Zoom lens transmission, T-number, F-number ?

[bernard_L]

I have been cleaning (fungus) a Tamron 85-210 zoom in Adaptall - Minolta MC mount. Some trial and error process... Once done, I wanted to check whether the diaphragm linkage was properly re-assembled.

With the full-aperture metering, the diaphragm linkage tells the body the intended f-stop relative to the full aperture of the lens. So I only needed to check that the f:4.5 Tamron, fully open, would push the body cam in the same position as, e.g., a 55/1.7 Rokkor, also fully open. And it did. So I might have stopped there; but I nevertheless proceeded to compare (same ISO, same shutter speed) the f-stop requested by the Tamron zoom with that requested by the 55/1.7 Rokkor, each mounted on a SRT-101. Uniform wall, overcast day, repeated measurements to exclude flukes. Outcome: the Tamron zoom consistently requests a faster f-stop, by 1/2 to 1 stop, than the prime lens.
Possible cause: lower transmission of the complex zoom optics, requiring a faster f-stop to achieve the same amount of light (i.e. the same T-number) in the focal plane.
Question: Is the order of magnitude (0.5 to 1 stop) plausible for a 12-element 9-group zoom?
Note: As some members on this forum are prompt to snipe at perceived weaknesses, the uncertainty (0.5 to 1 stop) arises from making measurement at various light levels, and, for each level, from the body's photocell operating at different light levels, with maybe some non-ideal behavior, through the f:4.5 Tamron and the f:1.7 Rokkor.
Remark: If the above is correct, it puts in question the validity of using a hand-held light meter together with a complex zoom whose T-number might be significantly different from its displayed F-number.

 

[Mr Bill]

You said that the Tamron zoom is f/4.5. Is this "rating" true throughout the zoom range, or does the lens actually say something like "4.5-5.6?" If it's a variable f-stop zoom, you expect the long end to be darker.

 

[bernard_L]

You said that the Tamron zoom is f/4.5. Is this "rating" true throughout the zoom range, or does the lens actually say something like "4.5-5.6?" If it's a variable f-stop zoom, you expect the long end to be darker.

This is a constant aperture zoom. Well built, I should say.

 

[Mr Bill]

Hmm... seems like it would be due to all the elements, then, maybe some internal haze too?

If you wanted to verify the working apertures, you COULD rig up a crude test jig (assuming you have some loose lens parts lying around). Basically you rig up an eyepiece and some random lens to focus on something like a foot or so away - a sort of close-working telescope. Set this, on some sort of moveable platform, in front of the Tamron lens so it is focused on the diaphragm. Now, you basically slide the "telescope" from one side of the diaphragm to the other; the distance traveled is the diameter, roughly, of the "entrance pupil," which is what you want to use to calculate the f-number. (Focal length divided by diameter = f-number.)

The more precise your jig, the more precise the result. I forgot to say, you ideally have a reticle in the eyepiece so as to have a reference when measuring; otherwise you have to rig up a pointer where the eyepiece is focused (could be easy ot hard, depending). I've done this sort of thing using a small precision rack-and-pinion setup for the motion, measuring the movement with a machinist's dial indicator, reading to a thousandth of an inch. But if you rig up a guide to slide the telescope side to side you can measure pretty good with a rule.

Anyway, if you're curious enough, this is a possible method.

 

[John Koehrer]

You shouldn't expect the same exposure from the lenses. You already know that
the f stop is just a ratio of aperture to focal length.
F stops are measured from the front of the lens actually measuring the diaphragm of the aperture.
Calculate the f stop @ 5.6 or 8 and measure from the front of the lens with all the elements in place.
Measuring a T stop actually measures the light coming through the lens, nothing to do with FL.

 

[ic-racer]

If it is one stop off comparing stopped down vs open metering then the open-aperture calibration is not correct.

 

[Bill Burk]

I would take the same picture with the two different lenses at either the same f/stop or at the camera’s recommended f/stop and compare the results to each other.

