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Calculate aperture for 3.5/28 lens remounted in shutter
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PostPosted: Thu Mar 13, 2014 12:00 am    Post subject: Calculate aperture for 3.5/28 lens remounted in shutter Reply with quote
Hi folks

I have remounted the cells of a Mamiya-Sekor E 3.5/28 in a Pronto shutter for use on a diy camera I am building.

I'm trying to work out how to rescale the aperture. The aperture is 14mm at f3.5 and closes down to 1.5mm.

I think that the aperture in f is calculated by dividing the focal length by the diameter of the aperture, is this correct?

If so, to find f8, should I divide 28 by 8 and that will give me the diameter of the aperture for f8?

28/8 = 3.5, so should the aperture be 3.5mm wide at f8?

28/3.5 = 8
28/5.6 = 5
28/8 = 3.5
28/11 = 2.5
28/16 = 1.75

Are my figures correct or am I missing something?


PostPosted: Thu Mar 13, 2014 12:36 am    Post subject: Reply with quote
iangreenhalgh1 wrote:
I think that the aperture in f is calculated by dividing the focal length by the diameter of the aperture, is this correct?

Well, no.
The nominal opening of a lens is defined as the number obtained dividing the focal length by the diameter of the entrance pupil of the lens and the entrance pupil of the lens is difficult, if not impossible, to measure.
For instance the entrance pupil of your lens is FocalLength / f.no. = 28/3.5 = 8mm.

I use this method that is quite precise:

1.
I measure the diameter (the diameter of the inner lens facing the diaphragm or the initial diaphragm maximum opening diameter, if it's available) corresponding with the nominal f/stop of the lens.
In your case diameter is 14mm for f/3.5. =>> D=14mm.

2.
I consider a virtual lens that would have the entrance pupil of 14mm and aperture of f/3.5.
The focal length (Fl) of this virtual lens should be obtained from the ecuation :
Fl / 14mm = 3.5 =>> Fl = 3.5 x 14 = 49mm
So, for a lens with the focal length of 49mm and the entrance pupil of 14mm you should have the nominal aperture of f/3.5

3.
Now, how much would be the entrance pupil of this virtual lens if the aperture is f/4?
4 = 49/x =>> x = 49/4 = 12.25 mm for f/4.
Your diaphragm diameter, having the same opening with the entrance pupil of the virtual lens, should be diminished to 12.25mm for the f/4 aperture, too.

4.
And so on:
5.6 = 49/x =>> x = 49/5.6 = 8.75 mm for f/5.6
8 = 49/x =>> x = 49/8 =~ 6.13 mm for f/8
11 = 49/x =>> x = 49/11 = ~4.45 mm for f/11
16=49/x =>> x = 49/16 =~ 3.06 mm for f/16
22=49/x =>> x = 49/22 =~ 2.23 mm for f/22

5.
I draw this concentric circles using a CAD program, print it, cut out by the largest circle (14mm, in your case) and place it behind the diaphragm petals. By closing the diaphragm to each of the circles I can mark the F/stops.

Last edited by dan_ on Thu Mar 13, 2014 4:24 am; edited 4 times in total


PostPosted: Thu Mar 13, 2014 12:58 am    Post subject: Reply with quote
Many thanks Dan, I shall scale the aperture using your figures and try it out.


PostPosted: Sat Mar 15, 2014 12:37 am    Post subject: Reply with quote
I've got another one to figure out.

A Mamiya 4.5/180 lens for an RZ67 I have remounted into a Copal #1 press shutter from a Polaroid MP-3 Oscilloscope camera.

Sadly, this shutter has a restricted aperture opening compared to a standard #1 and this opening is smaller than the diameter of the rear glass of the Mamiya lens by a small amount. The max opening of the aperture is 33mm.

Because the rear glass of the front cell (it only has one cell) is slightly larger than the max opening of the aperture, does this throw off the calculations?


PostPosted: Sat Mar 15, 2014 5:57 am    Post subject: Reply with quote
iangreenhalgh1 wrote:
... does this throw off the calculations?

No, the calculations are the same but at point1 the diameter D="diameter of the inner lens facing the diaphragm" (you should measure it) and not "maximum opening diameter of the diaphragm", F=4.5 and FocalLength=180mm.
If you continue and count the focal length of the virtual lens and then the diameters for all f-stops the diameter for f/4.5 could be greater then 33mm and your remounted lens won't be 180mm f/4.5 anymore.
Count then the f-stop for a diameter of 33mm to find out the maximum aperture of your lens as remounted in that shutter with a smaller (33mm) maximum opening.

If you still have the old Mamiya shutter then D should be its maximum opening.


PostPosted: Sun Mar 16, 2014 4:15 am    Post subject: Reply with quote
Thanks Dan. Sadly I don't have the old Mamiya shutter. I made a mistake, the maximum opening of the Copal is 23mm.

Yes, due to the restricted aperture, the lens isn't 4.5/180 anymore, but I can live with that.

What confuses me though is if I divide 180 by 23 I get 7.8, and that can't be right, surely, I would expect the maximum aperture to be around 5.6, but 7.8 is way too low it seems to me.


