[nandadevieast]

Hi,

Its said that the SLT's are 1/3 to 1/2 stop behind the normal SLR's, because less light reaches the sensor.

What exactly the "less light reaches the sensor" means, and what are the implications?

Does this mean that if i shot at 1/250, F8 and ISO 1600 on both the cameras, the SLT image will be 1/3 stops underexposed compared to the normal SLR?

Are the implications limited to high ISO noise only?

best regards,

anurag

[Klaus]

[quote name='nandadevieast' timestamp='1342900060' post='19525']

Hi,

Its said that the SLT's are 1/3 to 1/2 stop behind the normal SLR's, because less light reaches the sensor.

What exactly the "less light reaches the sensor" means, and what are the implications?

Does this mean that if i shot at 1/250, F8 and ISO 1600 on both the cameras, the SLT image will be 1/3 stops underexposed compared to the normal SLR?

Are the implications limited to high ISO noise only?

best regards,

anurag

[/quote]



yes, this is roughly correct.

However, there is no mirror slap which compensates a bit of the loss due to the lack of mirror-induced vibrations.

[Brightcolours]

It actually means that you get a little bit more noise at for instance ISO 1600. The ISO settings are adapted (and they should be adapted).



So, you still get the same exposure with the same ISO/exposure time/aperture setting. Just a bit more gain in the signal, which will lead to a little bit more noise compared to a DSLR with the same sensor.



ISO settings are not about how sensitive the sensor is, but at defined to equalize exposure times no matter how sensitive a sensor is, in standard JPEG settings with the by the manufacturer chosen tonal curve. Quite freely chosen.



So, to answer your question:

Does this mean that if i shot at 1/250, F8 and ISO 1600 on both the cameras, the SLT image will be 1/3 stops underexposed compared to the normal SLR?



No, it does not mean that the SLT image is underexposed. It means the ISO settings are adapted to the lesser light.



If you want to study the this ISO standard (ISO 12232:2006) yourself, you can buy the documentation here: http://www.iso.org/iso/catalogue_detail....mber=37777

[nandadevieast]

[quote name='Brightcolours' timestamp='1342904058' post='19527']

It actually means that you get a little bit more noise at for instance ISO 1600. The ISO settings are adapted (and they should be adapted).



So, you still get the same exposure with the same ISO/exposure time/aperture setting. Just a bit more gain in the signal, which will lead to a little bit more noise compared to a DSLR with the same sensor.



ISO settings are not about how sensitive the sensor is, but at defined to equalize exposure times no matter how sensitive a sensor is, in standard JPEG settings with the by the manufacturer chosen tonal curve. Quite freely chosen.



So, to answer your question:

Does this mean that if i shot at 1/250, F8 and ISO 1600 on both the cameras, the SLT image will be 1/3 stops underexposed compared to the normal SLR?



No, it does not mean that the SLT image is underexposed. It means the ISO settings are adapted to the lesser light.



If you want to study the this ISO standard (ISO 12232:2006) yourself, you can buy the documentation here: http://www.iso.org/iso/catalogue_detail....mber=37777

[/quote]



If the exposure/histogram is similar, then does that mean the ISO 1600 is actually ISO 2400 (1/2 stop) on a SLT??

Also, less light hitting the sensor should have implications across the ISO range, why only high ISOs?

[Brightcolours]

[quote name='nandadevieast' timestamp='1342936764' post='19528']

If the exposure/histogram is similar, then does that mean the ISO 1600 is actually ISO 2400 (1/2 stop) on a SLT??

Also, less light hitting the sensor should have implications across the ISO range, why only high ISOs?

[/quote]

No, ISO 1600 is ISO 1600, no matter how much amplification is applied.



Like I said, ISO with digital is NOT about sensor sensitivity. With film it was, but with digital it is about exposure time.

ISO settings are about equalizing exposure times more or less. How long it will take to reach clipping. And that in SRGB space (so TIFF or JPEG depending on what method the manufacturer chooses to "determine" ISO settings), so the chosen tonal curve can make big differences too.



Just think of a 6mp and a 24mm APS-C sensor. The 24mp sensor has pixels that are 4 (!!!!) times as small pixels. So, per pixel, the sensor gets less light over the same period of time. Yet, both will take about the same time to make a photo at ISO 100 (or 1600).

Why? Because ISO is not about sensitivity, but about exposure time.