 

[bernard_L]

You shouldn't expect the same exposure from the lenses. You already know that
the f stop is just a ratio of aperture to focal length.
F stops are measured from the front of the lens actually measuring the diaphragm of the aperture.
Calculate the f stop @ 5.6 or 8 and measure from the front of the lens with all the elements in place.
Measuring a T stop actually measures the light coming through the lens, nothing to do with FL.

???
Ignoring for one second the absorption of light, the light flux reaching the focal plane is directly tied to the F-number f/D, where f is the focal length and D the diameter of the entrance pupil. And, btw, the entrance pupil is generally not at the "front of the lens". In the absorption-less case, T-number and F-number are equal.
When absorption is present (as it always is) the amount of light is less than derived from geometry alone (F-Number); the T-number takes this into account and allows accurate exposure; Especially important in movies when successive scenes must merge seamlessly.

 

[bernard_L]

If it is one stop off comparing stopped down vs open metering then the open-aperture calibration is not correct.

I did not use stopped down metering and nowhere in my post did I mention it. I compare apples and apples: open-aperture metering with two different lenses.
Remember, in such a mechanical system, the body does not know the actual aperture, either full or intended for exposure. What the body knows is:

• How much light goes through with the lens fully open, as it is during the measurement
• How many f-stops the user intends to close down relative to full aperture

So basically, the match needle system is telling the photog: hmmm 100 ISO, 1/60s, you propose to close down 3 and 1/2 stop from full aperture, that sounds right. Only the photog knows, looking at the lens barrel (or the peephole in the v/f) what actual aperture is "3 and 1/2 stop from full aperture". And, once again, all that time the diaphragm is fully open; it will close down only to take the picture.

 

[bernard_L]

I would take the same picture with the two different lenses at either the same f/stop or at the camera’s recommended f/stop and compare the results to each other.

Believing as I do that the measurement is correct and that differences between lenses reflect differences in transmission, I don't want to invest time into a film test. Besides, with currently fashionable CI's around 0.55, a 1/2 stop would result in a density difference approx 0.08. Still above the error margin of my densitometer, but close.

 

[bernard_L]

OK, not what I expected so far.
The distinction between T-number and F-number is not academic, since cine people worry about that. I was hoping that someone more knowledgeable than me would say the difference between the two numbers for a typical tele-zoom (is/is not) likely to be significant at the 1/2...1 stop level.

 

[bernard_L]

Hmm... seems like it would be due to all the elements, then, maybe some internal haze too?

Apologies to Mr Bill, I seemed to have missed this post. I believe (as my eyes show) that this lens is haze-free, after cleaning away the fungus on two elements, and carefully (and gently) cleaning surfaces as I re-assembled it.

If you wanted to verify the working apertures, you COULD rig up a crude test jig <SNIP>

(please refer to post #4 above for full text)
I agree this is a correct method to measure the diameter of the entrance pupil. The F-number follows if the focal length is known : believe the zoom scale or measure (gasp!) it.
So, if I understand well, your proposed hypothesis is that the whole F-number scale of the zoom is shifted. The real max aperture is not 1:4.5, but maybe 1:6.3. So, when I meter (say) f:11 with the 55/1.7 prime Rokkor, and then I meter f:8 with the 1:4.5 Tamron zoom, the real numbers should be shifted by one stop to f:11 because the full aperture is actually 1:6.3.
An intriguing possibility. So Tamron got away with cheating all these years? Actually I have access to an optics lab with a very nice collimation telescope and translation stages. All the ingredients to perform your proposed experiment. Possibly I do the experiment on Friday, unless the zoom was sold today at the charity for which I refurbished that lens.

 

[AgX]

From another T-stop thread:

Transmission Tests

Not quite... I just checked current tests. A Canon EF 70-200mm F2.8 was tested at T3.3, thus half a stop loss.

