Extra stop on Super TAKUMAR 150 2.8

[hudef]

<p>After seeing how astonishing was the performance of my SMC 135 Macro 4.0 on my Canon 5DII using an adaptor, I picked up a pristine glass Super-Multi-Coated TAKUMAR/6X& 1:2.8/150 on ebay for a song($91). The thing that puzzles me is that it has a stop(no marking) that is wider than the 2.8. My guess from just playing is that it becomes an effective 1.8 at that setting. Again, just guessing, at that setting it would vignette badly on a 67 body(mine has a stuck mirror lifted up..but that's for another forum). But it takes great pix on the Canon. Can someone kindly critique my understanding and tell me what aperture I am actually achieving. By the way, the shots are remarkably sharp and distortion free despite the wide aperture. The crop factor is our friend.</p><div></div>

[hudef]

<p>The shot was taken with an MC 50 1.4 Minolta (sorry for the typo) using an adapter mounted on a 40D.</p>

[hudef]

<p>On playing around some more it appears that the lens is actually a bit soft at the extended aperture. But good at 2.8 and excellent by 4.0 on the 5DII body.</p>

[d_purdy]

<p>if you set the lens for manual stop down (depth of field pre check) you should be able to see if it opens wider after going past 2.8. I don't have that lens but it sounds strange to me. I would think that when you set it to 2.8 and look in the front of the lens with the manual stop down set, the aperture should be as wide open as it is possible to be. On some of my LF lenses I can go past the smallest fstop opening and get nearly a stop smaller fstop opening but I have never seen anything like that on a pentax lens.</p>

[hudef]

<p>I have done that. Yes it definately goes wider and the outline of the shutter blades dissapears so you have a round opening. It looks like a stop bigger. I don't have this on my 135 4.0.</p>

[jerry thirsty]

<p>A lens that is designed as an f/2.8 lens isn't going to magically open up to 1.8. Do you think 2.8 is at the little tick mark to the left of the number "2.8"? Because that's f/4 (but not labeled because there isn't room to fit all the numbers on the ring). If the aperture ring really is turning well past 2.8 to achieve a wide-open aperture, then I would say somebody took the lens apart at some point in the past and did some weird things putting it back together.</p>

[bueh]

I say the numbers on the ring are not correct in regards to the true aperture. Wide open = f/2.8. If you have an "extra stop" on your aperture ring, this means that the numbers are incorrectly marked, and aperture-ring-f/2.8 is really f/4. I wonder what happens when you try to set the aperture to the the smallest f-stop on the ring.

 

As the others said, it is physically not possible to make an f/2.8 lens any faster. I would be a neat trick, though.

[steve_rasmussen]

<p>I have this lens and at full aperture the indicater mark on the barrel points to the middle of 2.8. That's as fast as this lens goes. Another indicater that this is an f/2.8 lens is that it has a 54mm diameter front element combined with a focal length of 150mm. The math tells us that it is f/2.78. This 5 element Double Gauss lens may be the best bargain in Medium Format lenses. Pretty sharp at f/5.6. If it only had an f/45 stop like their 90-180 zoom!</p>