In theory, if we had the actual optical formula for a lens (curvature, spacing, and glass type) for each lens we could put it into one of the lens design programs and get a mathematical model of what ideal would be. Then if we were really bright, we could calculate distance from center, plug in each of those aberrations, do the calculations for each one, overlay them, and have an idea of the what the point should look like in the ideal lens. Of course, then we’d have to go back find acceptable range of variation for each element (because perfect only happens in computer programs), redo all the calculations overlay all the various combinations of those calculations and get a picture of what the point would look like for a realistically good lens.

In reality, we grab 6 or 8 known good copies, put them on OLAF, and take pictures so we know what it should look like.

RC

]]>Of course OLAF is a great tool for getting information from real instance of lens, but as far as I understand it doesn’t tell you anything about how should look reference image. In other words it doesn’t tell how close this tested instance to mathematical model of it’s optical scheme. It’s obvious that even mathematical model should have some aberrations and reference image would not look like simple grid of dots.

So it’s quite interesting how do you see (or calculate) how close tested instance to it’s mathematical model? Do you have some software which can render pictures similar to OLAF’s but use only mathematical model of optical scheme? Or you simply define best instance of lens as reference?

Best regards, Max

]]>Every type of lens is different qand we’ve learned some tendencies with many of them. But also every copy of a given lens is different. We often get some hints – for example, the center point tends to smear with decentering, all points tend to smear with spacing problems, and tilt messes up the sides with less effect on the center. With zoom lenses certain adjustable elements may effect the long end more than the short end, etc.

But adjusting one thing often messes up another, so we often find fixing a tilt problem at the long end makes a decentering at close focal lengths, fixing that makes the tilt change again, etc. etc. Then when we have it looking great on OLAF (which works at infinity) we Imatest it at 15 feet and find there’s a problem there. This is why it can take so long. It’s also why I’d pay good money to get my hands on some factory service manuals that would clarify which adjustments have which effects. For example, we’ve probably spent 80 hours with Canon 16-35 lenses just figuring out the algorithms for which adjustments have which effects and what other adjustments each adjustment messes up.

]]>Thanks for the very informative posts, I’m learning so much about lenses. I have that enquiring minds need to know thing going on but only just found out from you that I would like to know, do you adjust all types of lenses in the same order, ie centre sharpness first then move on to check and adjust for decentered elements or are some lenses done in the reverse order to other lenses, is that dependant on type zooms in one order primes in another, or by manufacturer, canon one way Nikon the other, just random depending on ease of access to each adjustment.

Oh what order would the adjustments be made, or is that something that you have worked hard to learn that gives you a commercial advantage that you wish to retain? BTW I fix cars not lenses so it won’t help me!

And yes I got the date thing even though you put it right, I like your reasoning for the correction too!

Thanks again,

Cheers Graham.

]]>What’s a “roung point” – another Cicala invention?

]]>Of course, on the superetelephotos that this is more interesting for, the aperture of the collimator is smaller than that of the lenses. But it would probably work for things like comparing a naked 300/4 to that with a 1.4x teleconverter.

]]>I was time reversed

Now I get it.

]]>Things we sell through Lensauthority.com are all tested, but not necessarily adjusted. We are starting to pick out some especially sharp copies, though, and listing them as such.

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