Different effective exposure from same settings on 2 DSRLs

**akptc** · 11-15-2018, 08:28 PM

This is probably a really silly question and I feel like should know the answer but am stomped. I've been playing my Pentax K3 and Nikon D700 side by side and I noticed that at identical settings - F4, 1/30, ISO 800, no exposure compensation, the image from the Nikon is rather well exposed while the same shot from the K3 is very under-exposed. The Pentax lens is clean.. Why is this? Can this be explained by the difference in sensor size?

.. I should add that on the Nikon I was using the 24-120 F/4 lens and on the Pentax I used the Sigma 85/1.4

Puzzled

AsahiGlass · 11-15-2018, 08:32 PM

Using the same lens? Also the base ISO is different , Nikon is 200 Pentax is 100...

Get an adapter and use the same lens to do your test.

Good luck...lol

**Bruce Clark** · 11-15-2018, 08:38 PM

Are you looking at RAW images or JPEGS processed by camera?

I suggest you post both images with EXIF data intact for comparison.

**clackers** · 11-15-2018, 08:46 PM

Originally posted by akptc

This is probably a really silly question and I feel like should know the answer but am stomped. I've been playing my Pentax K3 and Nikon D700 side by side and I noticed that at identical settings - F4, 1/30, ISO 800, no exposure compensation, the image from the Nikon is rather well exposed while the same shot from the K3 is very under-exposed. The Pentax lens is clean.. Why is this? Can this be explained by the difference in sensor size?

.. I should add that on the Nikon I was using the 24-120 F/4 lens and on the Pentax I used the Sigma 85/1.4

Puzzled

Show us, Akptc - put 'em both up!

**dms** · 11-15-2018, 09:06 PM

Lenses can be off in f-stop especially at fully open, and that Nikon is a very wide range to be a constant aperture.
I would use a few other lenses, preferably not zoom, and a few clicks (f stops) closed down.

stevebrot · 11-15-2018, 10:15 PM

Part of the difference might be in how Nikon defines ISO for their cameras vs. how it is done by Pentax. The full explanation is fairly involved, but it is enough to say that they are not fully equivalent. Pentax cameras use the so-called Standard Output Sensitivity (SOS) option of the ISO 12232:2006 standard. The SOS method is 18% gray traceable from rendered pixels and allows direct comparison with other brands using the SOS method. IIRC, Nikon uses the Recommended Exposure Index (REI) method, which allows the manufacturer to broadly tune the ISO for their products to reflect design and performance goals. Nikon has come under some criticism in the past for rating base ISO a stop or more over/under the actual sensor response. As a result, a particular set LV (ISO/shutter/aperture) on a Nikon body may or may not result in comparable pixel results to the same LV on another brand's body for in-camera images.

Originally posted by akptc

Different effective exposure from same settings on 2 DSRLs

Your choice of language is quite accurate for this case. The effect of exposure (EV, the amount of light) for a given ISO (sensitivity) is the important consideration and that often varies by brand. In actual practice, the metering systems tend to normalize user results between brands, as does RAW capture with camera-specific processing by available tools.

Steve

P.S. It should be noted that the ISO 12232:2006 standard never applies to RAW capture and always applies to in-camera JPEG or TIFF only.

sqrrl · 11-15-2018, 11:00 PM

Different manufacturers measure ISO differently (as steve mentioned). Or at least they have different standards as to what 'correctly exposed' means. Contrast ranges and other stuff like that.
also,
Aperture is basically just how wide the diaphragm is open, not how much light gets through (that is affected by coatings, the number of elements and so forth), that's why video cameras have T-stops (fairly sure T means transmission). Different lenses also transmit different spectrums of light to varying degrees.

Fogel70 · 11-15-2018, 11:46 PM

I would expect the aperture mechanism on a third party lens to be less accurate. So the aperture size might be a little too small on the Sigma lens when stopped down.