Other current zooms were tested for losses of over one stop.

 

[bernard_L]

half a stop loss
losses of over one stop.

Thank you. Just the kind of info I was hoping for. And the right ballpark. Although, of course, it does not prove that this is what is happening in the case of the Tamron.

 

[bernard_L]

So one incurs exposure errors "of over one stop" if one measures (like a Real Photographer) with a hand-held meter and transfers the reading to the F-stop scale.
Meanwhile, some are worrying about film ISO rating within 1/3 stop.
At least the open-aperture TTL metering is immune from
(a) a "cheat" in the geometrical aperture scale (f-number)
(b) less-than-perfect transmission.
Next you might discuss the effect of vignetting, that is more pronounced at full aperture, but I'll excuse myself from that discussion :/)

 

[AgX]

Of course a "real photographer" would not use a zoom lens. Which brings this whole discussion at its end...

 

[ic-racer]

I did not use stopped down metering and nowhere in my post did I mention it. I compare apples and apples: open-aperture metering with two different lenses.
Remember, in such a mechanical system, the body does not know the actual aperture, either full or intended for exposure. What the body knows is:

• How much light goes through with the lens fully open, as it is during the measurement
• How many f-stops the user intends to close down relative to full aperture

So basically, the match needle system is telling the photog: hmmm 100 ISO, 1/60s, you propose to close down 3 and 1/2 stop from full aperture, that sounds right. Only the photog knows, looking at the lens barrel (or the peephole in the v/f) what actual aperture is "3 and 1/2 stop from full aperture". And, once again, all that time the diaphragm is fully open; it will close down only to take the picture.

You indicated you wanted to check the linkage after you cleaned the lens. If it is one stop off comparing stopped down vs open metering then the open-aperture calibration is not correct.

 

[bernard_L]

You indicated you wanted to check the linkage after you cleaned the lens. If it is one stop off comparing stopped down vs open metering then the open-aperture calibration is not correct.

I am making statements based on the lens in front of me.

• The arm on the SRT101 body that receives the "intended aperture" from the lens is in the exact same position when the 55/1.7 and the 85-210/4.5 are at their respective maximum apertures. Remember, in case you missed that in an earlier post, the body only knows: "intended aperture minus maximum aperture"; that is all it needs to know.
  
• I might make the discrepancy (in apertures required to make the camera's body happy) worse by shifting the gears in the Adaptall in one direction, but I cannot reduce it by an opposite action because when the f-stop scale on the lens main body is at maximum (4.5) the actuating lever (linkage to the body) on the Adaptall is at the mechanical end of allowed travel, against the end of a slot in the Adaptall.

The two observations consistently tell me that I have re-assembled the lens correctly.

 

[bernard_L]

If you wanted to verify the working apertures, you COULD rig up a crude test jig (assuming you have some loose lens parts lying around).

Possibly I do the experiment on Friday

Well, I've done the measurement. I did "have some loose lens parts lying around". On the left an alignment scope that can focus from infinity down to a few inches, with the reticle tracking a true straight line within xx microradians. And on the right the DUT (device under test) mounted on a micrometric translation stage. Before some one asks, yes, the scope was focused on the entrance pupil of the lens. And before someone asks, I know that is not located at the beauty ring of the lens.

Results as follows:

Focal Length	Pupil diameter	f/D
85	19.01	4.47
100	21.96	4.55
150	32.94	4.55

I could not make the measurement at f=210mm, because the travel of the translation stage was not large enough to measure the pupil diameter. Nevertheless the measurements seem to support that the lens is indeed f:4.5 constant.

And so the problem must be transmission, consistent with the evidence mentioned by Agx in post #13 above. Now I'm putting the lens back on display for sale at the charity. As I already said, it is quite well built. I did not test the image quality, but at least one blog author is enthusiastic.

 

[Mr Bill]

Cool. Your setup is not quite as crude as I had in mind, though (wink).