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03-20-2012, 10:55 PM   #1
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Why do some lenses expose low?

I had noticed this before, but with a new lens I picked up (Pentax 24mm 2.8), it is quite pronounced - all shots expose about 1 stop lower.

Here is my setup: K-5, shooting RAW, center point exposure, center point focus, never auto WB.

I shoot on Manual (mostly with manual lenses, and expose using the green button). 3 lenses in particular, Pentax K 24 f2.8, Pentax M 50 f1.7, and Vivitar 70-210 f4 expose dark (with the 24 being a full stop darker than I would like). The 18-55, a FA 50 1.4, and a Sigma 70-300 don't expose low using the exact same exposing technique.

Now, I understand that some old lenses may not be as clear or light effective as newer ones and this symptom is hardly an issue (and easy to correct for), but I would like to understand the mechanics of it. Whatever the light efficiency of the lens is, wouldn't the exposure meter correct for it? I mean, if I put on an ND filter, the exposure will adjust, and under the same circumstances I would get a somewhat same brightness eventually with different shutter speeds.

Just trying to understand what is going on inside the camera's mind when exposing with these lenses.

Argun

03-21-2012, 01:18 AM   #2
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Yeah, I've been wondering about that too (can't help you here but I hope someone will enlighten us), especially about manual lenses, where the green button actually activates the diaphragm, there should be no error?
Another question in the same style: any idea why some lenses have a tendency to front- or back-focus when in fact the correct focus is determined by the camera body? Shouldn't the focus be independant from the lens and the error (if any) be the same with all?
Sorry, I'm not try to hijack your thread, just thinking that if we do get someone to answer one of the question they might know about the other
Just curious...
03-21-2012, 01:33 AM   #3
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its a mix of things, but it all points back to exposure technique.

note that you're using center focus. If you're going to expose for a scene using this method, make sure whatever is in the middle is as close to the middle tone as possible. It could be that whatever is in the middle is actually brighter than the rest of the scene

next, remember that lenses show vignette at larger apertures/wide open. so using center exposing, while the scene might be correctly exposed for the central portion, it may be underexposed for everything else because of the vignette.

also, remember that you're shooting pentax. from my experience, pentax bodies like to underexpose by half a stop to a full stop when shooting RAW. I'm assuming the reason for this is to take full advantage of the sensor. The digital sensor has a decent amount of shadow detail it can recover, however, blown highlights are very difficult if not impossible to pull back. I find that if i tell the meter to overexpose by half a stop or so, images straight out of camera are much more properly exposed
03-21-2012, 03:27 AM   #4
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Interesting subject! With my K-5 I normally use spot, or center-weighed, metering, and when using long lenses (say Sigma 150-500, or Pentax-K 400/5.6) I tend to have to overexpose shooting RAW, while on wider lenses I even at times need to underexpose 1/3, to 2/3 stops.

I hate washed out highlights, like what happens easily, when I shoot swans. With my NEX-5N, using the same 400 lens, the problems when shooting white objects is gone, while with the Nikon V1 it is somewhere inbetween (sadly the V1 can't use the Pentax lenses, but using Nikon lenses on the NEX or the Pentax is doable, using the right adapter)

03-21-2012, 03:41 AM   #5
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QuoteOriginally posted by adpo Quote
its a mix of things, but it all points back to exposure technique
Considering I use the same technique with all 6 lenses I mentioned, that should yield the same results, no?

I thought of vignetting, but seeing how the problem lenses are FF lenses, the crop sensor is mostly in the center of the FOV.
03-21-2012, 04:03 AM - 1 Like   #6
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You need to map the lens performance against aperature. I have found that metering with non A lenses is highly variable and changes from camera model to camera model.

See this thread as an example
https://www.pentaxforums.com/forums/general-technical-troubleshooting/179542-...pentax-k7.html

The K5 ism much better than most recent cameras but it still is not pfect for manual aperture lenses
03-21-2012, 04:57 AM   #7
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Apart from Lowell Goudge's (main) explanation above it should also be noted that aperture alone doesn't tell, how much light actually reaches the sensor. There are transmission lossess and it is no coincidence that lenses in the professional movie business are characterized by T(ransmission)-stops rather than by F(ocal ratio)-stops.

