Use of extension tubes instead of macro lenses

[Overread]

Mine doesn't to that.... focus to minimum and it's still 2.8.

Hmm I thought Nikon did that with most lenses, both own and 3rd party ones. What lens are you using?

 

[480sparky]

Mine doesn't to that.... focus to minimum and it's still 2.8.

Hmm I thought Nikon did that with most lenses, both own and 3rd party ones. What lens are you using?

Micro Nikkor 105mm 1:2.8 AF-D. I just fired off two shots, one at infinity, other at minimum.. both report 2.8.

 

[Gavjenks]

It's not just macro lenses. ALL lenses should get slightly darker when you focus on things closer to the lens, unless it's specifically corrected for (which it might be on modern lenses, including macro ones).

When you focus on things nearer to you, the lens or a focusing group is moving further away, and the image circle is essentially getting slightly larger, thus diffusing the light out and causing less of it to fall on the sensor/film.



Think about a view camera. You focus by moving the entire lens forward and backward, right? This is precisely the same thing you're doing when you use extension tubes, for instance, and the same side effects of brighter or darker images will result without changing mathematical aperture. You just don't normally notice it, because the difference in lens distance for 10 feet versus infinity is very small, plus it may be corrected for in modern lenses.

Even if it isn't corrected for in the engineering of the lens, it would still effectively be corrected for automatically in modern cameras due to TTL metering. The camera will detect slightly darker scene, and lower your shutter speed by 1/3 of a stop or whatever, without you necessarily realizing why, and you'd still get a fine exposure.

Mine doesn't to that.... focus to minimum and it's still 2.8.

Hmm I thought Nikon did that with most lenses, both own and 3rd party ones. What lens are you using?

Micro Nikkor 105mm 1:2.8 AF-D. I just fired off two shots, one at infinity, other at minimum.. both report 2.8.

I would expect it to still always report 2.8. Because the aperture isn't changing, even if it may be getting darker. You need to check the histograms to address this question, not the reported aperture.

Aperture strictly defined is the focal length / the diameter of the restricting aperture. When you move the whole lens closer or further to achieve macro magnification, you aren't actually changing the focal length, nor the size of the opening. Thus, it's still technically 2.8 But it should still get darker if and when the lens is being moved further from the body

 

[Overread]

Except most modern macro lenses do change the focal length as they focus closer - The canon MPE 65mm even comes with a sheet detailing how the aperture and focal length change as it focuses closer - its something around 41mm when at the 5:1 magnification. As a result the lenses are not just getting darker, but the effective aperture is reducing. The reporting or not by the camera is just a technicality on how its reported to the user.


Note I'm using the word "most" since I'm not directly aware of a current macro lens that works differently - there might well be several out there that don't use these methods or older version that do things differently.

 

[Gavjenks]

Except most modern macro lenses do change the focal length as they focus closer

That's true, and it is yet another factor that messes confusingly with brightness. But that's separate from the fundamental focusing = darker/lighter effect. It's also not any sort of optical law or whatever. It's just a cheaper, non-parfocal design. It is quite possible to make lenses that do not change FL as you focus, but they just cost more (for instance, all cine lenses are usually parfocal, and that fact has a lot to do with them costing so much money).

The costs of making a parfocal lens could very well increase exponentially at the macro end of things, too. I wouldn't be surprised if they do. In any case, most macro photographers probably don't care enough to pay whatever the huge cost premium would be, so they just give you a chart instead to keep their product sanely priced.

Even a parfocal lens should still get slightly darker though when focusing on closer things.

 

[Derrel]

..........Also be aware that most modern macro lenses reduce the effective aperture as they focus closer, so whilst the lens might be listed and might show f2.8 as the maximum aperture, the actual aperture might be closer to f5.6 (this shift remains constant through the aperture range, so the smaller apertures are also affected).
Extension tubes and other methods of focusing closer can also have a similar effect upon the effective aperture.

Nikon cameras do report this effective aperture change on macro lenses; which is why Nikon lenses can't shoot at f2.8 and Canon ones can (remembering of course that the actual aperture in both cases is the same its just a difference in how the camera is reporting the aperture to the user).

Mine doesn't to that.... focus to minimum and it's still 2.8.

A macro lens that focuses closer and yet still remains at f/2.8 is losing effective focal length. That's the deal with the devil that's made to get around loss of aperture. There is no free lunch...if the lens aperture is to remain at f/2.8, then SOMETHING MUST be altered, and that 'something' is the focal length.

I believe my Tamron 90mm at 1:1 magnification is only 73mm in focal length.

One thing that has not been brought up is the main property of a dedicated macro lens: flatness of field. A good macro lens has a flat focus field, which is very helpful in close-in shooting. A lot of regular field telephoto lenses have some field curvature at close focus. ALso, a number of zooms at their tele-settings lose sharpness, and when focused at their minimum focus distance, many tele-zoom lenses,and a number of field telephotos, can be less-than-stellar performers. The same is not true of macro lenses, which are mostly single focal length lenses (an exception are handful of macro-zooms like the 70-180mm Micro-Nikkor), and which are VERY sharp at close distances, and do not have many compromises made.