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What's the deal with aperture stopping down during AF?
  
 
AGeoJO
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p.1 #1 · p.1 #1 · What's the deal with aperture stopping down during AF?


Coming from the DSLR world, I always assumed that each lens would only gets stopped down when the actual exposure is being made. Until about a year ago I didn't realize that the aperture of some Sony FE lenses does get stopped down to the shooting aperture even before hand and while the lens is trying to AF. Well, depending on the lighting conditions and aperture used, the AF may struggle. I noticed that a few times but I didn't think much about it until yesterday. Granted, this situation doesn't come up that often but the only advantage I could see from that design is for rapid shooting, like in sport or wildlife photography. Let's face it, the way the camera system is now, those are the two areas of photography that this mirrorless system is not known for .

Case in point, I was trying to get a sunstar of a fairly low setting sun in my composition by stopping down to f/16 yesterday. Yes, the lighting was not super bright, hence the low setting sun but plenty bright. I used my Batis 25mm for that and it struggled to lock on focus and apparently it gave up after a while. So, I opened up the aperture to f/8 and it locked focus without any issues and then I stopped down the camera. I did take a single shot when after the AF gave up and the image was blurry. And that at f/16 using a 25mm lens - what the heck? Yes, you can work around this "issue" but it shouldn't be an issue to begin with if the system just follows or copies that of the DSLR world. Stop down the lens when the actual exposure is being made but open it up prior to that. Unless this system is designed for a super fast camera system of the future. Yes, a futuristic super fast mirrorless camera without aperture stopped down for a 25 fps maybe.... .

Any thoughts on this, folks?

Edited on Mar 21, 2017 at 12:52 PM · View previous versions



Mar 20, 2017 at 02:42 PM
mogul
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p.1 #2 · p.1 #2 · What's the deal with aperture stopping down during AF?


AGeoJO wrote:
Coming from the DSLR world, I always assumed that each lens would only gets stopped down when the actual exposure is being made. Until about a year ago I didn't realize that the aperture of some Sony FE lenses does get stopped down to the shooting aperture even before hand and while the lens is trying to AF. Well, depending on the lighting conditions and aperture used, the AF may struggle. I noticed that a few times but I didn't think much about it until yesterday. Granted, this situation doesn't come up that often but the only advantage I could
...Show more

It really isn't cut and dried, I guess some Sony E mount lenses do open up and are usable in dark situations. Long thread on DPreview.



Mar 20, 2017 at 02:51 PM
kdrk888
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p.1 #3 · p.1 #3 · What's the deal with aperture stopping down during AF?


Would be interesting to know which FE lenses behave in that manner. I remember Guy saying he is getting the FE 85m f1.8 because the 85mm f1.4 G Master steps down.

Douglas



Mar 20, 2017 at 02:53 PM
AGeoJO
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p.1 #4 · p.1 #4 · What's the deal with aperture stopping down during AF?


mogul wrote:
It really isn't cut and dried, I guess some Sony E mount lenses do open up and are usable in dark situations. Long thread on DPreview.



Yes, like I mentioned, SOME FE lenses. The impression that I get is the command is coming from the camera and depending on the way the chip of the lens is programmed, it may stop down the lens or not during AF. Sorry, I don't go to the forum of DPreview .



Mar 20, 2017 at 02:57 PM
snapsy
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p.1 #5 · p.1 #5 · What's the deal with aperture stopping down during AF?


The only reason I can think of for Sony doing this is to correct focus shift from spherical aberration. That would explain why they only do it on certain lenses, ie specific lenses with spherical aberration issues. This theory could be verified by checking if the lenses they do it on actually have focus shift - manually focus wide-open then shoot at a smaller aperture to see if the focus plane shifts.


Mar 20, 2017 at 03:04 PM
AGeoJO
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p.1 #6 · p.1 #6 · What's the deal with aperture stopping down during AF?


snapsy wrote:
The only reason I can think of for Sony doing this is to correct focus shift from spherical aberration. That would explain why they only do it on certain lenses, ie specific lenses with spherical aberration issues. This theory could be verified by checking if the lenses they do it on actually have focus shift - manually focus wide-open then shoot at a smaller aperture to see if the focus plane shifts.


