Actual focal length (1 Viewer)

I have a SW80ED which is 600mm, add a 1.4x tc = 840mm and shoot on a 1.6 crop Camera (40D) and the total reach should be 1344mm (600 x 1.4 x 1.6). My question is does the 80mm extension tube that I use influence/increase the focal length and if so by how much?
I have just done some focal length test with the SW combo against a Canon SX40 superzoom and just wanted to check if the extension tube alters things before reporting my findings.

The extension tube will only let you focus closer. It does not change your focal length unless it has glass in it, but then it is a TC and not an extension tube.
;-)

DanC.Licks said:

The extension tube will only let you focus closer. It does not change your focal length unless it has glass in it, but then it is a TC and not an extension tube.
;-)

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Hi Dan, I know that extension tubes let you focus closer as I have used them many times with my DSLR and macro lens. But I am fairly certain that they also increase magnification slightly as well due to the sensor being further away from the lens. By using an extension tube you move the lens further from the sensor thereby increasing the magnification. Perhaps Paul can clarify.

If you have anything between the camera and TC then it will change but as Dan said, the 80mm tube just lets you reach the correct focus. The 80mm tube is needed because there is no prism or mirror diagonal in the back of the scope. The distance light travels through the prism or mirror diagonal would take up most of that 80mm.

Paul.

Paul Corfield said:

If you have anything between the camera and TC then it will change but as Dan said, the 80mm tube just lets you reach the correct focus. The 80mm tube is needed because there is no prism or mirror diagonal in the back of the scope. The distance light travels through the prism or mirror diagonal would take up most of that 80mm.

Paul.

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So in my example above Paul, I can be certain that the focal length is 840mm - so on a 1.6 cropper the FOV is equal to 1344mm on a full frame sensor (I have not got anything between the Camera and the tc). I just need to be certain of the facts before publishing a test I did with regards to the actual focal length/reach of a superzoom Camera.

The 600mm focal length is only correct when focused at infinity so there will be a change in focal length when focusing on something close. Just looking at some formulas to see how to calculate it. I think you would need to know exact distance of object to lens and also distance between film plane and lens.

Paul.

I assumed infinity focus. There was no mention of closer focus.

Infinity would be the stars, moon etc and anything less would alter the focal length to a degree. I should think at the distances we use the scope over then the focal length is varying all the time.

Paul.

I always knew that zoom lenses will only get the full long end of the zoom when focusing on infinity due to the way zoom lenses are constructed but I have never understood this to be the case with a prime lens |:S|. With zoom lenses the further you are away from infinity then the less the top end focal length will be.

Paul Corfield said:

The 600mm focal length is only correct when focused at infinity so there will be a change in focal length when focusing on something close. Just looking at some formulas to see how to calculate it. I think you would need to know exact distance of object to lens and also distance between film plane and lens.

Paul.

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If the distance between the film plane and the lens comes into the equation then surely the length of the extension tube would influence this! |:S||:S||:S|

Roy C said:

If the distance between the film plane and the lens comes into the equation then surely the length of the extension tube would influence this! |:S||:S||:S|

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If you add an extension tube then you have to move the focus in by that amount to focus on the same subject so the tube length would just be cancelled out.

Paul.

While we're on the subject of focal lengths and superzoom cameras...one of the things that always confuses me is the way many of these manufacturers report the camera as so many X zoom. For instance, the SX50 is reported as a 50X zoom camera, which it is in a way as it does zoom 50X the widest to the longest focal length (24-1200mm). But most of us are more used to thinking of 1X and equivalent to 50mm, so this 50X zoom camera is really only yielding 24X as many of us are used to thinking of it (a 500mm lens is generally considered to be 10X). Anyway, maybe I'm so confused I don't even know what I'm talking about...haha

Dear Roy C.
The only exact way to measure the true focal length of a lens or a lens and telephoto adaptor or a lens and telephoto and extension tube is too accurately measure the size of a known object at 'infinity' such as the Moon or known bright stars AT THE FILM PLANE.
Or in this case on the digital sensor.

The Moon's size changes from approx. 29 arcminutes to about 33 arcminutes and one can find the true size on any date from Astro Handbooks or Astro programmes.

Star postions don't change in this context and one could use say the separation of the outermost stars in Orion's belt which are 2.736 degrees apart from memory.

With something like a Swarovski the marked focal length will be quite accurate but not totally so.
The positioning of a teleconverter strongly influences the combined focal length.
It will not be exactly 1.4x even in a normal position.

With digiscoping I would think that there are enormous changes due to positioning along the optical axis.

Using diagonals etc. will also change things.

With things like Superzooms any focal lengths will only be approximate.
The qualification of a 35mm focal length is found by measuring the true diagonal size of the sensor and comparing it to the diagonal size of a 36mm x 24mm film camera, about 43mm diagonal.
Some cameras are say 23.9 x 35.9mm. Some old ones 24mm x 32mm. But an exact 24 x 36mm is used.
Manufacturers will give sensor sizes but maybe not to great accuracy.
With an accessible sensor this could be measured but could introduce dust.
With a superzoom it is not normally accessible.
Mirror scopes, i.e Schmidt Cassegrain, actually I think they are Schmidt Dall Kirkhams.
Or Maksutov Cassegrains or complex mirror derivatives have enormous focal length changes as they focus by altering the distance between main mirror and secondary.
Because of the amplification factor of about 5x inherent in these designs there are vast changes in focal lengths.

