Longest Extension Tube for EF lenses

OK, whatever the physics (OK, as a (geo)physicist I should say that), Tom does have a valid point that I completely understood. The alternative to extension tubes is to use a 1.4TC or 2xTC and back up. Very nice of you to offer TOM, but I do have one of each TC - although they are older versions and I'll probably be upgrading around Christmas. However, as pointed out, it's not quite the same, does lose some IQ and at least a stop (and AF except the center point in the case of the 2x). And sometimes backing up is not easy (e.g. in an aviary).

It sounds like 36mm is the longest conventional I am going to get (which I suspected when I started the thread), or may even be useful which did not really consider. 31mm extension is quite nice still because although the MFD is still a little longer than I'd like, I also retain focus 20-3ft away.

Based on what a couple of folks have said it sounds like anything more than 30-odd mm and the AF will struggle. I guess I'm hoping a single tube would play nicer than stacking them, but then again why should it if the contacts are fine?

I'll have to think about whether to have a go at pulling the optics out of my x2TC when/if I upgrade (I might be tempted to try for what it would sell for)l. That was not a bad idea Dave, and something I had not even considered. Not that I'm good at that sort of thing, but I might give it a go - hammer, screwdriver and a soldering iron? Even if it did not work for the DO, it would be a nicer option than stacking for the other lenses that still AF with 60mm of extension.

I might also still give the Kenko's a try for the extra few mm and possible better construction. At least I could give them a test run and return them if they are no better than the ones I have.

Thanks folks.

Tom, think about this for a moment...if you take a non-AF fixed FL lens from 1970 with a helical focus mechanism that racks the ENTIRE OPTICS as a UNIT out from the focal plane for close focus, and toward the focal plane for infinity focus, according to YOUR interpretation the lens ...

1. is not a 'fixed' focal length (the distance from lens to focal plane varies continuously from minimum focus distance to infinity!)
   
2. is a 'variable' focal length (the distance from lens to focal plane varies continuously from minimum focus distance to infinity!)
   

Interesting thread. Tom, focal length is as Wilt and Jon say. But for you, it can be as you say.

Three Kenko tubes (total 68 mm of extension) work fine mechanically and electrically with my 100-400mm II (on my 7D2). There might be a little play when you flex the combo, but I hadn't actually noticed this before encountering this thread, and I don't notice any loss of communication with the body.

Focusing behaves weirdly with these tubes on this lens, because the focal length of this lens (and most other modern lenses) varies as you focus. So with the tubes, the change in sharpness that the AF system perceives can be unexpected compared to what the algorithms expect when the AF system moves the elements in the lens. At some zooms and distances the AF works, but at others, the AF oscillates.

And the magnification varies in a complex way with zoom and distance, again because the focal length changes with focusing. So when doing macro work with this lens extended with the tubes, it is not easy to pick the right extension for the magnification that you need. That would be simpler with the 400 DO though, and might be worth trying.

For the record, with 3 tubes and at 400mm, the minimum working distance (from the end of the lens to the subject) seems to be around one foot. I haven't measured it, it's just a guess. The magnification is around 0.6 - 0.7x. At 100mm, the MWD is around 3.5 inches.

johnf3f wrote in post #17738118
Plastic mounts have far less play in them as they are much more accurately made (moulded) ...

There is no way. Plastic is much more flexible than metal, and moulding is an imprecise process. Plastic is injected hot and contracts a lot during cooling, causing shrinking. Metal deforms less under stress and metal has a lower coefficient of thermal expansion. In short, metal is much more dimensionally stable than plastic.

Lots of camera and lens parts are made of plastic, though, and those parts function fine. That does not say they are better than if they were made of metal.

The Kenko extension tube set seems well made to me. The mounts are made of metal and the bodies seem to be made of a sturdy plastic. If the set still has too much flex when used together, then you might consider cementing them together.

Tom Reichner wrote in post #17739637
Actually, if you go by the typical, most commonly accepted definition of focal length, the addition of an extension tube does change the focal length of a lens. You are putting xx millimeters between the optical center of the lens and the focal point.....how can that possibly not be changing the focal length when the focal length itself is defined as the distance between the optical center of a lens and its focal point?

