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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 08 Feb 2011 (Tuesday) 09:10
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How accurate are focal lengths?

 
Hoppy1
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Feb 12, 2011 09:37 |  #46

juanpafer wrote in post #11827799 (external link)
Agree.
f/ versus T-stops: That is another huge topic! and don't get me started with this (external link).

Haha yes! I think the jury is still out on a lot of this stuff, but collectively there are quite a few issues around focal length, T/stop, ISO and vignetting etc.

TBH I don't really care what the answers are, I just want to know the truth of what's going on, so I can make informed decisions. I don't like being left in ignorance, or worse, being taken for a fool with deceit.

Some of this stuff falls well outside 'acceptable tolerances' or hidden behind 'established standards'. Manufacturers are undoubtedly exploiting the lack of transparency here.


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gasrocks
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Feb 12, 2011 10:36 |  #47

So I finally got the 300mm lens that I can afford. Takes wonderful pix, high keeper rate, fun to use. It is everything I wanted. Later I find out it really is only a 280. Should I be upset?


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Wilt
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Feb 12, 2011 11:18 |  #48

melcat wrote in post #11827256 (external link)
No, there were something like 5 versions of this lens. They didn't even have the same number of elements and groups. That's not sample variation.

Thanks for pointing the versions out...it makes more sense why there would have been multiple results over the years. Now, given the multiple versions, it is more impressive that Olympus managed to keep variation within a few tens of a millimeter if the optical formula changed, less so if mere mechanical updates and cosmetic changes (like the silver vs. black front edge of the barrel) were made.


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Skaperen
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Feb 12, 2011 13:28 |  #49

Tony-S wrote in post #11826533 (external link)
Focal lengths are calculated at infinity focus. That's the only reliable metric.

That fails to tell you the complete nature of the lens. A lens with internal focus does change its focal length. This is an advantage when it shortens the lens for closer focusing, because the movement distance is smaller. But it is a disadvantage for working distance.

What would be best, IMHO, is for a "complete spec sheet" list the focal length range. A "very complete spec sheet" would show graphs of several factors as they vary across focus setting, including focal length (of prime lenses), rear element distance, rear pupil distance, rear pupil width, front element distance, front pupil width, working distance to front element, working distance to front pupil, and working distance to sensor/film plane. For a lens with no internal focusing, where focus is done strictly by movement, having the focal length of the lens, and a few other numbers, lets you calculate all the rest. But with internal focus, there would be quite a lot of numbers needed to do all this by calculation. Just having the numbers or graphs would show people like me all we need to know.

Not everyone wants all this. So it is adequate for the numbers to be available on the manufacturer web site, instead of from dealers. But dealers can still have them under a "specs" tab. In many cases, I would not need the numbers to decide what to buy, but might need them to know how to use for very technical shots.

Tony-S wrote in post #11826533 (external link)
Yes, even the best engineers cannot violate the laws of physics.

But they can be truthful about what the actual lens factors are. Personally, I think a 5% tolerance for the design vs. marketing is fine. So if it is marketed as 70-200, but actually is 66.5-190, I'm fine with that. For what I would use a zoom for, I generally won't need to know the numbers for usage. But in some cases I might need to know the numbers for some uses of prime lenses. So I would want to know if that 200mm prime lens is actually 203.75 at infinity and 178.25 focused down to 0.2x due to internal focus (just picking numbers arbitrarily). A lens that is designed so that its angle of view remains constant over focus would be nice. I'd have to do the maths to figure out what the focal length variations should be to accomplish that.

Tony-S wrote in post #11826533 (external link)
Everyone who is informed understands this. It's only the uninformed who assert that its some conspiracy by the companies that manufacture lenses.

I don't think there is any conspiracy to deceive. At most it is a conspiracy to simplify things for the average buyer, and a conspiracy to reduce the cost of marketing brochures and such. I do have some concern about the variation of these numbers between different units of the same model, as an indication of manufacturing consistency.


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Skaperen
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Feb 12, 2011 13:30 |  #50

Wilt wrote in post #11828508 (external link)
Thanks for pointing the versions out...it makes more sense why there would have been multiple results over the years. Now, given the multiple versions, it is more impressive that Olympus managed to keep variation within a few tens of a millimeter if the optical formula changed, less so if mere mechanical updates and cosmetic changes (like the silver vs. black front edge of the barrel) were made.

Aren't changes like that what "mark II" and "mark III" and such are for?


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Wilt
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Feb 12, 2011 14:15 |  #51

Skaperen wrote in post #11829169 (external link)
Aren't changes like that what "mark II" and "mark III" and such are for?

That kind of convention was not always followed. For example, Pentax made a normal in the Takumar series, and when they came out with multicoating (the first in the industry) the lenses were referred to as Takumar SMC. I'd have to look up the old lenses to know whether or not there was an optical formulation change (elements, groups) as well, or not.


