Lets try and clear up a common F value misunderstanding...

Started Sep 4, 2010 | Discussions thread
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MadsR Senior Member • Posts: 2,235
Lets try and clear up a common F value misunderstanding...

I have now seens several people on this forum confuse what the f value describes.

A common mistake is to belive that the f value on a µ4/3 would be doubled if it was on a FF camera. this is not the case

The f value descrives how much light is led through the lens pr. square so it is an absolute value that is comparable between lenses. Which is why it was invented in the first place.


An f1.4 lens on a compact camera will let exactly the same amount of light pr. square cm as an f1.4 lens on a FF camera. This have nothign to do with the length of the lens or the size of the sensor. The f value is common and comparable. A compact with a 9mm f1.4 lens will let the same amount of light pr. square to the sensor as a 50mm f 1.4 lens... This can be used to calculate the shutter speed by ISO. A good exposure is calculated from 3 values, f number, iso value and shutter speed. So in our case above if the sensor is set to ISO 100 in both the compact and the full frame, the shutter speed for a good exposure is exactly the same. With a lens 6 times smaller, the field of view would even be the same!

If you don't believe me. Get a light meter (They are not that expensive used on e-bay and it is a good investment for any serious photographer!) and punch in a shutter speed and an iso and meassure the light of a given scene and get a shutter speed. Now use this shutter speed on a compact (you need a compact with full manual control! So an LX3 or S90 or something) and on a µ4/3 and on a Canon 5D or Nikon D700... And then compare the pictures... All of them will be correctly exposed! If you want to be easy on the comparison shoot a simple scene... 3 pieces of white grey and black paper in a square and compare the values of each part in photoshop or print them and compare side by side.

What is the difference then?

First noted difference between a 9mm f1.4 and a 50mm f1.4 is the DoF... Ie. depth of the sharp area...

This value also have nothing to do with the sensor! It is a function of the lens itself! The difference between these are the focal length. A 9mm lens will always have a larger DoF than any lens that is longer.

The test to do here if you don't believe me is to use a comparable lens length and a measuring tape. Put the cameras at a 45degree angle to the measuring tape. Focus at the same number (say 50cm) Now try with a µ4/3 camera with a 50mm lens set to the same f stop as a 50mm lens on a FF camera and compare where the lines on the measuring tape becomes blurry. You need to take only the center of the FF camera and scale to the same image size as the µ4/3... The best comparison here would be to a Sony Aplha 900 as it have 24mp sensor exactly the double of the µ4/3 12mp, so you don't have to scale images. What you will see is that the DoF (The number of sharp lines on front and behind your focused area) is the same!

Now, why all these confusions?

First of all people confuse the f value with DoF. And into that confusion add the FoV (Field of View)... They then look at a 20mm f1.7 and compare it to a 50mm f1.8 on a FF frame (not too much difference between the FoV and f number is even greater on the FF) and see a much shallower DoF on the FF... Then they say "oh we can't compare the f numbers because it does not give the same effect" and these misunderstands are started.

Another common misunderstand is that the size of a FF sensor is larger, so somehow the lenses must let in more light to light up the same area. Therefor a FF lens must be more light than the same µ4/3 lens. I can understand the reasoning behind this, it seems logical.. If it was water let in to fill a bathtub, the bigger the bathtub the more light must be let in to fill it. This is where ISO comes in. ISO describes "the amount of water" in a comparable way (and quite easy... especially when understood) It is a std. that describes how much light needs to hit an area of a medium (sensor or film) to make it go 80% grey. This is good news for us, because we don't have to know intricate details on the light gathering of the sensor, how sensitive each pixel of the sensor is, the dispertion of the microlenses etc. It gives a comparable level between media.

An ISO400 kodachrome film needs the same amount of light to turn 80% grey as a µ4/3 sensor set to ISO400 and the same amount as a full frame sensor set to ISO400.

Am I then saying that ISO400 on a compact is the same as ISO400 on a full frame camera? Well yes and no! The amount of light (same f value, same shutterspeed) is the same to create a good exposure between the compact and the full frame... But there the equality stops... We all know that FF cameras give better pictures with comparable lenses... What many here don't seem to understand is how to compare lenses

All this information is available in books and on the net You can read up on it and learn it yourself... As always, be careful with accepting anything read on the net at face value... Which is why I always recomend books. Go to the local library or book store and get "Understanding exposure" or one of the other books on this subject. They are extremely good and will help you become better photographers as well as help you understand how to compare lenses and cameras.

 MadsR's gear list:MadsR's gear list
Olympus PEN E-P1 Panasonic Lumix G 20mm F1.7 ASPH +2 more
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