anotherview Loc: California

The aperture setting determines the amount of light allowed through the lens to reach the camera sensor.

The shutter speed setting determines how long the aperture remains open for passing light through it.

Note that the ISO setting determines the amount of amplification of the light that the camera sensor captures.

So in your test, you'd have to include these three settings as well as the focal length.

You may wish to study the exposure triangle to gain a better understanding of these three variables.

I hope this comment helps.

dsmeltz Loc: Philadelphia

One more shot at this. The OP never stated the full parameters of the test. He states the length used but does not state the f stop the lenses were set to. The 100-400 is a 4.5-5.6 and the 300 is a 2.8 lens. So the settings of the lens relative to f stop are rather important to this experiment and the OP does not mention them.

TriX Loc: Raleigh, NC

I agree. I recently tested a Canon EF 70-200 f2.8L zoom at 200mm against the equivalent FL and max aperture (f stop) L series prime and found the prime to have ~1/2 stop better light transmission. If you check DXOMark’s lens tests, you’ll also see that the transmission (t stop) varies between lenses of the same f stop. I also agree that a full stop would be a bit more of a difference than I would expect.

dennis2146 Loc: Eastern Idaho

I have never heard of T stops. Can you elaborate please.

My understanding is that no matter what lens I use, zoom or prime, if I meter the light coming from my subject and it indicates let's say, ISO 200, f16 at 1/250th then that will be the correct setting for any lens used. Are you saying this is not correct?

Dennis

dsmeltz Loc: Philadelphia

An f-stop has to do with the size of the aperture while the T-Stop measures actual transmitted light. T-stops are affected by light loss related to the number and quality of the elements in the lens. Zoom lenses have more parts and therefore a bit more light loss. DxOMark includes T-Stop measures in their reviews. The difference tends to range from 0.1 stops to around a half stop with the T-stop being the loser. It would be unexpected for two lenses at the same length and f-stop settings to be more than a few tenths of a T-stop different.

dennis2146 Loc: Eastern Idaho

Thanks. I will have to look into it more I think. As I only use f stops I don't think I am missing out on anything.

Dennis

Dik

OK, I was wrong about the amount of difference between my Canon 24-105L f/4, and my Voightlander Ultron 40 f/2 @ f/4.
I just tested with both lenses at f/4, 3 shot bracket via shutter speed @ 1/2 stop steps. 1 sec, 1.5, .7.

Zoom @ 1.5 sec read in Adobe Camera RAW R175 G155 B125.
Prime @ 1.5 sec read in Adobe Camera RAW R199 G179 B150.
Prime @ 1.0 sec read in Adobe Camera RAW R172 G154 B124.

That's almost exactly 1/2 f/stop.

Dik

Not true!
Lens f/stops do not take into account the absorbed and scattered light inside the lens.

Architect1776 Loc: In my mind

Unless you are doing movies it is not all that important.

Dik

The point to take from all this tech is that simple primes are likely to be about a half stop brighter than complex zooms at the same f/stop.
That extra half stop can be used for more DOF, or shutter speed, or lower ISO.

dennis2146 Loc: Eastern Idaho

Thanks. I never do movies.

Dennis

TriX Loc: Raleigh, NC

👍👍. A half stop in low light is not to be sneezed at. I have started using my EF 135 f2L + an EF 1.4EX MKII extender which provides approximately 195mm at f2.8 instead of my EF 70-200 f2.8L IS (MKI) when I need 200mm. It is almost 1/2 stop faster (measured light transmission), actually sharper, even with the extender, and a whole lot lighter.

amfoto1 Loc: San Jose, Calif. USA

An f-stop is an f-stop, regardless the lens involved. At least theoretically if you select f/5.6 on any lens... zoom or prime... it should allow exactly the same amount of light to pass. (There may be some slight variation between models and possibly even copies of the same model... but that's rare and minimal with modern lenses that are pretty accurately calibrated and consistent.)

Now, a Canon EF 300mm f/4L lens has a larger max aperture than a 100-400mm L does at 300mm setting. I don't know about the older version, but the 100-400 II is at f/5 at 300mm.... so that particular zoom has two-thirds of a stop smaller max aperture than the prime, in this particular case. In other words, it's "2/3 stop slower". However, the max aperture only matters if it's what you're wanting to use to make an image (i.e., shooting with the lens "wide open").... AND it effects the brightness of your viewfinder... PLUS it can be one factor effecting auto focus performance of a particular lens & camera combo. (Negligible difference in the case of the 100-400L II and the 300/4, both of which I use personally).

Keep in mind that your lens' aperture is maintained wide open all the time, except for the very instant of exposure when it will very briefly stop down to whatever aperture you've set or the camera's auto exposure system has selected. The aperture opens wide open again immediately after the exposure. (Note: This is true of both the lenses you mention and the vast majority of lenses you might use on your camera... however, vintage adapted lenses with fully manual, mechanical aperture control might actually stop down when set to smaller apertures... you'll see the viewfinder dim down when this happens. Old camera systems in the 1950s and early 1960s, prior to the innovation of "auto aperture", operated the same way. So did some "preset" and "manual aperture" lenses produced even later, though those were usually specialty lenses or lower cost alternatives. All these exceptions are quite uncommonly used or seen now and virtually all modern lenses maintain a wide open aperture most of the time.)

Comparing your two examples, the 300mm f/4 lens will make for a brighter viewfinder all the time. If you were comparing instead it to the Canon 200-400mm f/4 Extender lens or one of the 24-105L f/4s, the viewfinder brightness would be exactly the same.... since both the zooms and the primes have the same f/4 max aperture. Or, if you were comparing that 300/4 to a 70-200mm f/2.8 or 24-70mm f/2.8 zoom, the zooms would instead make for a brighter viewfinder.

But, if you set any of them to make an image at a smaller aperture they're all capable of.... say f/8 or f/11... they would all produce virtually the exact same exposure results, admitting the same amount of light, regardless whether a zoom or a prime. (Again, there might be some minor variations... but we're usually only talking fractions of a stop, with modern lenses.)

Where can it make a difference? Well, I use my 100-400 II quite a bit for outdoor, daytime sports. But indoors... covered arenas... I usually switch to the 300mm f/4 and use it wide open. For really dim situations, I also have a 300mm f/2.8 available, but that's a much bigger lens that's less hand-holdable and, more importantly, can render such shallow depth of field that I am usually wanting to use f/4 anyway. In other words, the amount of light a lens admits per the available or selected aperture only pertains to the exposure side of the discussion. Aperture size for any given focal length also effects depth of field, which can often be an equal or greater concern. In addition to focal length and aperture size, DoF is also effected by distance between you and the subject... a distant subject might have adequate DoF at a given aperture, while a closer one where DoF becomes much shallower won't have adequate DoF.

Dik

The MkIII converter has newer coatings, I wonder if it might yield a tiny bit more light...

I've got both, so I guess I'll try My 100-400 MkII with both and compare, before I sell the 1.4x MkII.

Dik

"At least theoretically if you select f/5.6 on any lens... zoom or prime... it should allow exactly the same amount of light to pass."
This is not true because it does not take into account the light absorbed and scattered.
3x as many elements makes about a half stop difference in the amount of light transmitted to the sensor.
Easy enough to run your own test.