# Can I use a 127.5mm focal length on a crop camera to see what the view would be like using an 85mm full frame lens?

I wish the concept of “crop factor” would disappear, it’s the source of so much confusion. Further, it’s main value, it helps people that have worked with a given format camera for years switch to a different format. For novices and those who never worked a specific format, it has little value.

If you mount an 85mm lens on a full frame camera the angle of view realized is 16° height, 24° length and 43° diagonal. Of these values the diagonal angle of view is the one most likely published. I think its the least valuable but keep in mind, TV sets are sold based on their diagonal measure. Why, its sounds better, it’s the biggest measurement.

If you mount 57mm lens on a DX, this lash-up will deliver approximately the same angles of view. Why 57mm?. The crop factor (magnification factor) is 1.5 so 85 ÷ 1.5 = 57 (rounded).

Bottom line – you can’t set your 70 – 300 to 57mm so your question is moot.

About angle of view – Every format has a “normal” focal length. This occurs when you mount a lens with a focal length approximately equal to the diagonal measure of the format. For the FX that’s about 43mm, for the DX that’s about 30mm. Such lash-ups deliver a horizontal angle of view of about 45° and that’s considered “normal”.

A shorter lens delivers a wide-angle, a longer angle of view is telephoto. Also, the crop factor 1.5 means 1/1.5 = 0.66 = 66% in other words, the DX format is smaller than an FX it is 66% of the size of a FX the apparent view through the viewfinder appears enlarged 1.5X as compared to the view through the viewfinder of an FX, both with the same focal length lens mounted.