I have Tamron 90mm f/2.8 Macro Lens on my Nikon D5500. I’m trying to get my aperture to go to its lowest which is 2.8, however, I can’t get it to go lower than 3.0 or 3.5 (its lowest changes often and I’m not sure why). I’ve tried it both Aperture priority mode and Manual. Any advice would be greatly appreciated.
The lens is reporting the effective aperture at your current focus distance. Note that as you focus closer, the reported maximum aperture reduces; the closer you focus, the higher the f-number you'll get.
This is actually a good thing, particularly if you are relying on external metering. It saves you having to calculate the true exposure based on the "bellows draw" (that is, how far the lens is being moved away from the sensor or film plane as you focus). If you are shooting things at low magnification, the difference between the exposure at infinity and the exposure at your focusing distance is negligible no matter how the lens works, but at higher magnifications - as you move into macro territory - the difference between an internal-focus (rear-focus) lens and a unit-focus (traditional-type) lens becomes important. If the lens is moved far enough away from the sensor or film to focus at 1:1, you lose two stops of effective aperture. So a "90mm f/2.8" lens focused at 1:1 is either not 90mm anymore, or not f/2.8 anymore.
If a lens intended for macro shooting keeps a constant reported maximum aperture, it could be telling you one of two things:
the focal length of the lens is being reduced as you focus closer, in which case the effective aperture for exposure purposes remains the same, but you lose working distance; or
the focal length of the lens is remaining constant, but you will be underexposing unless you figure out the bellows factor and compensate if you are not using TTL metering at the time of shooting.
Some lenses are a little bit of both, too, so you can't just look at the focus distance and make a quick calculation. If the focal length is being reduced, but not as much as in a "pure" IF lens, while the lens as a whole is effectively being moved further from the sensor, you'll have had to do some testing ahead of time to know what compensations to make with external metering, or take test shots as you go.
It's best, then, if the lens tells you what's really going on. Even if that seems somewhat counter to your expectations.
The question is: Does the Tamron 90mm f/2.8 Macro lens maintain a constant aperture as you close focus and if not, why not?
My believe is; a true macro lens should maintain a constant aperture regardless if working at infinity or unity (life-size otherwise called 1:1 or magnification 1).
A general purpose lens is optimized to work at infinity and close-in to about 1 meter (3 feet). A true macro is optimized to work at unity and slightly compromised when tasked to work at infinity. Let me add that it’s unlikely you will notice any degration.
That being said, when we work in close with a non-macro lens, f/numbers becomes invalided for close distances starting at 1 meter or shorter. This is because, as we work closer and closer, we must rack the lens further and further away from film/digital sensor, to achieve focus. You should know that the distance lens-to-film/sensor is called the back-focus distance.
Now the f/number is computed by dividing the focal length by the working aperture diameter. When we close focus, the back-focus distance replaces the focal length distance for this f/number calculation. The net result is, the further the back focus, the dimmer will be the image. The change in image brightness, as you approach unity is severe. Unity is magnification 1 and formula to compensate for the dimming of the image is called the bellows factor. This formula is (M+1) X (M+1). Thus at unity, M=1 and the math is (1+1) X (1+1) = 2 X 2 = 4.. This tells us that at unity we need to multiply the shutter speed by 4 or open up two f/stop to allow 4X more light to play on film or sensor.
As to a true macro: A macro is optimized to work at unity and thus should maintain a constant exposure as you close focus. A true macro accomplishes this by design. The upfront lens group magnifies the apparent size of the aperture hole, as seen from the front. This magnified view allows more light to enter the lens. As you close focus, the apparent size of the aperture changes and compensates for any light loss.
If the lens is not a true macro, compensation will be only partial or nonexistent. You might check the specifications of your lens to check. Don’t worry if it is not a true macro, modern cameras with thru-the-lens metering provide compensation for light loss. Usually this will be in form of lengthened exposure time and/or elevated ISO or both.
I know nothing about your specific lens. I have been wrong more than 1000 times. Because of this I usually label my responses as gobbledygook.