1. ## Verito f-stops

After a bunch of years, first post ever.wow
I have an 8 3/4" verito in barrel. I wanted to make some f-stop marks for the back cels When I measure the apature wide open, it is 1 3/4 diameter.when I do the math , it doesn't add up to F4. for a 83/4" lens .Is the f4 not an accurate number? Should I use the measured opening, and use that as my f-stop?
Thanks for any help, Lloyd

2. ## Re: Verito f-stops

Are you measuring the aperture or the entrance pupil? You should be measuring the entrance pupil which is the apparent diameter of the aperture when seen looking through the lens.

3. ## Re: Verito f-stops

I measured at the blades. If I measure at the diameter of all the glass that I can see, it is about 2 1/8" which is pretty close to f-4. Is that the entrance pupil?

4. ## Re: Verito f-stops

What is the focal length of the back cells you are using?

It is 14"

6. ## Re: Verito f-stops

Hi Lloyd

I read somewhere by someone much smarter than me that the converted fstops are about 1.25 stops different
so your f4 will be a smidgen more than f5.6 and so on. These numbers I was given were not exact but I figure my exposures don't need to be perfect.

You might be able to figure it out exactly with this chart because it has some of the "in-between" fstops no one ever talks about
https://en.wikipedia.org/wiki/F-numb...gm_Numbers.gif

1 stop and a little more
f4 > f6.3;
f5.6 > f8.66;
f8 > f12.25;
f11 > f 17.32

and so on ..

... it might be wrong though i did not use any math just estimation 1 number up from the "regular fstop" with the chart.

7. ## Re: Verito f-stops

Originally Posted by lloyd goode
I measured at the blades. If I measure at the diameter of all the glass that I can see, it is about 2 1/8" which is pretty close to f-4. Is that the entrance pupil?
Yes. If you look into the front of the lens through the glass, the aperture you see is the entrance pupil. Unless the lens you have has been modified somehow, the lens should be f/4 wide open.
For the sake of the converted mode (when the front cell is removed and using only the rear cell), the entrance pupil will be the actual aperture of the iris.

Since the standard lens (unconverted) is 8 3/4" FL and the design speed is f/4, the aperture should conform within reason to their quotient or 2.187" (which is too many significant digits, but it's for the sake of argument).
Plugging that number into the same equation for the focal length of the rear cell alone (14") gives f/6.4 which represents the speed of the rear cell alone. You can go on from there.

But to be clear, in the converted mode (i.e., rear cell only, 14" FL) the entrance pupil is the physical aperture diameter and it is that which you should use to calculate the series of stops for the rear cell. I hope I've not clouded the issue earlier on.

8. ## Re: Verito f-stops

William is got the right parameters.

9. ## Re: Verito f-stops

Yes, I understand that, but the problem lies in the fact that the apapture can not open farther than 1 3/4 ".
Is it possible that the barrel is for a 7" lens , and the parts were miss matched? It focuses at infinity at 8 3/4"
Thanks

10. ## Re: Verito f-stops

Never say "never". But I should think it highly unlikely that parts are mis-matched. That the lens is in a barrel suggests that it is the factory configuration. I would be more suspicious if it were in a shutter. But there is no telling.

When the aperture is wide open what is the indicated f/stop? It should be f/4. My own 8 3/4" Verito was transplanted into an Ilex shutter, so it's really not reliable as a comparison. But I have to ask: Does the aperture ring on the barrel open a far as f/4 (indicated)? Does the iris (in the barrel) feel at all like something is blocking it? Does it operate smoothly or does it feel as though there is some resistance, possibly indicating a mechanical issue?

Perhaps someone on the forum who has an 8 3/4" Verito in barrel would be kind enough to measure their aperture wide open with the front cell removed.

I agree that it sounds a bit constricted at the wide-open setting. That would equate to f/8 for the rear cell (in the converted mode) which is a little slow, but it's not far outside of the norm.

By my personal experience, fwiw, in practice you're going to find an aperture that gives you the look you want and then you're going to adjust everything else around it, light, film speed, shutter speed.
But I understand that's not the question here. It's hard to make any further judgement without seeing the actual lens.

Final thought: You did indicate previously that the entrance pupil measured at about 2 1/8" which would put the overall lens at f/4.1 —> f/4. That's a good indication that your lens has the correct parts. Otherwise it would be a mess.
Keep us posted.

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