PIX 2015

There is no magical size/weight advantage

Started Mar 5, 2014 | Discussions thread
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You missed something...
In reply to myzel, Mar 6, 2014

myzel wrote:

OK, lets play that equivalency game.

First, i don't like the concept of equivalency because it's futile. It tries to make something equivalent that isn't and while doing so makes so much assumptions that it is reduced to a nice feeling.

"Futile"? Why? Because you can't understand it or because you don't WANT to understand it?

Anyway lets play.

In the spirit of equivalency and simplification lets make some assumptions:

OK, as long as not too simplified...

We use single element lenses and assume that they are optical perfect.

That's too much simplification... real lenses are better than fictional single element lenses.

We use cameras without lens mount and no flange distance.

Wow... OK, I believe in Santa also...

We take a 135 format sensor ("full frame") and a 1" sensor, booth with identical sensor technology and the same amount of pixels.

Same amount of pixels? Why? If something should be assumed same then it is the pixel density, not the number of pixels.

Lets see what we need to get equivalency.

The FoV has to be the same, the DoF has to be the same, the image noise has to be the same and the shutter speed has to be the same. (I guess that's enough for the beginning).

OK. But what about ISO noise? Your smaller sensor has more noise at the same ISO than the FF you compare with.

Now lets look at a 50mm 135-format lens. To get the same FoV with CX we need a 18.5mm lens. Last time I checked 50 mm is much bigger than 18.5 mm. No magic involved, smaller format = shorter focal length = shorter lens barrel.

No magic, a 50mm real lens is in most cases larger, but it is possible to make a 50mm pancake, though nobody is really interested in those, so they are not made, at least not for Nikon. The last Nikon pancake was the E-series, and that was actually almost identical in size with the CX 18.5, both having the SAME aperture, not equivalent. I believe that an FF lens with equivalent aperture would be possible to make really tiny, even with built in AF motor, which the Nikon E did not have. So you see, what is there is not the same as what is possible.

OK, now lets look at the DoF. We want a f/2.8 135-format lens, so we need a f/1.0 CX lens (yea, not exactly but close enough). That would result in a 17.9 mm diameter entrance pupil and just lets assume that the front lens element has the same size (yea, simplification).

Actually... the 18.5mm lens would demand a front element of minimum 18.5mm in diameter. Not a lot of difference, but if you are typing decimals lets use the right approximation.

Now we have a 50x17.9mm lens and a 18.5x17.9mm lens. Don't know about you, but that CX lens is still smaller in my opinion.

This is totally wacko to me. Where did you get the 50x17.9 from? A 50mm lens would NOT be 50mm thick, and an 18.5mm would not be 18.5mm thick, not even if you use a single element lens, so what is this exactly? The diameter would be 17.9 for the 50mm and 18.5 for the 18.5mm, but the thickness will not match the focal length.

Lets take a look at the other things. To get the same DoF we need to use that "equivalent" aperture, this results in a higher shutter speed on CX. Faster shutter speed results in a different image - so no equivalency unless we change the ISO accordingly.

Wrong. It will result in higher ISO which is actually good and it makes the image indeed equivalent, since ISO noise will be reduced. If you have higher shutter speed or not, depends on your selected mode and in my opinion, letting the camera select shutter speed is the wrong approach. You must let the camera select the ISO because that way you actually make the image equivalent.

Changing the ISO only works if we assume that that change of ISO is linear to the crop factor - that is that a CX sensor has 3 times more noise than a 135-format sensor.

Yes, but this is actually what you have to assume, because that is almost exactly what is happening. The same ISO has considerably more noise, if it is exactly 2.7 time or not is totally irrelevant in a theoretical discussion with all your assumptions. So why would you like to ignore this assumption?

This obviously only works if we use the 135-format not at it's base ISO, if we use it at base ISO we can't get "equivalency".

For the sake of discussion, this is totally irrelevant.

OK, so we use the CX camera at ISO 100 and the 135-format camera at ISO 800. Both have the same DoF and the same FoV, both have the same lens barrel diameter and the CX lens is shorter.

Wrong again. The ISO difference is not 3 stops, but 2.7x, i.e. multiply the ISO100 and you get ISO270. Quite a bit of difference from ISO800.

Since I made some simplifications at the beginning, lets make it a bit more "real world".

Yes, far too many simplifications, some are totally wrong.

Cameras have a flange distance and interestingly the flange distance of the F-mount and the Nikon 1 mount are scaled versions of each other. This results in similar lens constructions at equivalent focal lengths. I guess it's save to assume that the lens lengths simply scale.

The flange distance is constant so the lens length differences will be: Lens length - FT1, regardless of focal length, meaning that the gain is largest at shorter focal lengths.

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