400mm mft prime?

Started 2 months ago | Discussions thread
cameron2 Veteran Member • Posts: 4,053
Re: 400mm f/4 or better 500mm f/4 or go home

WileEC_ID wrote:

It's not as simple as that. Theory suggests it is but when one actually shoots various platforms, with an intent to learn, rather than an intent to justify - you discover that what you state is rooted in theory - it varies in practice. Also the actual T-stop of a lens comes into play. An F-stop rating is a general area - there is more slop in the tolerance if you will. A T-stop is a very specific measured detail. The better f/4 lenses often have a T-stop that is lower, while the lesser f/4 lenses often have a T-stop that is higher. This is one of the reasons DxO measures the actual transmission of a light through a lens when they review a lens - just because it is an f/4 lens doesn't mean it transmits the same amount of light as another f/4 lens. This is also why lenses designed for video typically list their T-stop, as matching the light from different lenses is a lot easier if you use lenses with the same T-stop.

So, with respect, I do have a grasp of the basics - and the practical impacts of real life - not just simple theories.

This is a reasonable point, and it would be helpful if all manufacturers provided T stop values in addition to F stop values.

However, the T stop values approach the F stop values, so in the absence of published T stop values, we use F stop values as an approximation thereof.

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