How should I resolve my birding GAS?

nnowak wrote:

User1303423862 wrote:

The acid test will be at 300mm + 2X TC downsampled to compare with 300mm cropped to half resolution to get the equivalent FOV. That's what this is really about.

Start simple and compare 150mm plus the 2X converter versus the bare lens at 300mm. This will eliminate any variance from upsampling/downsampling and will most clearly show what the 2X converter is doing to the image. If you need a repeatable target with lots of fine detail, I would suggest a child's stuffed animal.

I decided to go with my original idea of testing with and without the TC at 300mm, and comparing the cropped (non TC) result with the downsampled (with TC) result. This is more 'real world' for my purposes. The comparison is done in a quick and easy fashion with windows photo viewer - there may be better ways to downsample the TC image. All suggestions welcome.

The sky was dull and overcast with a slight mist. Typical British autumn weather. More testing in other conditions is needed, but this first attempt at comparing the EF 70-300 IS II USM lens with and without TC would indicate @nnowak is probably right about contrast - better results can be obtained through reduced ISO requirements and reduced glassware in optical path without it, given the resolution required for a small print or viewing on a hi-def TV screen - the two main ways I enjoy looking at my photos. To my eye, resolution looks fractionally better with the TC, but there's little in it.

In the following screenshots, the lens with 2X TC attached is on the left, bare lens on the right.

Lens + TC: f/5.6 1/320s ISO 1250 ............ Bare lens: f/5.6 1/320s ISO 250

Lens + TC: f/5.6 1/320s ISO 800 .............. Bare lens: f/5.6 1/320s ISO 200

All shots are taken at full zoom, 300mm. The swan shots are directly comparable. The subject had moved closer and the light had improved slightly in the bare lens duck shot: Displayed at 85% of full resolution to make the duck a similar size in both samples.

To reduce the colour fringing visible in yesterday's shots, I put a piece of card in front of the TC optic with an aperture hole appropriately sized for the APS-C sensor. This definitely helped, you can see there is less colour fringing on the edge of the protruberance just above the swan's bill in the TC shot than the bare lens shot. Also under the duck's bill and the back of its head/neck, although focus may be fractionally better on the bare lens shot. I'll be experimenting with a similar aperture card in the EF-EFM adapter to get the best out of using the FF lens on the APS-C M series camera.

My thanks to @nnowak for challenging my assumptions and encouraging me to test this. I'll be doing more tests in various conditions to see what works best.

My thanks also to Larry Rexley for his excellent TC, it gives me great enjoyment filling the frame with my subjects, rather than relying on later cropping. It'll still have a place in my kit bag, no matter what the further results under different conditions/processing lead me to conclude.