"I Need More Focal Length"



Are you sure that you really do?

Anyone with any type of long focal length telephoto lens will know the mixed pleasures of shooting through hazy, dusty, turbulent air - i.e. any less than perfect air: Image quality degrade rapidly over distance. Besides, there are questions of portability, ease of use and price!!!

So, do you really need that 400 mm f/2.8 lens plus 2 X teleconverter? And, if you get it: How often will you use it and for what???

Think about it........


But here is a little fun project that you may play with. It IS for fun mostly, and as presented here pretty much insane / ridiculous but with a little bit of seriousness in the end, as we shall see.

Now then here is an image shot out of my window with my Tamron SP 350 mm f/5.6 lens. (Adaptall-2 System, Model 06B). The target is the red poster some 300 meters away.


Figure 1

And here is an image of the same target taken immediately after and with the same lens plus a little extra:

Figure 2

This is the full, "raw" JPEG image straight-out-of-the-box - no crop and no processing; Direct comparison with Figure shows that the Effective Focal Length, (EFL) with which this image was taken is a bit more than 2800 mm - an 8-fold increase in focal length!.

That "little extra" is a 17 mm f/1.4 Euprogon projection lens from an old Eumig 8 mm movie projector inserted in a projection tube made as shown in the following image.


Figure 3

The Eumig lens is placed inside the part called Carton Eyepiece Adapter. This is "merely" the principle of Eyepiece Projection Photography well known in Astrophotography, albeit with some more unusual components and parts here. For the "theory" behind Eyepiece Projection you may refer to the Extreme Macro Section, page 1 on this site.

If you would like to copy this idea, you may rather easily find some similar adapters and tubes. There is a healthy market for astro- and microscope photographical accessories. Normally, one would use an astronomical eyepiece, such as a Plössl or an orthoscopic eyepiece in the 10 mm FL range instead of a movie projection lens. However, I have found that my Eumig Euprogon 17 mm f/1.4 lens produces a very decent image quality, designed as it is to blow up the tiny 8 mm movie frames on a large screen. The microscope adapter has a freely rotating and sliding tube inside with a locking ring. Nice! I can vary the projection distance considerably and achieve even larger EFLs (up to about 4000 mm).

Do note that most astro- and microscope photographic accessories are based upon the standard T2-mount and -thread, (M42, 0.75 mm) while photographic accessories of this type are based upon the M42-1.0 mm thread. Joining lens and adapter as shown above is at your own risk! The microscope adapter above also had another female thread and I have had to glue on a small T2 extension tube in order to join eyepiece and microscope adapter.

After I took my first 2800 mm picture (Figure 2) above, my plan was to take some more for stacking (in my case in Registax), as any astrophotographer would normally do. However, I only got 4 pictures when the batteries of my radio remote release were depleted ----- and you DON'T want to touch your gear for shutter release (not even with mirror lock-up) with such long EFLs!!! So, four images it became and here's the resulting stacked image, (which would normally be based upon many more images):


Figure 4


Note that I have inserted a 100% crop, the crown from the first 350 mm image (Figure 1) and you surely see the dramatic effect of eyepiece projection. But one also notes a (to me at least) surprisingly good image quality so, maybe this is a bit more than just fun? Maybe it can be used for some "serious fun"???
Át this point you may ask: So, the small lens merely captures a small part of the image from the primary lens and magnifies the image simply by projecting it over a longer distance. What then is the big deal? Why not just take the 350 mm image, crop and enlarge it?

The answer to that is that my pixels on my Pentax K200D - or any DSLR for that matter - are not small enough to utilize the full resolution of my Tamron lens. The resolving power of a lens only depends upon the physical size of its aperture (not the f-ratio) and my lens can resolve finer detail than my sensor can "see", and that you can convince yourself by looking at the following composite, where I have inserted an 810% crop of my 350 mm image, (Figure 1), corresponding to the image size achieved at an EFL of 2800 mm:


Figure 5

This should really have been the full, uncompressed image in order fully to appreciate the difference, but it ought to be clear enough, anyway. You may find a somewhat lengthy and tedious discussion on sensor- and pixel sized versus focal lengths, sampling and image quality on the sensor page at this site. Suffice to say here, that for a given pixel size, nothing beats a corresponding "fine-tuned" focal length & physical aperture when it comes to actually capturing the finest detail possible.

You might then object that this is all very academic. Who would ever dream of producing such large images of so small details in practice? Well, when it comes to tiny craterlets on the Moon, cloud bands on Jupiter and similar, I would dream for one. Anyone who has tried to capture the Moon with an otherwise respectable 200 mm or thereabout lens and enlarge the image afterwards will know - or should know - what I am talking about.

So, as soon as the weather improves, as soon as I get the time and as soon as Jupiter and/or the Moon comes into a favourable position as seen from my balcony, I shall be out there with my simple rig - just for the fun of it..............




Copyright © 2011 - Steen G. Bruun