[popo]

If we are talking about strict equivalence, as best we can, then you DO have to multiply the f number.

Effective focal length, aperture, and ISO need to be multiplied. Shutter speed has to remain unchanged. Reducing the aperture means it would get darker, but you offset that by increasing the ISO too. Noise would be about the same since you have a bigger sensor.

Of course, if you don't care about strict equivalence, you can ignore that. But it doesn't stop it from being the case.

[JJ_SO]

What kind of equivalence? These number acrobatics are just ridiculous - for what reason has the ISO be multiplied? Does the sensor gets less sensitive or more just because of the sensor size?

 

You're saying

 

a 50mm becomes a 75mm (not true)

f/1.8 becomes f/2.8

ISO 200 becomes 300

 

Left numbers FF, right numbers APS-C

 

That doesn't make any sense

 

If you go on with that you end up with 500mm, f/18 and 2000 ISO for a normal point and shoot - what kind of equivalence that should be, I just fail to understand.   :blink:

[popo]

If you have two cameras of different sensor size and stood in the same spot, and wanted to get the same photo, you need to do what I said. If you don't care about making it match, you can do whatever you like.

One step at a time, you multiply the crop factor to the focal length so you get the same field of view.
Bit if you then set the same f number, you will get different depth of field. So you need to multiply that by the crop factor too.
But a higher f number would be darker right? So we have shutter speed and ISO left. We can't use a different shutter otherwise we'd have a different amount of motion blur. So we set a higher ISO.
In this case, you have look at the sensor in area terms. The bigger sensor has a bigger light collection area, offsetting what would have been higher noise.

As for a compact camera being equivalent to high f numbers and ISO on a DSLR, that's exactly the case. Point and shoots generally have massive depth of field, and have to apply a lot of noise reduction to get a nice looking image out.

[popo]

From another forum:

First pictures with the lens http://lcap.tistory.com/entry/Sigma-ART-...18-Preview

And on full frame http://lcap.tistory.com/entry/Sigma-18-3...5D-Mark-II

Looks good but confirms I'm NOT buying this lens due to the backwards zoom ring.

[Brightcolours]

Brightcolours, a f/2.8 is slower than f/1.8 no matter which sensor size.

f2.8 is only slower than f1.8 on the SAME sensor. Or film. The sensor/film defines the speed. The f-value only describes the hole size. If you change the sensor or film, the speed will change. 

That was the reason I wrote "you can't just multiply the aperture by the crop factor" - only if it comes to DOF, but not to a gain of 1.5 f-stops faster shutter speeds, not to a gain of a relatively narrow DOF in wide-angle (for APS-C). To compare that shutter speed, it has to be a 28-55/1.8 for FF!

No, you are wrong there. if you set the cameras to an equivalent ISO value, the exposure times will also be the same. If you think of ISO still as "film sensitivity", you must think and read a bit more on the subject. ISO says nothing about sensitivity in the digital age.

And that sounds much less cheap than a 28-55/2.8 - each manufacturer can do that.

 

I admit, 1.5 f-stops are easily done by contemporary VR/OS devices, one can handhold up to 4 f-stops - but one doesn't get short shutter speeds by VR//OS.

Just change the ISO setting to an equivalent ISO setting, If you use ISO 200 on APS-C, use the equivalent ISO setting of 200 x 1.5 x 1.5 (square) = ISO 450. If you have a sensors with the same amount of sensels (pixels if you want), at ISO 200 on 1.5x crop and on FF with ISO 450 each sensel will register the same amount of light during the same exposure time.

It looks long and thin in the pictures, but given the filter size 72mm, it's a huge piece of gear.

[Brightcolours]

What kind of equivalence? These number acrobatics are just ridiculous - for what reason has the ISO be multiplied? Does the sensor gets less sensitive or more just because of the sensor size?

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. 

You're saying

 

a 50mm becomes a 75mm (not true)

No one says that 50 mm "becomes" 75mm. It is like this (for 1.5x crop compared to 135 format):

To get a similar field of view, you have to use an EQUIVALENT focal length (one that gives you the same image). So if you use 50mm on 1.5x crop format, you must use 75mm on 135 ("FF") format.

f/1.8 becomes f/2.8

Again the same: to get a similar DOF, you have to use an EQUIVALENT f-value, which will give the same aperture size. So if you use f1.8 on 1.5x crop, use f2.8 on FF for a similar DOF.

