chaz lyon

These days you rarely get time to mark a fresh ring every time you change lens. and lets face it if a gear slips you're boned anyway. As far as primo's go you shouldn't need to eyefocus anything under 50 mm as you should have reasonable depth. Projecting your lenses and shooting tests will give you more confidence in them when possible. but I would def prefer eye focus over marks at lengths of 150mm upwards. ( If using a doubler or diopters then eye focus essential) Bottom line, if its sharp to eye its sharp, especially on zooms. The blue mark on primos is at its biggest distance on wide lenses and by the time you get to 75 or 100 there is little if anything in it. (though each lens is different.) The lenses without blue lines were simply too close to each other to engrave 2 lines. As far as working to the blue line on the preston is concerned the solution is simple. Use a tape triangle on the preston to change the datum line to match your lens. Your throw remains the same. Some ignore them and get by fine. I use them and if shooting at an intermediate stop, i.e T2.5, I move the datum to a half way mark.