Same with different sized sensors. When you take a 12mp 2x crop camera, like an Olympus E-30, and a 12mp FF camera, like a Nikon D700, both have about the same amount of photo diodes. The FF camera has a sensor that is 4x the surface area of the 4/3rds one. It will collect 4x the light in the same exposure period. The pixels themselves are 4x bigger for the FF camera too. They too collect 4x the light during the same exposure time.

Yes, when you set both cameras to ISO 200, the exposure times are similar. Why? Because ISO is not about sensitivity like it was with film. The 4/3rds sensor signal gets more gain to reach ISO settings. Same with the SLT.



And it has "implications" across the entire ISO range, not only high ISO settings. Also at ISO 100 less light reaches the sensor in the same exposure time on an SLT.

[nandadevieast]

[quote name='Brightcolours' timestamp='1342945975' post='19529']

No, ISO 1600 is ISO 1600, no matter how much amplification is applied.



Like I said, ISO with digital is NOT about sensor sensitivity. With film it was, but with digital it is about exposure time.

ISO settings are about equalizing exposure times more or less. How long it will take to reach clipping. And that in SRGB space (so TIFF or JPEG depending on what method the manufacturer chooses to "determine" ISO settings), so the chosen tonal curve can make big differences too.



Just think of a 6mp and a 24mm APS-C sensor. The 24mp sensor has pixels that are 4 (!!!!) times as small pixels. So, per pixel, the sensor gets less light over the same period of time. Yet, both will take about the same time to make a photo at ISO 100 (or 1600).

Why? Because ISO is not about sensitivity, but about exposure time.



Same with different sized sensors. When you take a 12mp 2x crop camera, like an Olympus E-30, and a 12mp FF camera, like a Nikon D700, both have about the same amount of photo diodes. The FF camera has a sensor that is 4x the surface area of the 4/3rds one. It will collect 4x the light in the same exposure period. The pixels themselves are 4x bigger for the FF camera too. They too collect 4x the light during the same exposure time.

Yes, when you set both cameras to ISO 200, the exposure times are similar. Why? Because ISO is not about sensitivity like it was with film. The 4/3rds sensor signal gets more gain to reach ISO settings. Same with the SLT.



And it has "implications" across the entire ISO range, not only high ISO settings. Also at ISO 100 less light reaches the sensor in the same exposure time on an SLT.

[/quote]



Hi,

Little confused here...

If you're saying ISO's are about managing exposure time...in that case the ISOs from different cameras should not be equal and same. For example, ISO 1600 from A57 is not the same as ISO 1600 from LX5, right?



I always thought that ISO 1600 is 4 stops from ISO 100, which should be true for all the cameras. Then if it is a fixed value, how can it be about exposure time??



Maybe i am unable to understand this <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='' />

[Brightcolours]

[quote name='nandadevieast' timestamp='1342958614' post='19531']

Hi,

Little confused here...

If you're saying ISO's are about managing exposure time...in that case the ISOs from different cameras should not be equal and same. For example, ISO 1600 from A57 is not the same as ISO 1600 from LX5, right?



I always thought that ISO 1600 is 4 stops from ISO 100, which should be true for all the cameras. Then if it is a fixed value, how can it be about exposure time??



Maybe i am unable to understand this <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='' />

[/quote]

Lets introduce an imaginary sensitivity rating called Meh.

Suppose the LX5 with its really small photo sensitive diodes has a sensitivity of 50 Meh.

And lets suppose the A57 with is quite a lot bigger photo sensitive elements has a sensitivity of 250 Meh.



The difference is made by the much bigger surface of the "pixels" from the A57,which simply can "catch" more photons during the same exposure time.



Now we want photographers to make photos with each camera. Suppose they have "comparable" lensing, with a similar field of view, and the same f-value (f4 for instance). With the A57 with a sensor of 250 Meh sensitivity, a "correct" exposure would for instance last 1/250th of a second.

When we make the same photo with the LX5, it would then take 1/50th of a second, just because the sensor is so much less sensitive due to its smaller photo sensitive elements.



As you can imagine, this would be a bit troublesome for photographers. With every camera, with the vast variety of sensors available, exposure times would be very hard to get a grasp on... All would need different exposure times with the same familiar lens settings (field of view, f-stop).



That is where "ISO" comes in. It is a standard to equalize all different sensitivities, and make exposure times predictable, over all the different cameras and sensors. Camera makers are free to amplify the signal to whatever factor they need, to reach a certain ISO setting.



For ISO 100, for instance A57 would need a 1.5x amplification factor (just a made up number) to get its 250 Meh sensor to reach ISO 100. The LX5 would then need a 7.5x amplification to reach ISO 100.



