CLF wrote:
I have a question concerning a difference I have noticed since I have been using Prime lenses more often.
When using a Zoom lens and compare the settings with a Prime lens of the same length (ie a 100-400 L set to 300mm vs a 300mm L) it seems the Prime lens lets in at least one/two stops faster than the Zoom lens.
The question:
Does a Prime lens allow more light to pass through than a Zoom?
Thank you in advance for your comments.
Greg
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I just compared the amount of elements between the Canon 100-400 L and the 300mm L and the zoom has 2x the amount of elements.
Greg
I have a question concerning a difference I have n... (
show quote)
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.