How to Measure whether a Lens is what it Claims to Be ???

I wanted to get this question asked while it was still fresh in my head.

I was reading the reviews on a lens and one guy made a claim that I had never before read in a review.
He rated the lens poorly and said that, although the lens was labeled 400mm, in actual use it was more like 250mm.

I had never really considered it before; but, how can one determine for absolute fact that a lens has the reach that it claims it has ?

What governs whether a lens is 50mm or 500mm ?

Thanks for reading and all help is appreciated.

From what I understand, in single lenses and telescopes, focal length is the distance from the center of the lens to the focal point. In photography it's more associated with the field of view and magnification. It would be interesting to see the review and what lens they were reviewing. I find it difficult to believe that any camera lens manufacture would label a lens as 400mm when it delivers the FOV of a 250mm lens.

There is an allowable percentage deviation in rated FL and in the size of the max aperture, that manufacturers adhere to, a mandatory +-5% as the standard by the JCII (Japan Camera Industry Institute) for lenses exported from Japan back then.

Way back in the 1990s, actual tests of lenses for FL by Popular Photography showed that lens FL were short of the rated FL by about 6% on average, in tests conducting in a 12 month period 1991-1992... for fractions of a percent under, to as much as -10% under. And WA lenses tended to not be as wide as they claimed But 250mm actual vs. 400mm rating is rather excessive even by allowable JCII standards.

BuckSkin wrote in post #19021488
I wanted to get this question asked while it was still fresh in my head.

I was reading the reviews on a lens and one guy made a claim that I had never before read in a review.
He rated the lens poorly and said that, although the lens was labeled 400mm, in actual use it was more like 250mm.

I had never really considered it before; but, how can one determine for absolute fact that a lens has the reach that it claims it has ?

What governs whether a lens is 50mm or 500mm ?

Thanks for reading and all help is appreciated.

Could this be a simple misinterpretation of something involving a cropped sensor? 250 x 1.6 = 400

Wilt wrote in post #19021518
... But 250mm actual vs. 400mm rating is rather excessive even by allowable JCII standards.

Exactly, which is why seeing the review would be beneficial. Did the reviewer actually measure FOV or just take a wild guess. A 10% deviation is acceptable. a 40% deviation is going to be noticed by more than I reviewer I think.

John from PA wrote in post #19021533
Could this be a simple misinterpretation of something involving a cropped sensor? 250 x 1.6 = 400

OP said lens was labeled as 400mm but looked to be 250 so it's the wrong way for a crop sensor. A 400mm labled lens would look like a 640mm on a crop. Of course, maybe they stuck it on a Pentax 645. That would be a -1.4 crop.

gjl711 wrote in post #19021539
OP said lens was labeled as 400mm but looked to be 250 so it's the wrong way for a crop sensor. A 400mm labled lens would look like a 640mm on a crop. Of course, maybe they stuck it on a Pentax 645. That would be a -1.4 crop.

I’m aware of all that, hence the reason I said “a simple misinterpretation of something involving a cropped sensor?” It is obvious by multi-page responses on this forum that field of view, focal length, cropped factor, etc. can be confusing to many individuals.

I always try to avoid posting EBay links as they are so short-lived; but, here is the link to where I saw the review; the reviews are toward the bottom of the page; and, when I looked, it was the second from the top:

https://www.ebay.com …ksid=p2060353.m​1438.l9372

I don't know just how scientific the reviewer's calculations are.

Reading the review only prompted my question, regardless of whether the guy knows what he is talking about or not.

You can determine focal length by measuring the width of the field (at infinity ideally) and distance to the front nodal plane (or roughly the middle of the lens). Bryan Carnathan did something like that at closer distances and came up with a rough focal length of 360mm. The guy on eBay maybe made a typo or exaggerated or lied.

https://www.the-digital-picture.com ….3-Di-II-VC-HLD-Lens.aspx

BuckSkin wrote in post #19021488
.
I was reading the reviews on a lens and one guy made a claim that I had never before read in a review.
He rated the lens poorly and said that, although the lens was labeled 400mm, in actual use it was more like 250mm.
.

.
Any time there is that large of a discrepancy, it is not due to inaccurate specifications, but it is due to something often called "focus breathing." . It is nothing to worry about, because it only affects the field of view toward the extreme end of the focus range - at or close to minimum focus distance.

For instance, many 100-400mm zooms will actually be something like 105mm to 386mm, or something like that. . But if you are zoomed all the way out to 400mm and then focus on something real close, like just 3 feet away, then the angle of view may be only that which you would expect from a 250mm lens. . But when you focus on something further away it goes back to giving you the 386mm field of view that it normally provides.

This isn't really a fault of the lens or a deceptive practice by the manufacturers. . The very definition of focal length includes the qualifier "when focused at infinity". . Therefore, by definition, it is understood that a different angle of view may be realized when not focused at infinity, and that the further from infinity one gets, the angle of view has the greatest likelihood of falling away from that which it normally yields.


