There is no magical size/weight advantage

Started 6 months ago | Discussions thread
myzel
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Re: There is no magical …
In reply to IVN, 6 months ago

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.

Anyway lets play.

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

We use single element lenses and assume that they are optical perfect. We use cameras without lens mount and no flange distance. We take a 135 format sensor ("full frame") and a 1" sensor, booth with identical sensor technology and the same amount 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).

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.

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).

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.

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.

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.

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".

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.

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

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.

What I haven't taken into account is all the other stuff that affect the size of a lens or camera. The manufacturing, the different lens design needed to make the lens smaller, the higher precision needed for the smaller system… There are to many variables and to many things different, that in my opinion that whole concept of "equivalency" isn't very helpful (like all over simplified concepts).

Simply get a large(er) format camera if you like shallow DoF and want the best image quality you can afford. Simply get a small(er) format camera if you want small and light weighted cameras and lenses and don't care if the DoF is fairly large (or if you prefer large DoF).

Just use the system that gives you the results you want, use the tool for the task and don't try to make a hammer into a screwdriver.

And just in case: A CX 70-300 isn't going to be that much smaller than a FX 70-300 - in the worst case it could be bigger. But it would be smaller than a 189-810 if you don't mind the larger DoF

 myzel's gear list:myzel's gear list
Nikon 1 V1 Nikon 1 Nikkor 10mm f/2.8 Nikon 1 Nikkor 18.5mm f/1.8 Nikon 1 Nikkor 32mm f/1.2
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