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Author Topic: Full Frame Myth -- Question  (Read 3259 times)

bdosserman

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Full Frame Myth -- Question
« on: December 20, 2013, 09:56:45 am »

Hi,
   I was fascinated by the assertion in the article that part of the reason for larger lenses on SLRs is the greater rear register distance. If I understand correctly, this is saying that when one compares lens sizes between DSLRs and eg M4/3 cameras, the difference in lens size is due to a combination of the smaller sensor and the smaller rear register distance. Can anyone elaborate on this? How much of the difference is due to which factors? Does the difference from the rear register difference vary by lens type, or is it relatively constant?
   Thanks,

Brian
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fike

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Re: Full Frame Myth -- Question
« Reply #1 on: December 20, 2013, 10:14:41 am »

I am also kind of curious about this.  I will say that both moving the lens closer to the sensor AND having a smaller sensor both contribute substantially to the smaller lens. 

There have been some discussions about the problems inherent with small lenses on large sensors due to the extreme angle that light is required to go in order to hit the edge of the sensor.  This could be a potential problem with getting compact lenses for the Sony A7r.

I have also been curious to learn a bit more about the older Leica rangefinder lenses that were full frame, high quality, AND more compact. 

People talk about a full-frame sensor on a micro four thirds camera. Looking at the M43 lenses, I am not convinced it can be done.  I would imagine that the only way to make it work would be to pull the lens very far away from the sensor so that it projected a wider image, but I am not always aware of everything that can be done. 
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Vladimirovich

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Re: Full Frame Myth -- Question
« Reply #2 on: December 20, 2013, 01:00:55 pm »

the difference in lens size is due to a combination of the smaller sensor and the smaller rear register distance.

small registration distance more affects the design of wide angle lenses.. long tele - not so much... and indeed you need less diameter to illuminate smaller sensor... look @ 50x constant f2.8 zooms in P&S cameras w/ tiny 1/2.3" or so sensors.
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Christoph C. Feldhaim

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Re: Full Frame Myth -- Question
« Reply #3 on: December 20, 2013, 01:13:34 pm »

Talking about Leica M glass size don't forget  you get neither OIS nor AF with it.

Petrus

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Re: Full Frame Myth -- Question
« Reply #4 on: December 20, 2013, 01:45:54 pm »

and indeed you need less diameter to illuminate smaller sensor.

Not really. If we take a 20mm lens with maximum aperture of f/2, it will illuminate any size sensor equally well, as long as the image circle can cover it. Small sensor must have at least 20/2 = 10mm opening, same as a large one.

What you possibly mean is that small sensors use shorter focal length lenses, and short focal length lenses of certain aperture are smaller than longer focal length lenses of the same maximum aperture.
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fike

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Re: Full Frame Myth -- Question
« Reply #5 on: December 20, 2013, 01:56:01 pm »

... as long as the image circle can cover it. Small sensor must have at least 20/2 = 10mm opening, same as a large one.
...
Isn't that the rub.  With current small M43 lenses, for example, you would have to move the lens much further away from the sensor to get the lens to  cover a full-frame sensor.
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Vladimirovich

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Re: Full Frame Myth -- Question
« Reply #6 on: December 20, 2013, 03:59:51 pm »

Not really. If we take a 20mm lens with maximum aperture of f/2, it will illuminate any size sensor equally well, as long as the image circle can cover it. Small sensor must have at least 20/2 = 10mm opening, same as a large one.

What you possibly mean is that small sensors use shorter focal length lenses, and short focal length lenses of certain aperture are smaller than longer focal length lenses of the same maximum aperture.

that's is certainly correct note about focal distance (w/o consideration of eq. FOV for different sensor sizes) & aperture
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Petrus

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Re: Full Frame Myth -- Question
« Reply #7 on: December 20, 2013, 04:11:26 pm »

Isn't that the rub.  With current small M43 lenses, for example, you would have to move the lens much further away from the sensor to get the lens to  cover a full-frame sensor.

No, Focal length is the theoretical measure how far the lens is optically form the sensor. What the real distance is a matter of design and construction, and that does not affect the max aperture calculation. 20mm lens with f/2 max aperture looks like a 10 mm hole at 20 mm distance to the sensor no matter how it is designed, how far it is from the sensor physically or how large the image circle is.
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Vladimirovich

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Re: Full Frame Myth -- Question
« Reply #8 on: December 20, 2013, 05:22:04 pm »

No, Focal length is the theoretical measure how far the lens is optically form the sensor. What the real distance is a matter of design and construction, and that does not affect the max aperture calculation. 20mm lens with f/2 max aperture looks like a 10 mm hole at 20 mm distance to the sensor no matter how it is designed, how far it is from the sensor physically or how large the image circle is.
except of course that there are other factors influencing the lens size, that hole dictates just the minimum diameter of the optics
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Petrus

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Re: Full Frame Myth -- Question
« Reply #9 on: December 21, 2013, 01:49:56 am »

Like I said, lens size is a matter of design and construction, not only the maximum f-stop. Inverted telephoto design typical of SLR wide-angle lenses (necessity because of the mirror box) makes the lens much bigger than a symmetrical design we see in old rangefinder lenses for example. Making a lens with large image circle naturally makes the lens more complicated = larger than making same focal length normal or long lens (same focal length lens can be WA, normal or long lens depending on the system it is designed for).
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bjanes

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Re: Full Frame Myth -- Question
« Reply #10 on: December 21, 2013, 08:52:54 am »

No, Focal length is the theoretical measure how far the lens is optically form the sensor. What the real distance is a matter of design and construction, and that does not affect the max aperture calculation. 20mm lens with f/2 max aperture looks like a 10 mm hole at 20 mm distance to the sensor no matter how it is designed, how far it is from the sensor physically or how large the image circle is.

Because of optical vignetting, as seen from the sensor, that hole may not appear round but partly cut off with a cat's eye appearance. Mitigating this optical vignetting requires a large lens barrel and front element of the lens, and a compact large aperture lens such as used on compact cameras may suffer from this defect.

The Sigma 50 mm f/1.4 lens has a very large front element and has less vignetting as compared to the more compact 50 mm f/1.4 Nikkor as shown by the Photozone data. The sigma lens would not be awkwardly out of place on a large camera such as the Nikon D4, but might appear out of place on a compact full frame camera, and such a large lens would negate the portability advantage of the smaller camera.

Bill
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bdosserman

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Re: Full Frame Myth -- Question
« Reply #11 on: December 27, 2013, 12:58:30 am »

Thanks all for your replies. It sounds like there are a lot of variables that go into. I'll be curious to see where Sony's lenses end up!

Brian
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