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Are you familiar with crop factor times aperture?
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Jun 6, 2017 10:40:22   #
BebuLamar
 
TriX wrote:
But doesn't the luminance per area cd/m2 remain the same? If that were not the case, why does a FF f2.8 lens produce the same exposure on both a FF and crop body camera? According to your theory, the exposure from the crop body would be a stop less than the FF, but actual practice shows otherwise I believe.


It's the illuminance but yes it's the same for the same aperture.

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Jun 6, 2017 10:56:13   #
GoofyNewfie Loc: Kansas City
 
SS319 wrote:
A quick look at the ray diagram makes this an easy question.

THIS ONLY AFFECTS THE WIDE OPEN APERTURE! when you impose an iris and close the lens down one stop, and from there to f/36, there is no further change in the f stop.

So, on a f/2.8 FF lens, on a crop sensor, your f stops would look like this:

Full Frame F/2.8 F/4 F/5.6
Crop Sensor F/4 F/4 F/5.6


Not true.
Again and as others have stated- There is no adjustment/compensation for format or "crop factor" when using a hand-held meter.
The reason? It's not necessary because the aperture setting remains the same no matter what format camera a lens is mounted on.

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Jun 6, 2017 10:59:13   #
TriX Loc: Raleigh, NC
 
BebuLamar wrote:
It's the illuminance but yes it's the same for the same aperture.


Yes it is (illuminance). Damn spell checker or poor typing (I do know the difference, just missed it...)

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Jun 6, 2017 11:02:58   #
hrblaine
 
"Even Tony Northrup wouldn't agree with you."

But would Ken Rockwell??? :-)

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Jun 6, 2017 11:08:27   #
TriX Loc: Raleigh, NC
 
rehess wrote:
My brother, who worked for an astronomer at one time, tells me that they make reflecting mirrors by rotating molten glass - and physics will form the parabola for them...


So much easier. We used to grind mirrors from a flat blank by hand with abrasives and rouge and then "figure" them to create a parabolic curve prior to silver or aluminum plating them. If memory serves, we used a Foucault test to check them...

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Jun 6, 2017 11:15:30   #
ecurb1105
 
To repeat, the size of a sensor or film does not change the exposure for s given lens. If I set up an 8x10 camera with a 14 inch lens shooting a tabletop lit by hot lights so the exposure is constant. I can shoot 8x10, 5x7, 4x5 or smaller film and the exposure is constant no matter the size of the film. You are just using less of the lens image circle. This applies to sensors as much as film. If you had 100mm lens you could mount on a Fuji medium format, a D5, a D500 or a V1, the lens would be 100mm but the images would have different image size because your using smaller sections of the image circle.

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Jun 6, 2017 11:18:05   #
BebuLamar
 
hrblaine wrote:
"Even Tony Northrup wouldn't agree with you."

But would Ken Rockwell??? :-)


No! Do you think those two guys are stupid? KR tried his best to mislead the reader without being wrong. TN did not really said that the aperture changes when you use smaller sensor.

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Jun 6, 2017 12:15:25   #
SS319
 
BebuLamar wrote:
Your ray diagram is wrong and thus all what you said about the effective aperture is wrong. Even Tony Northrup wouldn't agree with you.



It is easy to say someone is wrong, it becomes much harder to show that same person where they are wrong

I am an easy learner - show me where I am wrong.

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Jun 6, 2017 12:50:28   #
fran120
 
I was also was confused about that Tony Northrup video. I asked a question about
the Zeiss Batis 25mm f/2 Lens for Sony E Mount (full frame) for the A 6300 on the B&H site
This is the question and answer:
Does this lens remain at F2 on the A6300 body?
The camera has no influence over the aperture of the lens. The ZEISS Batis 2/25 features an f/2 aperture regardless of whether it is used on a FF camera, or one with an APS-C sensor.
Answered by ZEISS ZEISS EXPERT on Mar 13, 2017

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Jun 6, 2017 12:59:15   #
rehess Loc: South Bend, Indiana, USA
 
Gene51 wrote:
But aperture has three main functions - controlling the amount of light getting through the lens to the sensor, and controlling the depth of field, and ultimately diminishing overall sharpness when diffraction starts to limit sharpness. Smaller aperture = greater depth of field and less light.

If you use a 1.4X TC, you add one stop of exposure, and using a 2X TC requires 2 stops extra exposure.

What does change, and it has already been mentioned and shown in Cambridge in Color DoF page - is that if you use the same composition a 100mm lens on a crop camera at F5.6 will have the same DoF as a 150mm lens on a full frame camera at F8.4 give or take.

But you are correct - using a crop sensor camera and a full frame lens will provide the same amount of light compared to the same lens on a full frame camera.
But aperture has three main functions - controllin... (show quote)
Gene's post gets to the heart of why "equivalent aperture" is such a confusing subject. Aperture is used as a proxy for all three of these
(1) intensity of light hitting sensor
(2) DOF
(3) diffraction
In the days when most of us used 35mm film, aperture provided a direct connection between these three, but once we get away from 35mm/FF that connection becomes iffy at best, which is why this discussion could go on for many days and many pages.

