www.ukbutterflies.co.uk

I confess I'm totally lost. I would understand that effect if the focus point got inside the lens, but apparently that is not the case here. Can you at least adjust the focus (without achieving it) or is that locked too?

I suspect that there may be some confusion here between 250mm focal length setting and 250mm working distance. I think Gruditch is pointing out that you can't get closer than 250mm distance whatever you do. There's a good discussion of the 100 - 400 for close-up work at http://www.birdforum.net/showthread.php?t=71054

Don't forget, also, the possibility of a teleconverter. These are particularly useful with shorter focal length macros - the older version of the Tamron 90mm even had scales to show magnification with a teleconverter. I use a 1.4x converter with my 90mm when I need a longer working distance.

Mike

That's what I thought first, but then it was mentioned that

Gruditch wrote:it it will only focus between about 30ft and 4ft, and you can't focus at all under 250mm

Christ alive.

My fault sorry

Without the tube fitted, the Canon 100-400mm, will focus to infinity at all focal lengths, and has a minim focus distance of 1.8m.

Fit the tube, you can't focus to infinity. At 400mm the minimum focus distance go's down a bit, for argument sake lets say 1.5m

At the now 1.5 minimum focusing distance, you can zoom from 400mm to 250mm, after that ( 250mm - 100mm ) you have to move progressively closer to the subject. At the 100mm focal length, you can get as close as 50cm.

Regards Gruditch

That makes more sense.

Does that actually mean that with the tube your min focus distance @ f=400 equals max focus distance @ f=250mm? You should be able to focus closer @ f=350 than @ f=400. Just don't touch the focus and move to focus to check this.

The question is what focal length gives you the best magnification with the tube? If it is 400, there's no point in using anything else.

JKT wrote:The question is what focal length gives you the best magnification with the tube? If it is 400, there's no point in using anything else.

Probably

Regards Gruditch

I have been playing around with my 100-400 and some extension tubes today.
From my crude experiments, there appears to be no significant difference in the actual magnification regardless of the focal length used.
e.g. The "in Focus" area at closest focus is from 6.2cm across a crop sensor body with a 32mm tube with zoom at 400 and 100 and all points in between.
Obvioulsy you are at a quarter of the distance away at 100mm - 25cm from the lens element at 100mm, and 98cm at 400mm
The big difference is the distance of focus at "infinity". At 100mm, it is just 38cm, but at 400mm it is about 5m (yes - metres - 500cm).
Mine still autofocuses with up to 32mm of extension tube fitted (20 + 12).
They are Kenko DG with 12cm, 20cm and 36mm tubes. Full AF and Exposure communication.
off E-Bay. I forget the price
HTH

Interesting. I would have thought you'd get better magnification with shorter focal length, but apparently not. So the only answer is stacking more tubes and going manual.

I've put two of those 36 mm Kenkos after 100/2.8 L IS, but the added benefit from the second is not that great and the minimum magnification is becoming limited. One tube is optimum for that lens. Besides, there's always MP-E...

Quick diversion and a question mainly for Gary but also to others who use the Canon 100-400L. Do you use it with a protector filter or UV filter or similar? The reason I ask is that I've found mine is noticeable less sharp/able to accurately focus with one on,

Bill

I have used it with a polarising filter, and it worked just fine. There does however seem to be a variation in the quality, from lens to lens.

Regards Gruditch

As you guys like playing around with lenses, maybe you can answer this question...
Although you can't fit a Sigma 150mm f2.8 lens onto a Canon 2 x II extender, you apparently can if you use a 12mm extension tube between them. Does that sound correct and if so, what are you likely to finish up with - focal length and F stop wise - apart from brain ache trying to figure it out in your head?
part b). Is it likely to be worth doing if you have them?

Trev

Jumping to part B, no probably not worth it. I think you will end up with a very restricted working distance, and terrible AF.

Regards Gruditch

I'd have to disagree here. A 12 mm. extension tube on a 150 focal length will allow focusing quite far - not infinity, but far enough for any butterfly. The AF would be lousy, but who uses that anyway? You would get a 300 f/5.6 macro lens, which would work to a bit over 2x magnification. How well it would work is another question entirely. The canon extender is designed for tele, so it might not be optimal for close focus. The other drawbacks would be quite dark viewfinder and (due to focal length) short time required for hand held pictures.

