Hello everyone, this is my 2nd dumb question of the day. I have a question about variable aperture lenses, specifically a (cheap) Tamron 70-210mm F4-5.6 which came with the Nikon FM2 I just bought. I have only ever used primes and I am not sure how variable aperture works in terms of using an external meter.
So with this lens zoomed to 210mm the largest effective aperture is F5.6. This is presumably the amount of light which is let in when the lens aperture is set to F4 on the aperture ring.
However, if I set the lens aperture to F5.6 on the ring and then zoom to 210mm there is clearly less light coming in than when zoomed to 210mm and set to F4. This is clear when looking through the lens and is confirmed by the camera's TTL meter. So would the effective aperture actually be F8 with the lens at 210mm and the aperture ring at 5.6? Do I have to compensate a whole stop throughout the entire range of the lens?
If using TTL metering then this obviously isn't a problem, but I use an external meter so I need to know what the actual effective aperture is. Also, how do I tell at what point in the lens's zoom range the aperture switches from F4 to F5.6?
Any advice is appreciated!