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Jun 6, 2017 13:40:58   #
SKAN Loc: Chennai INDIA
 
This video from Tony Northup explains it better:

Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses
https://www.youtube.com/watch?v=f5zN6NVx-hY

I would also like to quote one viewer's comment

[gamerguy00 2 years ago]
So in short, just so people don't misunderstand this - you're actually talking about the the effective DoF, not the actual amount of light coming through the lens. Logically, an f/2.8 on a full-frame is still a f/2.8 on a m43-camera, however the effective depth of field is equivalent of a f/5.6. This has to do with the size of the sensor, as you very well explained. Great explanation, just some bits and pieces that could've been misunderstood.

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Jun 6, 2017 19:36:54   #
BebuLamar
 
SS319 wrote:
A quick look at the ray diagram makes this an easy question.The outer blue lines are the primary rays for a given lens at a given distance from the red arrow forming the image inverted on the left. With the same lens, focused at the same distance, the focal point (where the rays cross) remains the same. Now, you can see how the crop size of the snsor creates the multiplication factor of the lens focal length, and you can see how the outer portions of the lens are not effectively used and the light coming through that part of the lens is lost to the image, thus the equivalent of a smaller aperture.

THIS ONLY AFFECTS THE WIDE OPEN APERTURE! when you impose an iris and close the lens down one stop, and from there to f/36, there is no further change in the f stop.

So, on a f/2.8 FF lens, on a crop sensor, your f stops would look like this:

Full Frame F/2.8 F/4 F/5.6
Crop Sensor F/4 F/4 F/5.6
A quick look at the ray diagram makes this an easy... (show quote)


I don't seem to be able to post the diagram but 1 proof of what you say is wrong. You said that with an f/2.8 FF lens when mounted on an APS-C camera the aperture at fully open is f/4 and then when you stop down 1 stop it's still f/4. You can simply take 2 pictures with the same shutter speed and ISO one with the lens set at f/2.8 and the other at f/4 and see if there is a difference. If there is a difference then both the wide open and 1 stop stopped down can't be both f/4.

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Jun 6, 2017 23:26:52   #
SS319
 
BebuLamar wrote:
I don't seem to be able to post the diagram but 1 proof of what you say is wrong. You said that with an f/2.8 FF lens when mounted on an APS-C camera the aperture at fully open is f/4 and then when you stop down 1 stop it's still f/4. You can simply take 2 pictures with the same shutter speed and ISO one with the lens set at f/2.8 and the other at f/4 and see if there is a difference. If there is a difference then both the wide open and 1 stop stopped down can't be both f/4.


Perhaps this will show you what I mean about the crop sensor (orange dashed ray lines) affecting the Full frame (blue lines) effective aperture - the red plate is the iris at f/2.8 top, and f/4 bottom.

With a ff lens on a crop body, the opening of the iris wider than the effective aperture of the crop sensor do not do anything optically for the camera system!



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Jun 6, 2017 23:54:02   #
GoofyNewfie Loc: Kansas City
 
SS319 wrote:
Perhaps this will show you what I mean about the crop sensor (orange dashed ray lines) affecting the Full frame (blue lines) effective aperture - the red plate is the iris at f/2.8 top, and f/4 bottom.

With a ff lens on a crop body, the opening of the iris wider than the effective aperture of the crop sensor do not do anything optically for the camera system!


Where did you get this diagram?
That's not how apertures work.
The aperture is usually located farther back, closer to where the light from the image crosses and only affects the brightness (and dof)
It is not dependent on sensor size.
It makes no difference what format the camera has.
The aperture (image brightness) DOES NOT ChANGE if you use a different format. I've been a pro for over 40 years, shot formats up to &x10 and know this is a fact.
If you don't believe us posters with more experience, go out and shoot photos to prove it to yourself. It's a great way to learn.

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Jun 7, 2017 00:50:35   #
anotherview Loc: California
 
I do not believe that "aperture . . . must be multiplied by the crop factor." The aperture is a ratio and thus a numberless dimension.

The range of aperture is physically fixed. Its ratio involves the length of the lens in relation to the size opening of the aperture.

Wikipedia says this about aperture: "The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter." (found at: https://en.wikipedia.org/wiki/Aperture]
Rab-Eye)

I suggest you consult this article on aperture.
[quote=Rab-Eye wrote:
I had never heard of this before. If accurate, it means my FX 70-300 f/4.5-5.6 zoom is an f/8.4 at 300mm. This led me to investigate the Nikon DX 70-300, which I have now rejected for several reasons, including but not exclusively because of its plastic mounting ring.

I assume that not only focal length but aperture as well must be multiplied by the crop factor is accurate because Nikon itself is the source of the information. Still, it's odd to me that it is virtually never mentioned; in my own experience it has never been referenced except by this Nikon video. Have I been living under a rock, or is this news to you as well?
I had never heard of this before. If accurate, it ... (show quote)

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