I am a new Nikon user, with a D850 and the 105f2.8 VR macro. In playing around with this lens the combination does not allow me to select f2.8, it only goes F3, why is that?

As you get closer to your subject, aperture narrows. This is a normal behavior of large-aperture macro lenses.

Just focus on a more distant object and you should get f/2.8.All macro lenses I own (105mm f/2.8 VR and the 200mm f/4) and other macro lenses I know have this property to reduce the aperture the smaller the distance to the focus point gets. Or in other words the higher the magnification gets.This might help you with your question (section "Light loss with Microphotography"):https://www.nikonimgsupport.com/eu/BV_article?articleNo=000006513&configured=1&lang=en_GB&setRedirect=trueEdit: maybe I should add that some manufactures like Canon (if I remember correctly) did not report the effective aperture back correctly. So instead of showing f/5.0 at 1:1 which is the actual, effective aperture (even on e.g. a Canon 100mm f/2.8 macro) they show only the selected aperture of f/2.8. Nikon on the other side always shows the actual, effective aperture, e.g. the real aperture the lens currently has depending on the magnification, ...

As others have mentioned the display only shows f2.8 at longer focus distances.Byminimumfocus the readout changes from f2.8 to f4.8.Also f32 at infinity becomes f51 at minimum focus.Taking good pictures with your camera/lens combination is often more relevant than the idiosyncrasies of Nikon top plate readouts using Nikon macro lenses.

Think of it as the lens is moving the elements further from the sensor as you focus closer and closer.   You get the same type of light loss and smaller aperture readings when you put an extension tube between the camera and a lens to focus closer.  Bottom line is that the more tube you add the more light is lost as the light has to travel further to get to the sensor.Not the first time this question has been asked when someone gets a new macro lens.

Great answers everyone.

Joseph K Boston wrote:As you get closer to your subject, aperture narrows.The aperture doesn't narrow; the exit pupil moves away from the image plane changing the effective f-number.See PhotonsToPhotosOptics Primer - Effective Aperturefor some visuals and further explanation.

Thanks for the explanations, I am not used these changes with my Canon system.

So aperture blades maintain their position? Interesting. I learned something new today.

seyin wrote:Thanks for the explanations, I am not used these changes with my Canon system.It goes back a while. In at least one of the old MF Micro-Nikkors, the aperture dial read out in effective aperture, unlike the standard lenses of he day. If you stopped it down with the DOF preview button, you could see the blades open up as you focused closer.Jim

JimKasson wrote:seyin wrote:Thanks for the explanations, I am not used these changes with my Canon system.It goes back a while. In at least one of the old MF Micro-Nikkors, the aperture dial read out in effective aperture, unlike the standard lenses of he day. If you stopped it down with the DOF preview button, you could see the blades open up as you focused closer.Pretty sure the blades don't "open"; the virtual image would appear to change size, of course.Or are you saying there was an attempt to maintain the effective aperture?

bclaff wrote:JimKasson wrote:seyin wrote:Thanks for the explanations, I am not used these changes with my Canon system.It goes back a while. In at least one of the old MF Micro-Nikkors, the aperture dial read out in effective aperture, unlike the standard lenses of he day. If you stopped it down with the DOF preview button, you could see the blades open up as you focused closer.Pretty sure the blades don't "open"; the virtual image would appear to change size, of course.Or are you saying there was an attempt to maintain the effective aperture?Yep. 55 mm f/3.5 Micro-Nikkor, first gen. I had one.Jim

JimKasson wrote:bclaff wrote:JimKasson wrote:seyin wrote:Thanks for the explanations, I am not used these changes with my Canon system.It goes back a while. In at least one of the old MF Micro-Nikkors, the aperture dial read out in effective aperture, unlike the standard lenses of he day. If you stopped it down with the DOF preview button, you could see the blades open up as you focused closer.Pretty sure the blades don't "open"; the virtual image would appear to change size, of course.Or are you saying there was an attempt to maintain the effective aperture?Yep. 55 mm f/3.5 Micro-Nikkor, first gen. I had one.Me too. I recall it focused by extension but I didn't notice anything about the aperture changing.Hard to believe. As the exit pupil is pulled away from the image plane making the physical aperture larger would almost certain just get a hard vignette from the rear lens elements.Regards,

