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Autofocus is the ability of a camera/lens system to automatically adjust the focus of the lens via a motor attached to the focus group of the lens. Contrast with manual focus.

learn more… | top users | synonyms

46
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4answers
12k views

What is the best way to micro-adjust a lens?

I have a Canon 7D with a 50mm f/1.4 lens, and I think the auto-focus of the lens is off. How can I test and adjust this reliably? Will this approach work with all of my lenses? If I had a different ...
35
votes
3answers
9k views

What apertures are required to enable autofocus, including cross-type or high-precision focusing, on Canon DSLR cameras?

This is a follow-up to Why does Canon and Nikon limit or disable autofocus beyond certain f-numbers? I'm doing this to separate the AF aperture requirements from the linked question above. My goal ...
30
votes
5answers
3k views

Focus problem vs. motion blur vs. camera shake - how to tell the difference?

I'm not sure if the title of the question captures what I mean, but it's hard to be succint in the space allowed. Suppose you have a picture that is not as sharp as you think it should/could be. ...
28
votes
3answers
3k views

How does autofocus work?

How does auto focus work on modern cameras? How accurate is it?
27
votes
8answers
3k views

What is the advantage to back-button autofocus?

I've heard some advanced photographers talk about using back-button autofocus. As I understand it, this involves changing one of the camera's custom setting to engage the AF when a button on the rear ...
27
votes
4answers
35k views

What is back-focusing?

What is back-focusing? Is it something I need to be worried about, or can I just live with it? How can I tell if my camera/lens is sufferring from it?
26
votes
3answers
122k views

What does STM mean on a Canon lens?

The Canon EF 40mm f/2.8 has a designation of STM on the lens. What does this mean? What are the advantages of having it and does it replace an older technology? We have a terminology thread that ...
25
votes
3answers
959 views

What is the difference between a linear and a circular polarizer?

When I bought my polarizer, a friend told me that I should get a circular one, because the linear ones can mess with the autofocus. Is this true? What should each be used for?
24
votes
6answers
4k views

Can I damage the focus system by turning the focus ring when the lens isn't mounted?

I have a couple lenses that don't have an internal focusing system, and whenever I want to put them back in my bag, I turn the focus ring to make the lens shorter. I noticed that when the lens is set ...
21
votes
9answers
3k views

Why would I want to select an autofocus point?

I'm trying to understand why I would ever want to select an auto focus point other then the center point. I have a Canon XTI Rebel DSLR which has 9 auto-focus points, but I'm also looking into getting ...
19
votes
6answers
12k views

What is the practical difference between phase-detect and contrast-based autofocus?

What are the main differences in field use between phase detect AF and contrast based AF? I understand the technical differences, but I am not sure what the implications from a photographer ...
19
votes
2answers
1k views

Why am I allowed to turn my focusing ring beyond infinity?

Is there a good reason why I can turn the focusing ring on my lenses beyond infinity mark on focusing scale? Following recent questions here and here, it sounds like a real usability issue. ...
18
votes
4answers
790 views

How to speed up autofocus on moving subjects under low-light conditions?

Since I am usually taking pictures under low light ambient conditions I have discovered the nifty 1.4 lenses (such as the Sigma 30mm/1.4 or the Canon 50mm/1.4 or also the Canon 85mm/1.8) for my Canon ...
18
votes
3answers
2k views

What is the advantage of the huge number of AF points?

If the camera has only 1 central AF point, it is difficult to focus on an off-center subject ( without recomposing after autofocus ) so having 3 AF points seems really useful. Similarly I understand ...
17
votes
5answers
2k views

When would one need manual focus override?

In lens reviews, ability to override auto-focus manually is often referred to as a good (and important) feature. I have rarely, if ever used that feature, so I feel I might be missing out on an ...
17
votes
2answers
38k views

What's the difference between cross-type autofocus points and regular ones?

What's the difference between different autofocus point types (cross vs point and such)? Cross points seem to be a "feature" — why?
17
votes
4answers
8k views

How do camera body motors compare to in-lens motors for focusing?

What is the difference between using a motor on the camera body and the motor built into lenses? Aside from how both compare in terms of speed, what differences would I notice if I use a body with a ...
17
votes
3answers
912 views

Why don't cameras have autofocus points in the edges and corners?

