Today I do tests to my Photo Attachment after I do not use for along time. I only use once since I bought. These are the results from the test. With the attachment it gives you between 1088 to 1600 mm of focal length (calculated with camera 1.6 crop factor). With long focal length like this, your photography set up must be steady and you may need Remote Switch for your camera to snap your subject ( RS-80N3 for my 20D) to avoid shake to your system. Happy birding.