Shuitter Lag: Why worse in digital than 35mm? And pre-focus

[Erik Neu]

I can probably be fairly described as a moderate amateur (short of dSLR), and have an engineering background. I am somewhat baffled as to the underlying reason for shutter lag in digital cameras as compared to 35mm auto-focus point-and-shoot cameras. First, it does seem like shutter lag is worse with digital than the comparable film auto-focus, but why? In either case, the camera has the overhead of auto-focusing. If anything, I would have though that Moore's law would have made the auto-focus on today's $250 digital camera better than the equivalent-priced auto-focus of 10 years ago.

Second--pre-focusing by holding the shutter release button half-way down seems to 80% mitigate the shutter lag problem. In my limited experimentation, it's not quite instantaneous, but very close. So, for the life of me, I don't understand why this fact isn't more well-known to the average consumer. I find that, in most cases, pre-focusing is feasible for the shot I am taking (e.g., child is opening present, and I am trying to capture the expression that crosses their face when they realize what it is). I think if most amateurs knew about pre-focusing, it would greatly diminish their complaints about shutter lag.

(Tangent: I understand that shot-to-shot lag is an inherent problem for digital, and worse at the low end. But for most casual photographers, I think that is a problem they only notice because they have gone digital. What I mean is, in the film days, most of us were pasimonius with our photos. So, I would try to get one photo of a child opening a present, and just hope it was a good one. I wouldn't try to squeeze off four in a row. So even though, with film, I could have squeezed off those four quickly, I rarely would have taken advantage of that ability. Of course, burst mode, in the better point-and-shoots, greatly mitigates even this problem.)

[photosbyvito]

very good points......i have a G2...which i've been told has "terrible shutter lag" but....i can prefocus most of my shots...and there isn't a problem...it is just the learning of how to anticipate....and how to hold it half-way down..lol

[LewTwo]

I can not speak to film vs digital. All the film cameras I ever used were manually focused and could snap shots off as fast as you could crank 'em.

However with the number of digital cameras I have used I think it depends on the level of sophistcation of the camera's focusing system. Some of these systems will try one method and fail and go on to another and they have to make the decison to use focus assist (if available). More complicated sytems have to decide what to focus on. How quickly can it move whatever ellements of the lens it must move ... back and forth ... till it is happy. Then someone is going to compalin that half the time it gets it wrong, so lets build in a verification sequence. All these addition things we ask the camera to do for require some amount of effort. I believe they have actually gotten better and faster over time.

My SONY F707 was supposed to have one of the best autofocus systems available, but on long shots I always went to manual focus because it never got it right.

[NHL]

Originally Posted by Erik Neu

I can probably be fairly described as a moderate amateur (short of dSLR), and have an engineering background. I am somewhat baffled as to the underlying reason for shutter lag in digital cameras as compared to 35mm auto-focus point-and-shoot cameras. First, it does seem like shutter lag is worse with digital than the comparable film auto-focus, but why? In either case, the camera has the overhead of auto-focusing. If anything, I would have though that Moore's law would have made the auto-focus on today's $250 digital camera better than the equivalent-priced auto-focus of 10 years ago.

There's a simple explanation for this:
- In order to save cost, and not necessary passing on to the consumers, digital cameras tend not to have a separate autofocus system. They used their CCD to detect sharpness mostly by contrast detection, and as you are probably aware, this sensor is a serial device: Portion of the frames are shifted out for the camera innards to perform the AF algorithm prior to the actual shot... BTW this same sensor is used for white balance and any pre-flash info, so there's no actual photocell as well as on their film cousins.
- dSLRs on the other end (like their film counterparts) used a separate sensors, usually phase detectors embedded in the prism, partially because their CCD itself is blocked by the mirror prior to the shot...
- Some newer point and shoots digicam (like some Kodaks) do have separate infrared autofocus like the film cameras and theses tend to be faster in AF!

[JimC]

http://www.howstuffworks.com/autofocus.htm

[eric s]

Having worked in the consumer electronic industry for several years, NHL gives the same answer I would have. It's cost.

Camera manufacturers have proven, by the capabilities of the higher end cameras, that they can make faster focusing-quicker metering cameras. But I beleve they don't do this more often as a way to keep costs down and profit margins reasonable, and to differentiate models.

If customers demanded better (by only buying cameras with better AF and metering systems) then that would be heard and faster/better AF cameras would be made. It takes awhile, but the feedback loop is there.

Eric

[NHL]

Originally Posted by Erik Neu

Second--pre-focusing by holding the shutter release button half-way down seems to 80% mitigate the shutter lag problem. In my limited experimentation, it's not quite instantaneous, but very close. So, for the life of me, I don't understand why this fact isn't more well-known to the average consumer. I find that, in most cases, pre-focusing is feasible for the shot I am taking (e.g., child is opening present, and I am trying to capture the expression that crosses their face when they realize what it is). I think if most amateurs knew about pre-focusing, it would greatly diminish their complaints about shutter lag.

Also some cameras notably the Minolta's D7s and A's series, one can fine-tune the focus manually by turning the focus ring after the camera has locked AF by holding the shutter release partway. This operation (called Direct Manual Focus) is very similar to Ultrasonic lenses where ones can quickly overide the camera AF without having to switch the camera to manual focusing 1st... which is very convenient and quick as well!

[bradg]

Kinda of the subject here but, what kind of enginer are you? From reading your writing, it seems to be extremly well thought out, most of us dont even check for errors in our posts... if i had to guess, i would say an ee, but i am probably way off...
Just curious.

Thanks,
Brad

[eric s]

If you're asking me, I am a software designer. I have a BS in Computer Science and have been programming in some form since I was about... 14 or so. Currently I write parallel processing engines for Fortune & Global 500 companies. So I try to be careful and deliberate. I dislike stating things in absolutes and I choose my words carefully.

But I've also done firmware design for a product sold in Radio Shacks across the country (and other big electronic chains), including a project where the product downloaded a new version of its flash ROM and then upgraded its self. So I had to learn a lot about flash chips and how to program them. Useful when answering questions about CF cards.

And I got to work with people who did the hardware design, so I learned stuff about that too just by being around them.

But I do make mistakes, and I am very grateful when people correct me. NHL has done so on several occasions (either correcting or adding more info) which I find to be very helpful.

Eric

[RoyG]

This is all very interesting and I'm learning from these posts but I'd like to go ahead and ask a question that certainly others have already tackled. Which camera, or type of cameras provide the least amount of lag? I had a Sony Cyber-shot P92, that I had bought to replace my Sony Mavica (can you say 640x480). The time between shots was awful in the Cybershot but I miss the 10x zoom of the Mavica. Am I asking too much to have a quick turnaround time in a 5 megapixel camera? I'd like 10x zoom, at least 3.2 megapixel and be able to take about a pic every 2 seconds. My daughter helped me discover that the Cybershot doesn't bounce on concrete so I'm in the market for a new camera. Suggestions?