Just google "definition of focal length" and see for yourself. Here are several of the definitions that pop up:

The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the subject (at infinity) is "in focus".

fo·cal length
noun
noun: focal length; plural noun: focal lengths
. . the distance between the center of a lens or curved mirror and its focus.

The distance between the focal point of a lens (that is, the point at which the lens will focus parallel rays of light) and the lens itself.

Focal length (shown in red) is the distance between the center of a convex lens or a concave mirror and the focal point of the lens or mirror — the point where parallel rays of light meet, or converge.

The distance of a lens or mirror to its focal point. Also called focal distance, focus.

Focal length means burn distance. That's what it's actually called in German (focal length = Brennweite). Brenn = burn, weite = distance.

Take a magnifying glass and use it to focus the rays of the sun to burn a piece of paper or wood. The distance from the glass to the burning point is the burn distance, the focal length.

If you raise the magnifying glass, the burning point is in the air somewhere. The location of the burn point has not changed, you just can't see it any more.

You can take a photographic lens off the camera and do the same thing. Burn a piece of paper with your 400mm DO. People will wonder what you are doing, but never mind. The distance from the lens (the rear nodal plane of the lens to be exact) to the burning point is the burn distance = focal length. As long as you don't change the positions of the glass elements inside the lens (by zooming or focusing), the focal length stays the same, even when it is off the camera.

Modern lenses are complex things and have unexpected behavior. Typically when you focus closer with a prime lens, the focal length becomes shorter, probably a lot shorter. So when you are doing macro work with a lens like this, focusing closer and using extension tubes, the focal length might be only around 200mm instead of the marked 400mm. How is this possible?!? It says 400mm right on the barrel and now I'm saying it might really be around 200mm. Well, such are the mysteries of photographic optics.

(BTW, the aperture also changes from the marked aperture!)

Archibald wrote in post #17741218
You can take a photographic lens off the camera and do the same thing. Burn a piece of paper with your 400mm DO. People will wonder what you are doing, but never mind.

Well, I'd be wondering why he wasn't using an f/1.4 lens, because that would start a fire much, much, quicker. An f/1.4 24, 35, or 50mm would put less total heat from direct sun on the focal plane than an f/4, [Edit:that should be 400mm f/4, or 100mm] but they would be much more intense, in a much smaller area, facilitating combustion.

If you're just trying to warm something larger than the projected sun and conducts heat well, then the most important thing is the physical (mm) size of the aperture.

f-number determines local intensity of sun (exposure per unit of time), and physical aperture determines total projected heat from the sun ("duck photons", or sun photons).

Wow!

Ed, you could check out the Canon Life Size Converter.
https://photography-on-the.net/forum/showthre​ad.php?t=93064


Made to work with the 50mm compact macro it combines extension tube and 1.7x or so T-con in one unit.
AF stinks, but it will fit your lens and function with very good image quality.

Archibald wrote in post #17740805
And the magnification varies in a complex way with zoom and distance, again because the focal length changes with focusing. So when doing macro work with this lens extended with the tubes, it is not easy to pick the right extension for the magnification that you need. That would be simpler with the 400 DO though, and might be worth trying.

Yes, using extension tubes with zooms can be nice or it can be hairy. Either way the FL (zoom) has much more effect than the focus ring. If you choose the wrong extension you can get the subject in focus, but you may not get the magnification you want.

Thanks for the link Jake, I was completely unaware of this. That's worth knowing about in any case.

Is there a formula to calculate how much a given extension tube will reduce the MFD of a given lens? I have a 7DII with a 600 F4 and I want to shorten up the MFD from the stated 18' for shooting perching birds. Any reason to go with the more expensive Canon tubes vs the Kenko's?
Any help is appreciated.

Quack Me Up wrote in post #18008000
Is there a formula to calculate how much a given extension tube will reduce the MFD of a given lens? I have a 7DII with a 600 F4 and I want to shorten up the MFD from the stated 18' for shooting perching birds. Any reason to go with the more expensive Canon tubes vs the Kenko's?
Any help is appreciated.

The formula IIRC is 1/(focal length) = 1/u + 1/v, where u is the subject distance and v is the sensor-lens distance. You need to run the equation a few times to see what the difference in v is. One problem is that the focal length usually changes when not at infinity, so the formula won't be accurate.

There is an online calculator here.
http://www.cambridgein​colour.com …tension-tubes-closeup.htm

Also check the lens manual.