How to shoot through curved glass

[MarkoD]

Originally Posted by sphelps

Same reason why macro lenses are prime. With a fixed focal length a prime lens can be designed to better reduce distortion and typically have better optical quality. For the most part the wider the zoom the more distortion you get resulting in a softer image. Not always an issue but trying to use a zoom lens in a macro type environment doesn't always work well, combine that with curved glass and you're really in trouble. Just something to keep in mind if you're trying to reduce one issue you may be introducing another one.

Now with that said one can't compare the 70-300 with the 70-200 you have, those are much different when it comes to optical quality and design, hence the price difference.

anything higher than 70 cm wont really distort.

i've shot 70-200 with extension tubes at f8 (on a tripod) with perfect sharpness.

what i would recommend for the OP is to put the camera on a tripod, turn on the live view mode and zoom in to 100% on the screen. Then manually adjust the focus