@bwv812 Yes, the multiplication factor gives 150mm.
But no, these are really not physical properties of the lenses as you might imagine. Changing the focal length of a lens is easy, thats what you do when you focus it. We use one particular measure of focal length as a surrogate for field of view. But that is also an equivalence to an ideal model of a camera.
The point is that ISO equivalence is a measure designed to allow setting of the exposure in a camera and not a measure of low light performance. It is sensitivity per unit area. But more unit area does not give more sensitivity. So if you want the sensitivity you need to correct for the collection area.
Drawing a comparison based on swimming pools vs buckets is misleading because its not the size of the storage area that makes a difference, its the size of the collection area. If yoy have a roof of a given size running down a drainpipe it will put just as much water into a bucket, a barrel or a swimming pool.
The collection area is the aperture, not the focal ratio of the lens. If you want better response in low light you need bigger lenses or a more efficient sensor technology.
It really sounds as if you need to get a copy stand, an external meter, and two or more late model cameras of different sensor sizes.
the copy stand is so you can standardize lighting and camera position.
the external meter, combined with ām modeā will let you standardize the shot conditions. Set each camera to a fixed aperture, and vary the exposure time (according to your external meter), and note which cameras have the highest usable ISOs before chroma noise and the like ruin the image.
Iād do it myself, but Iād be comparing a aps-C dlsr to a 1/2.5" p&s. Not a very fair contest.
Steve Jobs would have described an iCamera with a high ISO like that to the public. I mean this as a compliment to you, btw.
Of course, thereād be technical bickering from the pedantic peanut gallery over that statement, but like usual, theyād be missing the bigger point.
I keep thinking I should do a podcast showing it on my D800 which is an FX camera with a DX mode. But I donāt think its necessary. Consider the following thought experiment.
Lets imagine that I setup the camera on a tripod and point it at a scene with controlled lighting. I switch between DX and FX mode without changing the lens. Hopefully we both agree that the settings for ISO and speed and exposure should all stay constant, right?
The only thing that changes is the field of view and the resolution goes down from 36 MP to 17MP but lets call it 18 to make the math easy.
So the manufacturerās ISO equivalence stays the same in this particular case. But what happens if we take both of the pictures and downsample them to 12MP in Photoshop? The 36MP sensor has collected more information to start with and so it is going to be able to do a better noise reduction job. Which is why it is important to correct for resolution when comparing different resolution sensor of the same size.
The whole point of doing a real experiment, rather than a thought experiment, would be to confirm or, alternatively, to disprove, your mental model of how the outside world works.
Again, this is wrong. As Iāve said before, the focal length has a set definition: the distance from the focal node to the film/sensor plane when focused at infinity. Focus at anything other than infinity and youāre no longer measuring the focal length. Note that the field of view, however, will decrease (become narrower) the closer you focusāthis does not change the focal length, either.
But this changing in field of view does not change with sensor size. For example, a 1:1 reproduction ratio is always achieved (in a simple lens) when the focal node is twice the distance from the film plane as the lens was when focused at infinity. So, in a 100mm lens, you get 1:1 focus when the lens is 200mm from the film plane. This is true regardless of the sensor/film size. Thatās simply a physical property of the lens.
Also note that while less light reaches the film when focuses at 1:1, the aperture does not change size. the 100/1.4 lens is still an f/1.4 lens when focused at 1:1, even though only one-quarter of the light reaches the film at this focusing distance as did at infinity. We might say the lens has an effective aperture, or working f- stop, of āf/2.8,ā but it physically retains the aperture of f/1.4 by definition.
The problem is thatās not how things work. A drain-pipe collects all of the water from an area and concentrates it in one area. Lenses donāt do this. Letās take two lenses that have a physical aperture of 50mm. The 50mm lens will have an aperture of f/1.0. The 100mm lens will have an aperture of f/2.0. If the only thing that mattered is the physical size of the opening, they would have the same exposure. But they donāt, because the aperture in the 100mm lens is twice as far away as in the 50mm lens and gives only one-quarter the intensity of light to the sensor: itās like being twice as far from a light bulb. Itās not the matter of simply having a longer drainpipe. This is why teleconverters change the f-stop despite keeping the physical aperture size.
