Kostas writes: > hi all! Hi Kostas! :-) > i just purchased (for almost nothing) an old russian screw-mount Jupiter > lense, nice to hear you got it cheap :-) > and i wonder if you have prior experience with such lenses. no, but some Russian lenses I _have_ had experience with have proved to be really good - unfortunately their reputation for quality control is poor resulting in inconsistencies between lenses - that is to say, from the same batch one may be very good while the next is very poor. > i donot know if it is used 'cos the plastic cylindrical box and the lense > itself smell of grease. in fact there is old grease between the elements > module and the rest of the lense body which screw off. there are even traces > of grease among the 14 shutter leafs. grease or oil in the shutter blades isn't a good thing - it can slow down the speed at which the iris actuates and even prevent the iris from stopping down all the way to the correct aperture. I expect that you stop this lens down manually so this shouldn't be a problem, as long as none of the oil gets on the lens elements! > tripod and run a comparison test with all my lenses. > here are the results: > > f/22 f/16 f/11 f/8 f/5.6 f/4 f/3.5 f/2.8 f/2 > jupiter 85mm = -- 1/15 1/30 1/90 1/180 1/350 -- 1/1000 1/1500 > makinon @85mm= 1/8 1/15 1/30 1/90 1/180 -- 1/750 -- -- > pentax @80mm= 1/15 1/30 1/60 1/125 1/250 -- -- -- -- > > note that the shutter speeds run close with my jupiter and makinon ones, and > are one stop faster than the pentax(though in practice overexposure of 1 > stop is needed for the makinon). also note the ONE and a HALF stop > difference between f/11 and f/8 in both the non-Pentax lenses. that i cannot > explain... > > so what do you think? three thoughts occur to me: a)Is your pentax lens a zoom lens with a variable aperture? this will lead to different readings unless you take the _actual_ aperture into account and not just the mark on the lens barrel. b) different lenses have slightly different errors in aperture as a result of manufacturing variations, wear on the iris actuation mechanism, or oil on the blades (!) c) the optical characteristics in respect of light transmission will vary with the materials used to make the lens or materials used to coat the lens. This is why apertures marked in 'f stops' are not considered good enough for the movie industry where they rely on lenses tested to take the optical characteristics into account and give the 'light gathering ability' of lenses in 't-stops' (transmission stops). Lens flare will also contribute to light meter readings.. the way to eliminate that variation is to use a concertina lens hood, masking the lens to the exact frame format of the scene in realtion to the film (sorry if this sounds confusing).. what I mean is that you have to eliminate all light that is NOT used to make the image from entering the lens and bouncing around the lens elements and lens barrel. In reality, this will only give you a guide as to what the lenses optimal characteristics are but you should probably ignore that and test the lenses in a 'real world' setting - under the conditions you intend to use them, making comparitive shots on positive film or black and looking at them critically and objectively. hope this helps Karl