In old tests in say Modern Photography the true focal lengths of lenses at infinity were measured on a test bench.
They are never exactly as marked except for military lenses where they may be marked to 0.01mm with say Zeiss or Wild lenses.
Lenses for Hassleblad and Linhofs may also be marked individually as may top quality movie lenses.
The normal 50mm standard lens used to be 52 or 53mm but eventually settled near to 50mm.

So for normal folk you need to measure the astro objects mentioned above at your particular setting or combination setting to know the final true focal length.

I hope this is not too technical, but if you want accuracy that is how you get it.

In addition when using an eyepiece or afocal adaptor which is perhaps how digiscoping works? the eyepiece does not have a rectinear field. In other words there is distortion across the focal plane.
So the magnification is variable across the field as is the total focal length at any one position in the field.

So although the distance between say the outermost belt stars in Orion is known.
At the film plane it may differ depending which part of the film plane you are using.

What I am basically saying is that in normal bird photography it probably does not matter what the exact focal length is.

Just take beautiful pictures and enjoy them.

For my astro photos I knew the focal length of various optics as I measured them.
But for normal photos it was not important.

Looking again I see SW must be Skywatcher not Swarovski.
I.e. an astro refractor.
This is easier to measure the same as above.
But the teleconverter focal lengths are highly dependent on positioning along the optical axis.
2x to 3x teleconverters are the same lenses often 4 to 6 or 7 glass elements but with extension tubes sometimes variable to change the magnification which is highly dependent on position.

Computing actual focal length

The actual focal length when you focus near objetcs is a bit longer that the featured in specs, which is computed focusing to infinity.

When the focused area is near the scope, the focal plane goes backwards and the effective focal length increases a bit.

The best way to know is to directly measure it; you only have to take a picture to a rule at a certain distance and check in the picture the width of your shoot.

If you measure the distance to the rule in meters and the width in cms, you will have your actual 35mm equivalent focal length with this simple formula:

Focal length (mm) = 36 * 100 * Distance (meters) / Picture Width (cm)

For example if you take a picture at 12 meters, and you have a picture of the rule showing 20 cms. of width, your equivalent focal length is:

Focal length (mm) = 36 * 100 *12 / 20 = 2160 mm of equivalent focal length.

The "36" factor is related to the width of the negative in a 35mm system (which size is 36 * 24mm).

So if you measure the vertical height of the picture instead of the width, you have to use a "24" factor:

Focal length (mm) = 24 * 100 * Distance (meters) / Picture Height (cm)

And finally if you consider your diagonal (43.27mm) you have to use:

Focal length (mm) = 43.27 * 100 * Distance (meters) / Picture Diagonal (cm)


If you use a 4/3 sensor, it is better to compute with the diagonal size because of the different aspect ratio.

Roy C said:

I always knew that zoom lenses will only get the full long end of the zoom when focusing on infinity due to the way zoom lenses are constructed but I have never understood this to be the case with a prime lens |:S|. With zoom lenses the further you are away from infinity then the less the top end focal length will be.

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With a prime the focal length increases the further away from infinity you are. Adding macro tubes to a 50mm lens to allow close focus is a good example. Some formulas here that can help. http://www.panohelp.com/thinlensformula.html

I tested the thin lens formula on a subject 4m away from my SW this morning and it came somewhere close to predicting how much back focus I would need. As an example for my 4m range using the formula and using centimeters as my units. The 1/60 is the scopes standard focal length (60cm = 600mm).

(b=1/(1/60-1/400)) = 1/(0.0167-0.0025)) = 1/0.0142 = 70.4cm of back focus. Measuring from the middle of the objective back to the film plane should be approx 704mm at 4m range and I suppose this is then the focal length at this range. My Kenko Pro 1.4X is parfocal so I don't have to alter focus when using it. I suppose then you just multiply the thin lens formula by 1.4X and at 4m I would be getting 985.6mm focal length.

Paul.

Paul Corfield said:

With a prime the focal length increases the further away from infinity you are.

Paul.

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That confirms what I am getting, whereas with a zoom lens the focal length decreases the further away from infinity you are (with zooms the top end focal length is only measured at infinity).
I have seen loads of test between long primes and zoom lenses supposedly at the same focal length whereby when shooting at typical bird photography distances the Prime is always significantly 'longer'.

BTW guys I have just started a thread on the Canon forum with regards to the actual focal length of superzooms when compared against a prime set-up HERE. The result did not surprise me but the amount of difference certainly did.

The focal length of a lens is typically fixed and a known quantity.
It is the BACK FOCUS that changes as you focus.
The focal length of a lens changes slightly with temperature.
It changes with wavelength particulary into red and infra red.
It may change a bit with internal focus lenses.
It may change with stress although eventually the glass will break.

However, mirror lenses that focus by altering the separation of primary and secondary change focus a lot at close distance.
And also if you put a diagonal or upright prism or extension they change a lot.
The smaller the mirror lens the greater the focal length changes.

A simple doublet refractor does not change focal length noticeably.
It is usually within 1% of the marked focus.
Teleconverters can actually be a few percent wrong.
The 1.6x Canon conversion may be slightly out.
Hope this helps.

It is good to know that the DSLR astro scope combo is more or less the correct focal length when shooting at typical distances for bird photography (around 15-30 feet).
After numerous comparisons I conclude that DSLR zoom lenses all suffer from percieved focal length loss the further you are from infinity much more so that prime lenses.
So called superzooms or bridge Camera suffer immensely and much more than DSLR zooms Which was the point I was trying to prove.