dyximan

It seems like we could beat this one to death but we could agree that it will be relatively close from one Lens to another. but bottom line if you like the end result great if you don’t bump it up or down a stop or 2 to achieve what you would like. Or bracket your photos if your camera allows it. Unless you need super low light stuff 1.4 1.8 I don’t think it would matter much

TonyBot

You've gotten a lot of good answers, especially regarding T-stop v F-stop. On *that* subject, a 7.1 on an all-in-one zoom is not *exactly* the same as a prime lens of the equivalent focal length(s). The difference is usually minimal, and going by your built-in or hand-held meter is gonna be pretty close.

As for the "sweet spot" - it seems the majority feel/believe that the aperture about two stops down from the max is that spot (and I am in that camp), and for your f/5.6 zoom f/11 is ideal by that philosophy. Since my fixed aperture zooms are also f/4, the f/8 is what I use mostly. But ... don't let any of this info chain you to a particular practice - use what you think will give you the best photo - whether the lens is wide open or stopped all the way down.

cambriaman Loc: Central CA Coast

I recommend the Ansel Adams series on Photography: The Camera, The Negative and The Print. Learn from the best!

jaycoffman Loc: San Diego

I made a beginner bonehead mistake on my questions--something I know better than. I reversed how wide the opening of the lens is compared to f-stop numbers. Thanks for your answer and I think you still answered my question.

I should have stated it as: Shutter speed and ISO constant--using a 4-5.6 lens; If your are striving for the maximum amount of light hitting the sensor (assume DOF not an issue) and you set your camera to f2.8 what controls the amount of light hitting the processor the f4 of the lens or the f2.8 of the camera? I'm assuming in this case it would be set by the lens as the camera can't force the lens to open any wider. Conversely, if you set the camera to f8 then the camera would set the smaller f-stop as f8? I think this matters in knowing how to set your camera with various lenses and I'm getting a bit confused by my own thoughts.

Elmerviking

It depends...If you shoot in manual mode with auto ISO and chose the same aperture and shutter time ISO will be the same. On the other hand, if you shoot in aperture mode (Av) with auto ISO the shutter time will change automaticly. A longer zoom lens will set a shorter time which will increase ISO with the same aperture! A shorter lens will set a longer time resulting in a lower ISO. (At least this is how it works on my Nikon D7100)
Try it out and tell all of us your findings!

Naptown Gaijin

That was an arrogant snarky answer.... uncalled for. I suggest you read some books on manners and on the English language.

mwsilvers Loc: Central New Jersey

Actually the OP was specific when he asked about the amount of light hitting the sensor, not just just going through the lens.
He said, "Another way to state this... Does a 7.1 aperture setting on a short tele lens allow the same amout of light onto the sensor as a 7.1 aperture setting on a long tele lens?"

IBM

TonyBot

In most cameras I know of, the f/ cannot be set higher than the maximum (the lower numbers) of the lens: In other words, if you want f/2.8 and the max aperture is f/4, you'll get f/4. Most vari-focal/zoom lenses of the 'variable aperture' variety will automatically lower the f/stop to the max available - you set the max, say f/4.5 at 100mm, and then you zoom out to 600, the f/4.5 drops to, say, f/6.3. If you had set the lens to f/5.6, it will drop to f/6.3 when you zoom out. (and, if your original setting was f/6.3, it would stay there)

As you accurately stated - the lens can only go to a certain f/ stop - and no matter what you'd like, it won't open any further. And, although not part of your OP, the *minimum* aperture will change also. On the hypothetical 100-600mm, f4.5-f/6.3, the range of stops available would probably go like: 100mm, f/4.5 to f/22; 300mm, f/5.6 to f/32; and at 600mm, f/6.3 down to f/45. The camera/lens combo makes all the appropriate calculations and *should* be displaying the actual info to you. (Your *lens* manual should have the most accurate info, regardless of the mount you have. The same would apply if it is a "lens-in-camera" device, such as some of the "super-zoom" items out there.) But to repeat my earlier post, if you chose something like f/8 or f/11 or f/16 or f/22, the selection will hold through out the range.

HTH

mwsilvers Loc: Central New Jersey

DXO lens tests often shows significantly more than a 1/4 stop difference. Perhaps your university test used too small a sampling. As an example, the DXO test of the Sigma 30mm f/1.4 DC HSM ART lens indicates a light transmission t-stop of 1.9 on a Canon 7D Mark II. That is getting close to 1 full stop of light less than that lens' maximum f/1.4 aperture would suggest. Far more than the 1/4 stop difference you indicate and quite noticeable depending on the subject and available lighting. In fact my Canon 35mm f/2 IS USM with a t-stop of 2 lets in almost as much light as the Sigma with a aperture 1 stop faster.

Elmerviking

No! The op asked if ISO would be the same!😊

Wmetcalf Loc: Rogersville, Mo

F8 is F8 no matter what lens

Apaflo Loc: Anchorage, Alaska

It is almost two full stops less, not one.

mwsilvers Loc: Central New Jersey

I don't agree at all. The light transmission of some primes at their maximum aperture can be as much as 2/3 stop less than the size of the aperture would suggest. That can be very significant in the right circumstances. The problem here, as is often the case, is that too many people are guessing based on general assumptions and not enough people are actually researching the question.

mwsilvers Loc: Central New Jersey

How do you figure that? From f/1.4 to f/2 is one stop, therefore a t stop of 1.9 would be slightly less than one stop less light transmission than the maximum aperture would suggest.