On all the DSLRs I've used, there are never any autofocus points near the edges and corners. Why is that?
16
votes
5answers
1k views

How to improve focus accuracy?

I've noticed that quite a few of my photos aren't terribly well-focused. This particularly happens at long focal lengths. I'm using a Canon 500D and (mostly) a Tamron 18-270mm f/3.5-6.3 lens. I find ...
16
votes
3answers
1k views

How can I get better focus when a subject is moving towards or away from the camera?

When shooting events or action photos, engaging people, I often find that frontal (or rear) perspective may be very expressive, but my problem is focusing when the subject moves towards the camera (or ...
15
votes
4answers
1k views

Why can't my SLR autofocus on certain parts of a scene?

I had my Nikon D7000 on single-point auto focus and it just would not focus on the spots I wanted. I have marked the spots that could not be focused on with red circles, and the parts that could be ...
15
votes
2answers
818 views

Where can I find advice on using an advanced AF system for wildlife and bird photography?

I recently purchased a Canon 7D, as my previous 450D had become a severely limiting factor in my endeavors to wildlife and bird photography (particularly bird photography, which I LOVE, but for which ...
14
votes
2answers
3k views

When to use a particular auto-focus mode

I have a Nikon D90, and after taking a photography course that recommended single point auto-focus I predominately use that. I was testing some of the modes today (single point, dynamic area, ...
13
votes
7answers
945 views

How can I improve focus using both manual and auto on a candle flame?

I got my new tripod and 50mm F/1.4 sigma on my Nikon D7000 and was testing it out doing some shots of a flame on a candle. The below image is the sharpest i was able to produce. In review of my 50+ ...
13
votes
3answers
1k views

How does the lens influence the autofocus?

I think I've more or less understood how AF works now. However, I'm not sure how the lens influences the whole process. In my recent research for my next lens I found reviews stating the one lens' AF ...
12
votes
4answers
722 views

Why doesn't auto-focus work with an all-white subject (like a wall)?

I am trying to compose a shot using my Nikon D60 and 18-55mm lens towards a white wall in a auto-focus mode. It looks like I am not able to take the photo. I feel the camera is not able to find any ...
12
votes
4answers
1k views

How do you use the different autofocus points on your DSLR?

My Nikon D300 has quite a few autofocus points. There's a button I can operate with my thumb to change the autofocus point but I find that quite difficult to use. So I almost exclusively use the ...
12
votes
5answers
991 views

Why don't most cameras use infrared for focusing?

All auto focus assist lights are either red or white. Why do DSLRs not use infrared instead? It seems it would be beneficial to a photographer to not blind their subject before taking a picture. ...
12
votes
1answer
8k views

How does focus work on mobile phone cameras?

How does focusing work on mobile phone cameras? Aren't their optics simple enough to not have any moving parts whatsoever? If so, how come most smartphones have an auto-focus, and with various ...
11
votes
2answers
4k views

Why does Canon and Nikon limit or disable autofocus beyond certain f-numbers?

One thing that I've noticed with Canon DSLR camera is they will disable or limit autofocus when the lens is below certain aperture values, which are specified in my answer to What are the aperture ...
11
votes
6answers
6k views

How can I get everything in focus?

I have a Canon 550D with 70-200 f4 and 50mm prime lenses. Although the camera has built-in 9 focal points, I sometimes face a strange problem. Here's a sample photo: In reference to the above pic, ...
11
votes
4answers
548 views

Why aren't lenses designed with extra large apertures for autofocus beyond what's used for imaging?

Historically, designing large-aperture lenses has been difficult because correcting the optical aberrations that arise from large apertures requires complex designs and very large amounts of glass. ...
11
votes
3answers
3k views

Why do mirrorless cameras have quicker autofocus than SLRs using live view?

With early 2013 cameras, it's generally accepted that using contrast detection autofocus on SLRs (ie live view on most SLRs) is something which is really suitable only for static, or close to static, ...
11
votes
3answers
15k views

Will the Nikon D3100 autofocus with the 50mm f/1.8D lens?

