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04-22-2014, 07:11 PM   #1
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Do most camera's af systems struggle with fast lenses?

I used to shoot with 2 different Sony DSLR's, I currently use a Pentax K30, and over the weekend I got to rent a D7100 with 105mm f2.8 macro lens for free for a few hours at this tulip festival nearby (was a promo a camera store was doing). In the past both of my Sony's would sometimes miss the exact area of focus at f1.8 (Sony a330 and a55), sometimes my K30 will too at f1.8, but surprising the well regarded D7100 missed quite a bit of close up portraits of my wife at only f2.8 (not all shots, but quite a bit) and they were out of focus despite that I usually aim the af point as precisely at a person's eye as I can. I wasn't too wowed by the results when I opened them up on my computer either, so although it was a nice camera it made me realize that my much lighter K30 is all I need. I have been shooting for years and would like to think I use proper technique, so here's the question I have: Is Nikon's af system hyped up to be more than it really is? Are a few mis-focused shots to be expected with any camera's autofocus system since the depth of field is very narrow? Thoughts?

04-22-2014, 07:17 PM   #2
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QuoteOriginally posted by stillshot2 Quote
I used to shoot with 2 different Sony DSLR's, I currently use a Pentax K30, and over the weekend I got to rent a D7100 with 105mm f2.8 macro lens for free for a few hours at this tulip festival nearby (was a promo a camera store was doing). In the past both of my Sony's would sometimes miss the exact area of focus at f1.8 (Sony a330 and a55), sometimes my K30 will too at f1.8, but surprising the well regarded D7100 missed quite a bit of close up portraits of my wife at only f2.8 (not all shots, but quite a bit) and they were out of focus despite that I usually aim the af point as precisely at a person's eye as I can. I wasn't too wowed by the results when I opened them up on my computer either, so although it was a nice camera it made me realize that my much lighter K30 is all I need. I have been shooting for years and would like to think I use proper technique, so here's the question I have: Is Nikon's af system hyped up to be more than it really is? Are a few mis-focused shots to be expected with any camera's autofocus system since the depth of field is very narrow? Thoughts?
Narrow DOF is tough to work with. You have to make sure the lens is properly calibrated as any FF/BF count be very destructive to your photos, and the same goes for accidentally focusing on the wrong spot.

Generally speaking, faster lenses allow AF systems to focus quicker and more accurately thanks to their greater light transmission, but some lenses suffer from low contrast wide-open, which can backfire with certain subjects.

If you want to be 100% sure the focus is spot on, I recommend blowing up the image in live view. Nikon makes it very easy as the AF mechanism can be operated while zoomed in, and then you can manually make fine adjustments before releasing the shutter.

Adam
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04-22-2014, 07:18 PM   #3
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That's a fair question, but your story also invites a bit of scrutiny about your technique. . .was the Nikon on a tripod, what were the shutter speeds used, how was the lighting. . .you know the usual 20 questions. Maybe you could post the best and the most problematic shots. It would also show us perspective. Depending on the face, the distance between the eye and the tip of the nose may be enough for DOF issues. A macro lens especially.

M
04-22-2014, 07:47 PM   #4
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I've found that macro lenses can be a bit more temperamental and difficult to focus at normal focal distances than non macro lenses. It's the price to pay for all that focal range.

04-22-2014, 07:52 PM   #5
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I would think things would improve with experience with that camera. I know I would not be as good with strange gear as with something I have shot with a while.

And without personally dialling in the focus I would not trust any camera.
04-22-2014, 08:06 PM   #6
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Do most camera's af systems struggle with fast lenses?


I found that there are quite a few misses with the K5 and, for example, my Zeiss 85/1.4 ZK.
On the other hand, the Fuji 56/1.2 (on the Fuji X-E2) rarely misses focus, if ever, wide open.

Adam brings a good point: "If you want to be 100% sure the focus is spot on, I recommend blowing up the image in live view. Nikon makes it very easy as the AF mechanism can be operated while zoomed in, and then you can manually make fine adjustments before releasing the shutter."

Different cameras with different lenses will behave differently from what little experience I have with this, but LV seems to be the token theses days.

I know that my Cosina 55/1.2 is a real PITA to focus wide open, and that LV resolves the problem.

Miguel also discusses technique, which could be another "fact of life".

JP
04-22-2014, 08:51 PM   #7
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The camera was handheld, VR on, it was a bright sunny day, and my speeds were around 1/2000 and above. When taking pictures I stabilize cameras with my arms up against my chest and the camera held up to my face, use smooth shutter pressing movements, and control breathing as if shooting a scoped rifle. I didn't personally dial in the focus, but some photos were super sharp and others not, kind of hit and miss at farther distances. This makes me wonder why it would say it's locked on but miss when I calculated that I had about 1 foot of depth of field at the distances I was shooting and there was nothing near the af point to throw it off? It makes sense that Macro lenses might be more temperamental when used for portraits but if any camera says it's in focus, shouldn't it be? Is there a margin of error with any system? I swear sometimes I am using my K30 and the green focus confirm light will come on and I can even tell it's not in focus and then push the af button again and watch it improve and then capture the picture, review it, and it's good. So I had to force it to refocus and get it together. I would post examples of poor Nikon shots but I already deleted all of the bad ones.

