A question I admit I should know the answer.

[Patrick]

Focal length of a lens remains constant regardless of the size of the capture (film or sensor) medium, for example 50mm lens is the same be it 35mm film or a 6x6 frame or a 5x4. I know it behaves differently with the different formats eg 50mm is standard on a 35mm, but wide angle on a 6x6 and is super wide on a 5x4.
Does this still hold true for the aperture? by this I don't mean the exposure I mean the physical size of the hole itself. Put another way, when designing a 50mm for 35mm is there a difference in the measurement of the actual hole for any given aperture when compared with a 50mm lens designed for 6x6 for the same aperture
The reason I ask, is logic says for example: f8 on 35mm does not have as much area to cover as the f8 for a 6x6, and has even more area to cover for a 5x4.

I hope I have asked the question clearly.

Patrick

[Ian]

Re: A question I admit I should know the answer.

Focal length of a lens remains constant regardless of the size of the capture (film or sensor) medium, for example 50mm lens is the same be it 35mm film or a 6x6 frame or a 5x4. I know it behaves differently with the different formats eg 50mm is standard on a 35mm, but wide angle on a 6x6 and is super wide on a 5x4.
Does this still hold true for the aperture? by this I don't mean the exposure I mean the physical size of the hole itself. Put another way, when designing a 50mm for 35mm is there a difference in the measurement of the actual hole for any given aperture when compared with a 50mm lens designed for 6x6 for the same aperture
The reason I ask, is logic says for example: f8 on 35mm does not have as much area to cover as the f8 for a 6x6, and has even more area to cover for a 5x4.

I hope I have asked the question clearly.

Patrick

Aperture and focal length remain unchanged. The brightness of the image projected by the lens onto the sensor or film plane is unchanged by the size of the frame - the distance of the aperture to the film/sensor plane is unchanged. All you are doing is cropping that frame.

Of course if you use a lens designed for a larger frame than that of the camera it is fitted to, the image circle will be much greater than the frame requires. This has made me think that mirror box light reflection control effectivness must be important for APS-C and DX small sensor cameras when using full frame lenses.

Ian

[lumix]

Re: A question I admit I should know the answer.

Aperture and focal length remain unchanged. The brightness of the image projected by the lens onto the sensor or film plane is unchanged by the size of the frame - the distance of the aperture to the film/sensor plane is unchanged. All you are doing is cropping that frame.

Of course if you use a lens designed for a larger frame than that of the camera it is fitted to, the image circle will be much greater than the frame requires. This has made me think that mirror box light reflection control effectivness must be important for APS-C and DX small sensor cameras when using full frame lenses.

Ian

Next question. Based on the above, dose it mean that the physical size of f8 on a 50mm lens is double the size of f8 on a 100mm lens.

[Stephen]

Re: A question I admit I should know the answer.

Next question. Based on the above, dose it mean that the physical size of f8 on a 50mm lens is double the size of f8 on a 100mm lens.

Surely f8 is f8 or it makes a mockery of the exposure system that lightmeters use. If you take a reading with a meter and it gives you f8 at a given speed then that will apply to any lens.

[lumix]

Re: A question I admit I should know the answer.

Surely f8 is f8 or it makes a mockery of the exposure system that lightmeters use. If you take a reading with a meter and it gives you f8 at a given speed then that will apply to any lens.

Agreed as far as exposure goes that holds true. What I'm questioning is the physical diameter of f8. is it the same in all lenses? IE if it measures 1/8" in a 50mm lens, would it be 1/8" in all focal lengths. I believe it would have to change depending on the focal length. The reason I think this is that the more lens elements etc. the less light transmitted through the lens, so the physical diameter would have to change to compensate.

[Stephen]

Re: A question I admit I should know the answer.

Agreed as far as exposure goes that holds true. What I'm questioning is the physical diameter of f8. is it the same in all lenses? IE if it measures 1/8" in a 50mm lens, would it be 1/8" in all focal lengths. I believe it would have to change depending on the focal length. The reason I think this is that the more lens elements etc. the less light transmitted through the lens, so the physical diameter would have to change to compensate.

You make a reasonable point Ron, but of course shutter speed can also be influenced in order to compensate for lower light transmission. But doesn't the same principle apply here to my original point that a light meter, lets say it was the old Weston Master, would give a light reading of both aperture and shutter speed which was then transferred to the camera and lens. Didn't matter what the lens was the setting would be the same. However field of view may well have to be considered and this was where the spot meter would come in

[lumix]

Re: A question I admit I should know the answer.

