# If I could place the RX100 sensor in A500 for bird shooting, .........

Started 6 months ago | Discussion thread
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 Re: If I could place the RX100 sensor in A500 for bird shooting, ......... In reply to Allan Olesen, 6 months ago

Allan Olesen wrote:

Dave Oddie wrote:

The light gathering ability of any lens (its speed) is determined by the physical characteristics of the lens only. That is the ratio between the entrance pupil size and the lens focal length. It is focal length / diameter of the entrance pupil size (which is the effective aperture). Nothing else.

This will give you the same amount of light per sensor area.

It will not give you the same total amount of light if the sensors are of different area.

It has nothing to do with sensor area.  A lens speed  is not, never has been and never will be based on the size of the sensor or film.  It is rated on the size of the physical characteristics of the lens - that ratio of is maximum entry pupil size to its focal length.

Your post implies a 300mm F5.6 lens loses almost 4 stops of light gathering ability because you mounted it on a small sensor camera. That is clearly not the case. Whether that is what you intended to say I don't know but that is how it reads.

The total amount of light which hits the sensor will be reduced with a factor of 2.7^2 = 7.3x.

If you want to compare that in stops, it is ln(7.3) / ln(2) = 2.9 stops. I don't know where you got "almost 4".

I was saying F15 is nearly four stops slower than F5.6 is where "4" came from.  I should have said nearly 3 but its not relevant anyway because the implication of your post is that the F5.6 lens loses 3 stops of light because you put in on a smaller sensor camera is nuts.

I think you are looking at this the wrong way around. If you want an 810mm F5.6 lens you need to build one that allows you to set a pupil size of approximately 145mm.

Which is exactly what I wrote in one of my other replies.

But that does not alter the characteristics of a 300mm F5.6 lens on any sized sensor.

The fact when you set your 810mm lens to F15 you end up with a pupil size of 54mm which is close to the same size of pupil you get if you set a 300mm lens to F5.6 does not mean an 810mm F15 lens on a full frame camera is as fast as a 300mm F5.6 lens mounted on a small sensor camera.

It will gather exactly the same total amount of light.

No it won't.  In order for the longer lens to gather the same amount of light as the shorter one it needs a larger entry pupil size.

For ISO 100 a light level of EV15 requires an exposure of 1/1000 set at F5.6.  You do not have to modify that to be approx 1/125 sec because you put your F5.6 lens on a smaller sensor camera.

At ISO 100 a full frame user would have to set their 810mm F5.6 lens to F5.6 (and shutter to 1/1000) to get a correct exposure at EV15.

At ISO 100 a 1 inch sensor user would have to set their 300mm F5.6 lens to F5.6 and a shutter of 1/1000 to get a correctly exposed shot at EV15.

The implication of what you are saying as I read it is that you are suggesting that for a light level of EV15 an exposure of F15 and around 1/1000 sec will yield a correct exposure for the full frame user with their 810mm lens.

If so that is clearly wrong as that would give an under exposed shot.  You would have set the exit pupil to 54mm and that isn't large enough on a 810mm lens to give a correct exposure at a light level of EV15 if you set the shutter to 1/1000.

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