Adam, that explains it . But that should not apply to a 25mm lens like the Batis, right? The GM 85mm, a short tele with a large aperture, yes, I understand.



Mar 20, 2017 at 03:11 PM
Parariss
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p.1 #7 · p.1 #7 · What's the deal with aperture stopping down during AF?


A reason Sony lenses stop down is focus shift:

https://photographylife.com/what-is-focus-shift/

The main problem is that when you need to stop down a lens in order to focus accurately you consequently have less light available in order to focus rapidly.

Sony seems to favor accurate focusing, which makes sense - it doesn't matter how fast focusing is if the result is out of focus. So, lenses which exhibit focus shift necessarily have stop down focusing. I haven't seen a master list of which lenses stop down, or whether all Sony lenses do (but I'd like to).

Additionally, you can't have live view without stopping down, by definition. Perhaps there are situations/lenses where you can gain a better user focusing experience by turning off live view.



Mar 20, 2017 at 03:20 PM
GMPhotography
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p.1 #8 · p.1 #8 · What's the deal with aperture stopping down during AF?


Douglas that is correct . My issue with the GM 85 in the studio came full force when I was shooting a catalog of clothing. The GM is a touch slow to begin with but in low light it was not locking down focus because of this stop down. I was using F8. I saw a article about the 85 1.8 that does not act this way. I sent the article to Fred as well and we talked about it in emails and I decided after bouncing this off him to get the 1.8. As much as I love the rendering of the GM I just can't have that behavior in the studio. I'm think the 50 1.4 does it as well and a few others. For me this is a important lens in the studio more than anything else so I need this speed or I need a 135 that does not do this. The other way to get around this is flood the studio with ambient light so it can see better but that means I'm back to studio lights instead of my Godox battery lighting which I really like. I did buy 2 studio mono lights with 150 watt modeling bulbs but I need to bump them up to 250. That would have helped the cause but again the GM on its own is a little slow. So I was fighting two battles at once. This should help with the 1.8 focusing at least close to wide open. It may go to F2 to focus but I need to check that for sure.

kdrk888 wrote:
Would be interesting to know which FE lenses behave in that manner. I remember Guy saying he is getting the FE 85m f1.8 because the 85mm f1.4 G Master steps down.

Douglas




Mar 20, 2017 at 03:33 PM
artur5
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p.1 #9 · p.1 #9 · What's the deal with aperture stopping down during AF?


We need a option on the menu for choosing either stop down or full open focus, no matter the lens used, and that option should be assignable to one the custom buttons of the camera. This way we could have the best of both worlds.
Maybe if enough people keeps crying and moaning in Internet forums Sony will take the trouble of adding this feature to future firmware updates, like they did with the uncompressed raws ( although they ignored, so far, the demand for lossless compressed raws )



Mar 20, 2017 at 04:01 PM
snapsy
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p.1 #10 · p.1 #10 · What's the deal with aperture stopping down during AF?


AGeoJO wrote:
Adam, that explains it . But that should not apply to a 25mm lens like the Batis, right? The GM 85mm, a short tele with a large aperture, yes, I understand.


Longer focal lengths and large apertures make focus shift more obvious (thin DOF) but I think the prevalence of hr phenomena is based on the optical design.



Mar 20, 2017 at 04:04 PM
 

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tntcorp
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p.1 #11 · p.1 #11 · What's the deal with aperture stopping down during AF?


artur5 wrote:
We need a option on the menu for choosing either stop down or full open focus, no matter the lens used, and that option should be assignable to one the custom buttons of the camera. This way we could have the best of both worlds.
Maybe if enough people keeps crying and moaning in Internet forums Sony will take the trouble of adding this feature to future firmware updates, like they did with the uncompressed raws ( although they ignored, so far, the demand for lossless compressed raws )


not sure how having this option will resolve the issue of focusing-shift exhibited on certain lenses!? and, should the option is made available, user will complain of af inaccuracy with their lenses. not a win-win situation from sony's perspective.




