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10-01-2017, 02:26 PM   #1
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Tamron 70-200 - true focal length values?

So I wanted to know if there's a way to calculate true focal values for the lens (for portraits) keeping in mind the focus breathing this lens is prone to (as well as many other 70-200's for other systems).
Reason is - trying to figure out if it's worth getting a faster prime to play with (something like SY 135 f2, 85 1.4 or Mitakon 85 1.2), or just stick with the 70-200 and avoid spending (boring I know).

the tamron lens is sharpest at 135mm focal length, but is it a true 135mm, or it's in fact shorter, closer to say a 100mm? If it is shorter, then I guess Samyang 135mm would give me more background blur even at 2.8, and especially at f2?
Mainly talking about portrait distance, maybe waist up format, head and shoulders etc. I believe focus breathing is much less prominent at longer distance, so I guess environmental portraits aren't as affected by it.

Basically, doing some theory crafting and researching while I'm waiting for my camera to come back from repairs lol, and maybe start an interesting discussion?

Also would be interested to know if there's a way to calculate dof with different focal lengths while keeping the subject the same size. Not sure if I'm phrasing it correctly, it's just dof calculators only work with distance from lens to the subject, and not subject's actual size. 135mm f2 seems to give shallower dof than 85 1.4 or even 1.2 at 3m distance to the subject (0.054m vs 0.099m), but I'm guessing it might be too close to the subject with the 135mm, so I'll need to go further back.

10-01-2017, 02:45 PM   #2
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QuoteOriginally posted by awscreo Quote
he tamron lens is sharpest at 135mm focal length, but is it a true 135mm, or it's in fact shorter, closer to say a 100mm? If it is shorter, then I guess Samyang 135mm would give me more background blur even at 2.8, and especially at f2?
Not for nothing is the term "circle of confusion" used when refering to DOF

1. If your lens suffers from focus breathing it is likely (I do not know the lens) to be apparent at longest FL and at closest focussing distance. So i doubt your Tamron at 135mm will differ much...perhaps someone else can assist.

2. The biggest effect on DOF is not focal length but camera to subject distance. So even if your Tamron was short-changing you at 135mm , just take a step or two closer to your subject so he/she/it fills the frame in the same way any other 135mm lens would, and hey presto , for a given aperture your DOF is the same.
10-01-2017, 03:34 PM   #3
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Tony Northrup says 200 at headshot range is about 140-160. Which makes me wonder at 4x it breathes 1/4. At 2x where i assume breathing is less is it still 1/4 or maybe closer to 1/8 or 1/16? Anyone know if there is some formula given a lens focal length with maximum breathing of x?
10-01-2017, 04:13 PM   #4
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QuoteOriginally posted by pschlute Quote
Not for nothing is the term "circle of confusion" used when refering to DOF

1. If your lens suffers from focus breathing it is likely (I do not know the lens) to be apparent at longest FL and at closest focussing distance. So i doubt your Tamron at 135mm will differ much...perhaps someone else can assist.

2. The biggest effect on DOF is not focal length but camera to subject distance. So even if your Tamron was short-changing you at 135mm , just take a step or two closer to your subject so he/she/it fills the frame in the same way any other 135mm lens would, and hey presto , for a given aperture your DOF is the same.
Actually, I just remembered that I have an older 135mm I can test it against once I get my camera back. Haven't used that lens in a while.

Regarding your second point - theoretically speaking, if 135mm is not true 135mm on the Tamron, and I move closer, would it give a slightly different "look" compared to a true 135mm? I mean the compression of the facial features etc.

---------- Post added 10-01-17 at 04:15 PM ----------

QuoteOriginally posted by swanlefitte Quote
Tony Northrup says 200 at headshot range is about 140-160. Which makes me wonder at 4x it breathes 1/4. At 2x where i assume breathing is less is it still 1/4 or maybe closer to 1/8 or 1/16? Anyone know if there is some formula given a lens focal length with maximum breathing of x?
Yup, that's what I'm trying to find out too, if there's a way to calculate true focal lengths of a lens.

10-01-2017, 04:25 PM   #5
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QuoteOriginally posted by awscreo Quote
Regarding your second point - theoretically speaking, if 135mm is not true 135mm on the Tamron, and I move closer, would it give a slightly different "look" compared to a true 135mm? I mean the compression of the facial features etc
Correct, the perspective would change, but not the DOF.
10-01-2017, 05:19 PM   #6
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QuoteOriginally posted by pschlute Quote
Correct, the perspective would change, but not the DOF.
I'm really curious now to do the comparison) But my camera is in prison of Precision Camera for over 3 weeks now with no end in sight

---------- Post added 10-01-17 at 05:20 PM ----------

The samyang 135 looks like a stunning lens optically, maybe worth getting it anyway
10-02-2017, 05:02 AM   #7
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The first question in this interesting discussion is: what do you mean by true focal length? Do you define it by how much of the subject is in the image and how close you are to the subject? (And if it's defined by closeness to the subject, then do you use the distance to the front of the lens or the camera body?)

There's mathematical formulas for magnification (size of the subject), focal length, and distance but they are only valid for "simple" lenses. That means they don't work for zooms and they don't work for complex modern primes.


It's also worth noting that all prime lens show some focus breathing unless they have been very carefully designed to avoid it -- if the entire lens changes length as you focus, it will almost certainly breath. A 135 mm prime will shift to narrower angles of view (essentially like a higher focal length) at closer focusing distances. In contrast zooms tend to breath in the opposite direction especially on the long end.
10-02-2017, 08:20 AM   #8
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QuoteOriginally posted by photoptimist Quote
The first question in this interesting discussion is: what do you mean by true focal length? Do you define it by how much of the subject is in the image and how close you are to the subject? (And if it's defined by closeness to the subject, then do you use the distance to the front of the lens or the camera body?)

There's mathematical formulas for magnification (size of the subject), focal length, and distance but they are only valid for "simple" lenses. That means they don't work for zooms and they don't work for complex modern primes.


It's also worth noting that all prime lens show some focus breathing unless they have been very carefully designed to avoid it -- if the entire lens changes length as you focus, it will almost certainly breath. A 135 mm prime will shift to narrower angles of view (essentially like a higher focal length) at closer focusing distances. In contrast zooms tend to breath in the opposite direction especially on the long end.
Wouldn't you define it by the field of view? It seems it's the only thing that should in theory stay constant, rest can be changed - size of the subject, distance to the subject etc. I guess correct angle of view for a 135mm would help to identify how the subject would look like - perspective compression etc. Could be wrong on that one

Found this on the web - 135mm (values in degrees) - 15.2 (horizontal) - 10.2 (vertical) - 18.2 (diagonal)

I have no idea how my 135mm prime behaves lol, it's a vintage 3.5 prime I picked up at a flea market for peanuts. Weirdly enough it has like 13 aperture blades lol, but unfortunately the Asahi m42-K adapter I have seems to let the light in through the holes on the adapter (used to turn the adapter and lock it in place) as this (and one another lens I have) lens is not wide enough to cover the entire mount plate. I get nasty light leaks with it)

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