You make a reasonable point Ron, but of course shutter speed can also be influenced in order to compensate for lower light transmission. But doesn't the same principle apply here to my original point that a light meter, lets say it was the old Weston Master, would give a light reading of both aperture and shutter speed which was then transferred to the camera and lens. Didn't matter what the lens was the setting would be the same. However field of view may well have to be considered and this was where the spot meter would come in

The shutter speed only controls how long the censor is exposed to the available light. Agreed. For a given exposure of say 1/100th sec at f8 the censor must receive the same amount of light no matter what the focal length of the lens is. If a 50mm lens is capable of passing more light than a 100mm lens, which by it's construction of more elements and greater spacing of elements, must have some affect on its transmission of light. Surely to compensate this the physical size of the aperture must also change. Are you with me so far? Getting more and more complicated the more I think about it.

[Stephen]

Re: A question I admit I should know the answer.

The shutter speed only controls how long the censor is exposed to the available light. Agreed. For a given exposure of say 1/100th sec at f8 the censor must receive the same amount of light no matter what the focal length of the lens is. If a 50mm lens is capable of passing more light than a 100mm lens, which by it's construction of more elements and greater spacing of elements, must have some affect on its transmission of light. Surely to compensate this the physical size of the aperture must also change. Are you with me so far? Getting more and more complicated the more I think about it.

I see what you are getting at, but the one flaw in your hypothesis is that the lens has already compensated. If a lens lets light through because of the number of elements etc. then it becomes lets say an f4 lens instead of say an f2.8 You can see in the viewfinder that a long lens, lets say a 300mm f5.6 is darker than an equiv f2.8 lens. Though the lens will have a much bigger front element to allow more light through, and be a lot more expensive F8 will still be f8 though

[Patrick]

Re: A question I admit I should know the answer.

Agreed as far as exposure goes that holds true. What I'm questioning is the physical diameter of f8. is it the same in all lenses? IE if it measures 1/8" in a 50mm lens, would it be 1/8" in all focal lengths. I believe it would have to change depending on the focal length. The reason I think this is that the more lens elements etc. the less light transmitted through the lens, so the physical diameter would have to change to compensate.

Ron I think you have put it better than I did, the question you ask is in many ways the same question I am asking.

For example a standard lens for 35mm is as we know 50mm
For a 6x6 its 75mm or 80mm
For a 5x4 its 135 -150mm

Any of these cameras set at f8 (or other any given aperture) has to throw enough light on to the sensitive surface to give correct exposure with a f8 reading, we can for the sake of this discussion accept shutter speed as given.

Now if as Ian suggests focusing distance is always the same at a given focusing point on a given focal length from the film/sensor plane then surly the 6x6 and the 5x4 require more light to cover the entire sensitive surface, because the light entering the camera has to spread itself over a larger area.
Lenses are designed for the format they use, 50mm lens designed to have a circle to cover a 35mm would badly vignette a 6x6 never mind a 5x4. The 6x6 and 5x4 would each have 50mm designed to give the appropriate larger circle befitting the format.

If the lens being used is one of the accepted standard lenses as outlined above for the different formats, then the distance from lens to film plane increases with focal length. We all know that increasing the distance between lens and film plane requires more light therefore the longer the focal length the larger the hole will have to be if it is to represent f8.
This if I read it correctly is confirmed by a rather long explanation in my trusty Focal Encyclopedia of Photography it uses the term effective aperture, which confirms IMO the actual hole size can vary to suit the individual lenses and formats to give a constant f8.
I perhaps should have consulted the encyclopedia in the first place, , but if I had we wouldn't be having this interesting discussion.

Patrick

[Patrick]

Re: A question I admit I should know the answer.

I see what you are getting at, but the one flaw in your hypothesis is that the lens has already compensated. If a lens lets light through because of the number of elements etc. then it becomes lets say an f4 lens instead of say an f2.8 You can see in the viewfinder that a long lens, lets say a 300mm f5.6 is darker than an equiv f2.8 lens. Though the lens will have a much bigger front element to allow more light through, and be a lot more expensive F8 will still be f8 though

I just found an explanation on this site.

http://www.punitsinha.com/resource/aperture_focal_length.html

Patrick

[lumix]

Re: A question I admit I should know the answer.

I just found an explanation on this site.

http://www.punitsinha.com/resource/aperture_focal_length.html

Patrick

Thanks Patrick, that has made it very clear. I was working on the fact that Zoom lenses have different max f Numbers from end to end, which makes you think focal length affects aperture size.

[Ian]

Re: A question I admit I should know the answer.

Next question. Based on the above, dose it mean that the physical size of f8 on a 50mm lens is double the size of f8 on a 100mm lens.

Yes - it's very simple - focal length divided by diameter of iris = aperture value (f/x).

At f=50mm, f/8 the diameter = 6.25mm, and at 100mm, f/8 the diameter = 12.5mm

This is one reason why constant aperture zoom lenses are expensive; the aperture must be altered in size with the focal length to maintain its value - which requires some additional precision engineering.

