Can someone explain this lens phenomenon? Darker BG wide open.

[Perry Ge]

I've noticed on occasion when shooting portraits, that one can have the exact same exposure on two shots where the face looks the same exposure-wise, but where the background is darker when the lens is wide open.

I just tried an experiment where I shot a test subject (chair) wide open on my 85 1.8, and then stopped down to f/2.5, and halved my shutter speed, took the same shot, and the blurred background was indeed brighter in the f/2.5 shot than the f/1.8 shot. I don't think it's just a vignetting effect either.

Can anyone explain this? I remember back in the day, when schmoelzel posted his 85 1.8 vs 85 1.2 comparisons, that at f/1.8, the 85 1.2's backgrounds were way brighter than the 85 1.8 at the same exposure (I'll try find a link) too. [Edit: Found it: http://photography-on-the.net/forum/showthread.php?t=143497&highlight=85+1.8+vs+1.2]

I don't have any samples of my own at the moment but the sample images here of his daughter also display the phenomena (brighter BG stopped down than wide open):

http://www.the-digital-picture.com/Reviews/Canon-EF-200mm-f-2-L-IS-USM-Lens-Review.aspx

[TheHoff]

The lens reporting an aperture that it really can't hit? (ie, fudging the numbers)

That thread certainly showed that one of the 85s is not calibrated to the same f/stops as the other.

[Perry Ge]

But how come it seems that when the lens is stopped down, the background gets brighter, even with the same exposure? The 85 1.2 was stopped down to match the 85 1.8 in schoelzel's shots, so it was one lens stopped down vs one wide open, with the wide open one displaying the darker BG.

And again, in the 200 f/2 IS L review, it's only once stopped down that the background suddenly gets brighter - the exposure on the face seems to stay the same throughout. I don't think it's an isolated phenomena to a particular lens - I think you can replicate it with just about any lens.

[TheHoff]

Apertures are calibrated (I'm assuming) electronically by the lens. If at f/1.8 the lens is open to 100% of its capability, maybe they are fudging 1.8 and it is actually f/1.92. That would be enough to throw off the apparent exposure if you're comparing it to 1.8 on a lens that is actually giving you 1.8.

Now when you say "go down to 5.6" it closes down the appropriate amount and gives you a true 5.6... so your exposures match.

I'd have to see the 200 example to see again if it happens across all lenses but I wouldn't be surprised if most wide open numbers were fudged to make them seem faster. Wasn't Canon's famous 50/0.95 actually measured to be a 50/0.99 or similar?

[Perry Ge]

That makes a lot of sense Hoff, but wouldn't quite explain why it's the backgrounds in particular, and not the entire frame including the subject, that looks darker, right?

[chauncey]

Perry my friend, I'm going to be watching this thread.
I haven't been shooting long enough to have noticed this phenonomen and your links were interesting.

Got to have something to do with the more even distribution of light due to the narrower aperture. That's a stab in the dark.

[airfrogusmc]

I did notice the density on the skin looked to me to be darker on the ones with the darker b/gs and there is some tolerance in apertures from the factory. In the old film days Nikon had a half stop on both shutters and apertures. This tolerance was one reason you did actual density test on the negs when starting with the zone system.

I would imagine with digital there needs to be a tighter tolerance but there still has to be one. Could explain the slight inconsistency.

[apersson850]

If you shoot to get the same exposure, but with a larger aperture, then your background is more out of focus. When a part of the image is out of focus, the brighter spots grow to consume the darker areas. Thus the image may seem brighter, in spite of really being the same.

[Perry Ge]

If you shoot to get the same exposure, but with a larger aperture, then your background is more out of focus. When a part of the image is out of focus, the brighter spots grow to consume the darker areas. Thus the image may seem brighter, in spite of really being the same.
But that would entail that shooting wide open = brighter background. In these instances, it's clear that the opposite is happening: stopping down is giving brighter backgrounds than shooting wide open.

[apersson850]

Yes, you're right. That's what he said. I misread it to mean the other way around.

Then I have no reasonable explanation.

