[Brightcolours]

With all this math mambo-jambo you, popo and brightcolours, just seem to forget:


There's not such a lens for APS-C like there are some very pricey ones for FF - and to use them, you just need to get an also pricey FF-body. Which, following Klaus' post are overrated and overpriced anyway, so all the people buying these are just wasting money?

 



I don't get the point trying to calculate equivalence for FF, this is not a question for an APS-C shooter how "lame" this lens is compared to what is available for FX. At the moment it's just outstanding for APS-C and if it can be used at open aperture, it still gives 1.5 f-stops more than the usual 17-50/2.8 "fast standard zooms" which I don't find that overpriced (except a Nikon 17-55/2.8, ok, point taken).


And DoF? At 18mm and open aperture it's 0.86m, not breathtakingly narrow. f/2.8 (with APS-C and same focal length) would give 1.47m.

You can not talk about DOF as something set like you do. There is no such thing as a DOF of 0.86m at 18mm, or something like that. DOF is really always only immeasurably thin, and it will appear bigger depending on many factors, which include projection size and spectator distance. Without stating those two, one can't put a figure at how deep DOF appears to be. Any DOF calculators that do are not right.

One can ask if a Sigma 17-70/2.8-4 OS couldn't be a bit more useful, because of the OS which compensates more than the 1.7 missing f-stops (I was interpolating until 35mm) - but there's no other way to get a narrow DoF and short shutter speeds.


And, by the way, it's not correct to calculate those ISO differences like you did: the borders between the sensels keep the same size, more or less, so in reality the sensitive area decreases more when the sensors get smaller with the same pixelcount.


"The sensor gets MORE sensitive. If you have more surface, the sensor will catch more light. If you have bigger pixels, the pixels will catch more light. Hence: the FF sensor is MORE sensitive by definition."


So a film 8×10 inch and 100 ISO is per your definition much more sensitive than a film 135 with 100 ISO?

So you keep thinking ISO on digital says something about the sensitivity of the sensor? 

 

1. With film, the grain size changes with the sensitivity. Low sensitivity film has a much finer grain than high sensitivity film. 

2. With digital, we can set whatever ISO-setting we want. Film... well, film has its own sensitivity.

3. With 8x10 inch plates/film and 135 format film of the same era you get similar sized grain. Think about that. 

4. In the film era the sensitivity was not a variable. In digital the amount of signal amplification is a variable. Why feel the need to stay stuck in your ways which made working with film easier?

5. Back to that grain size of equal ISO films on different formats. You really want to talk about sensel size here? Then how do you handle a 6mp APS-C sensor, a 10mp APS-C sensor, a 12mp, 16mp, 24mp APS-C sensor? All those will have different sensitivities.

6. ISO in the digital age is NOT about the sensitivity of the sensor. 

 

A FF sensor is more sensitive than an APS-C sensor, no matter how you look at it. If you look at it from a pixel POV, and take similar sensel counts (for instance 12mp): the FF sensor will have 1.5x1.5 times the surface.

If you look at it from an image POV too, the image gets formed with 1.5x1.5 the amount of light over the same exposure period.

 

That you can't get your head around that ISO is not about sensitivity in the digital age, and around the idea of equivalent ISO settings for different format sensors, does not mean it is wrong.

Strange - my old spotmeter didn't list film formats as parameter. Must've been a cheap one...

It does allow you to use different ISO settings to get appropriate exposure time figures. So it is not yet useless, even in the digital age. In the film age, different sized grain would give different ISO ratings for films. Ask yourself why that is not the case with digital cameras with different sized pixels/sensels (big hint to what has changed in the ISO standard).

It will capture more photons, in absolute numbers - but not more on the same area than the smaller frame will catch, too.

In the film era, ISO/ASA was about the film getting exposed right. With digital that is not the case at all, anymore. Manufactures can apply whatever amplification they deem necessary, and top that with extra amplification via "ISO settings". One camera's ISO 100 may mean way more amplification than an other camera's ISO 100.

 

So stop placing importance on the ISO figure, it has no importance. 

 

For equivalent images (FOV and DOF wise), multiply the focal length and f-value by the crop factor.

If you also want a similar exposure time, use an equivalent ISO setting by multiplying the ISO setting value by the crop factor squared.