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Skaperen
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Feb 12, 2011 19:29 |  #52

Wilt wrote in post #11829388 (external link)
That kind of convention was not always followed. For example, Pentax made a normal in the Takumar series, and when they came out with multicoating (the first in the industry) the lenses were referred to as Takumar SMC. I'd have to look up the old lenses to know whether or not there was an optical formulation change (elements, groups) as well, or not.

That would come across as a whole new product with a specific designation. My focus was on the idea of changing the lens design (configuration) substantially, but not in any way changing any indications of it being different. I didn't mean that "mark II" was the only way to do it. "SMC" works, and even sounds meaningful (although "Super" is a marketing weasel word). Software tends to have versions (like "2.0"). I guess it all depends on how radical the change is.


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melcat
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Feb 12, 2011 19:56 |  #53

Skaperen wrote in post #11829169 (external link)
Aren't changes like that what "mark II" and "mark III" and such are for?

Well, they didn't. There are subtle differences in the cosmetics and the markings on the bezel (stuff like "f=50mm" v. "50mm", I kid you not). I suppose each company had to balance the desire to let the market know there was an improvement against the desire not to have a large number of old pattern lenses stuck in the channel.




  
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Skaperen
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Feb 13, 2011 07:35 |  #54

melcat wrote in post #11831165 (external link)
Well, they didn't. There are subtle differences in the cosmetics and the markings on the bezel (stuff like "f=50mm" v. "50mm", I kid you not). I suppose each company had to balance the desire to let the market know there was an improvement against the desire not to have a large number of old pattern lenses stuck in the channel.

Isn't that what rebates are for, to clear the old crud out? We end up with confusion when two or more different models masquerade as one model. In many cases the new version suffers from the bad reputation of the previous. What a manufacturer should want, at least long term, is to get people to replace their old one with the new one.


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Mk1Racer
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Feb 13, 2011 10:50 |  #55

gasrocks wrote in post #11801098 (external link)
How big is a 2X4? Numbers given are nominal they will say. Some 500mm lenses are really 478mm, etc. Especially shows up when comparing images taken by 2 different zooms with overlapping ranges.


Please tell me you didn't just use the accepted practices for dimensional framing lumber to justify why the advertised focal length of a piece of precision optical equipment may not be correct.

And while a 22mm variance may seem like a lot, it's a lot different on a 500mm lens than it is on a 200mm lens.

I don't expect the focal length to be accurate down to the millimeter, but I do expect it to be w/in a reasonable percentage, and to me, double-digit percentage variations are not reasonable.

The other issue, is that it seems to vary from mfg. to mfg (and certainly varies from lens to lens). For example, I'm weight a Sigma 120-300 f/2.8 vs. a Canon 300 f/2.8. All other differences (IQ, etc.) notwithstanding, I expect the advertised focal lengths to be close. If the Sigma is 280mm at the long end, compared to 295mm for the Canon, that may not be that much of an issue. If it's 260mm vs. 295mm, that's pretty significant.

It would be nice if the mfg's published actual focal length at say 3 different distances, MFD, Infinity, and say 50'. Or maybe just MFD and infinity, and you could assume that a rough interpolation would be close enough.


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Skaperen
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Feb 15, 2011 05:46 |  #56

Mk1Racer wrote in post #11834330 (external link)
Please tell me you didn't just use the accepted practices for dimensional framing lumber to justify why the advertised focal length of a piece of precision optical equipment may not be correct.

Which practice are you referring to? The standard dimensions, which are not 2 inches and 4 inches, or the fact that from batch to batch, and even to some degree from stick to stick, the dimensions vary.

Mk1Racer wrote in post #11834330 (external link)
And while a 22mm variance may seem like a lot, it's a lot different on a 500mm lens than it is on a 200mm lens.

Don't you mean that the other way around?

Mk1Racer wrote in post #11834330 (external link)
I don't expect the focal length to be accurate down to the millimeter, but I do expect it to be w/in a reasonable percentage, and to me, double-digit percentage variations are not reasonable.

I'd agree. There is the design point, where the design might be off by some percentage from what is marketed. Then there is the unit to unit variation around that design point. The latter is what concerns me more. It should be a lot less.

Mk1Racer wrote in post #11834330 (external link)
The other issue, is that it seems to vary from mfg. to mfg (and certainly varies from lens to lens). For example, I'm weight a Sigma 120-300 f/2.8 vs. a Canon 300 f/2.8. All other differences (IQ, etc.) notwithstanding, I expect the advertised focal lengths to be close. If the Sigma is 280mm at the long end, compared to 295mm for the Canon, that may not be that much of an issue. If it's 260mm vs. 295mm, that's pretty significant.

Agreed.

Mk1Racer wrote in post #11834330 (external link)
It would be nice if the mfg's published actual focal length at say 3 different distances, MFD, Infinity, and say 50'. Or maybe just MFD and infinity, and you could assume that a rough interpolation would be close enough.

MFD might get tricky to compare. Maybe a standard magnification? I might suggest 1 meter, but some teles probably can't focus that close. Maybe 10 meters? Whatever is chosen, it can be extreme for some lenses.


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How accurate are focal lengths?
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