Optics Information Required

[Cloneranger]

Come on Gentlemen stop pulling my leg. The focal length of a lens is only 'as marked' when it is focused at infinity. when focused closer the focal length increases ... it doesn't matter if you focus closer with the focusing mechanism of the lens or by physically moving it forward, increasing the extension. This was one of my photo exam questions back in the fifties which I passed with distinction

I agree, to a point. I shoot a lot of manual focus lenses and according to your explanation "focal length of a lens is only 'as marked' when it is focused at infinity", when an extension tube is put on a lens it will not focus to infinity thus it would decrease the focal length of the lens, would it not? An extension tube is used to decrease the magnification ratio of the lens (which increases magnification), thus Alex has the correct formula which is (FL + et)/FL. So, if the above assertions hold true, adding an extension tube to a lens will decrease the lens's focal length.

[jcuknz]

As a practical empiracle photographer I avoid formula and simply know that the f/stop, assuming a fixed constant kind, effectively gets smaller with extension with the well known result that the f/5.6 aperture at infinity becomes an f/11 aperture at double extension. So if you believe that f/aperture means something then obviously the focal length has changed, doubled.

When I want to go close I add my CU lens or insert my extension tube[s],bellows until I get the required magnification possible with my gear and take the photo. I do have an opinion that adding CU lens to short lenses is a pointless effort as the change is minimal, as also using short extension tubes. Both add focusing restrictions without gaining much magnification, but each to their own.

I wasn't questioning Alex's formulae, which I ignored, only the statement below it I should have deleted it from my quote, sorry!

[jcuknz]

ajohnw ...I am trying to sort out a macro lens for my pen. I have a 100mm Pentax one that I can use on an eos but it's a bit of a problem on a pen so I bought a 50mm one - and even that one is still a problem. The ultimate macro lens is probably the old 200mm nikon with it's kit. Maybe others do something similar now but they are likely to be incredibly expensive.

A possible solution is to get the Panasonic 14-140 lens* and add a moderate powered CU lens, say 2 or 4 dioptre, and get away from the preoccupation with coming in close for tight framing and use the narrow angle of view of the longer lens to achieve the BCU. There is no difference in depth of field, only perspective, in using the longer lens. The 014140 has a switch on it to disable the OIS when used on a Pen and is a straight forward lens change. I have both Pany and Oly M4/3 cameras.

*280 AoV at full zoom versus the 160 of the 100 on the Canon.
My conclusion is to use automatic lenses rather than mess with legacy lenses. I prefer to work with a bit less reach in the case of telephoto in auto than manual to gain less than double the reach. So my purchase of my Oly PEN was pointless until recently I discovered it did quite a reasonable job of IR, which no Pany camera I have had did or does.

[ajohnw]

It's hard to say what an extension tube between a lens and a converter will do. It should just reduce the power of the converter if it's a thin tube but the image coming out of the converter wont be where it usually is. It may fall short of the sensor.. Changes to the focus so that the distances no longer match the scale may cure that or may not. I'd guess that there will be problems even if only poor images.

One thing that might help. I have a sigma 170-500mm apo and bought a 2x converter to go with it by Canon. It wouldn't fit so took it back and exchanged it for a Jessop's branded one and it all fitted together. I knew some one who used Jessop's converters professionally even on medium format and found them a lot better than he expected. Perfect was the way he put it, He used them on occasional jobs that cropped up where it wasn't worth buying a specific lens.

John

[Inkanyezi]

I should have guessed that it would cause more confusion.

And of course the barlow lens doubles the focal length of the prime lens, but what I said before is that when you focus the prime lens, that is mounted in front of a negative system, the focal length of the combined system is decreased. Hence you get double focal length of the prime lens when you put it on the extender and it is focused at infinity. Then when you focus it, the total focal length of the system prime lens plus barlow lens is somewhat decreased, causing it to focus closer, without moving the entire system farther from the sensor.

So - a tele extender is essentially a barlow lens. It multiplies the focal length by the enlargement factor.

Those macro focusing tele extenders that have been in the market are actually very good optical instruments. I have a Vivitar extender that is excellent, and with it, my 35 mm f/2.8 Carl Zeiss Distagon becomes a very good 70 mm f/5.6 macro lens. Of course, on a µ4/3 camera, only the central part of the image circle is used.

[Glenn NK]

Adding an extension tube to a lens and putting that combination on a camera will have no effect on the focal length of the lens, but alas, there's a fuzzy concept of "back focal length" that has indeed increased.

Thanks Urban:

I hope I never implied that putting an extender or tube behind a lens would change its focal length (any more than a crop camera changes the FL of the lens). I was trying to determine what the NET focal length of the COMBINATION would be.

Glenn

[ajohnw]

On F number changes with tubes the effect is similar to 2x etc converters. The lens which STILL has the same focal length is producing a larger image with the extension tubes on so the lighting levels on the sensor are "diluted" accordingly. A typical 2:1 macro lens for instance tubed to give 1:1 has the same effect a a 2x converter on a lens.

Thanks for the mention of the macro extenders I may give one a try. On my problems re a Pen and macro lenses the problem is focusing. The newer 14-42mm zoom will focus down to about 250mm and gives about 80mm across the sensor but af consistently misses small subjects and focuses on what's behind them. To manually focus the magnified view is needed. That's 7x or 5x min now I have an e-p3. Hand held in 35mm terms a 100mm macro lens = 100x2x7 = 1400mm. Rather difficult to keep steady or to check the entire subject out for focus. The Pen image stabilisation only works when the actual shot is taken. It copes with the magnifications easily. A panasonic lens is tempting for it's built in image stabilisation but the question is which one. 2:1 or higher mags are true macro to me.

This is the sort of result I have obtained so far. I'm about to start carrying some depth of field figure for the manual pentax 100mm macro lens about in my head that might help.

http://www.23hq.com/ajohnw/photo/8163926/original That is hand held, full sensor resolution and cropped. The spider was about 6-8mm long in the body.

John