Do you see the same using a Pentax lens?

joergens.mi · 11-16-2018, 12:20 AM

You should check wether both cameras are metering the same exposure value. (have a look at the exif's). If yes, the manufacturers have different definitions of the "ISO" value. On the other hand every lens have a different transmission of the light, therefore in the video camera word they a using the T-Stop value. With a T-Stop two lenses with the same value put the same amount of light to the sensor

Wikipedia: A T-stop (for transmission stops, by convention written with capital letter T) is an f-number adjusted to account for light transmission efficiency (transmittance). A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N. A particular lens' T-stop, T, is given by dividing the f-number by the square root of the transmittance of that lens:

normhead · 11-16-2018, 08:52 AM

Originally posted by joergens.mi

You should check wether both cameras are metering the same exposure value. (have a look at the exif's). If yes, the manufacturers have different definitions of the "ISO" value. On the other hand every lens have a different transmission of the light, therefore in the video camera word they a using the T-Stop value. With a T-Stop two lenses with the same value put the same amount of light to the sensor

Wikipedia: A T-stop (for transmission stops, by convention written with capital letter T) is an f-number adjusted to account for light transmission efficiency (transmittance). A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N. A particular lens' T-stop, T, is given by dividing the f-number by the square root of the transmittance of that lens:

T-stop is pretty much irrelevant in AV mode, because the light meter is behind the lens. The exposure should be calculated from the actual light available. This has been true since the spotmatic in 1965, and it's why DSLR users usually pay no attention at all to t-stop. For cameras without through the lens metering it's more of an issue.

AsahiGlass · 11-16-2018, 11:52 AM

You need a constant. Different bodies, different lenses. So do the same test with the same lens. Get an adapter then do your test.

You can't do an a test with so many variables.

At least similar lenses, u have a prime vs a zoom?

**UncleVanya** · 11-16-2018, 12:00 PM

To be perfectly frank - this is why TTL metering is good. Using an external meter and setting the two cameras to the same exposure on different lenses with different actual ISO performance of the sensors you get different results - but I'd guess if you let each camera pick the best exposure you'd be fine with both.

joergens.mi · 11-16-2018, 12:26 PM

Originally posted by normhead

T-stop is pretty much irrelevant in AV mode, because the light meter is behind the lens. The exposure should be calculated from the actual light available. This has been true since the spotmatic in 1965, and it's why DSLR users usually pay no attention at all to t-stop. For cameras without through the lens metering it's more of an issue.

He used F4, 1/30, ISO 800, on both cameras, therefore the Transmission is important especially when used two really different optics.
A second thing is, that the ISO 800 of an Camera 1 must not be the same as an ISO 800 on another camera 2, because you can use one of the 5 different interpretations of the specifications (Film speed - Wikipedia wikipedia)

swanlefitte · 11-16-2018, 12:49 PM

The t stop of the sigma is 1.8 much larger than the zoom so in this case that is not an issue.

**UncleVanya** · 11-16-2018, 01:14 PM

T stop variation from F stop is generally not even a full stop from what I recall. Typical values are more like 1/3 of a stop at least for typical primes. My guess is the lion's share of the exposure delta is the way the two cameras rate ISO and/or accuracy in the aperture linkage ability to set the aperture correctly. One other factor could be the JPG engine - if the Nikon applies any corrections to the in camera produced JPG it would do so for the Nikon branded lens I assume. The Pentax would not be doing so for the Sigma 85 as it only corrects Pentax branded lenses. Last but not least - while I have not heard of a K-3 with problems with the aperture block - I would at least test to see that the Sigma is able to shoot wide open correctly before assuming the f/4 selected by the lens was actually not more stopped down.

