Focal length 35mm equivalent, but not F-stop?

Started Jul 24, 2014 | Discussions thread
Allan Olesen Veteran Member • Posts: 3,391
Re: Focal length 35mm equivalent, but not F-stop?
5

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

In fact, a 50 mm f/8 on full frame will have the same effective lens area as a 12.5 mm f/2 on 4x crop.

This is why we refer to lenses with wide apertures as fast, not shallow.

"Fast" is very vague in comparisons between different sensor sizes. You can always get a fast shutter speed by increasing the ISO. You don't want to do this because of noise. So the only relevant comparison is at ISOs which produce the same amount of noise. Usually, a modern FF camera will have the same amount of noise at a 16x higher ISO than a modern 4x crop camera.

See how the numbers compare?

The difference between ISOs with the same noise (4 stops) is the same as the difference between equivalent f-stops (4 stops).

So at the same shutter speed, you can expect the FF camera at f/8 and ISO 1600 to have the same noise as the 4x crop camera at f/2 and ISO 100.

So equivalent f-stops tells us much, much more about a camera's low light abilities.

And it tells us much, much more about how fast the lens really is: Which shutter speed can we get with a given amount of noise?

Comparing real f-stops across different sensor sizes is a waste of time as it tells us nothing about the low light abilities.

(If you do a comparison between real cameras, you may find that the numbers are 1/2 stops off because small sensors of the same generation are usually slightly more efficient than large sensors. But this 1/2 stop error is nothing in comparison to the 3.5 stop error you would make if you just looked at the real f-stops.)

Sadly, this simple truth is rejected by large groups of forum members which appear to either...:

  • ...feel threatened on their photographic knowledge.
  • ...or feel threatened on their small sensor cameras.

So they make these kneejerk reactions in the same way as a creationist being told about Darwin.

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