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

Started Jul 24, 2014 | Discussions thread
flakin stephen Contributing Member • Posts: 752
Re: Whoops!

Great Bustard wrote:

Tim Tucker wrote:

Great Bustard wrote:

Wheatfield7 wrote:

Allan Olesen wrote:

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.

This is what is known as wrong.

Um, it is absolutely correct.

Any lens will gather the same amount of light at any given aperture. A 50mm f2 lens on a 35mm format will gather exactly the same amount of light as a 12.5mm lens on a micro four thirds lens if both are set to the same f/2 aperture.


Peoiple really need to learn what they are talking about before they start to spread the sort of misinformation that is quoted above..

Yes, they do. The fact of the matter is that 16x as much light will fall on the sensor for a given perspective, framing, and shutter speed with 50mm f/2 on FF as will fall on the sensor at 12.5mm f/2 on 4x.

- SNIP -

WHOA!!! Hang on there. I don't quite understand the point here, is this different with digital than it was with film?


Let's say I have one lens and I put it on my FF camera and take a picture at a shutter and aperture combination X1.

If I take the same lens and put it on a camera with a smaller sensor then it still transmits the same amount of light and the correct exposure would still be X1 or an equivelant of X1. However the smaller sensor would collect less of this light. If I used film it would be a simple crop to obtain the equivalent photo. The exposure would be the same. Yes?

What I'm getting at is that if you put film behind each of the lenses you discuss then with the exposure of X1 you will get correct exposure for all the film, just the coverage of the different lenses would change, yes? (the total light hitting the film would be the exposure - X1 times the total area exposed).

Here we are:

Given four cameras, one with...

  • mFT (4/3) sensor,
  • ...another with a 1.6x sensor,
  • ...another with a 1.5x sensor,
  • ...and another with a FF sensor...


  • ...a photo of a scene from the same position with the same focal point and the same settings (e.g. 25mm f/1.4 1/200 ISO 400) with all cameras,
  • ...the photos cropped to the same framing as the photo from the mFT (4/3) camera,
  • ...and the photos are displayed at the same size...

...then the resulting photos will be Equivalent. In addition, if...

  • ...all the sensors are equally efficient, then all the photos will also have the same noise,
  • ...the pixels are all the same size, the AA filter the same strength, and the lens is the same sharpness, then all the photos will also have the same detail,
  • ...the exact same lens is used and the sensors are of the exact same design with the exact same size pixels, AA filter, CFA, and processing...

... then the photos will not merely be Equivalent, but be identical.

Are we just talking about noise generation in digital sensors here?

Works for film and digital.

(Not forgetting that the aperture marking system is not limited to digital camera lenses, and it measures transmittance and not DoF. DoF is not even an absolute science as certain assumptions are made about enlargement and viewing distance.)

Next up:

Neither the focal length nor the f-ratio of a lens change as a function of sensor (for example, a 50mm f/1.4 lens is a 50mm f/1.4 lens, regardless of the sensor behind the lens). However, the effect of both the focal length and the f-ratio on the visual properties of the photo very much depend on the sensor, and scale in direct proportion to the size of the sensor:

25mm f/1.4 on mFT (4/3) is equivalent to 31mm f/1.8 on 1.6x (Canon APS-C), 33mm f/1.9 on 1.5x (APS-C for everyone else), and 50mm f/2.8 on FF (FX), where "equivalent to" means:

  • The photos all have the same diagonal angle of view (25mm x 2 = 31mm x 1.6 = 33mm x 1.5 = 50mm) and aperture diameter (25mm / 1.4 = 31mm / 1.8 = 33mm / 1.9 = 50mm / 2.8 = 18mm).
  • The photos all have the same perspective when taken from the same position.
  • The photos all have the same DOF (as well as diffraction softening) when they are taken from the same position with the same focal point and have the same display size.
  • The photos all have the same motion blur for the same shutter speed (regardless of pixel count).
  • The same total amount of light falls on the sensor for the same DOF and shutter speed.
  • The same total light falling on the larger sensor will result in a lower exposure than the smaller sensor (the same total light over a larger area results in a lower density of light on the sensor).
  • The larger sensor system will use a concomitantly higher ISO setting for a given brightness of the LCD playback and/or OOC (out-of-the-camera) jpg due to the lower exposure.
  • The same total light will result in the same noise for equally efficient sensors (regardless of pixel count and regardless of the ISO setting).
  • If the 25mm lens at f/1.4 is 2x as sharp as the 50mm lens at f/2.8, 1.33x as sharp as the 33mm lens at f/1.8, 1.25x as sharp as the 31mm at f/1.8, the sensors have the same pixel count, and the AA filter introduces the same blur, then all systems will also resolve the same detail.
  • Other elements of IQ, such as bokeh, color, distortion, etc., as well as elements of operation, such as AF speed/accuracy, size, weight, etc., are not covered in this use of the term "equivalent".

In fairness, the OP's questions was about lenses. You could have a 40mm f2.8 lens designed for APS-C, 35mm, or medium format. Even though the density of light on the sensor would be the same, each would transmit a different total amount of light, as the area it will illuminate is designed to be larger for the larger formats.

Absolutely everything else could be exactly the same (f stop, aperture size, noodle points). The variables are all to do with sensor size, so are irrelevant to a question about lenses. Surely measuring photons by the proxy of noise levels (or vice-versa) is a minority interest ?



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