11-15-2018, 08:28 PM	#1
akptc Site Supporter Join Date: Feb 2008 Location: USA Photos: Gallery Posts: 771	Different effective exposure from same settings on 2 DSRLs This is probably a really silly question and I feel like should know the answer but am stomped. I've been playing my Pentax K3 and Nikon D700 side by side and I noticed that at identical settings - F4, 1/30, ISO 800, no exposure compensation, the image from the Nikon is rather well exposed while the same shot from the K3 is very under-exposed. The Pentax lens is clean.. Why is this? Can this be explained by the difference in sensor size? .. I should add that on the Nikon I was using the 24-120 F/4 lens and on the Pentax I used the Sigma 85/1.4 Puzzled

11-15-2018, 10:15 PM	#6
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Part of the difference might be in how Nikon defines ISO for their cameras vs. how it is done by Pentax. The full explanation is fairly involved, but it is enough to say that they are not fully equivalent. Pentax cameras use the so-called Standard Output Sensitivity (SOS) option of the ISO 12232:2006 standard. The SOS method is 18% gray traceable from rendered pixels and allows direct comparison with other brands using the SOS method. IIRC, Nikon uses the Recommended Exposure Index (REI) method, which allows the manufacturer to broadly tune the ISO for their products to reflect design and performance goals. Nikon has come under some criticism in the past for rating base ISO a stop or more over/under the actual sensor response. As a result, a particular set LV (ISO/shutter/aperture) on a Nikon body may or may not result in comparable pixel results to the same LV on another brand's body for in-camera images. Originally posted by akptc Different effective exposure from same settings on 2 DSRLs Your choice of language is quite accurate for this case. The effect of exposure (EV, the amount of light) for a given ISO (sensitivity) is the important consideration and that often varies by brand. In actual practice, the metering systems tend to normalize user results between brands, as does RAW capture with camera-specific processing by available tools. Steve P.S. It should be noted that the ISO 12232:2006 standard never applies to RAW capture and always applies to in-camera JPEG or TIFF only. Last edited by stevebrot; 11-15-2018 at 10:44 PM.

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11-15-2018, 08:32 PM	#2
AsahiGlass Forum Member Join Date: Oct 2018 Location: Florida WPB Posts: 54	Using the same lens? Also the base ISO is different , Nikon is 200 Pentax is 100... Get an adapter and use the same lens to do your test. Good luck...lol

11-15-2018, 08:38 PM	#3
Bruce Clark Loyal Site Supporter Join Date: Mar 2010 Location: Ocean Grove, Victoria Photos: Gallery \| Albums Posts: 4,458	Are you looking at RAW images or JPEGS processed by camera? I suggest you post both images with EXIF data intact for comparison.

11-15-2018, 08:46 PM	#4
clackers Otis Memorial Pentaxian Join Date: Jul 2013 Location: Melbourne Photos: Albums Posts: 16,397	Originally posted by akptc This is probably a really silly question and I feel like should know the answer but am stomped. I've been playing my Pentax K3 and Nikon D700 side by side and I noticed that at identical settings - F4, 1/30, ISO 800, no exposure compensation, the image from the Nikon is rather well exposed while the same shot from the K3 is very under-exposed. The Pentax lens is clean.. Why is this? Can this be explained by the difference in sensor size? .. I should add that on the Nikon I was using the 24-120 F/4 lens and on the Pentax I used the Sigma 85/1.4 Puzzled Show us, Akptc - put 'em both up!

11-15-2018, 09:06 PM	#5
dms Site Supporter Join Date: Aug 2011 Location: New York, NY Photos: Gallery Posts: 2,192	Lenses can be off in f-stop especially at fully open, and that Nikon is a very wide range to be a constant aperture. I would use a few other lenses, preferably not zoom, and a few clicks (f stops) closed down.

11-15-2018, 11:00 PM	#7
sqrrl Veteran Member Join Date: Mar 2017 Location: Otago, New Zealand Posts: 422	Different manufacturers measure ISO differently (as steve mentioned). Or at least they have different standards as to what 'correctly exposed' means. Contrast ranges and other stuff like that. also, Aperture is basically just how wide the diaphragm is open, not how much light gets through (that is affected by coatings, the number of elements and so forth), that's why video cameras have T-stops (fairly sure T means transmission). Different lenses also transmit different spectrums of light to varying degrees.

11-15-2018, 11:46 PM	#8
Fogel70 Pentaxian Join Date: Sep 2009 Location: Stockholm, Sweden Photos: Albums Posts: 3,062	I would expect the aperture mechanism on a third party lens to be less accurate. So the aperture size might be a little too small on the Sigma lens when stopped down. Do you see the same using a Pentax lens?

