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Measuring speed booster lens boost ratio?
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PostPosted: Fri Mar 14, 2014 5:53 am    Post subject: Measuring speed booster lens boost ratio? Reply with quote
Hello.

I've got a nice speed booster lens, removed from some surplus observation equipment. Tried it on various lenses, and results are as follows:

The object is 1410 pixels wide on 1000mm lens without the booster, and 1050 pixels wide on same lens with booster. The boost ratio is 1410/1050=1.34, which means lens now is 1000/1.34=750mm. But what about F number? this lens is F10, and when shoot gray panel in controlled lightning conditions, it sets 1/13 sec exposure without booster, and 1/20 with booster.

So, if I'm correct, this booster is 0.75x, right? I'm asking this because it was sold to me as 0.45x booster, which it definetly is not, so I'm going to talk seller about that, but I have to be sure in my calculations.


PostPosted: Fri Mar 14, 2014 9:53 am    Post subject: Re: Measuring speed booster lens boost ratio? Reply with quote
CuriousOne wrote:
But what about F number?

That's difficult to say.
Your speed booster may act as an additional diaphragm for the new lens+SB optical system and the calculated F number for the system may not be correct.
But if it doesn't and if your SB is placed at the back of the lens then you can calculate it.

Entrance pupil diameter (EP) = FL/F (focal length / F number) = 1000/10 =100mm
By adding the SB (at the back of the lens) the entrance pupil doesn't change. The new focal length (fl) of the system is, you say, 750mm. So, the new F number (f) should be:
f = fl/EP and fl = 0.75FL =>> f= 0.75(FL/EP) =0.75F
That makes almost one F stop (for instance f/16 becomes f/(16x0.75)=f/12).

The reduction and the boost ratio may vary by changing the distance between the SB, the lens and the sensor. That mean that your SB may act with a 0.45 ratio in a certain position behind the lens (the one it was made for initially), position allowed only by the lens/lenses it was made for.

Last edited by dan_ on Fri Mar 14, 2014 11:28 am; edited 1 time in total


PostPosted: Fri Mar 14, 2014 11:05 am    Post subject: Reply with quote
Yes, it is placed on back of the lens. I've tried to add/remove some additional extension tubes, to see how it would perform. The change of position had not much impact on image dimensions, it had most effect on focusing distance - infinity was lost, but macro mode become on, when moved booster further from the lens. When moved as close as possible, I've got infinity overshoot.


PostPosted: Fri Mar 14, 2014 11:39 am    Post subject: Reply with quote
CuriousOne wrote:
...macro mode become on...

When macro mod becomes on that mean the system focal length is less then the flange distance of your camera allows (focal length reduced more then 0.75x).
It could go this way to a 0.45x factor if your camera had very little a flange distance. If it was made for an eye observation equipment the eye flange distance permits such little distance between the lens and retina. If it was an observation camera attached to it that camera could permit such little distance, too.
My advice is that, if you are content with the IQ it produces, just say "thank you" to the seller.


PostPosted: Fri Mar 14, 2014 11:42 am    Post subject: Reply with quote
Yes, IQ is excellent, actually, it might be eye fooling, but sharpness appear to be better with booster than without. It was made for observation equipment+film camera, but I suspect it was made for rangefinder camera (has M39 mount).

btw, 1000mm lens on 50cm macro is just awesome Smile
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