question on effective f-number variation

[davros]

can someone help explain what effective F-number variation is? I bought a new Nikkor 105mm f/2.8 micro lens today. By it's description I would expect that it would be able to reach an aperture of 2.8 at any focal length. After taking it for a test drive I've found that aperture closes down to f/3.5 at the shorter focal lengths. Is this correct or do I have a dodgy lens?

[ricktas]

Umm, it only has one focal length: 105mm. Are you talking about distance to subject here?

[davros]

sorry, i'm obviously not using the correct terms. Yes I am talking about distance to the subject. At 3m+ I can set the aperture to f/2.8, but at 1.5m the best I can get is f/3. Every time I half the distance to the subject the aperture closes by .2 - give or take. So at the closest distance of .3 meters the widest aperture I can achieve is f/4.8.

[Speedway]

What mode are you using? In AP or full manual mode the aperture should stay as set. In other modes it will change according to available light.
Keith.

[fillum]

Davros I assume that you have the current model (AF-S VR), so I can't be specific about that lens as I have the previous AF-D model. The maximum aperture on my lens reduces as it approaches a 1:1 macro ratio (i.e. min focus distance) down to f/5 at 1:1 even though it's nominally an f/2.8 lens.

As far as I'm aware this is normal macro lens behaviour. I think all lenses might behave this way to some degree but on most it's negligible. So why don't they write it on the barrel? Marketing I guess...


Edit: Oh, your lens is new today. Then that would definitely be the AF-S VR model. I should read more carefully :)

Cheers.

[TOM]

no macro lenses are technically constant aperture, and yes what you experience is normal.

[peterb666]

It is a characteristic of macro lenses that as you move closer and the front element moves forward in focussing, the f-stop decreases. It can be, and normally is, well over a stop. I think it may be due to the focal length actually changing with really close focussing distances in order to achieve focus.

This is not the same as the difference in light transmission (known as T-stops) and f-stops which is a mechanical relationship of aperture to focal legth. The difference in that relationship is generally less than 1/3rd stop even in the most complex lens.

[kiwi]

Interesting, I was not aware of this "phenomena" before

[arthurking83]

I think there are a few exceptions to the issue, but as TOM said. They all do it to varying degrees.
The reasoning behind the issue is something that actually makes a lot of sense too. As you focus closer the effective aperture should be reduced for some depth of field, and your terminology (confusing Rick) should have been closer focus distance not shorter focal length.. never mind..

As you focus closer and closer, the DOF is becoming thinner and thinner to the point where it'll basically be non existent.. so with the design of these lenses, it makes sense to force the lens to produce some DOF.

I can't exactly remember which lenses have the ability to maintain a constant max aperture as they focus closer, but I think(I'm sure they're smaller initial aperture lenses anyhow).

That is, if the 105VR was designated an f/4.8 aperture lens by Nikon, then at MFD(minimum focus distance, and hence 1:1 macro) it'd then be a constant aperture lens all the way through the focus distance scale. But what use would that be, if you wanted portraits(at a distance), or lower light imagery(which wider apertures help with). We know that the 105VR is capable of close to f/2.8 at beyond 3m focus distance so it's best to take full advantage of this ability.

I think the actual maximum open aperture rating of this lens is accurately described as f/2.92(??) anyhow(don't quote me on this tho!)... so even the f/2.8 rating is(may be) misleading.

anyhow.. the end point is that as you focus closer the lens is automagically lending you a helping hand.. AND reveals this phenomenon. Some Canon lenses still report the max aperture value rather than the real value(this would lead to exposure errors!!).

The other aspect you haven't yet considered is that the actual focal length of this lens also shortens as you focus closer too! I think the end result is that the effective focal length is more like 85mm or so. But this again is not an issue either(as they all do it, bar a few exceptions.. and again, this has to do with both the internal focus design and the need to keep an aperture value that's still quite largely usable(for viewing through the viewfinder).
As the max aperture value gets smaller, the image in the viewfinder also gets a lot darker.

it's all about lots of compromises.

at MFD the maximum aperture on this lens is f/4.8... you'll be stopping down to f/11 or smaller to get any depth in your focus anyhow... so the value is basically academic.
(this of course doesn't take into account the topic of focus stacking!!).

[davros]

Thanks everyone for your input. Aurthorking83, you explanation makes a bit of since to me. I've clearly got a lot to learn about lenses. Now I just need get out for a play with the new toys!