[figmented]

I'm gonna say that what happens is the background is blurred more and therefore the camera is trying to expose (you shooting av/tv/m?) everything correctly when the background blurs more more of the exposure is calculated from the subject so it doesnt over expose. (kinda like ettl flash)

u can go back and forth from a 1.8 image to a 2.8 image )same settings otherwise) and see that the background is def different and therefore will have a diff exposure.

edit: apersson850 is right, he just had it backwards.

[airfrogusmc]

I'm gonna say that what happens is the background is blurred more and therefore the camera is trying to expose (you shooting av/tv/m?) everything correctly when the background blurs more more of the exposure is calculated from the subject so it doesnt over expose. (kinda like ettl flash)

u can go back and forth from a 1.8 image to a 2.8 image )same settings otherwise) and see that the background is def different and therefore will have a diff exposure.

edit: apersson850 is right, he just had it backwards.

I would hope anyone doing a test would be shooting manual.

[Perry Ge]

That doesn't make sense, obviously everything is done in M. Why would having narrower DOF (bg more blurred) = darker?

Also, there isn't a noticeable increase in brightness as you continue to stop down, the noticeable increase seems to only happen when you compare a shot wide open to any stopped down aperture, and not as you continue to stop down.

[Quad]

In the link the first comparison looks like a change in total exposure to me the girl gets brighter as well so I would put that down to the lens not being a true f 2. She also tilts her head a bit differently so that makes things change as well.

The second comparison looks like a fair bit of vignetting going on as the background right by her head does not seem to change much in density.

[Perry Ge]

I wanna resurrect this thread because I'm still looking for an answer.

I've found this phenomenon on every single lens I've owned, including the SMC Takumar 55 1.8 - surely not every single lens on the planet has a falsely advertised max aperture, and this still doesn't explain the fact that only the brightness of the background changes, not of the subject.

Someone out there's gotta know what causes this! I'm so curious, no matter how technical the explanation might be.

[DStanic]

Would the longer shutter speed of stopping down simply allow more ambient light to reach the background?

[Perry Ge]

But again you're in M and you're stopping down at the same rate as you slow the shutter so your exposure should be exactly the same for both background and subject.

[walternewton]

I'd personally suspect it's vignetting...I also think these things are deceptively difficult to judge by eye alone; are you comparing actual pixel values across the frame for various test exposures at constant lighting/camera position/EV or just "eyeballing" it?

Photozone test results for the 85/1.8 shows 0.7 stop vignetting on a 1.6X crop camera for example - pretty significant, performance would likely be worse on a full frame camera.

[Perry Ge]

It's clearly not vignetting because it's affecting the background across the image, and not just the corners.

Having said that, I certainly am just 'eyeballing', but it's a pretty clear and distinct phenomenon that is easy to reproduce with any lens.

[walternewton]

Well, I'd suggest taking actual measurements on photos taken under controlled conditions to confirm your perceptions - just a simple object against a plain white background, perhaps before and after running through the vignetting correction built into DPP if its performance is of interest.

[eddarr]

I wonder if it more of an optical illusion than reality. Because the background is in better focus the light gets through "cleaner" which gives the appearance of more light or more pinpoint light. When wide open the light appears "blurred" because of lack of focus which diffuses the light in the image.

[Perry Ge]

I thought that too, but why does it change so dramatically when you go from wide open to a little stopped down, but not after that? That's what I can't figure out.

[S.Horton]

Post the pictures, perhaps?

In the meantime, I'll go play someday to see what I get........ Never thought about this.

[DrPablo]

It may be a matter of contrast. Contrast tends to be lower when lens is wide open.

If you're using flash it could be a different issue, as the flash calibrates its output based on aperture, but it tends to be the other way around -- when using flash a narrow aperture will result in a darker background.

[eddarr]

One other thing to consider is that 1 stop of aperture is not as accurate as 1 stop of shutter speed. Aperture numbers are rounded to a point that makes them easier to remember, although it doesn't help me. Also even identical lenses are known to not close the blades to the exact same amount when both are set to the same aperture.

In english that means that one less stop of aperture (from f/2.8 to f/4) is not as accurate as one more stop of shutter speed (from 1/50 to 1/100) to get the same exposure.

Either that or the lens manufactures are lying to us. An f/1.8 will not actually get to 1.8 but they sell more than if they call them what they really are which is a f/1.89. That fractional difference in aperture from wide open to 1 stop does affects the shutter speed so that the exposure is not actually the same. However your eye fixes the image where the focus is sharp but not where the background is blurred.