We as camera users do not see how much the signal is amplified, we just get easily handled ISO "standard" settings, a nice abstraction layer to keep things simple and usable.



Stop thinking of "ISO" as a sensitivity rating, it is not (with DIGITAL). It is an exposure index rating.

[nandadevieast]

[quote name='Brightcolours' timestamp='1342962438' post='19532']

Lets introduce an imaginary sensitivity rating called Meh.

Suppose the LX5 with its really small photo sensitive diodes has a sensitivity of 50 Meh.

And lets suppose the A57 with is quite a lot bigger photo sensitive elements has a sensitivity of 250 Meh.



The difference is made by the much bigger surface of the "pixels" from the A57,which simply can "catch" more photons during the same exposure time.



Now we want photographers to make photos with each camera. Suppose they have "comparable" lensing, with a similar field of view, and the same f-value (f4 for instance). With the A57 with a sensor of 250 Meh sensitivity, a "correct" exposure would for instance last 1/250th of a second.

When we make the same photo with the LX5, it would then take 1/50th of a second, just because the sensor is so much less sensitive due to its smaller photo sensitive elements.



As you can imagine, this would be a bit troublesome for photographers. With every camera, with the vast variety of sensors available, exposure times would be very hard to get a grasp on... All would need different exposure times with the same familiar lens settings (field of view, f-stop).



That is where "ISO" comes in. It is a standard to equalize all different sensitivities, and make exposure times predictable, over all the different cameras and sensors. Camera makers are free to amplify the signal to whatever factor they need, to reach a certain ISO setting.



For ISO 100, for instance A57 would need a 1.5x amplification factor (just a made up number) to get its 250 Meh sensor to reach ISO 100. The LX5 would then need a 7.5x amplification to reach ISO 100.



We as camera users do not see how much the signal is amplified, we just get easily handled ISO "standard" settings, a nice abstraction layer to keep things simple and usable.



Stop thinking of "ISO" as a sensitivity rating, it is not (with DIGITAL). It is an exposure index rating.

[/quote]



Ok, i see.

So coming back to original question, lets see how this pans out:

Sony A57: shot at F8, 1/250, ISO 1600

Nikon D5100: shot at F8, 1/250, ISO 1600

You're saying that the exposure will be same. Ok.

You're saying that A57 receives less light so if it has to maintain correct exposure, it will have to increase the sensor gain by 1/2 stop. So because it has amplified the sensor signal or whatever you call it, at 1600 ISO, it will have more noise by 1/2 stop, compared to D5100 which has a less amplified signal at that ISO.

Which means, for noise to be equal, we have to shoot at ISO 1200 on A57, but that will underexpose our picture by 1/2 stop.

Wonder that's what Klaus meant when he replied to my question....

BC, am i getting it right now?

By the way, to my eyes at least, the noise difference looks something like 1 stop. I see lots of noise in A57 files. Is that how it's supposed to be in SLT vis a vis DSLR? I have red something like 1/2 stop.

[Brightcolours]

[quote name='nandadevieast' timestamp='1342974311' post='19536']

Ok, i see.

So coming back to original question, lets see how this pans out:

Sony A57: shot at F8, 1/250, ISO 1600

Nikon D5100: shot at F8, 1/250, ISO 1600

You're saying that the exposure will be same. Ok.

You're saying that A57 receives less light so if it has to maintain correct exposure, it will have to increase the sensor gain by 1/2 stop. So because it has amplified the sensor signal or whatever you call it, at 1600 ISO, it will have more noise by 1/2 stop, compared to D5100 which has a less amplified signal at that ISO.

Which means, for noise to be equal, we have to shoot at ISO 1200 on A57, but that will underexpose our picture by 1/2 stop.

Wonder that's what Klaus meant when he replied to my question....

BC, am i getting it right now?

By the way, to my eyes at least, the noise difference looks something like 1 stop. I see lots of noise in A57 files. Is that how it's supposed to be in SLT vis a vis DSLR? I have red something like 1/2 stop.

[/quote]

One is a Nikon, the other a Sony. You can not just assume they will show the same noise just because their sensors both come from Sony. Best is to compare it with a NEX.



And the image will only be underexposed if for some reason you feel you have to keep the exposure times exactly the same. Which of course you do not have to want to.



Anyway. we do not shoot to get noise equal, so that is a bit odd a debate <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='' /> .



I do not exactly know what Klaus meant, but probably he meant that the SLT's are that half a stop behind (and was not talking about your underexposure idea). And you are getting it right, now, I think.

[nandadevieast]

BC, thanks a lot.