Ian

[Ian]

Re: A question I admit I should know the answer.

Ron I think you have put it better than I did, the question you ask is in many ways the same question I am asking.

For example a standard lens for 35mm is as we know 50mm
For a 6x6 its 75mm or 80mm
For a 5x4 its 135 -150mm

Any of these cameras set at f8 (or other any given aperture) has to throw enough light on to the sensitive surface to give correct exposure with a f8 reading, we can for the sake of this discussion accept shutter speed as given.

Now if as Ian suggests focusing distance is always the same at a given focusing point on a given focal length from the film/sensor plane then surly the 6x6 and the 5x4 require more light to cover the entire sensitive surface, because the light entering the camera has to spread itself over a larger area.
Lenses are designed for the format they use, 50mm lens designed to have a circle to cover a 35mm would badly vignette a 6x6 never mind a 5x4. The 6x6 and 5x4 would each have 50mm designed to give the appropriate larger circle befitting the format.

If the lens being used is one of the accepted standard lenses as outlined above for the different formats, then the distance from lens to film plane increases with focal length. We all know that increasing the distance between lens and film plane requires more light therefore the longer the focal length the larger the hole will have to be if it is to represent f8.
This if I read it correctly is confirmed by a rather long explanation in my trusty Focal Encyclopedia of Photography it uses the term effective aperture, which confirms IMO the actual hole size can vary to suit the individual lenses and formats to give a constant f8.
I perhaps should have consulted the encyclopedia in the first place, , but if I had we wouldn't be having this interesting discussion.

Patrick

Lenses do have to be designed for the frame size they are catering for, well, at least for the optimum (largest) frame size. Putting a lens designed for a larger frame onto a camera with a smaller frame is OK, but vice-versa and you will get vignetting because the image corcle will be too small.

What determines the image circle? The designer of the lens determines this because the exit lens of the design will form a (usually) circular cut-off so the projected image is circular.

This doesn't mean alll 100mm lenses will have the same size circular projection onto the film/sensor plane. A 100mm lens designed for a 6x6cm lens will be designed to accommodate the whole usable film frame.

The lens designer will have to design the lens accordingly, to factor in natural reduction in brightness towards the corners of the frame. This can be done by using larger lens elements, but this won't affect the actual aperture diameter for a given focal length, no matter what the frame size is.

Naturally, if you accommodate a larger frame size, you are cropping less, so the lens has a wider field of view.

The brightness per scare millimetre on the film/sensor plane, remains constant. All you are doing is changing the coverage of the lens.

Hope that makes sense

Ian

[Patrick]

Re: A question I admit I should know the answer.

Lenses do have to be designed for the frame size they are catering for, well, at least for the optimum (largest) frame size. Putting a lens designed for a larger frame onto a camera with a smaller frame is OK, but vice-versa and you will get vignetting because the image corcle will be too small.

What determines the image circle? The designer of the lens determines this because the exit lens of the design will form a (usually) circular cut-off so the projected image is circular.

This doesn't mean alll 100mm lenses will have the same size circular projection onto the film/sensor plane. A 100mm lens designed for a 6x6cm lens will be designed to accommodate the whole usable film frame.

LThe lens designer will have to design the lens accordingly, to factor in natural reduction in brightness towards the corners of the frame. This can be done by using larger lens elements, but this won't affect the actual aperture diameter for a given focal length, no matter what the frame size is.

Naturally, if you accommodate a larger frame size, you are cropping less, so the lens has a wider field of view.

The brightness per scare millimetre on the film/sensor plane, remains constant. All you are doing is changing the coverage of the lens.

Hope that makes sense

Ian

The reason I asked this question in the first place, came from the Breaking Waves thread, and diffraction softness.

Large format cameras 5x4 and up to 10x8 offer very high f numbers much higher than we expect from our DSLR's I remember when I had a MPP 5x4 (I never got along with the camera and didn't really test it logically) the 150mm lens went to f64. If diffraction softness is a problem with f22 whats it going to be like at f64.

Patrick

[Ian]

Re: A question I admit I should know the answer.

The reason I asked this question in the first place, came from the Breaking Waves thread, and diffraction softness.

Large format cameras 5x4 and up to 10x8 offer very high f numbers much higher than we expect from our DSLR's I remember when I had a MPP 5x4 (I never got along with the camera and didn't really test it logically) the 150mm lens went to f64. If diffraction softness is a problem with f22 whats it going to be like at f64.

Patrick

There is a digital sensor element to diffraction softness as well, but that's for another discussion.

But with large format cameras you will need much longer focal length lenses for the equivalent field of view, so larger diameter irises for the same aperture (and brightness) value and so diffraction softness will only be an issue at comparatively smaller apertures.

Ian