I have the Nikon D3100 and was wondering if the autofocus on the Nikon 50mm f/1.8D lens would work with it. I've read that the AF is not compatible with the D5000, and that the AF is rather slow in ...
11
votes
1answer
3k views

What is a focus limiter?

What is a focus limiter and how does it work? What is the advantage of it? For instance Tamron 90/2.8 Macro has a focus limiter. According to the documentation: In the limit position, the Tamron ...
10
votes
4answers
472 views

Focusing on moving target with large aperature?

I have read about difficulties getting focusing right when using a large aperature (because of depth-of-field issues). If I am using a fast prime wide open (say F1.4 or F1.8) to shoot a target in ...
10
votes
3answers
1k views

What type of lens will auto-focus on Nikon bodies without a focus motor?

I know my D40 doesn't have a motor in it to auto-focus lenses. What type of lens has the motor in it? How can I know which lenses will and will not work?
10
votes
1answer
280 views

When focusing in Live View, where is the point of focus?

I use Canon SLRs. When using live view focusing, there is a rectangle which turns green when the focus is achieved. My question is: Is there a single focus point at the center of the rectangle? Or ...
10
votes
2answers
839 views

How can I determine if my Sigma 10-20mm lens copy is appropriately sharp and focusing right?

I just purchased a Sigma 10-20mm lens (Canon mount) and based on the reviews I was aware that many people receive copies of the lens that are soft or don't focus properly. I purchased from B&H, so ...
10
votes
2answers
2k views

Is it possible to measure distance to an object by adjusting the focus?

This is maybe not a 100% on topic photography, but I couldn't find a better stackexchange site to post this on and maybe you guys can help me out. I have a camera that takes pictures of a production ...
9
votes
2answers
698 views

What is manual-priority autofocus?

I have been looking at the Nikon AF-S 35mm f1.8 lens and it has two focus settings: Manual Focus Manual-Priority Autofocus What is "manual-priority autofocus"?
9
votes
2answers
967 views

Can lens focus speed affect continuous shooting speed?

I was using a 50mm f/1.8 II lens on my Canon 40D yesterday, and I was trying to capture some action shots using continuous high-speed drive mode and AI Servo autofocus mode. I was using a shutter ...
9
votes
8answers
3k views

Should auto-focus need adjusting?

I own a Canon XTI, and it always seems my shots are slightly out of focus. I usual have my camera set to the center focus point, and usually half press the shutter button to establish the focus on the ...
9
votes
3answers
743 views

How does color affect auto-focus?

I've noticed that the subject color seems to affect auto-focus, and am curious if I am just imagining this, or if there is a reason for it. For instance, I shoot a lot of figure skaters, and ...
9
votes
3answers
910 views

Is the color of the AF assist light important?

What are the differences between white and this darkroom red/orange active autofocus assistance lights?
8
votes
5answers
3k views

How to get focused pictures for action shots?

My daughter just started playing Tee-Ball and I was out there with my Digital Rebel XT and a Sigma 300mm. I know enough to be dangerous and I was shooting in "Auto mode" with the action preset (the ...
8
votes
5answers
421 views

Why is the main sensor not used instead of the separate AF sensor to focus a DSLR?

From this answer I understand that the reflex mirror of a DSLR is actually not reflecting all the light, but it passes a certain amount on to the AF sensor. So if the reflex mirror can pass light, ...
8
votes
4answers
4k views

What is the best auto-focus mode to use for bird photography?

Consider the image below: How would the shot be focused here? Using Auto-focus or manual? If there was a cluttered background, will auto focus detect the small bird and sharply focus on it?If manual ...
8
votes
3answers
790 views

When a camera auto-focuses, is the current focus distance reported from the lens to the camera?

The question says it all -- does the camera "know" how far the subject is from the lens, i.e. what is the current exact focus distance set at the lens?
8
votes
2answers
570 views

Why the AF confirm lights won't blink when the spot is in-focus while using an all-manual lens with an adapter without any connectors?

I understand that my Canon 450D's (XSi/Kiss X2) passive autofocus system knows a spot in the view is in-focus when its nearby contrast is adequate. Then the camera blinks AF confirm lights to inform ...