Sven44: It's refreshing to see more and more people on this thread "getting it", and fewer and fewer coming out with rubbish like "f2=f2=f2".
One last time:
Bob shoots with a Panalympus (2x 'crop factor') with a 28mm lens at f/1.8, 1/60s at ISO 400
AND HIS PICTURES ARE DAMN NEAR IDENTICAL TO
John's 'full frame' 35mm camera with a 56mm lens shot at f/3.6, 1/60s at ISO 1600.
Same FOV. Same DOF. Same brightness of image. Same noise even - notice how Bob's shot was at ISO 400, but his sensor is smaller so intrinsically noisier - meanwhile John cranked up to ISO 1600 because he used a slower aperture.
The difference then? Bob's camera, and especially his wide angle lenses, are smaller. Hurrah for Bob! But sadly, his lens is slower (it's labelled 1.8 but shoots just like f/3.6), while John's lens really does open up to 1.8 to give him more blurred backgrounds and cleaner images at ISO 400. Hurrah for John!
Take your pick, then take lots of pics! :-)
Great Bustard, the notional sense is to try and relate a lens to a set system. In order to provide a way of producing a similar image using a different system.
However as the focal length doesnt change the aperture doesnt change. The field of view does change because you are effectively cropping the image.
Lets assume you crop each image, in the centre to a factor of 2. You shoot using a 35mm camera and each image is compose so that you can crop the middle. Effectively doubling the focal length. Does that mean you have change your equivalent f-ratio because you have changed the equivalent focal length needed to replicate the shot?
Sven44, light intensity does not change. Light intensity is amount of light per set area.35mm film has 4 times the area of four thirds, thus total light input is greater. Intensity per actual f-stop remains the same.
If you compose the scene correctly at f/8 on 35mm, medium format and four thirds. Assume the ISO is 100, the shutter speed for each identical scene would be the same. The light intensity is the same, just the bigger formats use bigger lenses to gather more light in total.
example:If you have 1000 lumens of light per cm^2 leaving an f/2 lens then at f/2.8 it would only be 500 lumens for the same area. Thus meaning you need to double the ISO to obtain the same brightness. Attach this lens to 35mm, four thirds, aps-c, nikon 1, etc... the intensity of light leaving the lens remains at 1000 lumens per cm^2 at f/2.
Please do not confuse light intensity and light gathered. Its a rate of flow (intensity) and accumulation of flow (gathered).
I appreciate that the resulting image would require different settings on different systems, but that does not change what f/2 is.
f/2 is the ratio between the focal length and aperture diameter. The focal length remains (the equivalent focal length changes), the aperture diameter remains (the diameter remains the same on all systems) and thus f/2 lets a ratio of light through the lens.
this sets light intensity and thus forms the basis of your statement. (ignoring you saying f2=f2=f2 is rubbish, as it stands as fact for the actual focal length and aperture)
To get identical images you would need the settings you state, however if you only cared about the light intensity at f/2 (ignoring depth of field, total light, etc...) then any lens at f/2 would work.
Mssimo: When did 1024 x 768 8" screen become "low-res." This article needs a new title. It may suck for photographers but for other reasons like its crap 5MP camera and low storage.
its not designed for photography. It is aimed for web browsing, reading and occasional gameplay.It is also a "cheap" Apple tablet. So the lower resolution is the trade-off. If it had the high resolution of the 3rd generation iPad then it would be more expensive and not be placed in the lineup as it is.
this makes me smile, knowing that its not just my local area that fails with its cycle routes