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09-19-2010, 11:41 AM   #1
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75mm f/4.5 SMC Takumar Lens Stops

Sorry to sound like a dimwit, but something that has been coming up from time to time is wondering what the first half f-stop is on the 75mm f/4.5. I have the Takumar version and I am very happy with it. I use is often for astrophotography and use it wide open without difficulty. It's a great lens. When I quote f-stops, I'd like to be accurate.

I sometimes stop down to what seems like the first half stop. Most refer to this as f/5.6. I question if this is truly f/5.6 or is it f/6.2 or f/6.7?

Trying to calculate the aperture by using a millimeter ruler is difficult, but I seem to be coming up nowhere near f/5.6 and more like f/6.7. Using this same method to measure f/4.5 and f/8 confirms that the first click on this lens is indeed f/6.7.

One must think one-half stop back from f/8 and not one-half up from f/4.5.

Has anyone seem any published literature to confirm this?

I suppose one could meter a uniform object with say, the 75mm f/2.8 at the questioned f/stop and switch to the 75 F/4.5, thereby confirming stop to stop. I do not use a metered prism.

Thank you.

Jim

09-19-2010, 02:48 PM   #2
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I have the SMC Pentax 67 version of that 75mm. I can clearly see by looking at the aperture flutes alignment with the f-stop numbers that when I stop down from wide open to the first "click" and then move it again to f8 and beyond, the distance the ring moves is not the same. That is, the first click moves more which would support the idea going from f4.5 to f5.6 being more than a 1/2 stop.

Last edited by tuco; 09-19-2010 at 02:53 PM.
09-19-2010, 05:18 PM   #3
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QuoteOriginally posted by tuco Quote
I have the SMC Pentax 67 version of that 75mm. I can clearly see by looking at the aperture flutes alignment with the f-stop numbers that when I stop down from wide open to the first "click" and then move it again to f8 and beyond, the distance the ring moves is not the same. That is, the first click moves more which would support the idea going from f4.5 to f5.6 being more than a 1/2 stop.
Thanks,

Yea, I see the same thing. The ring turns a bit more than half a stop, but nowhere near a whole stop. Perhaps f/5.6. f/4.5 to f/6.7 would be more than one stop.

But looking at it backwards, opening up from f/8 to the unidentified stop is the same distance as the other half stops. Ergo f/6.7

Clear as mudd.
09-20-2010, 11:25 AM   #4
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I used the 75 Takumar for years but no longer own one. I do recall however that the f/4.5 stop is not a half stop between f/4 and f/5.6. I believe the true half stop is closer to f/4.7. So when you are at f/4.5 with the 75 and move to the next click, it is more than a half stop and is probably f/5.6

09-20-2010, 04:28 PM   #5
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Thanks. Yea, it's 4.5 wide open with only one click between 4.5 and 8. The first click down is a "longer" turn than the remaing half stops. I'll call it f/5.6, but looking at the actual aperture blades, it appears smaller. The math would be simple if I could just get a measurement of the blade opeings. Eyeing a ruler in front of the opening measures about f/6.7, or about 11mm, but looking through the front element might be giving me a false reading.

I shall not persue the matter further. Thank you both for entertaining me.
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