[DrPablo]

One other thing to consider is that 1 stop of aperture is not as accurate as 1 stop of shutter speed.Alternatively, the opposite may be true. Shutter speed is determined by a complicated mechanical mechanism, whereas aperture is simply a diameter. 100% manual shutters, as we use in large format and some medium format lenses, are FAR more likely to need their shutter speed recalibrated by the manufacturer.

The thing is, if there is a discrepancy or an inaccuracy of either one of those variables, then all you need to do is stick the camera in Av or Tv mode and take shots of a uniform target at different combinations. If you're holding the exposure value constant, then the images should look the same if there is true reciprocity between shutter speed and aperture.

[eddarr]

That crossed my mind also. The better way to say it may be that 1 stop of aperture may not be as accurate as 1 stop of shutter speed. Being that they are both mechanical actions it would be fairly safe to assume that both could be about equally inaccurate.

It may simply be that the inaccuracies themselves are the cause of the phenomenon. Those variables are within acceptable tolerance levels for consumer and professional level cameras.

[cdifoto]

Looks like she's in open shade. Perhaps the skies cleared a touch after the initial shot. Open shade wouldn't be noticeably affected as much as the exposed area behind her.

[Perry Ge]

Looks like she's in open shade. Perhaps the skies cleared a touch after the initial shot. Open shade wouldn't be noticeably affected.
But Don, this is easily reproducible, it's not isolated to the two examples I posted - try it for yourself, it's kinda trippy.

[cdifoto]

I can't. It's dark here & by the time it's daylight, I'll have forgotten. :D :p

[DDCSD]

Solar wind. (http://en.wikipedia.org/wiki/Solar_wind)

[cdifoto]

I had solar wind once. I'll never go to that hot dog vendor again.

[DDCSD]

Well, in order to make Perry sound like a loon, I went ahead and tested this. I am now going to bed. Hopefully I wake up.......

All images taken at ISO 1600, 1/100s, and f/2.8 on my 1DMkIIN in M and Auto WB. Handheld, didn't bother to get the FL's the exact same. Close enough for me.

70-200 f/2.8 93mm
http://derekcecil.smugmug.com/photos/389324646_pmVLj-M.jpg
85 f/1.8 at f/2.8
http://derekcecil.smugmug.com/photos/389325687_ofRQ9-M.jpg
85 f/1.8 at f/2.8
http://derekcecil.smugmug.com/photos/389321796_2TVn6-M.jpg
85 f/1.8 at f/2.8
http://derekcecil.smugmug.com/photos/389322811_CuKZR-M.jpg
And finally put the 70-200 f/2.8 back on to verify it wasn't a fluke. 78mm
http://derekcecil.smugmug.com/photos/389323742_37Jgn-M.jpg

The BG is a bit different in the last one than the first one, but it was a 15mm difference and I had moved a tiny bit.

Not scientific, but something to think about. Worth looking into more.

[Perry Ge]

But you shot at f/2.8 the whole time. The idea here is that when you go from wide open to a little stopped down, this phenomenon occurs.

[milorad]

This thread is useless without pics.

[Perry Ge]

There are some links in the first post to a few. I'll try get some more up tomorrow.

[milorad]

poppycock... the pics in the first thread were shot with different lenses, evaluative metering, and all sorts of other BS that impact upon the shots.

The subject wasn't static, and so even a slight angle adjustment can throw off the exposure by 1/3rd stop, which is going to have more impact on the background, than it will on the foreground.

post the pictures, exif intact, then we see if it's worth getting up out of this comfy seat and cracking open the tripod :)

[DDCSD]

But you shot at f/2.8 the whole time. The idea here is that when you go from wide open to a little stopped down, this phenomenon occurs.

I though the point was that one lens stopped down will have a different looking BG than a lens that is wide open at the same aperture. That was what was in the link that you posted.......

Perhaps you should post an example of what you are talking about then. Of course the BG will look different with the same lens at a different aperture, you're letting in a different amount of light.......

[Perry Ge]

I though the point was that one lens stopped down will have a different looking BG than a lens that is wide open at the same aperture. That was what was in the link that you posted.......