.

BuckSkin wrote in post #19021617
I always try to avoid posting EBay links as they are so short-lived; but, here is the link to where I saw the review; the reviews are toward the bottom of the page; and, when I looked, it was the second from the top:

https://www.ebay.com …ksid=p2060353.m​1438.l9372

I don't know just how scientific the reviewer's calculations are.

Reading the review only prompted my question, regardless of whether the guy knows what he is talking about or not.

18-400mm F/3.5-6.3 Di II VC HLD ...Ken Rockwell summarizes

"Like most modern zooms, this lens only gets to 400mm when focused at infinity.

At closer distances, like 30 feet (10 meters) and closer, it cheats and the 400mm setting doesn't get anywhere near 400mm. It's the same for the Nikon 18-300mm f/3.5-6.3: as you get closer, neither lens really gets to the focal lengths you expect.

While I call this cheating, it's actually a clever optical design technique used to allow closer focusing that you'd get otherwise. This way you get 400mm where you actually need it for shooting very far away things, and also can focus super-close at all zoom settings. Just don't expect to see the image zoom-in much as you move the ring from 200mm to 400mm at close distances."

...but he did not mention 250mm actual vs. 400mm rating.

The-Digital-Picture.com clarifies the issue about what Field of View to expect from this lens on APS-C camera
[INDENT][/INDE"Keep in mind that this is an APS-C-only lens (the "Di II" portion of the name indicates this) as the image circle it projects is not wide enough to cover full frame imaging sensors. Thus, the angle of view presented by this lens on an APS-C/1.6x camera approximates a 28.8-640mm zoom lens on a full frame model."NT]

They go on to state:

"The 18-400's 400mm test target (the chart used measures 600mm x 400mm) framing distance was 26.96' (8.218m) while the 100-400's same 400mm setting required 33.78' (10.296m). I'm not going to declare this comparison scientific and focal length is perhaps best measured at even longer distances, but that the Tamron's distance was only 80% that of the Canon's raises suspicion for me. Using these two referenced lenses side-by-side focused at about 92' (28 m), it seems that the Tamron is still not reaching a true 400mm at this distance with perhaps 360mm being a rough estimate of what I see."

But I do not know how to interpret this last statement above. If the target is 600 x 400mm, with a 400mm lens on APS-C body, the program that I have says that one should need to be 12' away, to capture that 600 x 400mm target in the APS-C and fill the frame?!

Wilt wrote in post #19021638
The-Digital-Picture.com clarifies the issue about what Field of View to expect from this lens on APS-C camera
"Keep in mind that this is an APS-C-only lens (the "Di II" portion of the name indicates this) as the image circle it projects is not wide enough to cover full frame imaging sensors. Thus, the angle of view presented by this lens on an APS-C/1.6x camera approximates a 28.8-640mm zoom lens on a full frame model."NT]

>Groan<

We are talking focal length. It doesn't help to start talking about focal length equivalence.

Someone already brought up the fact that 250mm actual FL on ASP-C has the FOV of 400mm FL on FF (Post 6). I am not the one who brought up that issue!
And, discussions were speculating why apparent confusion about 250mm vs 400mm might have existed in the other review's mind to confuse things.
And the Digital Plcture discussion, after mentioning 28.8-640mm, did bring up that 400mm Tamron did NOT EQUAL the FOV of 400mm Canon...that was the point that I was trying to emphasize, in quoting that article! Butts seems to have grasped that point.

Interesting, I did a quick search and found THIS:-

" In our image quality test, a target is precisely framed at each tested focal length and the sensor plane-to-chart laser measurement is recorded. While there can be some variation in this measurement due to focus breathing (focal lengths are measured at infinity), distortion and potentially slight imprecision of focal length selection, the 400mm test for both the Tarmron 18-400mm VC Lens and the Canon EF 100-400mm f/4.5-5.6L IS II USM Lens were completed at their longest focal lengths. Thus, both lenses exhibit rather similar pincushion distortion and both were set to as close to 400mm as possible.

What do the numbers tell us in this case? The 18-400's 400mm test target (the chart used measures 600mm x 400mm) framing distance was 26.96' (8.218m) while the 100-400's same 400mm setting required 33.78' (10.296m). I'm not going to declare this comparison scientific and focal length is perhaps best measured at even longer distances, but that the Tamron's distance was only 80% that of the Canon's raises suspicion for me. Using these two referenced lenses side-by-side focused at about 92' (28 m), it seems that the Tamron is still not reaching a true 400mm at this distance with perhaps 360mm being a rough estimate of what I see. "

So yes, 400mm appears not to be a true representation (based on comparison with Canons own 100-400) but is closer to 360mm.
Certainly NOT 250mm in the review you referenced.

Wilt wrote in post #19021660
Butts seems to have grasped that point.

Butts just repeated what I said.