Transmission losses with old MF lenses may be somewhat brand specific due to glass and coating types used as well as design specific due to number of elemets used, losses due to scatter and internal reflexions. My own experience is that Pentax MF lenses tend to underexpose a bit while Tamron Adaptall-2 lenses tend to do the opposite. Also, colours differ due to the different coatings used and Pentax colours from that time seem to be more saturated.

Finally the (epoxy) bonding between glass elements may change somewhat over time. A well documented example is that of old Takumars that use very slightly radioactive (VERY slightly - no danger here!) thorium glass. Over many years that very sligth radiation will actually make the expoxy cement turn darker and more yellowish.
03-21-2012, 05:23 AM   #8
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QuoteOriginally posted by Stone G. Quote
Apart from Lowell Goudge's (main) explanation above it should also be noted that aperture alone doesn't tell, how much light actually reaches the sensor. There are transmission lossess and it is no coincidence that lenses in the professional movie business are characterized by T(ransmission)-stops rather than by F(ocal ratio)-stops.

Transmission losses with old MF lenses may be somewhat brand specific due to glass and coating types used as well as design specific due to number of elemets used, losses due to scatter and internal reflexions. My own experience is that Pentax MF lenses tend to underexpose a bit while Tamron Adaptall-2 lenses tend to do the opposite. Also, colours differ due to the different coatings used and Pentax colours from that time seem to be more saturated.

Finally the (epoxy) bonding between glass elements may change somewhat over time. A well documented example is that of old Takumars that use very slightly radioactive (VERY slightly - no danger here!) thorium glass. Over many years that very sligth radiation will actually make the expoxy cement turn darker and more yellowish.
while your explanation is relevent to why the camera esposure does not match an independent light meter, it is not relevent to TTL metering,

The issue at hand is that the TTL metering and the true exposure on the sensor do not match with a particular lens.

this means that for some reason, the light hitting the metering sensor appears to be higher intensity than the light hitting the image sensor. THis is the only explanation.

We have found that with many old lenses, the metering can be quite variable (i will not say eratic because we have through multiple members and different lens / camera combinations demonstrated the behavior is repeatable)

the only point I can really stress is that part of being a competent photographer is understanding how each lens works on the camera. That means a simple investment of perhaps 10 minutes per lens and body to map the exposure.

Note that this is not just for legacy lenses, but ALL lenses. I cannot stress this point enough. My Tamron 28-75/2.8 for example, even in A mode shows an exposure error, starting at no error wide open, with a gradual but linear drift upwards of about 1/2 to 1 stop by F32. As a conseqquence, I generally set EV comp to -1/2 stops to minimize the average error. My sigma 70-200/2.8 has perfect exposure on my K10 alone, but add a 1.4x TC and I need to set EV comp to -.7 and with a 2x TC I need -1.3 stops of EV comp. this is all because the TC does not correct the aperture, and the K10D metering needs to absolutely know the true aperture to correct for the non linearities in the metering.

part of your job is to understand how your kit works, especially when using legacy lenses, third party equipment etc... It is not pentax's job to make the camera work perfect for every permitation and combination of lens and attachment you can dream up. If they tried they would go bankrupt. It would be nice if things were better, but I also have to point out that pentax is far better than canikon already in this regard.

03-21-2012, 06:04 AM   #9
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QuoteOriginally posted by Lowell Goudge Quote
part of your job is to understand how your kit works, especially when using legacy lenses
Although I agree wholeheartedly that one should know their tools and their habits, I still would like to find out what the mechanics (or rather, optics) of this lens+aperture based difference in the metering.

Reading the article and examining the charts, I am wondering if incident angles could be the cause, and certain angles increase the difference of light received by the two mechanisms.
03-21-2012, 06:24 AM   #10
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your guess about incident angles is as good as any other. I have not mapped out a 24mm lens yet, but do have both M42 and K mount Fast lenses (F2 and F2.5)

This is on my to do list, but I have added 2 bodies and 20 lenses since I spent a lot of time doing this and really need to update my data.

Although it only takes about 10 minutes per lens, when you have 45 lenses and add a new body it does take some time,

I have somewhat gone away from my own advise, and only map one or two lenses, to see any relitive change which I then assume will track, for all lenses (perhaps this may not be 100%)
03-21-2012, 06:30 AM   #11
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QuoteOriginally posted by Lowell Goudge Quote
The issue at hand is that the TTL metering and the true exposure on the sensor do not match with a particular lens.
this means that for some reason, the light hitting the metering sensor appears to be higher intensity than the light hitting the image sensor. THis is the only explanation.
I understand - but couldn't it be then, that the wavelenght sensitivities/responses differ systematically from light meter sensor to image sensor? That could vary from camera model to camera model, but as a general observation my (old) SMC coated lenses tend to me much more saturated and a bit under exposed while the opposite holds for my Adaptall BBAR coated lenses. And this is, generally speaking, true with both my *ist DL, my K200D and my K-5.

Just a thought....
03-21-2012, 06:43 AM   #12
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QuoteOriginally posted by Stone G. Quote
I understand - but couldn't it be then, that the wavelenght sensitivities/responses differ systematically from light meter sensor to image sensor? That could vary from camera model to camera model, but as a general observation my (old) SMC coated lenses tend to me much more saturated and a bit under exposed while the opposite holds for my Adaptall BBAR coated lenses. And this is, generally speaking, true with both my *ist DL, my K200D and my K-5.

Just a thought....
It could be possible, but i am not sure this would give errors when you stop down a lens, to a different aperture, I think as atekant suggested that it is more to do with the angles of incident light, and also perhaps scatter, off the focusing screen, and what light hits the exposure sensor, because it was demonstrated somewhat irrefutibly that changing the focusing screen from the K10D to the same screen as the *istD made the K10D perform from a metering perspective with manual lenses, identically to an *istD.

The only explanation is the manner that light reflects off the focusing screen, and a fast lens, wide open will have a lot more light at different angles than a lens stopped down, where the small aperture ensures most light is nearly normal to the focusing screen. Wide angle lenses might also have more light at low angles hit the focusing screen. Edit note, considering all wide angle lenses are retro focus or inverted telephoto designs in order to achieve the distance necessary for the mirror, this design will to some extent also determine the worst case angles that light can hit the sensor, to those similar to longer lenses. As a result it may not have an impact, only time and testing will tell
It is something I could perhaps test, simply by taking my lenses out and performing the same test using the following lenses

Vivitar (kiron) 24/2, Vivitar (kiron) 28/2, SMC-M 35/2, and comparing the results to my K50/1.4 which I already know

Last edited by Lowell Goudge; 03-21-2012 at 06:51 AM.
03-24-2012, 06:20 PM   #13
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QuoteOriginally posted by Lowell Goudge Quote
The issue at hand is that the TTL metering and the true exposure on the sensor do not match with a particular lens.

this means that for some reason, the light hitting the metering sensor appears to be higher intensity than the light hitting the image sensor. THis is the only explanation.
The focus screen behaviour is the source of the mismatch between viewfinder and liveview metering. Metering off the focus screen is a kludge and I'm surprised that it works as well as it does in practice.

What is really needed is a mode where the camera would flip the mirror out of the way, meter using the sensor (stopping down with a manual lens if needed) and drop the mirror back down again, holding the metered reading - similar to the liveview phase detection focus sequence but the other way round
03-24-2012, 07:42 PM   #14
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That would leave the mirror constantly flopping like a fish, putting the light sensor anywhere else in the body would put it out of the main path of the light (bouncing off the mirror in this case) and it would meter like crap, so its either mirrorless or through the focusing screen, rather half assed compromise either way but what can you do. I specifically got my katzeye screen without optibrite because apparently its not just you that sees a brighter image with optibrite and it makes the underexposure even worse, though its negligible according to katzeye people. I think the darker than stock screen actually helped slightly correct the underexposure, plus it looks just like my old super program screen so its what I'm used to.
03-24-2012, 07:59 PM   #15
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QuoteOriginally posted by kh1234567890 Quote
What is really needed is a mode where the camera would flip the mirror out of the way, meter using the sensor (stopping down with a manual lens if needed) and drop the mirror back down again, holding the metered reading - similar to the liveview phase detection focus sequence but the other way round
Excellent idea!


Steve
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