Perhaps you should post an example of what you are talking about then. Of course the BG will look different with the same lens at a different aperture, you're letting in a different amount of light.......
The second link shows the same lens stopped down a little. The idea is that you keep the exposure the same (i.e. let in the same amount of light), so if you stop down from f/1.4 to f/1.6 obviously you're slowing your shutter speed by 1/3 of a stop too.

[DDCSD]

The second link shows the same lens stopped down a little. The idea is that you keep the exposure the same (i.e. let in the same amount of light), so if you stop down from f/1.4 to f/1.6 obviously you're slowing your shutter speed by 1/3 of a stop too.

Sorry, I meant that it changes the way the light gets to the sensor, not that it changes the amount of light coming in (assuming that the SS is adjust, as you said.).

[Perry Ge]

Could you explain why that affects the brightness of the background, when the exposure is meant to be the same?

Maybe I'm just insane.

[tonylong]

Perry,

I just ran your test -- I pretended that my 85L was an 85 1.8 -- and shot at f/1.8, 1/125, then at f/2.5, 1/60. Visually, aside from the background blur smearing some highlights and shadows, they appear very much alike, although in the histogram the f/2.5 shot is just a touch brighter and the colors are spread out a bit more (presumably due to the smearing).

I'd say that different lenses/bodies may have somewhat different variations that would account for slight differences between "equivalent" settings. Plus, I noticed on one of the kid shots you referred to when you started this thread, the background view had shifted, presumably to keep track of a moving kid, and was shifted toward the darker part of the background, showing just a bit of the lighter background. This kind of variation needs to be eleminated for a serious test.

[Perry Ge]

Thanks tony. What happens when you shoot with the 85L first at f/1.2, then at f/1.4 or f/1.6 though? My main interest was when you went from wide open to a little bit stopped down, regardless of the lens.

[tonylong]

Thanks tony. What happens when you shoot with the 85L first at f/1.2, then at f/1.4 or f/1.6 though? My main interest was when you went from wide open to a little bit stopped down, regardless of the lens.

OK, here's the results:

f/1.2 @ 1/250
1.4 @ 1/200
1.6 @ 1/160
1.8 @ 1/125
2.5 @ 1.60

The shots became progressively lighter as the aperture narrowed. The difference between f/1.2 and f/2.5 was about 1/3 stop EV. The most marked difference seemed to be between f/1.2 and f/1.4, but you can see more or less subtle differences between each step. And, BTW, this affected the foreground as well as the background.

Interesting enough, there was less difference between f/1.8 and f/2.5 (a whole stop) than there was in the 1/3 stop between f/1.2 and f/1.4.

Edit: It was nice to see how sharp the 85 is with the tiny depth of field at f/1.2! I normally use this lens without a tripod, but this series with a tripod definitely turns this lens into a show-off!

[Perry Ge]

I don't see any pictures, Tony, but now you know what I'm talking about eh?

[tonylong]

I don't see any pictures, Tony, but now you know what I'm talking about eh?

Right, I haven't done anything to post this series, but if you want me to I'll just have to mess with them...

But, as far as your original post you were concerned about the background exposure seeming to shift even if your subject was still comparable, is that right? Or are you asking about what I'm seeing, an overall shift of the exposure?

I don't understand much about lens optics, but I guess a 1/3 EV shift over two stops isn't something I'd fret about.

[Perry Ge]

Oh yeah it's not a problem - I've just seen this happen a lot and I'm really curious about why. I want a meaty, geeky, technical explanation with as much number crunching and jargon as possible :P. I hate having a photographic phenomenon that I don't understand.

[tonylong]

Oh yeah it's not a problem - I've just seen this happen a lot and I'm really curious about why. I want a meaty, geeky, technical explanation with as much number crunching and jargon as possible :P. I hate having a photographic phenomenon that I don't understand.

Heh! At this point I just like blowing up the shots and seeing how the threads of my jacket behave between f/1.2 and f/2.5:)!

Maybe we should get LightRules to ignore his famous thread long enough to expound on this!

[DDCSD]

I finally had a close look at your second link. I see what you mean on those examples. If you look, it is like the dark areas overtake the lighter areas in the BG when he shot wide open and the darker areas "shrink" as the background comes more into focus.

I certainly can't explain it though.