(Sigh) Sir, with all due respect, you are mistaken in your assumption and interpretation of the “rule of thumb” which you are citing. That guidance, as has already been explained, relates FL to shutter speed to prevent the effects of camera shake, NOT exposure. My strong suggestion is to research that “rule of thumb” and read the results carefully, rather than just repeating inaccurate information. Now as why your camera, in your specific situation, shows a change in ISO when you zoom has NOTHING to do with the FL, only changes in aperture (perhaps RELATED to the FL) or lighting or shutter speed. You are simply misunderstanding the relationship. You are certainly entitled to your own opinion, but not your own facts!

THANK YOU to "MOST" of you for taking the time to share what your learning and experiences have brought you. Ultimately, I am looking for a longer zoom lens than the Nikor AF-S 70-300 4.-5.6 VR E lens that I have now. I am looking at either the Nikor 200-500 or the Tamron 150-600 G2. My situation is this:

My back yard is heavily shaded. It has enough wildlife in it that often it looks like the trailer to a Disney movie. With the heavy shade, and only 300 in zoom (450 with the 1.5 factor) I am getting more noise (higher ISO settings) when I shoot in Manual+Auto ISO. I am seldom happy with an exposure with a higher ISO than 3600, but really like to keep it below 2000.

The Nikor has a fixed aperture of 5.6, the Tamron is variable between 5-6.3. SO, ultimately the question is, if I use the SAME shutter speed, and set BOTH lens to +/- 500mm, AND set the aperture to the SAME f/8. Will the camera's auto ISO sense similar light and will my ISO settings be set close enough that I will not notice a significant increase in noise? ALL VARIABLES THE SAME, WILL I GET SIMILAR EXPOSURES from all THREE lenses based on lighting alone? (please, I don't want to start a NIKON/TAMRON feud, just SHOULD THE NUMBERS BE SIMILAR?

I am in a small town, so I don't have the luxury of going to the local camera store to get "professional help".

Thanks again to all who have an opinion to help me decide which of these lenses I am going to RENT FIRST to get a feel.

1/ I never said this “ rule of thumbs” effects exposure. It is a recommendation what minimum shutter speed you should use to avoid camera shake. I said that if you use aperture priority with auto ISO the minimum shutter time seems to follow this thumb of rule, I.e. longer FL sets a shorter time. For Nikon: go to settings and chose autoISO, maximum ISO and minimum shutter speed. If you go to minimum shutter speed you can chose any time or “auto”. If you chose auto the minimum shutter time will vary depending on what Fl you use! You can even adjust this minimum auto time to be faster or slower. The factory setting very much resembles this rule of thumbs.....Shutter time to avoid camera shake should be 1/FL seconds.
So...ISO will change with different Fl because the minimum shutter time will change!
Is this soooo hard to understand?

It is not at all, and I see that you have (finally) understood WHY your shutter speed and ISO is changing. It is because with your particular camera, you have made a menu setting in auto ISO mode that relates shutter speed to FL to prevent blur from camera shake, and THAT is what’s causing the change in ISO. It is not the FL directly, it is the auto ISO setting you have chosen. If you’ll look back 2 pages, you will see that I specifically mentioned menu setting in auto ISO as one potential reason for the change. Either way, we are now (I hope) in agreement, and I wish you well.

It's actually a good question, but with a many-sided answer. Short answer, yes, every lens in a perfect world lets in the same amount of light at the same setting. As was stated, the f stop is a ratio of aperture size to focal length. So, F4 on a 50 mm lens opens up to 1/4 of 50 mm. F 16 to 1/16th of 50 mm. Capiche? The reason lenses of different focal lengths let in. The same amount of light at the same settings is: the ‘length’. Of any lens is defined as the distance between the rearmost element in the lens to the focal plane, that is, your sensor. So a 300mm lens has a distance of 300 mm between the rearmost lens element and your film or sensor plane. That said, that rear element is for all practical purposes, the light source for the sensor. As light sources get farther away, the light intensity falls off, hence the need for a larger diameter aperture to keep all things equal. Bottom line is Yes to an excellent question. This answer may have been more than you asked for, but it is what it is.