Making an f/stop scale

Jim Moore · Aug 18, 2004

I just purchased a 150mm G-Claron that I will be mounting in a Copal 0 shutter that I have.

I would like to try and make my own f/stop scale for it and I seem to remember reading something about measuring the f/stop opening in mm's and dividing that number into something to determine the f/stop.

Could someone help me out?

Thanks!

Jim

Mateo · Aug 18, 2004

Have you ever noticed that stops are written F/ ? It simply means focal length devided by. A 300mm lens at F/8 has an opening of 37.5 mm (300/8=37.5)

Jim Moore · Aug 18, 2004

Mateo said:
Have you ever noticed that stops are written F/ ? It simply means focal length devided by. A 300mm lens at F/8 has an opening of 37.5 mm (300/8=37.5)

I knew it was something simple like that but I couldn't remember.

Thanks Mateo!

Jim

gma · Aug 18, 2004

First you will need to remove one of the lens cells. Setting the iris diaphragm is the hardest part. I recommend using drafting dividers ( like a drafting compass for drawing a circle, but with points instead of lead ) with very sharp points to set the spacing to the correct dimension for each f/ stop using a good quality metric scale. Adjust the diaphragm to the exact spacing set on the dividers and mark on tape at the shutter rim. Repeat for each f/ stop.

Silverpixels5 · Aug 18, 2004

You can also get a cheap caliper from your local hardware store. the plastic ones don't cost much, but the metal ones can be rather expensive.

JHannon · Aug 18, 2004

If you have a copy of "The Print" by Ansel Adams, he describes calibrating apertures (Pg 66). He used a focal-plane exposure meter that has probes that can be inserted under the ground glass. If, for example, your lens is a f/8 (wide open) and you get a reading of 17, then it should read exactly 16 at f/11, 15 at f/16 etc. you need to have focus set to infinity and a white card as a target.

I have never tried it and have a lens I suspect of being marked incorrectly (in the smaller apertures).

I do not have a meter with the probes to connect to the GG. I wonder if a spot meter could be used to measure off the ground glass and get the same basic readings...

Just a thought, I would think measuring apertures to be too tedious (for me anyway)...

Tom Hoskinson · Aug 18, 2004

Yes, John. Andrew Glover (dagor77) has documented a spot meter based procedure. I have asked him to email me a copy. I'll ask him if he will share it on APUG.

bobfowler · Aug 18, 2004

JMoore said:
I would like to try and make my own f/stop scale for it and I seem to remember reading something about measuring the f/stop opening in mm's and dividing that number into something to determine the f/stop.

I have calibrated more than a few lenses that I have put into shutters myself, this is my procedure - YMMV. If I do this wrong, hopefully someone will tell me the error of my ways, but I do get excellent results with this method.

Before starting, I cut one of those 1x3 inch file folder labels to make a clean surface to mark my f/stop scale and affix it to the shutter. I take the camera outside and focus on infinity - LOCK the focus kobs when it's at infinity!

I move the camera in a darkened room with a diffused light source aimed directly at the lens (about 3 feet away), lens locked open on B or T, and focused at infinity. I keep my focus cloth handy to block any stray light.

With the focus panel removed from the camera, I use an incident exposure meter with a flat disk (I use a Liuna-Pro F with the copy attachment) and place the meter at the film plane. Using the focus cloth to block any stray light from the back of the camera, I make a baseline reading and null the meter on my Luna-Pro F. Since I know the max aperture of the lens, it just a matter of closing the aperture until the scale on my Luna-Pro F just starts to move, I the re-open the lens until the meter nulls and then I make a mark on the paper scale to indicate the max aperture. I then put the meter in the same place on the film plane and stop down the lens until the meter reads 1 stop less exposure and I make another mark on the F/stop scale. Just continue doing this until the whole scale is finished. For lenses of intermediate max aperture (such as f/4.5 or f/6.3), your first stop past max aperture will be a fractional stop, but the Luna-Pro F is calibrated in 1/3rd stops so it's easy. Uses of other meters will have to figure out ways that their meters would work in this situation.

Using this method with the Luna-Pro F, it's necessary to re-null the meter every 3 f/stops, but That's usually not a problem. The most important thing is to place the meter is the SAME place at the film plane EVERY TIME.

I've done a lot of lenses this way, it works for me!

Hope this helps.

JHannon · Aug 18, 2004

Tom Hoskinson said:
Yes, John. Andrew Glover (dagor77) has documented a spot meter based procedure. I have asked him to email me a copy. I'll ask him if he will share it on APUG.

Thanks Tom, I would like to see it.

Tom Hoskinson · Aug 18, 2004

bobfowler said:
I have calibrated more than a few lenses that I have put into shutters myself, this is my procedure - YMMV. If I do this wrong, hopefully someone will tell me the error of my ways, but I do get excellent results with this method.

Before starting, I cut one of those 1x3 inch file folder labels to make a clean surface to mark my f/stop scale and affix it to the shutter. I take the camera outside and focus on infinity - LOCK the focus kobs when it's at infinity!

I move the camera in a darkened room with a diffused light source aimed directly at the lens (about 3 feet away), lens locked open on B or T, and focused at infinity. I keep my focus cloth handy to block any stray light.

With the focus panel removed from the camera, I use an incident exposure meter with a flat disk (I use a Liuna-Pro F with the copy attachment) and place the meter at the film plane. Using the focus cloth to block any stray light from the back of the camera, I make a baseline reading and null the meter on my Luna-Pro F. Since I know the max aperture of the lens, it just a matter of closing the aperture until the scale on my Luna-Pro F just starts to move, I the re-open the lens until the meter nulls and then I make a mark on the paper scale to indicate the max aperture. I then put the meter in the same place on the film plane and stop down the lens until the meter reads 1 stop less exposure and I make another mark on the F/stop scale. Just continue doing this until the whole scale is finished. For lenses of intermediate max aperture (such as f/4.5 or f/6.3), your first stop past max aperture will be a fractional stop, but the Luna-Pro F is calibrated in 1/3rd stops so it's easy. Uses of other meters will have to figure out ways that their meters would work in this situation.

Using this method with the Luna-Pro F, it's necessary to re-null the meter every 3 f/stops, but That's usually not a problem. The most important thing is to place the meter is the SAME place at the film plane EVERY TIME.

I've done a lot of lenses this way, it works for me!

Hope this helps.

Yes Bob, this is very helpful. What this method gives you is a "T" number(Transmission) scale at infinity focus. IMO this is of more practical use (more accurate) than the f number scale.

I am going to try out your technique with my Pentax Spotmeter.

bobfowler · Aug 18, 2004

If you use a spot meter, don't remove the ground glass panel, just meter off of it.

Tom Hoskinson · Aug 18, 2004

Thanks, Bob!

Making an f/stop scale

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