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

Started Jul 24, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 43,680
Intentional obfuscation?

D Cox wrote:

Great Bustard wrote:

The f-ratio is an *indicator* of light gathering ability, but the same f-ratio on different formats does not gather the same amount of light.

That is, f/2 gathers the same amount of light for a given scene luminance, shutter speed, and lens transmission regardless of focal length *on a given format*, but it does not gather the same amount of light as f/2 does on a different format.

It does.

The number of photons arriving on each square mm is not affected by the size of the sensor. This is the whole point of using f numbers rather than apertures in mm -- a small camera such as 35mm and a large camera such as 10x8 inch, using the same film, need the same exposure time at the same f number.

The intensity of the light is the same.

Would you say that two pots filled to the same depth have the same amount of water? Then why would you say that the same density of light falling on the sensor means the same amount of light on the sensor?

To do so seems like an exercise in intentional obfuscation, as the conversation between Lee Jay and Tim Tucker below demonstrates.

Likewise for different focal lengths on the same camera (ignoring lens faults such as vignetting).


The difference with digital is that the design of sensors varies greatly, so you are not using "the same film".

Sensors of the same generation are usually rather close in terms of efficiency:

Post (hide subjects) Posted by
(unknown member)
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow