Different shutter speeds??

geoffw · 07-15-2014, 12:42 PM

Hi all. Can anybody explain this scenario please. K5, sigma 10-20 & tamron 17-50. On A priority @ f5.6. Shutter speeds are different for each lens. Sigma@ 1/60th tamron @ 1/20th. On tripod & exactly the same scene within seconds of each other. Thanks in advance. Regards.
PS both @ 17mm.

Last edited by geoffw; 07-15-2014 at 12:58 PM.

Adam · 07-15-2014, 12:56 PM

Some lenses can fool the light meter a bit- or you just got a different result because of the difference in FOV.

geoffw · 07-15-2014, 01:00 PM

Sorry Adam. Just added both at 17mm. But I take your first point. Thanks.

link81 · 07-15-2014, 01:04 PM

are both photo's (at different speeds) exposed the same? Could it be as simple as the 10-20 hood allowing more side light to hit the lens then the 17-50 hood?

**savoche** · 07-15-2014, 01:13 PM

Originally posted by link81

are both photo's (at different speeds) exposed the same? Could it be as simple as the 10-20 hood allowing more side light to hit the lens then the 17-50 hood?

I wondered about that, too. Is the ISO value the same for both shots? If it is, three times slower shutter speed sounds a bit much just for stray light - unless there is a strong light source near by.

Also, what metering mode did you use? With spot metering it's almost impossible to meter the exact same spot twice. And even center weighted and average/matrix can be picky about exactly where in the frame light sources are. Even with the same framing you still have to deal with different characteristics of the distortion from each lens.

**Just1MoreDave** · 07-15-2014, 01:31 PM

Distortion, vignetting, resolution, coatings, etc. mean that the meter is seeing something different with each lens. A solid color, evenly lit subject would help equalize some of that. I think the most likely culprit is vignetting, since that would be most likely to have the largest effect. Is this the Sigma 10-20mm f4-5.6? If so, it's wide open or close to it.

There are almost too many other factors to list, large and small impacts. The aperture is "the same" but is it the same shape? (Depends on Sigma model.) The lenses won't have the same transmission characteristics, depends on coatings, elements and materials. How close are the mechanical tolerances of the aperture linkage?

Lighting is sometimes forgotten. Fluorescent lights are flickering enough to cause this effect.

Then there are human factors. I have a bad habit of kicking the tripod at some point while changing lenses.

EarlVonTapia · 07-15-2014, 01:42 PM

Originally posted by geoffw

Hi all. Can anybody explain this scenario please. K5, sigma 10-20 & tamron 17-50. On A priority @ f5.6. Shutter speeds are different for each lens. Sigma@ 1/60th tamron @ 1/20th. On tripod & exactly the same scene within seconds of each other. Thanks in advance. Regards.
PS both @ 17mm.

There is a thing related to the f-stop called the t-stop, which is basically how much actual light a lens lets through to the camera sensor*. This is affected by things such as the number of elements in the lens, the quality of the elements, etc. And this can have an effect on the shutter speed. The Sigma 10-20 and the Tamron 17-50 have different optical formulas, so it's no surprise that they probably have different t-stops. Here's a brief discussion on the topic. lens - What is T-number / T-stop? - Photography Stack Exchange

For added mind-blowing fun, try that same test first when looking through the view finder, and then through live view. I just discovered this with my own Tamron 17-50. It overexposes by a bit when shooting in live view. I suspect it has something to do with the amount of light the camera reads when the mirror is up, but I can't say for sure.

*I don't know how to properly describe this relationship, so hopefully someone who is well-versed in this sort of thing doesn't crucify me.

fgaudet · 07-15-2014, 02:20 PM

+1 for different light transmittance, even at the same focal length and aperture, the Sigma probably allows more light through. F number only represents the size of the opening (aperture), where T number represents the amount of light going through.

geoffw · 07-15-2014, 02:22 PM

Mmm. Interesting replies. The shot was just after sundown in my back garden. Matrix metering. Same camera on tripod but changing lenses, carefully. 4 shots taken alternating lenses each time but getting same results each time too. Having read replies, I reckon it's a case of learning each of the lens' characteristics. Well, the tamron anyway. Not had it long but noticed the photos taken seemed brighter than what I've been used to. BTW where do you get all your technical info from? Just goes to show how much I DON'T know about photography.

Imageman · 07-15-2014, 02:27 PM - **1 Like**

Aperture is not what you think, from reading your question.

An aperture is not a physical opening size its a relationship.

The relationship is the aperture physical size to the focal length.

Therefore the physical size of an aperture opening will be different if its on different lenses of different focal length, even though its the same f number.

In other words an aperture of f5.6 on a 50mm lens will be quite different in physical opening size to an aperture of f5.6 on a 200mm lens.

I have not considered what this means in light transmittance through the lens and how that compares to other lenses of quite different focal length but I really dont care as long as the exposure is controlled satisfactorily and a well exposed image results.

I will say this, one of your lenses is a sigma 10-20, and the other is a tamron 17-50.

Consider the lenses at the longer focal length, one lens is a 20mm the other is a 50mm, at the long end.

This is a factor of nearly 3 times the focal length, and the exposure difference is 3 times.

Either this is a coincidence or the determining factor is the relationship between the focal lengths of these lenses that explains the huge difference in shutter speed.

It doesn't matter that both lenses were set to 17mm, the different manufacturers used a relationship algorithm to determine what f5.6 would physically mean on each lens, and finished up with a different physical aperture size, and that affected all focal lengths

The big question is, were both images correctly exposed.

amoringello · 07-15-2014, 02:27 PM

Shooting indoors or outdoors? Any other lights nearby?
- If outdoors, daylight can change a lot even if you don't notice it.

Since you're on a tripod, are you possibly stepping away from the camera?
- light through the eye-piece can alter metering. Be sure the eye piece is covered if you stand up and away from the camera. Any variance in ambient light from behind the camera (where you're standing, sunlight, car headlights, shadows from people walking by) can affect the metering.

SpecialK · 07-15-2014, 06:16 PM

Why don't you post the samples with exif?

**Lowell Goudge** · 07-15-2014, 06:44 PM

I have a few questions. What ISO, is it set to auto, and one lens shooting at higher ISO.

Also note that although F stops are the ration of apparent opening to focal length, lenses can vary in accuracy of setting, and light transmission. So those people who ask about exposure are spot on. Same scene,focal length and aperture should give same histogram,have you checked?

I also note that tamron lenses don't always have accurate aperture. Mu 28-75/2.8 starts wide open with accurate exposure but as you stop down over the full aperture range exposure drifts up by more than a full stop (I.e. The lens over exposes by a stop.)

I check calibration of my lenses by shooting a block wall, uniformly lit, at fixed ISO, over the full aperture range and check the grey scale value for each stop

Na Horuk · 07-16-2014, 11:14 AM

-different optical characteristics (vignetting, distortion, coatings)
-different transmittance. Aperture is useful for photographers, but it doesn't say exactly how much light gets through. You want to look at T-stops for that. T-stops are also used by videographers, which is why cinema lenses have a stepless aperture (no clicks) and only markings for T numbers.Two f2.8 lenses can have completely different max T stop
-different scenario. Don't know when you were doing this test, but environmental effects can change the light quickly. It is most dramatic during sunsets/sunrise and when clouds are moving. Artificial lights, of course, can also come into play.

This assumes the camera settings remained constant. As in, no auto-ISO, no EV-compensation, nothing.

geoffw · 07-16-2014, 12:53 PM

I'm back😜. Right, let me run this past you and see if I've sussed it. After reading all the replies I got to thinking I'd hook out some old kit. Using both the lenses on camera on spot metering on a grey card plus a soligor spot meter plus a lunasix 3s on incident I'm getting all reading within 1/3 of a stop. So, it must be the scene I was looking at that gave me different readings, somehow, for each lens. I think that one day I'll go out using the lunasix and see what happens. In other words, jump back on the learning curve. I hope this makes sense and thank You all again for your time. Best regards. Geoff.

07-15-2014, 12:42 PM	#1
geoffw Senior Member Join Date: Feb 2013 Photos: Gallery Posts: 193	Different shutter speeds?? Hi all. Can anybody explain this scenario please. K5, sigma 10-20 & tamron 17-50. On A priority @ f5.6. Shutter speeds are different for each lens. Sigma@ 1/60th tamron @ 1/20th. On tripod & exactly the same scene within seconds of each other. Thanks in advance. Regards. PS both @ 17mm. Last edited by geoffw; 07-15-2014 at 12:58 PM.