PostPosted: Sun Mar 16, 2014 10:01 am    Post subject: Reply with quote
iangreenhalgh1 wrote:
Thanks Dan. Sadly I don't have the old Mamiya shutter. I made a mistake, the maximum opening of the Copal is 23mm.

Yes, due to the restricted aperture, the lens isn't 4.5/180 anymore, but I can live with that.

What confuses me though is if I divide 180 by 23 I get 7.8, and that can't be right, surely, I would expect the maximum aperture to be around 5.6, but 7.8 is way too low it seems to me.


And so it is Ian because you tried to find it by the formula Fstop=focal length/diaphragm opening witch is incorrect.
The correct formula is Fstop=focal length/ entrance pupil where the entrance pupil is the image of the diaphragm opening as it can be seen through the front lens.
Quite confusing this entrance pupil concept, I agree Smile . And you can't measure it whit home instruments.

That's why in my 5 points procedure I tried to avoid the use of the entrance pupil dimension and still get correct calculations of the diaphragm openings by using that "virtual lens".
What's the dimension of the diameter of the inner lens facing the diaphragm? You'll need to use it as a close approximation of the diaphragm opening for f/4.5 on your 180mm lens in order to count the openings for any other f-stop or the f-stop for a given opening(23mm in your case).


PostPosted: Sun Mar 16, 2014 2:20 pm    Post subject: Reply with quote
Hi Dan

I very much appreciate your help.The diameter of the rear glass of the front cell is 32mm.

Therefore, should I calculate like this:

32mm = f4.5

FL / 32mm = 4.5 + >> FL = 4.5 x 32 = 144

144 / 23 mm = 6.2

Hmm, that's disappointing, the maximum aperture being f6.2. I won't be using the lens at wide apertures but f6.2 sounds like I will have a pretty dim ground glass.


PostPosted: Sun Mar 16, 2014 3:07 pm    Post subject: Reply with quote
iangreenhalgh1 wrote:
I very much appreciate your help.

I'm glad I could help, Ian.

iangreenhalgh1 wrote:
Hmm, that's disappointing, the maximum aperture being f6.2. I won't be using the lens at wide apertures but f6.2 sounds like I will have a pretty dim ground glass.


That's correct, but you are only losing a little less then 1 stop.
The ratio between 1 ; 1.4 ; 2 ; 2.8 ;....; 22 being an ~1.4 factor that means that 4.5 x 1.4 = 6.3 is one full stop loss from f/4.5.
It's still usable, albeit not so comfortable.

Last edited by dan_ on Sun Mar 16, 2014 10:59 pm; edited 1 time in total


PostPosted: Sun Mar 16, 2014 5:26 pm    Post subject: Reply with quote
Cheers Dan. 1 stop is fine, a little harder to focus on a dull day, but no great loss.

Here's the two Mamiya lenses I remounted - 3.8/90 and 4.5/180, I used shutters from Polaroid CU-5 Oscilloscope cameras that formerly had Rodenstock Eurygons front mounted on them. Not the ideal shutter, but #1 shutters are expensive things and I'm a poor man so these ones will have to suffice.









PostPosted: Sun Mar 16, 2014 6:01 pm    Post subject: Reply with quote
They are looking nice! And I'm sure they have no problem to cover 6x9.
Are these "Polaroid Prontor" shutters self-cocking?
I only have 2 Mamiya RB67 lenses, the 65mm and the SF 150mm. I initially acquired them to be remounted in Copal shutters for my 6x9 Linhofs but neither of them matched the Copal#1 threads. My conclusion was that the RB threads are non-standard or an weird Copal#2 standard and I gave up. Nice to hear that yours fit the Polaroid Prontor threads.


PostPosted: Sun Mar 16, 2014 7:19 pm    Post subject: Reply with quote
Cheers Dan. Yes, they cover perfectly as they are designed for 8x8cm.

The shutter threads inside them are, annoyingly, non-standard, they are approx 41-42mm front and back. So what I did was grind the threads of the cells down then use epoxy adhesive to secure them to the shutter. Hardy ideal, but it works. It would be possible to re-thread them to fit a #1 shutter but I don't have a lathe. I paid very little for these two Mamiyas so I didn't mind butchering them.

The shutters are self-cocking 'press' types. A little different to the other press shutters I have as they don't have an activating lever, so you have to use a cable release to fire them. They don't have a 'B' mode either, there is a second cable release socket that opens and closes them, a 'T' mode.

I prefer a normal shutter, but I only paid 40ukp for four of these (two Prontors and two Copals), two had Tominon 4.7/127mm lenses in them, one had a Rodenstock Ysaron 4.5/75 in it and the fourth had a Rodenstock Eurygon 4.5/35 mounted on the front. I am going to use one of the Tominon 127s, if, as I have read, it shoots well at distance. The Ysaron 75 I am going to try out, it's a 4/ tessar-type enlarging lens so I doubt it will offer me anything my other lenses don't do better. The shutter for the Ysaron needs cleaning as it's running slowly.

One set of Tominon 4.7/127 cells with clean glass is available for a very modest sum if they would be of use to you, they fit a #1 shutter, of course.


PostPosted: Mon Mar 17, 2014 10:32 am    Post subject: Reply with quote
Mamiya-Sekor 4.5/180 optical block has 42x0.75 thread (so like T2). You can screw it into any T2 adapter and shot with it on bellows.
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