11-16-2018, 12:20 AM	#9
joergens.mi Veteran Member Join Date: Jan 2012 Posts: 408	You should check wether both cameras are metering the same exposure value. (have a look at the exif's). If yes, the manufacturers have different definitions of the "ISO" value. On the other hand every lens have a different transmission of the light, therefore in the video camera word they a using the T-Stop value. With a T-Stop two lenses with the same value put the same amount of light to the sensor Wikipedia: A T-stop (for transmission stops, by convention written with capital letter T) is an f-number adjusted to account for light transmission efficiency (transmittance). A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N. A particular lens' T-stop, T, is given by dividing the f-number by the square root of the transmittance of that lens:

11-16-2018, 08:52 AM	#10
normhead Pentaxian Join Date: Jun 2007 Location: Near Algonquin Park Photos: Gallery \| Albums Posts: 40,451	Originally posted by joergens.mi You should check wether both cameras are metering the same exposure value. (have a look at the exif's). If yes, the manufacturers have different definitions of the "ISO" value. On the other hand every lens have a different transmission of the light, therefore in the video camera word they a using the T-Stop value. With a T-Stop two lenses with the same value put the same amount of light to the sensor Wikipedia: A T-stop (for transmission stops, by convention written with capital letter T) is an f-number adjusted to account for light transmission efficiency (transmittance). A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N. A particular lens' T-stop, T, is given by dividing the f-number by the square root of the transmittance of that lens: T-stop is pretty much irrelevant in AV mode, because the light meter is behind the lens. The exposure should be calculated from the actual light available. This has been true since the spotmatic in 1965, and it's why DSLR users usually pay no attention at all to t-stop. For cameras without through the lens metering it's more of an issue.

11-16-2018, 11:52 AM	#11
AsahiGlass Forum Member Join Date: Oct 2018 Location: Florida WPB Posts: 54	You need a constant. Different bodies, different lenses. So do the same test with the same lens. Get an adapter then do your test. You can't do an a test with so many variables. At least similar lenses, u have a prime vs a zoom?

11-16-2018, 12:00 PM	#12
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,455	To be perfectly frank - this is why TTL metering is good. Using an external meter and setting the two cameras to the same exposure on different lenses with different actual ISO performance of the sensors you get different results - but I'd guess if you let each camera pick the best exposure you'd be fine with both.

11-16-2018, 12:26 PM	#13
joergens.mi Veteran Member Join Date: Jan 2012 Posts: 408	Originally posted by normhead T-stop is pretty much irrelevant in AV mode, because the light meter is behind the lens. The exposure should be calculated from the actual light available. This has been true since the spotmatic in 1965, and it's why DSLR users usually pay no attention at all to t-stop. For cameras without through the lens metering it's more of an issue. He used F4, 1/30, ISO 800, on both cameras, therefore the Transmission is important especially when used two really different optics. A second thing is, that the ISO 800 of an Camera 1 must not be the same as an ISO 800 on another camera 2, because you can use one of the 5 different interpretations of the specifications (Film speed - Wikipedia wikipedia)

11-16-2018, 12:49 PM	#14
swanlefitte Pentaxian Join Date: May 2015 Location: Minneapolis Photos: Gallery Posts: 4,068	The t stop of the sigma is 1.8 much larger than the zoom so in this case that is not an issue.

11-16-2018, 01:14 PM	#15
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,455	T stop variation from F stop is generally not even a full stop from what I recall. Typical values are more like 1/3 of a stop at least for typical primes. My guess is the lion's share of the exposure delta is the way the two cameras rate ISO and/or accuracy in the aperture linkage ability to set the aperture correctly. One other factor could be the JPG engine - if the Nikon applies any corrections to the in camera produced JPG it would do so for the Nikon branded lens I assume. The Pentax would not be doing so for the Sigma 85 as it only corrects Pentax branded lenses. Last but not least - while I have not heard of a K-3 with problems with the aperture block - I would at least test to see that the Sigma is able to shoot wide open correctly before assuming the f/4 selected by the lens was actually not more stopped down.