalk]

Here's a question, I thought I thought I saw someone answer this or put the

formula out there earlier on this forum but how do you figure out a lens focal

length. I thought it had something to do with measureing the width of the lens

glass and multiplying or dividing by the marked f/stop. Does this sound

familiar? I have a lens off an old ansco folder that is in a sweet shutter

and I wanted to know if I could adapt it to my speed graphic and give it a

whirl, but I would like to figure out the focal length before I get to

involved. Thanks for any help you can give me. JW

[dave_s]

The width of the glass and the aperture have nothing to do with focal length.

 

A precise definition of focal length for a compound lens is a little complex, and I'll leave that to the lens pedants. For practical purposes what you need to do is the following.

 

First, put your lens on a lensboard (cardboard is fine), and attach it to your Speed Graphic. Second, open both shutters and focus on infinity using the ground glass (no, not the sun please). Third, measure the distance between the center of the lens and the ground glass. That's your focal length, within a few mm.

 

You may be able to save some time. If your Ansco folder was for 120 film, it has a focal length between about 80 and 105mm. If you are using 4 x 5 sheet film in the Speed, it won't come close to covering it.

 

Good luck-- tell us what you end up doing.

[dave_s]

<i>The width of the glass and the aperture have nothing to do with focal length.</i><p>Heck, I take that back-- sorry, brain not firing on all cylinders today. I see what you mean by this. You could measure the linear diameter of the aperture as seen from the front of the lens. Multiply that by the marked f-number. That's your focal length.<p>This will give you a pretty rough estimate of focal length, but it may be good enough for your purposes.

[alex_lofquist]

The most precise way is with a device called a nodal slide mounted on a calibrated optical bench and viewed with an aerial microscope. Unless you have access to such a system, your best bet is to photograph a subject of known size at a known distance. You could try a yardstick at 20 yards away. Then measure the size of the image of the yardstick, multiply it by the distance from the camera lens, devide it by the subject size and you will have your focal length. If you use the yardstick at 20 yds, just multiply the image size by 20!

This is not perfect, as the focal length of a lens is figured at infinity, measurements would be from the front nodal point of the lens. However, these errors would be insignificant (less than 1%) for your purpose. You could have the yardstick at 10 yds and multiply by 10 and your error would still be less than 2%.

[jwalk]

Thanks for your answers everyone. For dave specifically....If it is 105mm won't that cover a 4x5? Or is it the spread of light which would be off? Don't people use 90mm lens all the time on their speeds as wide angle lenses? I guess I am confused on that point but totally understand your method of finding the focal length (the first one). Thans again. JW

[minhnguyen9113]

Johnnie. I did try couple 105mm lens, but the image circle is about 125mm that can not cover 4x5, beside the lens from an Ansco folder is not really good, you should look for a lens from a Kodak 616 with 126mm 4.5 or at least 6.3 those will work. The 90mm mounted on 4x5 are wide angle lens, the lens on your Ansco is normal lens will fit 2x3 maybe 3x4 if you want it to. Let me know if you want to try I'll send you a 126mm lens. Minh

[jwalk]

That makes sense, thank you for the answer and the offer. Just thought I would give it a try sometime. It is a anstigamat 6.3 and seems to have at least 3 elements. It however is uncoated. Thanks Again!! JW

[richard_oleson]

105mm is the normal lens for a 2 1/4 x 3 1/4 inch negative; 4x5 usually had a 135, though a good 127 will cover it. if the lens is marked as an anastigmat it's undoubtedly a triplet, and it might not be a bad performer. lack of coating isn't a big problem with only 3 elements, especially in black & white....

[richard_oleson]

90mm is used as a wide angle on 4x5, but the lens has to be designed as a wide angle to cover that much film. the normal lens off of a folder would be a simpler lens with a narrower coverage angle.

[jwalk]

Thank you Richard,I already canned the idea of using it on a 4x5, maybe someday I will find a use for it. It's just so darn nifty and I am facinated by how something with a patent date of 1914 can still work so darn well. Besides being a photographer I am also a gadget freak :)