I donāt know if youāre confused by the question of a lensās rear coverageāwhich is different from its speedāor not. Take a wide angle LF lens. Even with the same focal length and maximum aperture, lenses will have very different prices because if differences in rear coverage, or how large an image circle they project onto the film. In the 35mm world, DX-only lenses lack the rear coverage to cover the entire FF format, which is why they can only be used on DX lenses. Lack of rear coverage doesnāt change the aperture of the lens, and less rear coverage doesnāt mean they are concentrating the light into a smaller image circle.
That might make sense if you had been doing that when comparing the 16.2 megapixel D4s with the 18.4 megapixel V3. In which case the D4s uses 7.29 times more physical sensor size to capture 12% fewer pixels, meaning that if we correct for resolution then each D4s pixel is about 8.16 times larger than each V3 pixel. Yet from this you somehow figure that their ISOs are similar and the D4s is only a little better. That totally makes sense.
Oh wait, itās actually the total opposite of what you said before. I mean, if your argument before was that the V3 sensor, on a per square mm basis, was just a little inferior to the D4s, then that would be fine, and we would all see that the D4s wins easily since it has 7.3 times the square mms to work with. But instead you suggested that on the whole the D4s is just a little better, which is rubbish.
Take a telephoto lens. Doesnāt matter which. The body of the lens which is by definition less than the quoted focal length. A 200mm lens is less than 200mm in length. Thats what makes it a telefocus lens.
The focal lengths quoted are for a normal (i.e. non tele, non retro) lens with the same field of view. It is only a fixed property of the lens if the field of view is fixed and we only do that because it is more convenient to use focal length than angle of view to calculate exposure.
If you are going to officiously correct people then get the physics right.
Iāve made it clear in the multiple other times Iāve discussed this that Iāve been talking about the simple-lens equivalent of any given lens, be it telephoto or retrofocus. Itās an obvious pain to include this caveat every single time I talk about it, since most people understand the definition of focal length.
And if quibbling over the definition of focal length is the best response you have, and you canāt defend the core of your argument, maybe itās time to stop making the same false statements over and over again in different threads. The V3, contrary to your statements, does not have similar high ISO performance and image quality to the D4sāand the difference in dynamic range is far from being the only differentiating factor.
By the way, angle of view is never used to calculate exposure. The reason is that angle of view of a given focal length varies from format to format, while to f-stop does not. If the required exposure is 1/100 at f/16 and ISO 100, it doesnāt matter what the focal length or angle of view is. You can keep the focal length the same and use a 300mm lens on a view camera, on medium format, on full frame 35mm, or with a DX sensor and the angle of view will change but the exposure will remain 1/100 and f/16. Or you could keep the angle of view the same and use different focal lengths on all of the above formats and the exposure will still be 1/100 and f/16.
When you were claiming that the focal length does not change when you focus you were rather obviously referring to an actual lens or it makes no sense.
I have not a fucking scoobie how to use a camera, but thereās a Canon 700D at work, work being situated in some astonishingly beautiful countryside with pretty much zero light pollution if I switch the house lights off, so Iāma use yours & othersā helpful info here to good effect and take lovely sky pictures. Always assuming that Perfidious Albion ever gets rid of its fucking cloud cover, that is.
If you donāt have a remote to get an exposure longer than 30 seconds, put your lens cap on, switch to B (bulb) and then tape a small pebble down on the button to hold it in. Take your lens cap off, start exposure, put cap on at the end and remove the pebble. Repeat as needed.
Every actual lens has a simple/thin lens equivalent. Focal length is measured based on this. Thin lenses equivalents can also focus, which should be obvious since I talked about simple lens equivalents when focusing down to 1:1. The fact remains that simply focusing at a different distance does not change the focal length of a lens because the focal length is defined as the distance (of the thin lens equivalent) from the lens plane when focuses at infinity.
Itās true that some macro lens designs have floating elements that actually change the optical formula when focusing close (The 60/2.8 micro-Nikkor effectively becomes something like a 55mm lens when at 1:1, despite having a much narrower field of view at 1:1 than it does at infinityāthe opposite of what you would expect if focal length is based on field of view), but their focal length is still measured by distance from the film plane (of the thin lens equivalent) at infinity focus and not by field of view.
Itās pretty telling that you are choosing to focus on this instead of actually trying to defend your core argument.