Mar 20, 2017 at 04:17 PM
snapsy
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p.1 #12 · p.1 #12 · What's the deal with aperture stopping down during AF?


tntcorp wrote:
not sure how having this option will resolve the issue of focusing-shift exhibited on certain lenses!? and, should the option is made available, user will complain of af inaccuracy with their lenses. not a win-win situation from sony's perspective.


DSLRs are presumed to account for focus shift by maintaining a database of lens models and their behaviors and adjusting the AF accordingly based on the shooting parameters. Sony could do the same. An alternate strategy could be to only stop down by one or two stops rather than stopping down fully since most focus shift is reasonably resolved in the first few stops.



Mar 20, 2017 at 04:40 PM
Parariss
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p.1 #13 · p.1 #13 · What's the deal with aperture stopping down during AF?



snapsy wrote:
DSLRs are presumed to account for focus shift by maintaining a database of lens models and their behaviors and adjusting the AF accordingly based on the shooting parameters. Sony could do the same. An alternate strategy could be to only stop down by one or two stops rather than stopping down fully since most focus shift is reasonably resolved in the first few stops.


Cool idea. Although, that doesn't account for lens copy variation. Fortunately, Sony has none of that, whatsoever...



Mar 20, 2017 at 05:35 PM
ecarlino
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p.1 #14 · p.1 #14 · What's the deal with aperture stopping down during AF?


AGeoJO wrote:
I was trying to get a sunstar of a fairly low setting sun in my composition by stopping down to f/16 yesterday.
.....
So, I opened up the aperture to f/8 and it locked focus without any issues and then I stopped down the camera.
.....
Any thoughts on this, folks?



Joshua, I'm not sure if this is what caused you some troubles, but I only mention it because you referenced starting out at f/16 and then moving to f/8 ---

on the a7r2, the OSPDAF turns off past f/8 and the camera only relies on contrast-detect AF.
at f/8 and wider, the AF acquires focus using PDAF and then perfects focus with the contrast-detect.



Mar 20, 2017 at 06:22 PM
AGeoJO
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p.1 #15 · p.1 #15 · What's the deal with aperture stopping down during AF?


ecarlino wrote:
Joshua, I'm not sure if this is what caused you some troubles, but I only mention it because you referenced starting out at f/16 and then moving to f/8 ---

on the a7r2, the OSPDAF turns off past f/8 and the camera only relies on contrast-detect AF.
at f/8 and wider, the AF acquires focus using PDAF and then perfects focus with the contrast-detect.


Eric, I deliberately chose f/16 first, again, to get the sunstar. But when the camera/lens had a fit with AF, I just opened it up and I happened to choose f/8 first. So, the f/8 was an arbitrary selection without taking into account on what AF mode the camera was selecting. So, when I pushed the AF button at f/8, it zipped into focus immediately. I just stopped down the aperture back to f/16 and I took the picture.

Yes, it is probable that the camera switched from PDAF to C-based AF starting at f/8 and for sure at f/16, and that it struggled or actually failed to acquire focus using contrast-based AF. The point I was trying to make is that the camera/lens does or did get stopped down in my case yesterday to f/16 in the focusing acquisition phase.



Mar 20, 2017 at 06:41 PM
DavidBM
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p.1 #16 · p.1 #16 · What's the deal with aperture stopping down during AF?


AGeoJO wrote:
Adam, that explains it . But that should not apply to a 25mm lens like the Batis, right? The GM 85mm, a short tele with a large aperture, yes, I understand.


Plenty of normal and wide lenses have a degree of focus shift, Joshua!

The best way out would be a database in the lens or camera which tells the lens how much to adjust focus when the taking aperture is different from the focusing aperture. Could be difficult with third party AF lenses - I suppose they could be asked to provide the data, otherwise the lens defaults to stop down focus. Probably one stop down is the best to focus - there's a tradeoff between amount of light, DOF, and the improved contrast stopped down a little.



Mar 20, 2017 at 08:43 PM
AGeoJO
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p.1 #17 · p.1 #17 · What's the deal with aperture stopping down during AF?


DavidBM wrote:
Plenty of normal and wide lenses have a degree of focus shift, Joshua!