07-15-2014, 12:56 PM	#2
Adam Administrator Site Webmaster Join Date: Sep 2006 Location: Arizona Photos: Gallery \| Albums Posts: 51,595	Some lenses can fool the light meter a bit- or you just got a different result because of the difference in FOV. Adam PentaxForums.com Webmaster (Site Usage Guide \| Site Help \| My Photography) PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating or purchasing one of our Pentax eBooks. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, KEH, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

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07-15-2014, 01:00 PM	#3
geoffw Senior Member Join Date: Feb 2013 Photos: Gallery Posts: 193 Original Poster	Sorry Adam. Just added both at 17mm. But I take your first point. Thanks.

07-15-2014, 01:04 PM	#4
link81 Veteran Member Join Date: Jan 2011 Location: 12822 Photos: Albums Posts: 459	are both photo's (at different speeds) exposed the same? Could it be as simple as the 10-20 hood allowing more side light to hit the lens then the 17-50 hood?

07-15-2014, 01:13 PM	#5
savoche Master of the obvious Join Date: Jun 2011 Location: Lowlands of Norway Posts: 18,311	Originally posted by link81 are both photo's (at different speeds) exposed the same? Could it be as simple as the 10-20 hood allowing more side light to hit the lens then the 17-50 hood? I wondered about that, too. Is the ISO value the same for both shots? If it is, three times slower shutter speed sounds a bit much just for stray light - unless there is a strong light source near by. Also, what metering mode did you use? With spot metering it's almost impossible to meter the exact same spot twice. And even center weighted and average/matrix can be picky about exactly where in the frame light sources are. Even with the same framing you still have to deal with different characteristics of the distortion from each lens.

07-15-2014, 01:31 PM	#6
Just1MoreDave Site Supporter Join Date: Feb 2007 Location: Aurora, CO Photos: Gallery \| Albums Posts: 6,340	Distortion, vignetting, resolution, coatings, etc. mean that the meter is seeing something different with each lens. A solid color, evenly lit subject would help equalize some of that. I think the most likely culprit is vignetting, since that would be most likely to have the largest effect. Is this the Sigma 10-20mm f4-5.6? If so, it's wide open or close to it. There are almost too many other factors to list, large and small impacts. The aperture is "the same" but is it the same shape? (Depends on Sigma model.) The lenses won't have the same transmission characteristics, depends on coatings, elements and materials. How close are the mechanical tolerances of the aperture linkage? Lighting is sometimes forgotten. Fluorescent lights are flickering enough to cause this effect. Then there are human factors. I have a bad habit of kicking the tripod at some point while changing lenses.

07-15-2014, 01:42 PM	#7
EarlVonTapia Veteran Member Join Date: Oct 2012 Location: Vancouver Photos: Gallery Posts: 1,207	Originally posted by geoffw Hi all. Can anybody explain this scenario please. K5, sigma 10-20 & tamron 17-50. On A priority @ f5.6. Shutter speeds are different for each lens. Sigma@ 1/60th tamron @ 1/20th. On tripod & exactly the same scene within seconds of each other. Thanks in advance. Regards. PS both @ 17mm. There is a thing related to the f-stop called the t-stop, which is basically how much actual light a lens lets through to the camera sensor. This is affected by things such as the number of elements in the lens, the quality of the elements, etc. And this can have an effect on the shutter speed. The Sigma 10-20 and the Tamron 17-50 have different optical formulas, so it's no surprise that they probably have different t-stops. Here's a brief discussion on the topic. lens - What is T-number / T-stop? - Photography Stack Exchange For added mind-blowing fun, try that same test first when looking through the view finder, and then through live view. I just discovered this with my own Tamron 17-50. It overexposes by a bit when shooting in live view. I suspect it has something to do with the amount of light the camera reads when the mirror is up, but I can't say for sure. I don't know how to properly describe this relationship, so hopefully someone who is well-versed in this sort of thing doesn't crucify me.

07-15-2014, 02:20 PM	#8
fgaudet Veteran Member Join Date: Oct 2013 Location: Ontario Posts: 726	+1 for different light transmittance, even at the same focal length and aperture, the Sigma probably allows more light through. F number only represents the size of the opening (aperture), where T number represents the amount of light going through.

07-15-2014, 02:22 PM	#9
geoffw Senior Member Join Date: Feb 2013 Photos: Gallery Posts: 193 Original Poster	Mmm. Interesting replies. The shot was just after sundown in my back garden. Matrix metering. Same camera on tripod but changing lenses, carefully. 4 shots taken alternating lenses each time but getting same results each time too. Having read replies, I reckon it's a case of learning each of the lens' characteristics. Well, the tamron anyway. Not had it long but noticed the photos taken seemed brighter than what I've been used to. BTW where do you get all your technical info from? Just goes to show how much I DON'T know about photography.