ISO 200 becomes 300

Wrong. The same applies again as above. To catch the same amount of light to form the image, and/or to take the image in a similar exposure duration, you will have to use an EQUIVALENT ISO setting. So if you use ISO 200 on 1.5x crop, you have to use ISO 450 on 135 format, for a similar amount of light forming the image and a similar exposure time.

Left numbers FF, right numbers APS-C

 

That doesn't make any sense

It does make sense in every way you look at it. But you have to not make thought process errors, like that f-values are linked to film sensitivity.

If you go on with that you end up with 500mm, f/18 and 2000 ISO for a normal point and shoot - what kind of equivalence that should be, I just fail to understand.   :blink:

I think you are confusing yourself here... "normal point and shoot" (I am guessing you mean compact digital cameras with a small sensor, here....) use lenses with super small focal lengths. Of course, if you take a compact digital ultra zoom model which reaches a 500mm FF EQUIVALENT, then yes, 500mm for FF for that 500mm equivalent setting on the compact digital. 

 

Lets take a "typical" compact digital as example, a Canon PowerShot S100. It has a 7.44 x 5.58mm sensor size. This S100 has a 5.2-26mm f2-5.9 lens.

And lets compare it to a "typical" 135 format FF DSLR, a Nikon D3s. It has a 36 x 23.9mm sensor size. Which lens would be equivalent on the D3s to the lens of the S100?

 

36 / 7.44 = ~4.8x crop. I take the width and not the diagonal, to get the same FOV when using the camera in horizontal orientation.

 

To get the same FOV, the D3s will need a 5.2x4.8= 25mm to 26x4.8= 124mm lens.

So, basically a Nikon 24-120mm lens.

To get the same DOF (same size aperture), we will need to use 2x4.8= f9.6 to 5.9x4.8= f28.3.

 

Of course, we can not get a lens with such small max. aperture on FF, so the lens will always have more capabilities than that of the compact digital camera (which of course is the reason both type of cameras exist). 

 

So, an equivalent lens to the S100's 5.2-26mm f2-5.9 on 135 format will be a 24-120mm f10-f32 lens, basically. One will have to settle for the Nikon 24-120mm f4 VR or its older sibling and stop it down accordingly.

 

To match the amount of light forming the image, and keep the same exposure time (both have nothing to do with the equivalence of the lens, we are talking about the equivalence of the sensor now), we have to set an equivalent ISO setting.

Suppose we use ISO 200 on the digital compact. We then have to use 200 x 4.8 x 4.8 = ~ ISO 4600 in the D3s.

This then will result in a similar exposure times, when we set the lenses at equivalent settings.

[vodanh1982]

What kind of equivalence? These number acrobatics are just ridiculous - for what reason has the ISO be multiplied? Does the sensor gets less sensitive or more just because of the sensor size?

 

You're saying

 

a 50mm becomes a 75mm (not true)

f/1.8 becomes f/2.8

ISO 200 becomes 300

 

Left numbers FF, right numbers APS-C

 

That doesn't make any sense

 

If you go on with that you end up with 500mm, f/18 and 2000 ISO for a normal point and shoot - what kind of equivalence that should be, I just fail to understand.   :blink:

Let's compare 50 f/1.8 vs 75 f/2.7

From http://en.wikipedia.org/wiki/F-number

 

D (crop)=f/N=(50/1.8) = 27.7

D (FF) = (75/2.7) = 27.7

 

So two lenses have the same "opening"

 

Because the FF is larger the light intensive is lower. At the same shutter speed and ISO the FF image is underexposed. To make 2 images at the same exposure, we need to increase ISO of FF. Now the FF has higher ISO than the crop.

The question: will the FF at higher ISO has more noise than the crop at lower ISO?

There are two factors that affect the noise

-The sensor technology.

-The total amount of light get to the sensor.

Assume two sensors are made with the same technology. Because of the same "opening", two sensors receive the same amount of light with the same shutter speed. In conclusion, two sensors has the same noise even at different ISO.

In reality, the result maybe a little bit off because the crop and the FF are not manufactured using the same technique, micro-lenses, spacing, light fall off sensor (non-rounded sensors), etc

 

Other factor that is not equivalent is the resolution. Lens is limited by their own resolution in term of LP/mm. Assume the same sharpness and even sharpness distribution for the whole area of the lens, the FF will have more resolution because it is bigger. The total resolution = lens resolution x picture height.

[Sylvain]

A possible game changer, go Sigma, go.

[Klaus]

A possible game changer, go Sigma, go.

Is it ? Wirh this focal length range ?


Fast standard zoom lenses are the most overrated lenses in general.

Not overly fast compared to primes, overpriced, heavy.

[JJ_SO]

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.


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?


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


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