The best way out would be a database in the lens or camera which tells the lens how much to adjust focus when the taking aperture is different from the focusing aperture. Could be difficult with third party AF lenses - I suppose they could be asked to provide the data, otherwise the lens defaults to stop down focus. Probably one stop down is the best to focus - there's a tradeoff between amount of light, DOF, and the improved contrast stopped down a little.



David, I got your point but in general the depth-of-field would take care of the focus shift when the lens is stopped down sufficiently, even for a normal lens. I remembered in my DSLR days, dealing with the 50mm f/1.2 lens, the issues we had was more between f/2 and f/4. At about f/5.6 or so, the effect of the focus shift became negligible. And in my case, we are talking about a 25mm lens...



Mar 20, 2017 at 09:12 PM
DavidBM
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p.1 #18 · p.1 #18 · What's the deal with aperture stopping down during AF?


AGeoJO wrote:
David, I got your point but in general the depth-of-field would take care of the focus shift when the lens is stopped down sufficiently, even for a normal lens. I remembered in my DSLR days, dealing with the 50mm f/1.2 lens, the issues we had was more between f/2 and f/4. At about f/5.6 or so, the effect of the focus shift became negligible. And in my case, we are talking about a 25mm lens...


Joshua you are of course right about the DOF covering up most of it.
But if you were using the 2/25 at f2.8 or 4 at close distances you might lose enough accuracy to be just visible if it focused at f2 and if there were substantial focus shift.

But I don't know if there is with that lens, and I agree if there were it would be subtle to say the least!

Actually I must have a look and se what aperture it focuses at. If it keeps focussung at the taking aperture past f4 that's madness....

I've noticed of late that my 1.8/55 seems to focus at taking aperture to f2.8 and thereafter keep focusing at f2.8 - at 5.6, for example, you press the remote, the aperture goes wide open, then stops to f2.8 for a moment to focus, and then stops down to f5.6 for the shot.



Mar 20, 2017 at 09:28 PM
JohnJ
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p.1 #19 · p.1 #19 · What's the deal with aperture stopping down during AF?


snapsy wrote:
...DSLRs are presumed to account for focus shift by maintaining a database of lens models and their behaviors and adjusting the AF accordingly based on the shooting parameters. Sony could do the same. An alternate strategy could be to only stop down by one or two stops rather than stopping down fully since most focus shift is reasonably resolved in the first few stops.


I think the operative word here is 'presumed', unless you have some evidence to support your case which would be nice to see.

Most fast Planars, double gauss lenses, have focus shift (there are a few with correction for this, but not many). So this includes virtually every fast lens in the 50-85 range. Obviously focus shift is not limited to planars however.

I don't have an AF 1.2/50 or 1.2/85 to test this, maybe you do. If set to F2-4 then according to your theory a slight (and visible) adjustment to focus would occur automatically before every exposure, especially when focused at close range. I have no proof either way but I don't think anything like that happens with DSLR's and lenses prone to focus shift. It would be interesting to know for sure.



Mar 21, 2017 at 12:00 AM
snapsy
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p.1 #20 · p.1 #20 · What's the deal with aperture stopping down during AF?


JohnJ wrote:
I think the operative word here is 'presumed', unless you have some evidence to support your case which would be nice to see.

Fast Planars, double gauss lenses, have focus shift, full stop. So this includes virtually every fast lens in the 50-85 range. Obviously focus shift is not limited to planars however.

I don't have an AF 1.2/50 or 1.2/85 to test this, maybe you do. If set to F2-4 then according to your theory a slight (and visible) adjustment to focus would occur automatically before every exposure, especially when focused at close range. I have no proof either way but
...Show more

I saw some evidence of correction in the 5DM3's AF system but it's been a long time so I wouldn't quote me on it. Btw a DSLR user can automatically correct for focus shift himself via AFMA by manually focusing in LV at the stopped-down aperture and then performing a Dot Tune AFMA. The AFMA will be done with the lens wide-open (since DSLR phase-detect always evaluates the phase differential with the lens wide-open) but since the user focused stopped-down the AFMA value will incorporate the focus shift differential. Of course this is only useful when the photographer plans to shoot stopped down, although he can disable the lens AFMA if he wants to switch to shooting wide-open.



Mar 21, 2017 at 12:08 AM
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