Here's a link to the good pics but I'm not sure it will help too much. There are close up flower shots posted too.
https://www.flickr.com/photos/adudenamedjosh/13940623614/

Last edited by stillshot2; 04-22-2014 at 09:07 PM.
04-22-2014, 09:02 PM   #8
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QuoteOriginally posted by stillshot2 Quote
The camera was handheld, VR on, it was a bright sunny day, and my speeds were around 1/2000 and above. When taking pictures I stabilize cameras with my arms up against my chest and the camera held up to my face, use smooth shutter pressing movements, and control breathing as if shooting a scoped rifle. I didn't personally dial in the focus, but some photos were super sharp and others not, kind of hit and miss at farther distances. This makes me wonder why it would say it's locked on but miss when I calculated that I had about 1 foot of depth of field at the distances I was shooting and there was nothing near the af point to throw it off? It makes sense that Macro lenses might be more temperamental when used for portraits but if the camera says it's in focus, shouldn't it be? I would post pics but I already deleted all the bad ones.
Well, post a few good ones.

M

04-22-2014, 09:08 PM   #9
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Just posted a link, edited in my post above
04-22-2014, 09:24 PM   #10
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The short answer is yes.

The long answer is complicated and has to do with how phase-detect AF (PDAF) systems work. Most of the AF sensor sites for most dSLR cameras have a working aperture of f/5.6. I say most because some cameras (the K-3 being the sole Pentax example) have at least some sensors optimized for f/2.8. The result is relatively poor precision when used with faster lenses at wider apertures.

Contrast-detect (CDAF) systems such as used in live view do not have this limitation and generally provide both greater precision and greater accuracy.


Steve

---------- Post added 04-22-14 at 09:28 PM ----------

QuoteOriginally posted by stillshot2 Quote
Just posted a link, edited in my post above
I was tempted to put the blame on the Nikon D7100 camera, but resisted

For what its worth, I was out Sunday morning shooting tulips and switched to manual focus because the AF system simply could not deal with the curved surfaces. It could not read my mind as to where I wanted to place focus.


Steve
04-23-2014, 12:29 AM   #11
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Well I sometimes use my FA*85mm on f1.4 and that isn't easy all the time. So missing shots is almost normal. Working with the K-01 does give a better result on average.
04-23-2014, 05:00 AM   #12
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How far off were the ones that were off? At those shutter speeds, something should be in sharp focus, nose, ears, back of the head. If it's close, it could be you or your wife moving even a few cm's to throw off critical focus, or it could be the focusing points aren't quite as small as you'd hope and it's not actually grabbing the eye.

DoF is a gradual thing, and is just the region of 'acceptable focus' at a given print size, viewing distance, etc.. At 2.8 it can be noticeable if the eyes are still in the calculated DoF but the nose is the thing in the plane of focus. Especially if you're magnifying a high megapixel image to 1:1.

Last edited by BrianR; 04-23-2014 at 05:24 AM.
04-23-2014, 05:15 AM   #13
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One additional thought: many macro lenses have a focus limiter. When I rent or just start to get to know a new lens--especially with a whole new platform--I sometimes overlook controls like limiters. Did you have the limiter disengaged if there was one?

M
04-23-2014, 07:13 AM   #14
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I did not have the focus limiter engaged. I wish they had a K3 to demo to see how it performs with it's f2.8 optomized sensors. Any K3 owners out there that notice a big difference over other Pentax models? I just pixel peeped the original size photos that were acceptable including some not on my flickr account and noticed in a lot of them my wife's earrings were consistently in better focus than her eyes, so the lens may have been slightly back-focusing a bit. The best way to describe the ones that were bad were my wife looking over my shoulder saying "delete, delete, delete." So they were pretty off with nothing near her to compare where the focus actually landed. Being a past Sony user, I find my K30's live view rather clunky and slow in comparison to how Sony implements it but would prefer not to use live view in general unless there a way to zoom in focus check using live view during manual focus on a K30? I could get it to focus check when using af but not when using manual focus. Is there a way?

Last edited by stillshot2; 04-23-2014 at 08:15 AM.
04-23-2014, 08:03 AM   #15
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QuoteOriginally posted by stevebrot Quote
Most of the AF sensor sites for most dSLR cameras have a working aperture of f/5.6.
Learn something every day. I have always been mystified by autofocus query/complaint threads since I rarely seem to have problems. Now I suppose I know why.


  • I default to f/5.6 (a habit that goes back to High School instruction)
  • Since I am older and lack the fine muscle control I had in my youth I use a tripod much more than in the past, especially for wide apertures
  • I use manual focus lenses much more than autofocus and have learned to ignore the AF indicator and focus in the VF or with LiveView.
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