Is 24-105L really f/4.0?

[lexonio]

Recently got the lens, loving it atm, however in a lot of user reviews I've seen that its f/4.0 setting is in fact closer to ~f/4.8. Those claims are based on the tests of the f/4.0 setting of this lens and other f/4.0 lenses like 100-400mm.
Is it true in your experience?

[marekjoz]

If someone has the broken lens, could measure it easily...

[neuroanatomist]

Recently got the lens, loving it atm, however in a lot of user reviews I've seen that its f/4.0 setting is in fact closer to ~f/4.8. Those claims are based on the tests of the f/4.0 setting of this lens and other f/4.0 lenses like 100-400mm.
Is it true in your experience?

Can you point to some of these reviews?  My 24-105mm f/4L IS gives exposures at f/4 that are consistent with other lenses.

[bvukich]

... other f/4.0 lenses like 100-400mm.

I would be suspect of opinions from anyone that thinks the 100-400 f/4.5-5.6 IS USM is f/4.

[lexonio]

Sorry bvukich, I may have been wrong.
Well, looking at that review, I am wrong The reviewer claimed to have it tested alongside Canon 24-70/2,8, Canon 24/2,8, Canon 50/2,5 and Canon 35-105/3,5-4,5.
Sadly I can't provide you with a link, since, well, it's a website in russian language. The reviewer (who seems to be credible, i.e. it's not his first forum post) claims that when pitched against those lenses it produces a darker image at f/4.0 than those other lenses produce at f/4.5.
There's a huge list of feedback there, and only 3-4 answers seem to claim that this f/4.8 is indeed an issue, but I just wanted to hear the CR community's point of view. Seems those 3-4 people got faulty lenses or w/e.

By the way, could anyone please provide me with a link to some website where people review lenses, bodies and etc.? I'm reluctant to trust Amazon user reviews since the majority of Am's users are not photo enthusiasts, even if they are trying to review L lenses and such. I'd be extremely grateful

[marekjoz]

Sorry bvukich, I may have been wrong.
Well, looking at that review, I am wrong The reviewer claimed to have it tested alongside Canon 24-70/2,8, Canon 24/2,8, Canon 50/2,5 and Canon 35-105/3,5-4,5.
Sadly I can't provide you with a link, since, well, it's a website in russian language. The reviewer (who seems to be credible, i.e. it's not his first forum post) claims that when pitched against those lenses it produces a darker image at f/4.0 than those other lenses produce at f/4.5.
There's a huge list of feedback there, and only 3-4 answers seem to claim that this f/4.8 is indeed an issue, but I just wanted to hear the CR community's point of view. Seems those 3-4 people got faulty lenses or w/e.

By the way, could anyone please provide me with a link to some website where people review lenses, bodies and etc.? I'm reluctant to trust Amazon user reviews since the majority of Am's users are not photo enthusiasts, even if they are trying to review L lenses and such. I'd be extremely grateful

I'm not saying the guy is not right, just want to point out one thing - darker image doesn't mean that the apperture is wrong. Apperture is a physical parameter calculated something like by dividing the focal length by apperture hole's diameter. So f4 in 50mm means that the hole is 12,5mm big. In 200mm it means it has 50mm diameter. But in first case there could be only 5 lens inside and in other 20 of them, not even to mention different antirelfex coatings. So the first can transmit more light that the other, so the picture would be darker in the second case.
Cinema lenses have another parameter "T" which says exactly what is light loss when going through a lens, while "F" parametrized lenses can have quite different "T" characteristics.

http://www.lenstip.com/ - that is quite credible for me, though there are plenty other

[scrappydog]

By the way, could anyone please provide me with a link to some website where people review lenses, bodies and etc.? I'm reluctant to trust Amazon user reviews since the majority of Am's users are not photo enthusiasts, even if they are trying to review L lenses and such. I'd be extremely grateful

Here are a few good ones:

Photozone: http://photozone.de/
Digital Picture: http://www.the-digital-picture.com/Canon-Lenses/Canon-General-Purpose-Lens.aspx
B&H (user reviews): http://www.bhphotovideo.com/
Adorama (user reviews): http://www.adorama.com/

[Mt Spokane Photography]

f/4 is a measured physical diameter of the aperture.  How did he measure it?  Did he disassemble the lens or what?  The amount of light a f/4 lens lets in is measured in "T" stops, and is always less than the physical f/4 diameter.

This is nothing new, its been discussed for years.

Cinema lenses are specified in T stops, and are up to one stop slower.  A "T" stop of f/4.8 seems reasonable.

DXO publishes the transmissive numbers for lenses they have tested, they do not list the 24-105, but here is the 16-35 which they give as having a T stop of 3.3 even though the physical aperture is f/2.8.

http://www.dxomark.com/index.php/Lenses/Camera-Lens-Database/Canon/EF16-35mm-F2.8L-II-USM/(camera)/436

[corntrollio]

f/4 is a measured physical diameter of the aperture.  How did he measure it?  Did he disassemble the lens or what?  The amount of light a f/4 lens lets in is measured in "T" stops, and is always less than the physical f/4 diameter.

This is nothing new, its been discussed for years.

Cinema lenses are specified in T stops, and are up to one stop slower.  A "T" stop of f/4.8 seems reasonable.

DXO publishes the transmissive numbers for lenses they have tested, they do not list the 24-105, but here is the 16-35 which they give as having a T stop of 3.3 even though the physical aperture is f/2.8.

http://www.dxomark.com/index.php/Lenses/Camera-Lens-Database/Canon/EF16-35mm-F2.8L-II-USM/(camera)/436

Yes, I would not be surprised if the T-stop is something greater than 4 because the large number of lens elements will necessarily lower the amount of light that gets through. Even with modern coatings, each additional element still blocks some light, although the percentages keep getting lower and lower.

However, that's T-stop, which is important for cinema because of the need for more precise light levels, and not F-stop which is more relevant for still photography.

Even when a lens is F4, it is not exactly F4. Check out the summary info regarding a patent filing for the 300 f4:

http://photorumors.com/2011/02/14/canon-patents-new-300mm-f4-lens/

You can see that the F-stop filed on the patent application is actually F4.14, and that the filed focal length is 294.00 mm instead of 300mm. This is normal and within tolerances.

4.8 is way off, and it's unclear how people actually measured that number, if they actually did measure it. It sounds like they were just comparing to other lenses and eyeballing the amount of light transmission, but that's not really the proper way to measure aperture. F-stop is a measure of the physical size of the aperture (relative to the focal length). T-stop is the measure of actual light transmitted. The two are different.