bclaff wrote:JimKasson wrote:bclaff wrote:JimKasson wrote:seyin wrote:Thanks for the explanations, I am not used these changes with my Canon system.It goes back a while. In at least one of the old MF Micro-Nikkors, the aperture dial read out in effective aperture, unlike the standard lenses of he day. If you stopped it down with the DOF preview button, you could see the blades open up as you focused closer.Pretty sure the blades don't "open"; the virtual image would appear to change size, of course.Or are you saying there was an attempt to maintain the effective aperture?Yep. 55 mm f/3.5 Micro-Nikkor, first gen. I had one.Me too. I recall it focused by extension but I didn't notice anything about the aperture changing.Hard to believe. As the exit pupil is pulled away from the image plane making the physical aperture larger would almost certain just get a hard vignette from the rear lens elements.Regards,https://www.photo.net/discuss/threads/compensating-55-micro-3-5.177529/#post-1945746http://nikongear.net/revival/index.php?topic=224.0https://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/6070nikkor/micro/index.htmThere were lots of versions and not all had the compensating diaphragm.Jim

JimKasson wrote:bclaff wrote:JimKasson wrote:bclaff wrote:JimKasson wrote:seyin wrote:Thanks for the explanations, I am not used these changes with my Canon system.It goes back a while. In at least one of the old MF Micro-Nikkors, the aperture dial read out in effective aperture, unlike the standard lenses of he day. If you stopped it down with the DOF preview button, you could see the blades open up as you focused closer.Pretty sure the blades don't "open"; the virtual image would appear to change size, of course.Or are you saying there was an attempt to maintain the effective aperture?Yep. 55 mm f/3.5 Micro-Nikkor, first gen. I had one.Me too. I recall it focused by extension but I didn't notice anything about the aperture changing.Hard to believe. As the exit pupil is pulled away from the image plane making the physical aperture larger would almost certain just get a hard vignette from the rear lens elements.https://www.photo.net/discuss/threads/compensating-55-micro-3-5.177529/#post-1945746http://nikongear.net/revival/index.php?topic=224.0https://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/6070nikkor/micro/index.htmThere were lots of versions and not all had the compensating diaphragm.Interesting. I'll have to see if I can find a patent so I can put an example into the Optical Bench.Regards,

My non-compensating successor lens 55/2.8 Ai-S has two long metal "popsicle sticks" that flank a tab on the iris actuator, such that the actuator stays at the same rotation for every focal distance. Does the compensating model simply have a cam curve in those sticks?The picture of that older lens on the Mir site makes it look like the front lens element is just about the same diameter as on my lens. That makes me wonder if the compensation isn't actually effective at the wide open setting.

just Tony wrote:... The picture of that older lens on the Mir site makes it look like the front lens element is just about the same diameter as on my lens. That makes me wonder if the compensation isn't actually effective at the wide open setting.Effective f-number has to do with how much light is delivered by the exit pupil, not how much is gathered at the entrance pupil. The compensation would be to make the exit pupil larger since it's moving away from the image plane and light is spreading out more. No more light would be gathered, the front lens element would become a field stop and limit that.

bclaff wrote:Hard to believe. As the exit pupil is pulled away from the image plane making the physical aperture larger would almost certain just get a hard vignette from the rear lens elements.Ive thought more about this, Bill, and I don't understand your point.Say you have a non-compensating macro lens. You set the focus ring for 1:2. You want f/22, so you set the aperture ring to f/16.Say you have a compensating macro lens. You set the focus ring for 1:2. You want f/22, so you set the aperture ring to f/22.In either case, the diameter of the aperture is one 16th of the lens focal length. I don't see how vignetting plays into the situation.Jim

bclaff wrote:just Tony wrote:... The picture of that older lens on the Mir site makes it look like the front lens element is just about the same diameter as on my lens. That makes me wonder if the compensation isn't actually effective at the wide open setting.Effective f-number has to do with how much light is delivered by the exit pupil, not how much is gathered at the entrance pupil.What comes out must first go in.The compensation would be to make the exit pupil larger since it's moving away from the image plane and light is spreading out more. No more light would be gathered, the front lens element would become a field stop and limit that.The last sentence in my quote above addresses that field stop aspect. Closing down the entrance pupil also closes down the exit pupil.

just Tony wrote:What comes out must first go in.Yep:https://en.wikipedia.org/wiki/EtendueJim