WHAT MAKES A GOOD ADAPTER

You probably already discovered that Ebay offers many lens adapters at varying prices points. Finding the right adapter requires some homework. ‘Google’ the seller’s Ebay ID and look for actual user reviews in photography forums such as Fred Miranda or the Manual Focus Forum. See what other users are saying about the adapter - and if possible about the lens(es) and Canon dSLR you will be using. Most likely the results will be mixed with some people writing XYZ lens works perfectly with their adapter, while the next poster states the complete opposite. Why the conflicting reports? One plausible explanation is - not everybody has a critical enough eye to spot the flaws. Other possible technical explanations include:

Proper Thickness - Adapter thickness is probably the most common quality control issue. If an adapter is too thick then the lens will not focus to infinity. If the adapter is too thin, then images may look out-of-focus. If the lens has a fixed rear element the distance from the rear element to the film plane is fixed, thus a too thick or too thin adapter may result in mis-focused images every time.

Some adapters are poorly designed, while others are designed correctly but fabrication variances cause problems, such as poor quality control during plating process could lead to increased thickness. The generally accepted for a Leica-R Canon EOS adapter is 3mm.

Uniform Thickness - Cheap(er) adapters may have poor quality plating with high spots and low spots, thus the lens may sit slightly crooked. A crooked lens may result in one side of the image looking sharper than the other. At F8, F11 and up depth of field usually hides the problem, but at faster apertures such as F1.4 and F2 this can result in one side of the image being sharper than the opposite side.

FOCUS CONFIRMATION ADAPTERS

My favorite adapters are the newer focus confirmation adapters sold by HappyPageHK on Ebay. As the focus ring is turned, the AF light blinks (beeps) when focus is achieved. In my experience the AF confirmation adapters have proven as accurate as split and prism focus screens.

In my opinion the focus confirmation adapters are the best option currently available. Without focus confirmation the next best options are focusing aids (especially on a Canon 40D or Rebel XSi) such as a split screen, prism screen and/or viewfinder magnifier. According to the Canon user manual the split and prism viewfinder screens can interfere with metering which has proven true (for me) for the Canon 1Ds and 1Ds Mark II.

The focus confirmation works better with the primes and not quite as well with zooms.  I think this because the adapter is coded for a specific focal length (usually 50mm). Using the 35-135 zoom as an example, the DOF is different at 35mm vs 135mm, so using 50mm to calculate the DOF at 135mm is not a tight enough tolerance.   For each prime I have specific adapter coded to the the lens’ exact focal length and the results have been very good.

ADJUSTING FOR BEST FIT

To ensure the best performance, adapters need to be snug against the lens. The picture to the left shows a slot than can be expanded to hold the lens tightly against the adapter (adapters usually have three such slots). Insert a small jewelers screwdriver into the slot, twist the screw driver slightly to expand the slot. Expand each slot an equal amount. The goal is even pressure at all points between the lens and the adapter.

If the slots continually collapse over time, try placing a small small piece of aluminum foil into the slot. The piece of foil should be repeatedly folder until it is .25 to .50mm thick. Over expand the slot slightly, insert the foil and then close the slot so the foil is held in place. With the tabs properly expanded, the adapter and lens should be snug with no fore or aft play or wiggle. These adjustments will not change the adapter’s performance, but it does keep the lens from spinning slightly which can be annoying. 

WHAT HAS WORKED

I have tried different adapters over the years on the Canon 1Ds and Canon 1Ds Mark II, and the following adapters worked well: 

1. •Cameraquest - these are made by Kindai and relabeled by Cameraquest (unless Cameraquest changed their supplier recently).  The Cameraquest price costs about the same as buying Kindai from Japan.  The adapters are nicely made and in 2005 time frame the best option when it came to infinity focus. At $175 per adapter these are quite expensive.
   

2. •HappyPageHK - the focus confirmation variety have worked with the Leica 50mm F2 Summicron (ROM) and Leica 90mm F2.8 Elmarit (current version - both CAM and ROM).
   

3. •Fotodiox - I did not have good results with Fotodiox adapters a couple years ago. They seemed to thick I think the adapter fell just shy of reaching infinity focus with the Leica 21-35mm ASPH, 21mm F4 Super-Angulon and 35-70mm F3.5 Elmarit.
   

4. •Zoerk (Zork) - Amazingly expensive and very poor build quality in my opinion. Considering how expensive the Zork adapters can be, I was very disappointed.
   

The above comments are not an endorsement or recommendation, just my first hand experience.