I have saved the best till last. The result from the F/16 test was the most promising and starts to show the real potential of what a plastic lens can do.
[50.1 PMMA @ F/16]
Let's get straight to the point. This is the best photo out of the lot. Finally we can see a bit of sharpness (in a vague sort of a way)! Things are looking good, however there is some bad news as well.
Lets get the bad news out of the way first... Taking photos at F/16 is simply unfeasible 75% of the time. With a simplet lens of a focal length 50mm, stopping it down to F/16 means that only a 3.125mm diameter disc of the lens surface is actually used. The light gathering capability of the lens has literally been divided by 16. Only a tiny bit of light actually passes through to the image sensor. This means one of two things, either the shutter speed has to increase (reducing the amount of practice applications the lens has) or the ISO speed has to increase (reducing image quality), or a combination of both. Let's show an example:
Lets say I am outside with my 50mm lens, and I take a photo with it set to F/1. For examples sake, lets say the shutter speed for a normal exposure is 1/250th of a second. If I then stopped the lens down to F/16, to attain the same exposure I would need to increase the shutter speed to 1 second. That is a huge amount of time! To prevent motion blur from occurring in your images, it is common practice to not let the shutter speed drop below 1/focal length, which in our case would be 1/50th of a second. And that's if you have steady hands! Put simply, to use the lens at F/16 at a shutter speed that won't blur, you need a f#*k load of light.
Now for the good news!!
The image is as close as I'm ever going to get to sharp with this particular optic. The centre of the image is rendering the details right the way down to the text on the bottle, which is exactly what I am after. The size of the usable area of the image has also increased, as we can see the halo ring of SA being pushed further and further out of the image. It is now only present at the very edges. This is possibly the best news, as I can now cheat a little.
Back when I started testing I wrote in bold that it was important to note the camera I was using to take the test photos. My Canon 5D Mark II is a full frame high end camera, and as a result it will see the entire image circle designed to hit the 35mm sized sensor. Why is this important? Because lenses on full frame bodies often suffer from image quality degradation towards the edges. Now I know my lens is showing errors more than a professional lens, and I've had to stop it down to F/16 just to get it that good, but the fact that the centre of the image is sharp give me an opportunity to exploit the best out of the lens as I possibly can. How? Use an APS-C based camera.
My lens has a Canon EF mount, and luckily for me I know plenty of people with crop sensor-ed Canon cameras. The smaller image space should align nicely with the sharp central area of the image that my lens renders on the image plane. Therefore if I mount my lens onto a crop body I should be able to take photos that have nearly no SA and should be relatively sharp across the entire image. Worth a shot!! The effective focal length will be increased (50mm x 1.6 FOVCF which means it will look like an 80mm lens) but this is something I'm willing to compromise on for the sake of getting the best out of the lens.Stay tuned for my next post where I will be showing off images taken out and about in the real world at F/16 on a crop body!
