1. ## Advice required as to how to diagnose an aperture.

I have done a search both on the forum and in Google and have a query as to the expected maximum aperture when using the front element only of a symmar covertable lens.

I realise that to get the converted value of the lens you remove the front element and use the rear element only for my 210/370mm f5.6/f12 symmar. But prior to knowing this I had removed the rear element and had a play but was surprised at the fact I was not receiving the focal length I was expecting. Since then I have been informed of the correct way to convert the lens but looking at the image on the ground glass and with an 8x lupe with just the rear element removed and it looks like a reasonably useable image (I would use for portraiture only). When focussed on infinity the length between the rear element and the ground glass is about 250mm (10"). Can I assume that the max aperture would be similar to that when using the rear element ie circa f12?

2. ## Re: Advice required as to how to diagnose an aperture.

Ed
I had a think about this last night , on the basis of using the thin lens equations and the data you already have, but concluded it was more tricky than I first thought . Today I have a better way, if you are up for a bit of maths and have a tape measure and some space in which to set this up ?

The f-number is the focal length divided by the entrance pupil diameter (epd) .
You need to determine these , it can be done as follows :
(a) EPD : fit the front lens (only) onto the shutter/aperture
Hold the lens , with lens facing you, and with a bright background ; open shutter & aperture up to max and using a plastic ruler ( transparent ) estimate the diameter of the circle of light .
(b) Focal length : more difficult ; doesn't relate all that well to the back focal distance in this case, so here's where we need a bit of practical work and simple maths .
This document -
http://www.ispoptics.com/pdfs/optical_design.pdf
has a diagram and an equation underneath :
f = ss' / ( s- s' ) - where 'f' is the focal length .
( the / symbol means 'divided by' if you're not familiar with maths )

So, set up the camera and focus on something like a wall about 2-3 m away, for instance. Then measure s and s' as shown on the diagram , and work out 'f ' .
The only tweak I would suggest , since the front of a Symmar is hardly an idealised 'thin-lens' , is to measure s and s' to the aperture stop, not the lens cell, or the front of the lens. This should be more accurate .
It may be a little tricky to measure s , with the bellows & stuff, but get the best estimate you can . You might take the lens off once the focus has been set, to measure the board to ground-glass distance.

As I said, then , f-number is f/epd .

3. ## Re: Advice required as to how to diagnose an aperture.

Hi Mark, Thanks, I think I'll have a go at it in the next few days.

4. ## Re: Advice required as to how to diagnose an aperture.

"When focussed on infinity the length between the rear element and the ground glass is about 250mm (10"). Can I assume that the max aperture would be similar to that when using the rear element ie circa f12? "

Why not set-up the camera and a target and pencil the target size on the GG for 210 and 370 then compare the size for the front only. Should get you into useable range with little testing.

5. ## Re: Advice required as to how to diagnose an aperture.

Originally Posted by Paul Fitzgerald
"When focussed on infinity the length between the rear element and the ground glass is about 250mm (10"). Can I assume that the max aperture would be similar to that when using the rear element ie circa f12? "

Why not set-up the camera and a target and pencil the target size on the GG for 210 and 370 then compare the size for the front only. Should get you into useable range with little testing.
Hi Paul, thanks for your comment, surely that would only give me an idea of the focal length which I am pretty sure is going to be circa 250mm, it's the max aperture I am trying to find.

6. ## Re: Advice required as to how to diagnose an aperture.

Ed, I'd say Paul's simpler idea is good, as long as you keep the object a few metres away , given that 370mm is a longish focal length .

I'd be surprised if your front section is 250mm F.L. , a quick calc suggests that this combined with a 370mm rear section would give you only 150mm for the complete lens . I'd expect it to be a lot longer, for example 450mm . On the other hand it might have quite a lot of spherical aberration, which could confuse the issue of finding the exact focal plane . With the front section, check if there's a lot of stop-down focus shift .

7. ## Re: Advice required as to how to diagnose an aperture.

If memory serves me right, the front cell alone should behave like a 420mm f:14.

Remember that in the same way as the rear cell alone requires a lot more extension than the focal length, so the front cell alone requires a lot less extension. Flange focal length is useless for determining focal length with wildly unsymmetrical lenses.

8. ## Re: Advice required as to how to diagnose an aperture.

Thanks Ole, I have some spare time today so will take some shots with all three variations, full lens, front element only and rear element only. It will be useful to see in picture form how they look.

9. ## Re: Advice required as to how to diagnose an aperture.

Okay folks, I had a try today at taking 3 images from the same position within a short time of each other with the 210 Symmar, Rear Group Only and Front Group Only.

From my field tests (or garden path tests to be accurate) I found that it looks like Ole was pretty near with his memory. The image from the Front Group was a bit thinner than the other 2 images and I suspect that the aperture is actually closer to f16 or even f18. In saying that the image was still very useable (thanks to the Caffenol) although it would not yield a truly 'fine print,' in future I would just give it an extra stop when using the Front Group only.

The really nice thing though was the Front Group only acted in the same way as a telephoto in that I only needed about 10" of bellows to focus whereas with the Rear Group only it acts like a long lens and I was struggling to get to infinity focus using the full length of the monorail and the focusing adjustment on each standard.

Images were shot on TMAX100, 1/2 sec @ f32 were developed in Caffenol CL (which has a great compensating action) were scanned at 3200dpi on a Epson V750 scanner and all were given exactly the same treatment in Photoshop, Auto contrast and some USM.

210mm Symmar

370mm using Rear Group only

??? suspect 420mm Front Group only

10. ## Re: Advice required as to how to diagnose an aperture.

The image below was taken using the front group only (assuming circa 420mm focal length) and at a marked aperture of f16 (on the secondary scale), Coby was approximately 8ft from the camera.

It seems plenty sharp enough especially for portraiture and the bokeh looks acceptable too.

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