07-15-2014, 02:27 PM - 1 Like	#10
Imageman Veteran Member Join Date: Dec 2013 Photos: Albums Posts: 461	Aperture is not what you think, from reading your question. An aperture is not a physical opening size its a relationship. The relationship is the aperture physical size to the focal length. Therefore the physical size of an aperture opening will be different if its on different lenses of different focal length, even though its the same f number. In other words an aperture of f5.6 on a 50mm lens will be quite different in physical opening size to an aperture of f5.6 on a 200mm lens. I have not considered what this means in light transmittance through the lens and how that compares to other lenses of quite different focal length but I really dont care as long as the exposure is controlled satisfactorily and a well exposed image results. I will say this, one of your lenses is a sigma 10-20, and the other is a tamron 17-50. Consider the lenses at the longer focal length, one lens is a 20mm the other is a 50mm, at the long end. This is a factor of nearly 3 times the focal length, and the exposure difference is 3 times. Either this is a coincidence or the determining factor is the relationship between the focal lengths of these lenses that explains the huge difference in shutter speed. It doesn't matter that both lenses were set to 17mm, the different manufacturers used a relationship algorithm to determine what f5.6 would physically mean on each lens, and finished up with a different physical aperture size, and that affected all focal lengths The big question is, were both images correctly exposed.

07-15-2014, 02:27 PM	#11
amoringello Veteran Member Join Date: May 2008 Location: Virginia, USA Photos: Gallery Posts: 1,562	Shooting indoors or outdoors? Any other lights nearby? - If outdoors, daylight can change a lot even if you don't notice it. Since you're on a tripod, are you possibly stepping away from the camera? - light through the eye-piece can alter metering. Be sure the eye piece is covered if you stand up and away from the camera. Any variance in ambient light from behind the camera (where you're standing, sunlight, car headlights, shadows from people walking by) can affect the metering.

07-15-2014, 06:16 PM	#12
SpecialK Pentaxian Join Date: Dec 2006 Location: So California Photos: Gallery Posts: 16,481	Why don't you post the samples with exif?

07-15-2014, 06:44 PM	#13
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,890	I have a few questions. What ISO, is it set to auto, and one lens shooting at higher ISO. Also note that although F stops are the ration of apparent opening to focal length, lenses can vary in accuracy of setting, and light transmission. So those people who ask about exposure are spot on. Same scene,focal length and aperture should give same histogram,have you checked? I also note that tamron lenses don't always have accurate aperture. Mu 28-75/2.8 starts wide open with accurate exposure but as you stop down over the full aperture range exposure drifts up by more than a full stop (I.e. The lens over exposes by a stop.) I check calibration of my lenses by shooting a block wall, uniformly lit, at fixed ISO, over the full aperture range and check the grey scale value for each stop

07-16-2014, 11:14 AM	#14
Na Horuk Veteran Member Join Date: Feb 2012 Location: Slovenia, probably Photos: Gallery \| Albums Posts: 11,186	-different optical characteristics (vignetting, distortion, coatings) -different transmittance. Aperture is useful for photographers, but it doesn't say exactly how much light gets through. You want to look at T-stops for that. T-stops are also used by videographers, which is why cinema lenses have a stepless aperture (no clicks) and only markings for T numbers.Two f2.8 lenses can have completely different max T stop -different scenario. Don't know when you were doing this test, but environmental effects can change the light quickly. It is most dramatic during sunsets/sunrise and when clouds are moving. Artificial lights, of course, can also come into play. This assumes the camera settings remained constant. As in, no auto-ISO, no EV-compensation, nothing.

07-16-2014, 12:53 PM	#15
geoffw Senior Member Join Date: Feb 2013 Photos: Gallery Posts: 193 Original Poster	I'm back😜. Right, let me run this past you and see if I've sussed it. After reading all the replies I got to thinking I'd hook out some old kit. Using both the lenses on camera on spot metering on a grey card plus a soligor spot meter plus a lunasix 3s on incident I'm getting all reading within 1/3 of a stop. So, it must be the scene I was looking at that gave me different readings, somehow, for each lens. I think that one day I'll go out using the lunasix and see what happens. In other words, jump back on the learning curve. I hope this makes sense and thank You all again for your time. Best regards. Geoff.