As mentioned above, the fractional f-stop difference is trivial except in low light, when it's minor. The Takumar is worth far more, produces better results, etc. The no-name Beck can be useful if you're shooting in hazardous places and you don't care if the lens is damaged or trashed. If the f-stop difference were greater, if the Beck were a f/2 for instance, then it would be worth keeping for situations where speed is more important than image quality, like blackmail.
But it all depends on what you want to shoot, and from how far away, and what else you want the lens to do, if anything. Sell both and buy a Takumar 135/2.5, or almost any 100/2.8 or f/2 macro, or...
It depends on the light you shoot in. For low-light, faster is better. For daylight, slower is often sharper. My GOLD STANDARDS in that range are an Enna Tele-Ennalyt 135/3.5 and especially the Jupiter-11 135/4, both with astounding optics. (Less than US$90 for the pair.) Some people rob minimarts to pay for a 200/2.8 beast; my light little Tele-Takumar 200/5.6 cost US$29 and is tack-sharp throughout its range. And 'pods are handy.
So it's all a matter of where you shoot and when and how hazardously, and how much glass you want to carry, and how much cash you want to drop, etc. I'll repeat my mantra -- Ask yourself: Where do I want to go? What will get me there? How much am I willing to spend to be happy?
|