I'm not sure what the point would be, if it focused to infinity, you would have a 300mm F5.6 that you could use for general wildlife, all be it with awful AF. But as it will have a restricted working distance, it would only be useful for macro work, and you would be better served using just the Sigma 150, or add a x1.4 converter.

Regards Gruditch

It would do two things that the Sigma can't do without it. It can go to 2x magnification and it can take a comparable picture of a skittish insect from twice as far. If one doesn't need either, it can be - and should be - taken off. On the average the 1.4x would probably be a better overall solution, but that wasn't the initial question. There is also the bokeh to consider: I don't much like the results of Tamron 180 with Kenko 1.4x. But that requires testing.

Some of the discussion in this thread has brought out the complex behaviour of modern lenses. If they were simple, then adding an extension tube would have the same effect as screwing the focus tube out a bit further.

Unfortunately, many lenses now use 'internal focus' systems, which actually 'cheat' by reducing the effective focal length as the focus ring is turned. The lens remains the same overall length but the focal length is shortened, thus bringing the point of focus closer.

A side effect of this is that the image magnification is not as great as one would expect from the published focal length. I don't know the actual figures but the maximum focal length of the Canon 100 - 400 is probably less than 300mm at its closest focus distance. This explains why extension tubes do not always do what you expect, espacially when used with zoom lenses.

Mike

MikeOxon wrote:I don't know the actual figures but the maximum focal length of the Canon 100 - 400 is probably less than 300mm at its closest focus distance.

That's easy to check if you have the lens at hand. Just set the lens at f=400 and minimum focus. Put a ruler horizontally where it is in focus. Measure the distance from the sensor to the ruler (L) and calculate magnification (m) from how long stretch of ruler is in the picture and sensor width. Then the actual focal length is: f = m*L/(1+m)^2

I've done the following:
- Tamron 180/3.5: 115 mm
- Canon 100/2.8 L: 74 mm
- Canon 60/2.8: 50 mm (at infinity 61...62 mm!!!)

From external data I've calculated:
- Sigma 150/2.8: 95 mm
- Sigma 180/3.5: 115 mm

and there I was worrying I might be being too 'techie' for this forum! I should have known better.

Mike

Bill S wrote:Quick diversion and a question mainly for Gary but also to others who use the Canon 100-400L. Do you use it with a protector filter or UV filter or similar? The reason I ask is that I've found mine is noticeable less sharp/able to accurately focus with one on,

I've had a Canon clear protect filter on mine since new. I checked it by taking identical shots with and without the filter and comparing them at full size on my PC. They looked identical. AF has never been an issue either (including photographing high speed aircraft). YMMV though - there's so many filters and filter manufacturers out there that the only way to be sure is to test for yourself.

NickC wrote:

Bill S wrote:Quick diversion and a question mainly for Gary but also to others who use the Canon 100-400L. Do you use it with a protector filter or UV filter or similar? The reason I ask is that I've found mine is noticeable less sharp/able to accurately focus with one on,

I've had a Canon clear protect filter on mine since new. I checked it by taking identical shots with and without the filter and comparing them at full size on my PC. They looked identical. AF has never been an issue either (including photographing high speed aircraft). YMMV though - there's so many filters and filter manufacturers out there that the only way to be sure is to test for yourself.

Thanks Nick and Gary, I sent my 100-400 back again to Canon. It went back just before my hols for calibration but I shot some test shots of an ISO chart at the weekend, it was tripod mounted, each shot focus was set to infinity before shutter activation via remote release. I took shots on the 7D and -20 to +20 MA in 2 MA unit increments and the results were all over the shop. Very hard to pick the best MA setting whereas other lenses I have have been easy. Then I took 20 repeat shots at a single MA setting and checked for consistent focus. Image stabilisation was off and shutter speed was around 1600. Again some pretty poorly focussed results in amongst some sharp ones. I don't know if I'm expecting too much but it's gone back anyhows...we shall see.

Cheers

Bill